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L_0553 | human sperm | T_3034 | A sperm has three main parts: 1. The head of the sperm contains the nucleus. The nucleus holds the DNA of the cell. The head also contains enzymes that help the sperm break through the cell membrane of an egg. 2. The midpiece of the sperm is packed with mitochondria. Mitochondria are organelles in cells that produce energy. Sperm use the energy in the midpiece to move. 3. The tail of the sperm moves like a propeller, around and around. This tail is a long flagella that pushes the sperm forward. A sperm can travel about 30 inches per hour. This may not sound very fast, but dont forget how small a sperm is. For its size, a sperm moves about as fast as you do when you walk briskly. This drawing of a sperm shows its main parts. What is the role of each part? How do you think the shape of the sperm might help it swim? | text | null |
L_0553 | human sperm | T_3035 | To make sperm, cells start in the testes and end in the epididymis. It takes up to two months to make sperm. The steps are explained below: 1. Special cells in the testes go through mitosis (cell division) to make identical copies of themselves. 2. The copies of the original cells divide by meiosis, producing cells called spermatids. The spermatids have half the number of chromosomes as the original cell. The spermatids are immature and cannot move on their own. 3. The spermatids are transported from the testes to the epididymis. Involuntary muscular contraction moves the spermatids along. 4. In the epididymis, spermatids slowly grow older and mature. They grow a tail. They also lose some of the cytoplasm from the head. It is here that the spermatids mature, becoming sperm cells. 5. When sperm are mature, they can swim. The mature sperm are stored in the epididymis until it is time for them to leave the body. Sperm leave the epididymis through the vas deferens. As they travel through the vas deferens, they pass by the prostate and other glands. The sperm mix with liquids from these glands, forming semen. The semen travels through the urethra and leaves the body through the penis. A teaspoon of semen may contain as many as 500 million sperm! | text | null |
L_0557 | immunity | T_3047 | In previous concepts, you learned about B and T cells, special types of white blood cells that help your body to fight off a specific pathogen. They are necessary when the body is fighting off an infection. But what happens to them after the pathogen has been destroyed? Most B and T cells die after an infection has been brought under control. But some of them survive for many years. They may even survive for a persons lifetime. These long-lasting B and T cells are called memory cells. They allow the immune system to remember the pathogen after the infection is over. If the pathogen invades the body again, the memory cells will start dividing in order to fight the pathogen or disease. These dividing cells will quickly produce a new army of B or T cells to fight the pathogen. They will begin a faster, stronger attack than the first time the pathogen invaded the body. As a result, the immune system will be able to destroy the pathogen before it can cause an infection. Being able to attack the pathogen in this way is called immunity. Immunity can also be caused by vaccination. Vaccination is the process of exposing a person to a pathogen on purpose in order to develop immunity. In vaccination, a modified pathogen is usually injected under the skin by a shot. Only part of the pathogen is injected, or a weak or dead pathogen is used. It sounds dangerous, but the shot prepares your body for fighting the pathogen without causing the actual illness. Vaccination triggers an immune response against the injected antigen. The body prepares "memory" cells for use at a later time, in case the antigen is ever encountered again. Essentially, a vaccine imitates an infection, triggering an immune response, without making a person sick. In many countries, children receive their first vaccination at birth with the Hepatitis B shot, which protects infants from Hepatitis B, a serious liver disease. Before vaccines, many children died from diseases that vaccines now prevent, such as whooping cough, measles, and polio. Those same germs exist today, but because babies are now protected by vaccines, we do not see these diseases nearly as often. Diseases you have probably been vaccinated against include measles, mumps, and chicken pox. How does a vaccine work? See How a Vaccine Works at and The History of Vaccines at . Click image to the left or use the URL below. URL: | text | null |
L_0570 | inflammatory response | T_3090 | The little girl pictured below ( Figure 1.1) has a scraped knee. A scrape is a break in the skin that may let pathogens enter the body. If bacteria enter through the scrape, they could cause an infection. These bacteria would then face the bodys second line of defense. The second line of defense is also nonspecific, fighting many types of pathogens. | text | null |
L_0570 | inflammatory response | T_3091 | The bodys second line of defense against pathogens includes the inflammatory response. If bacteria enter the skin through a scrape, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries. Inflammation is caused by chemicals that are released when skin or other tissues are damaged. The chemicals cause nearby blood vessels to dilate, or expand. This increases blood flow to the damaged area, which makes the area red and slightly warm. The chemicals also attract white blood cells called neutrophils to the wound and cause them to leak out of blood vessels into the damaged tissue. This little girl just got her first scraped knee. It doesnt seem to hurt, but the break in her skin could let pathogens enter her body. Thats why scrapes should be kept clean and protected until they heal. | text | null |
L_0570 | inflammatory response | T_3092 | What do these white blood cells do at the site of inflammation? The main role of white blood cells is to fight pathogens in the body. There are actually several different kinds of white blood cells. Some white blood cells have very specific functions. They attack only certain pathogens. Other white blood cells attack any pathogen they find. These white blood cells travel to areas of the body that are inflamed. They are called phagocytes, which means eating cells. Neutrophils are a type of phagocyte. In addition to pathogens, phagocytes eat dead cells. They surround the pathogens and destroy them. Sometimes it is said that the phagocyte engulfs the pathogen, and then destroys it. This process is called phagocytosis. White blood cells also make chemicals that cause a fever. A fever is a higher-than-normal body temperature. Normal human body temperature is 98.6 F (37 C). Most bacteria and viruses that infect people reproduce fastest at this temperature. When the temperature is higher, the pathogens cannot reproduce as fast, so the body raises the temperature to kill them. A fever also causes the immune system to make more white blood cells. In these ways, a fever helps the body fight infection. | text | null |
L_0589 | lymphatic system | T_3152 | If pathogens get through the bodys first two lines of defense, a third line of defense takes over. This third line of defense involves the immune system. It is called an immune response, and is a specific type of response. The immune system has a special response for each type of pathogen. The immune system ( Figure 1.1) is also part of the lymphatic systemnamed for lymphocytes, which are the type of white blood cells involved in an immune response. They include several lymph organs, lymph vessels, lymph, and lymph nodes. This diagram shows the parts of the im- mune system. The immune system in- cludes several organs and a system of vessels that carry lymph. Lymph nodes are located along the lymph vessels. | text | null |
L_0589 | lymphatic system | T_3153 | The lymph organs are the red bone marrow, tonsils, spleen, and thymus gland. They are described below ( Figure Each lymph organ has a different job in the immune system. | text | null |
L_0589 | lymphatic system | T_3154 | Lymph vessels make up a circulatory system that is similar to the cardiovascular system, which you can read about in a previous concept. Lymph vessels are like blood vessels, except they move lymph instead of blood. Lymph is a yellowish liquid that leaks out of tiny blood vessels into spaces between cells in tissues. Where there is more inflammation, there is usually more lymph in tissues. This lymph may contain many pathogens. The lymph that collects in tissues slowly passes into tiny lymph vessels. It then travels from smaller to larger lymph vessels. Lymph is not pumped through lymph vessels like blood is pumped through blood vessels by the heart. Instead, muscles around the lymph vessels contract and squeeze the lymph through the vessels. The lymph vessels also contract to help move the lymph along. The lymph finally reaches the main lymph vessels in the chest. Here, the lymph drains into two large veins. This is how the lymph returns to the bloodstream. Before lymph reaches the bloodstream, pathogens are removed from it at lymph nodes. Lymph nodes are small, oval structures located along the lymph vessels. They act like filters. Any pathogens filtered out of the lymph at lymph nodes are destroyed by lymphocytes in the nodes. | text | null |
L_0589 | lymphatic system | T_3155 | Lymphocytes ( Figure 1.3), a type of white blood cell, are the key cells of an immune response. There are trillions of lymphocytes in the human body. They make up about one quarter of all white blood cells. Usually, fewer than half of the bodys lymphocytes are in the blood. The rest are in the lymph, lymph nodes, and lymph organs. There are two main types of lymphocytes: 1. B cells. This image of a lymphocyte was made with an electron microscope. The lym- phocyte is shown 10,000 times its actual size. 2. T cells. Both types of lymphocytes are produced in the red bone marrow. They are named for the sites where they grow larger. The "B" in B cells stands for bone. B cells grow larger in red bone marrow. The "T" in T cells stands for thymus. T cells mature in the thymus gland. B and T cells must be switched on in order to fight a specific pathogen. Once this happens, they produce an army of cells ready to fight that particular pathogen. How can B and T cells recognize specific pathogens? Pathogens have proteins, often located on their cell surface. These proteins are called antigens. An antigen is any protein that causes an immune response, because it is unlike any protein that the body makes. Antigens are found on bacteria, viruses, and other pathogens. Your body sees these as foreign, meaning they do not belong in your body. | text | null |
L_0596 | meiosis | T_3162 | Sexual reproduction combines gametes from two parents. Gametes are reproductive cells, such as sperm and egg. As gametes are produced, the number of chromosomes must be reduced by half. Why? The zygote must contain genetic information from the mother and from the father, so the gametes must contain half of the chromosomes found in normal body cells. When two gametes come together at fertilization, the normal amount of chromosomes results. Gametes are produced by a special type of cell division known as meiosis. Meiosis contains two rounds of cell division without DNA replication in between. This process reduces the number of chromosomes by half. Human cells have 23 pairs of chromosomes, and each chromosome within a pair is called a homologous chromo- some. For each of the 23 chromosome pairs, you received one chromosome from your father and one chromosome from your mother. Alleles are alternate forms of genes found on chromosomes. Homologous chromosomes have the same genes, though they may have different alleles. So, though homologous chromosomes are very similar, they are not identical. The homologous chromosomes are separated when gametes are formed. Therefore, gametes have only 23 chromosomes, not 23 pairs. | text | null |
L_0596 | meiosis | T_3163 | A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. Most of the cells in a human body are diploid. A cell with one set of chromosomes, such as a gamete, is haploid, referred to as n. Sex cells are haploid. When a haploid sperm (n) and a haploid egg (n) combine, a diploid zygote will be formed (2n). In short, when a diploid zygote is formed, half of the DNA comes from each parent. | text | null |
L_0596 | meiosis | T_3164 | Before meiosis begins, DNA replication occurs, so each chromosome contains two sister chromatids that are identical to the original chromosome. Meiosis ( Figure 1.1) is divided into two divisions: Meiosis I and Meiosis II. Each division can be divided into the same phases: prophase, metaphase, anaphase, and telophase. Cytokinesis follows telophase each time. Between the two cell divisions, DNA replication does not occur. Through this process, one diploid cell will divide into four haploid cells. Overview of Meiosis. During meiosis, four haploid cells are created from one diploid parent cell. | text | null |
L_0596 | meiosis | T_3165 | During meiosis I, the pairs of homologous chromosomes are separated from each other. This requires that they line up in their homologous paris during metaphase I. The steps are outlined below: 1. Prophase I: The homologous chromosomes line up together. During this time, a process that only happens in meiosis can occur. This process is called crossing-over ( Figure 1.2), which is the exchange of DNA between homologous chromosomes. Crossing-over forms new combinations of alleles on the resulting chromosome. Without crossing-over, the offspring would always inherit all of the alleles on one of the homologous chromo- somes. Also during prophase I, the spindle forms, the chromosomes condense as they coil up tightly, and the nuclear envelope disappears. 2. Metaphase I: The homologous chromosomes line up in their pairs in the middle of the cell. Chromosomes from the mother or from the father can each attach to either side of the spindle. Their attachment is random, so all of the chromosomes from the mother or father do not end up in the same gamete. The gamete will contain some chromosomes from the mother and some chromosomes from the father. 3. Anaphase I: The homologous chromosomes are separated as the spindle shortens, and begin to move to opposite sides (opposite poles) of the cell. 4. Telophase I: The spindle fibers dissolves, but a new nuclear envelope does not need to form. This is because, after cytokinesis, the nucleus will immediately begin to divide again. No DNA replication occurs between meiosis I and meiosis II because the chromosomes are already duplicated. After cytokinesis, two haploid cells result, each with chromosomes made of sister chromatids. Since the separation of chromosomes into gametes is random during meiosis I, this process results in different combinations of chromosomes (and alleles) in each gamete. With 23 pairs of chromosomes, there is a possibility of over 8 million different combinations of chromosomes (223 ) in a human gamete. During crossing-over, segments of DNA are exchanged between non-sister chro- matids of homologous chromosomes. Notice how this can result in an allele (A) on one chromatid being moved onto the other non-sister chromatid. | text | null |
L_0596 | meiosis | T_3166 | During meiosis II, the sister chromatids are separated and the gametes are generated. This cell division is similar to that of mitosis, but results in four genetically unique haploid cells. The steps are outlined below: 1. Prophase II: The chromosomes condense. 2. Metaphase II: The chromosomes line up one on top of each other along the middle of the cell, similar to how they line up in mitosis. The spindle is attached to the centromere of each chromosome. 3. Anaphase II: The sister chromatids separate as the spindle shortens and move to opposite ends of the cell. 4. Telophase II: A nuclear envelope forms around the chromosomes in all four cells. This is followed by cytokinesis. After cytokinesis, each cell has divided again. Therefore, meiosis results in four haploid genetically unique daughter cells, each with half the DNA of the parent cell ( Figure 1.3). In human cells, the parent cell has 46 chromosomes (23 pairs), so the cells produced by meiosis have 23 chromosomes. These cells will become gametes. | text | null |
L_0602 | mitosis and cytokinesis | T_3180 | The genetic information of the cell, or DNA, is stored in the nucleus. During mitosis, two nuclei (plural for nucleus) must form, so that one nucleus can be in each of the new cells after the cell divides. In order to create two genetically identical nuclei, DNA inside of the nucleus must be copied or replicated. This occurs during the S phase of the cell cycle. During mitosis, the copied DNA is divided into two complete sets, so that after cytokinesis, each cell has a complete set of genetic instructions. | text | null |
L_0602 | mitosis and cytokinesis | T_3181 | To begin mitosis, the DNA in the nucleus wraps around proteins to form chromosomes. Each organism has a unique number of chromosomes. In human cells, our DNA is divided up into 23 pairs of chromosomes. Replicated DNA forms a chromosome made from two identical sister chromatids, forming an "X" shaped molecule ( Figure 1.1). The two chromatids are held together on the chromosome by the centromere. The centromere is also where spindle fiber microtubules attach during mitosis. The spindles separate sister chromatids from each other. | text | null |
L_0602 | mitosis and cytokinesis | T_3182 | During mitosis, the two sister chromatids must be divided. This is a precise process that has four individual phases to it. After the sister chromatids separate, each separate chromatid is now known as a chromosome. Each resulting chromosome is made of DNA from just one chromatid. So, each chromosome after this separation is made of "1/2 of the X." Through this process, each daughter cell receives one copy of each chromosome. The four phases of mitosis are prophase, metaphase, anaphase and telophase ( Figure 1.2). 1. Prophase: The chromatin, which is unwound DNA, condenses forming chromosomes. The DNA becomes so tightly wound that you can see them under a microscope. The membrane around the nucleus, called the nuclear envelope, disappears. Spindles also form and attach to chromosomes to help them move. 2. Metaphase: The chromosomes line up in the center, or the equator, of the cell. The chromosomes line up in a row, one on top of the next. 3. Anaphase: The two sister chromatids of each chromosome separate as the spindles pull the chromatids apart, resulting in two sets of identical chromosomes. 4. Telophase: The spindle dissolves and nuclear envelopes form around the chromosomes in both cells. An overview of the cell cycle and mito- sis: during prophase the chromosomes condense, during metaphase the chromo- somes line up, during anaphase the sister chromatids are pulled to opposite sides of the cell, and during telophase the nuclear envelope forms. This is a representation of dividing plant cells. Cell division in plant cells differs slightly from animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different stages of mitosis? | text | null |
L_0602 | mitosis and cytokinesis | T_3182 | During mitosis, the two sister chromatids must be divided. This is a precise process that has four individual phases to it. After the sister chromatids separate, each separate chromatid is now known as a chromosome. Each resulting chromosome is made of DNA from just one chromatid. So, each chromosome after this separation is made of "1/2 of the X." Through this process, each daughter cell receives one copy of each chromosome. The four phases of mitosis are prophase, metaphase, anaphase and telophase ( Figure 1.2). 1. Prophase: The chromatin, which is unwound DNA, condenses forming chromosomes. The DNA becomes so tightly wound that you can see them under a microscope. The membrane around the nucleus, called the nuclear envelope, disappears. Spindles also form and attach to chromosomes to help them move. 2. Metaphase: The chromosomes line up in the center, or the equator, of the cell. The chromosomes line up in a row, one on top of the next. 3. Anaphase: The two sister chromatids of each chromosome separate as the spindles pull the chromatids apart, resulting in two sets of identical chromosomes. 4. Telophase: The spindle dissolves and nuclear envelopes form around the chromosomes in both cells. An overview of the cell cycle and mito- sis: during prophase the chromosomes condense, during metaphase the chromo- somes line up, during anaphase the sister chromatids are pulled to opposite sides of the cell, and during telophase the nuclear envelope forms. This is a representation of dividing plant cells. Cell division in plant cells differs slightly from animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different stages of mitosis? | text | null |
L_0603 | mitosis vs. meiosis | T_3183 | Mitosis, meiosis, and sexual reproduction are discussed at . Click image to the left or use the URL below. URL: Both mitosis and meiosis result in eukaryotic cells dividing. So what is the difference between mitosis and meiosis? The primary difference is the differing goals of each process. The goal of mitosis is to produce two daughter cells that are genetically identical to the parent cell, meaning the new cells have exactly the same DNA as the parent cell. Mitosis happens when you want to grow, for example. You want all your new cells to have the same DNA as the previous cells. The goal of meiosis, however, is to produce sperm or eggs, also known as gametes. The resulting gametes are not genetically identical to the parent cell. Gametes are haploid cells, with only half the DNA present in the diploid parent cell. This is necessary so that when a sperm and an egg combine at fertilization, the resulting zygote has the correct amount of DNAnot twice as much as the parents. The zygote then begins to divide through mitosis. Pictured below is a comparison between binary fission (Figure 1.1), which is cell division of prokaryotic organisms, mitosis, and meiosis. Mitosis and meiosis are also compared in the table that follows (Table 1.1). A comparison between binary fission, mi- tosis, and meiosis. Purpose Number of Cells Produced Rounds of Cell Division Haploid or Diploid Daughter cells identical to parent cells? Daughter cells identical to each other? Mitosis To produce new cells 2 1 Diploid Yes Meiosis To produce gametes 4 2 Haploid No Yes No | text | null |
L_0612 | nerve cells and nerve impulses | T_3206 | The nervous system is made up of nerves. A nerve is a bundle of nerve cells. A nerve cell that carries messages is called a neuron ( Figure 1.1). The messages carried by neurons are called nerve impulses. Nerve impulses can travel very quickly because they are electrical impulses. Think about flipping on a light switch when you enter a room. When you flip the switch, the electricity flows to the light through wires inside the walls. The electricity may have to travel many meters to reach the light, but the light still comes on as soon as you flip the switch. Nerve impulses travel just as fast through the network of nerves inside the body. The axons of many neurons, like the one shown here, are covered with a fatty layer called myelin sheath. The sheath covers the axon, like the plastic covering on an electrical wire, and allows nerve impulses to travel faster along the axon. The node of Ranvier, shown in this diagram, is any gap in the myelin sheath; it allows faster transmission of a signal. | text | null |
L_0612 | nerve cells and nerve impulses | T_3207 | A neuron has a special shape that lets it pass signals from one cell to another. A neuron has three main parts ( Figure 1. The cell body. 2. Many dendrites. 3. One axon. The cell body contains the nucleus and other organelles. Dendrites and axons connect to the cell body, similar to rays coming off of the sun. Dendrites receive nerve impulses from other cells. Axons pass the nerve impulses on to other cells. A single neuron may have thousands of dendrites, so it can communicate with thousands of other cells but only one axon. The axon is covered with a myelin sheath, a fatty layer that insulates the axon and allows the electrical signal to travel much more quickly. The node of Ranvier is any gap within the myelin sheath exposing the axon, and it allows even faster transmission of a signal. | text | null |
L_0612 | nerve cells and nerve impulses | T_3208 | Neurons are usually classified based on the role they play in the body. Two main types of neurons are sensory neurons and motor neurons. Sensory neurons carry nerve impulses from sense organs and internal organs to the central nervous system. Motor neurons carry nerve impulses from the central nervous system to organs, glands, and musclesthe opposite direction. Both types of neurons work together. Sensory neurons carry information about the environment found inside or outside of the body to the central nervous system. The central nervous system uses the information to send messages through motor neurons to tell the body how to respond to the information. | text | null |
L_0612 | nerve cells and nerve impulses | T_3209 | The place where the axon of one neuron meets the dendrite of another is called a synapse. Synapses are also found between neurons and other types of cells, such as muscle cells. The axon of the sending neuron does not actually touch the dendrite of the receiving neuron. There is a tiny gap between them, the synaptic cleft ( Figure 1.2). The following steps describe what happens when a nerve impulse reaches the end of an axon. 1. 2. 3. 4. When a nerve impulse reaches the end of an axon, the axon releases chemicals called neurotransmitters. Neurotransmitters travel across the synapse between the axon and the dendrite of the next neuron. Neurotransmitters bind to the membrane of the dendrite. The binding allows the nerve impulse to travel through the receiving neuron. Did you ever watch a relay race? After the first runner races, he or she passes the baton to the next runner, who takes over. Neurons are a little like relay runners. Instead of a baton, they pass neurotransmitters to the next neuron. Examples of neurotransmitters are chemicals such as serotonin, dopamine, and adrenaline. You can watch an animation of nerve impulses and neurotransmitters at Some people have low levels of the neurotransmitter called serotonin in their brain. Scientists think that this is one cause of depression. Medications called antidepressants help bring serotonin levels back to normal. For many people with depression, antidepressants control the symptoms of their depression and help them lead happy, productive lives. | text | null |
L_0615 | non mendelian inheritance | T_3214 | In all of Mendels experiments, he worked with traits where a single gene controlled the trait. Each also had one allele that was always dominant over the recessive allele. But this is not always true. There are exceptions to Mendels rules, and these exceptions usually have something to do with the dominant allele. If you cross a homozygous red flower with a homozygous white flower, according to Mendels laws, what color flower should result from the cross? Either a completely red or completely white flower, depending on which allele is dominant. But since Mendels time, scientists have discovered this is not always the case. | text | null |
L_0615 | non mendelian inheritance | T_3215 | One allele is NOT always completely dominant over another allele. Sometimes an individual has a phenotype between the two parents because one allele is not dominant over another. This pattern of inheritance is called incomplete dominance. For example, snapdragon flowers show incomplete dominance. One of the genes for flower color in snapdragons has two alleles, one for red flowers and one for white flowers. A plant that is homozygous for the red allele (RR) will have red flowers, while a plant that is homozygous for the white allele will have white flowers (WW). But the heterozygote will have pink flowers (RW) ( Figure 1.1) as both alleles are expressed. Neither the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents. Pink snapdragons are an example of in- complete dominance. Another example of incomplete dominance is with sickle cell anemia, a disease in which a blood protein called hemoglobin is produced incorrectly. This causes the red blood cells to have a sickle shape, making it difficult for these misshapen cells to pass through the smallest blood vessels. A person that is homozygous recessive (ss) for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is homozygous dominant (SS) will have normal red blood cells. What type of blood cells do you think a person who is heterozygous (Ss) for the trait will have? They will have some misshapen cells and some normal cells ( Figure 1.2). Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a combination of the recessive and dominant traits. Sickle cell anemia causes red blood cells to become misshapen and curved unlike normal, rounded red blood cells. | text | null |
L_0615 | non mendelian inheritance | T_3215 | One allele is NOT always completely dominant over another allele. Sometimes an individual has a phenotype between the two parents because one allele is not dominant over another. This pattern of inheritance is called incomplete dominance. For example, snapdragon flowers show incomplete dominance. One of the genes for flower color in snapdragons has two alleles, one for red flowers and one for white flowers. A plant that is homozygous for the red allele (RR) will have red flowers, while a plant that is homozygous for the white allele will have white flowers (WW). But the heterozygote will have pink flowers (RW) ( Figure 1.1) as both alleles are expressed. Neither the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents. Pink snapdragons are an example of in- complete dominance. Another example of incomplete dominance is with sickle cell anemia, a disease in which a blood protein called hemoglobin is produced incorrectly. This causes the red blood cells to have a sickle shape, making it difficult for these misshapen cells to pass through the smallest blood vessels. A person that is homozygous recessive (ss) for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is homozygous dominant (SS) will have normal red blood cells. What type of blood cells do you think a person who is heterozygous (Ss) for the trait will have? They will have some misshapen cells and some normal cells ( Figure 1.2). Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a combination of the recessive and dominant traits. Sickle cell anemia causes red blood cells to become misshapen and curved unlike normal, rounded red blood cells. | text | null |
L_0615 | non mendelian inheritance | T_3216 | Another exception to Mendels laws is a phenomenon called codominance. For example, our blood type shows codominance. Do you know what your blood type is? Are you A? O? AB? Those letters actually represent alleles. Unlike other traits, your blood type has three alleles, instead of two! The ABO blood types ( Figure 1.3) are named for the protein attached to the outside of the blood cell. In this case, two alleles are dominant and completely expressed (IA and IB ), while one allele is recessive (i). The IA allele encodes for red blood cells with the A antigen, while the IB allele encodes for red blood cells with the B antigen. The recessive allele (i) does not encode for any proteins. Therefore a person with two recessive alleles (ii) has type O blood. As no dominant (IA and IB ) allele is present, the person cannot have type A or type B blood. What are the genotypes of a person with type A or type B blood? An example of codominant inheritance is ABO blood types. There are two possible genotypes for type A blood, homozygous (IA IA ) and heterozygous (IA i), and two possible genotypes for type B blood, (IB IB and IB i). If a person is heterozygous for both the IA and IB alleles, they will express both and have type AB blood with both proteins on each red blood cell. This pattern of inheritance is significantly different than Mendels rules for inheritance, because both alleles are expressed completely, and one does not mask the other. | text | null |
L_0618 | organelles | T_3222 | Eukaryotic cells have many specific functions, so it can be said that a cell is like a factory. A factory has many machines and people, and each has a specific role. Just like a factory, the cell is made up of many different parts. Each part has a special role. The different parts of the cell are called organelles, which means "small organs." All organelles are found in eukaryotic cells. Prokaryotic cells are "simpler" than eukaryotic cells. Though prokaryotic cells still have many functions, they are not as specialized as eukaryotic cells, lacking membrane-bound organelles. Thus, most organelles are not found in prokaryotic cells. Below are the main organelles found in eukaryotic cells ( Figure 1.1): 1. The nucleus of a cell is like a safe containing the factorys trade secrets. The nucleus contains the genetic material (DNA), the information needed to build thousands of proteins. 2. The mitochondria are the powerhouses of the cell. Mitochondria are the organelles where cellular energy is produced, providing the energy needed to power chemical reactions. This process, known as cellular respiration, produces energy is in the form of ATP (adenosine triphosphate). Cells that use a lot of energy may have thousands of mitochondria. 3. Vesicles are small membrane bound sacs that transport materials around the cell and to the cell membrane. 4. The vacuoles are like storage centers. Plant cells have larger vacuoles than animal cells. Plants store water and nutrients in their large central vacuoles. 5. Lysosomes are like the recycling trucks that carry waste away from the factory. Lysosomes have digestive enzymes that break down old molecules into parts that can be recycled. 6. In both eukaryotes and prokaryotes, ribosomes are the non-membrane bound organelles where proteins are made. Ribosomes are like the machines in the factory that produce the factorys main product. Proteins are the main product of the cell. 7. Some ribosomes can be found on folded membranes called the endoplasmic reticulum (ER), others float freely in the cytoplasm. If the ER is covered with ribosomes, it looks bumpy like sandpaper, and is called the rough endoplasmic reticulum. If the ER does not contain ribosomes, it is smooth and called the smooth endoplasmic reticulum. Many proteins are made on the ribosomes on the rough ER. These proteins immedi- ately enter the ER, where they are modified, packaged into vesicles and sent to the Golgi apparatus. Lipids are made in the smooth ER. 8. The Golgi apparatus works like a mail room. The Golgi apparatus receives proteins from the rough ER and puts "shipping addresses" on them. The Golgi then packages the proteins into vesicles and sends them to the right place in the cell or to the cell membrane. Some of these proteins are secreted from the cell (they exit the cell); others are placed into the cell membrane. Also, the cytoskeleton gives the cell its shape, and the flagella helps the cell to move. Prokaryotic cells may also have flagella. | text | null |
L_0625 | passive transport | T_3247 | Recall that the cell membrane is semipermeable. It does not allow everything to pass through. Some molecules can pass easily through your cell membranes, while others have more difficulty. Sometimes molecules need the help of special transport proteins to move across the cell membrane. Some molecules even need an input of energy to help get them across the cell membrane. The movement of molecules across a membrane without the input of energy is known as passive transport. When energy (ATP) is needed, the movement is known as active transport. Active transport moves molecules against their concentration gradient, from an area of low concentration to an area of high concentration. | text | null |
L_0625 | passive transport | T_3248 | One example of passive transport is diffusion, when molecules move from an area of high concentration (large amount) to an area of low concentration (low amount). Molecules are said to naturally flow down their concentration gradient. This type of diffusion proceeds without an input of energy. In simple diffusion, molecules that are small and uncharged can freely diffuse across a cell membrane. They simply flow through the cell membrane. Simple diffusion does not require energy or need the assistance of a transport protein. Other larger or charged molecules that diffuse across a membrane may need assistance from a protein. Oxygen is a molecule that can freely diffuse across a cell membrane. For example, oxygen diffuses out of the air sacs in your lungs into your bloodstream because oxygen is more concentrated in your lungs than in your blood. Oxygen moves from the high concentration of oxygen in your lungs to the low concentration of oxygen in your bloodstream. Carbon dioxide, which is exhaled, moves in the opposite direction - from a high concentration in your bloodstream to a low concentration in your lungs. | text | null |
L_0625 | passive transport | T_3249 | Sometimes, molecules cannot move through the cell membrane on their own. These molecules need special transport proteins to help them move across the membrane, a process known as facilitative diffusion. These special proteins are called channel proteins or carrier proteins ( Figure 1.1), and they are attached to the cell membrane. In fact, they go through the cell membrane, from the inside of the cell to the outside. Channel proteins provide an open channel or passageway through the cell membrane for molecules to move across. Many channel proteins allow the diffusion of ions. Ions are charged atoms. The charge makes it difficult to cross the cell membrane without assistance. Channel proteins are specific for the molecule they transport. For example a sodium ion crosses the membrane through a channel protein specific for sodium ions. Carrier proteins bind and carry the molecules across the cell membrane. These proteins bind a molecule on one side of the membrane, change shape as they carry the molecule across the membrane, and deposit the molecule on the other side of the membrane. Even though a protein is involved in both these methods of transport, neither method requires energy. Therefore these are still types of passive transport. | text | null |
L_0630 | plant cell structures | T_3261 | Even though plants and animals are both eukaryotes, plant cells differ in some ways from animal cells ( Figure organelles of photosynthesis. Photosynthesis converts the suns solar energy into chemical energy. This chemical energy, which is the carbohydrate glucose, serves as "food" for the plant. | text | null |
L_0630 | plant cell structures | T_3262 | First, plant cells have a large central vacuole that holds a mixture of water, nutrients, and wastes. A plant cells vacuole can make up 90% of the cells volume. The large central vacuole essentially stores water. In animal cells, vacuoles are much smaller. A plant cell has several features that make it different from an animal cell, including a cell wall, huge vacuoles, and chloroplasts, which photosynthesize. | text | null |
L_0630 | plant cell structures | T_3263 | Second, plant cells have a cell wall, while animal cells do not ( Figure 1.2). The cell wall surrounds the plasma membrane but does not keep substances from entering or leaving the cell. A cell wall gives the plant cell strength and protection. In this photo of plant cells taken with a light microscope, you can see green chloroplasts, as well as a cell wall around each cell. | text | null |
L_0630 | plant cell structures | T_3264 | A third difference between plant and animal cells is that plants have several kinds of organelles called plastids. And there are several different kinds of plastids in plant cells. For example, Chloroplasts are needed for photosynthesis, leucoplasts can store starch or oil, and brightly colored chromoplasts give some flowers and fruits their yellow, orange, or red color. It is the presence of chloroplasts and the ability to photosynthesize, that is one of the defining features of a plant. No animal or fungi can photosynthesize, and only some protists are able to. The photosynthetic protists are the plant-like protists, represented mainly by the unicellular algae. | text | null |
L_0635 | plant reproduction and life cycle | T_3275 | The life cycle of a plant is very different from the life cycle of an animal. Humans are made entirely of diploid cells (cells with two sets of chromosomes, referred to as 2n). Our only cells that are haploid cells (cells with one set of chromosomes, n) are sperm and egg cells. Plants, however, can live when they are are at the stage of having haploid cells or diploid cells. If a plant has a haploid chromosome number of 20, what is the diploid chromosome number? If the diploid chromosome number is 20, what is the haploid number? Plants alternate between diploid-cell plants and haploid-cell plants. This is called alternation of generations, because the plant type alternates from generation to generation. In alternation of generations, the plant alternates between a sporophyte that has diploid cells and a gametophyte that has haploid cells. Alternation of generations can be summarized in the following four steps: follow along in the Figure 1.1 as you read through the steps. 1. The haploid gametophyte produces the gametes, or sperm and egg, by mitosis. Remember, gametes are haploid, having one set of chromosomes. 2. Then, the sperm fertilizes the egg, producing a diploid zygote that develops into the sporophyte, which of course, is diploid. 3. The diploid sporophyte produces haploid spores by meiosis. 4. The haploid spores go through mitosis, developing into the haploid gametophyte. As we will see in additional Plants concepts, the generation in which the plant spends most of its life cycle is different between various plants. In the plants that first evolved, the gametophyte takes up the majority of the life cycle of the plant. During the course of evolution, the sporophyte became the major stage of the life cycle of the plant. In ferns, the sporophyte is dominant and produces spores that germinate into a heart-shaped gametophyte. | text | null |
L_0648 | prokaryotic and eukaryotic cells | T_3307 | There are two basic types of cells, prokaryotic cells and eukaryotic cells. The main difference between eukaryotic and prokaryotic cells is that eukaryotic cells have a nucleus. The nucleus is where cells store their DNA, which is the genetic material. The nucleus is surrounded by a membrane. Prokaryotic cells do not have a nucleus. Instead, their DNA floats around inside the cell. Organisms with prokaryotic cells are called prokaryotes. All prokaryotes are single-celled (unicellular) organisms. Bacteria and Archaea are the only prokaryotes. Organisms with eukaryotic cells are called eukaryotes. Animals, plants, fungi, and protists are eukaryotes. All multicellular organisms are eukaryotes. Eukaryotes may also be single-celled. Both prokaryotic and eukaryotic cells have structures in common. All cells have a plasma membrane, ribosomes, cytoplasm, and DNA. The plasma membrane, or cell membrane, is the phospholipid layer that surrounds the cell and protects it from the outside environment. Ribosomes are the non-membrane bound organelles where proteins are made, a process called protein synthesis. The cytoplasm is all the contents of the cell inside the cell membrane, not including the nucleus. | text | null |
L_0648 | prokaryotic and eukaryotic cells | T_3308 | Eukaryotic cells usually have multiple chromosomes, composed of DNA and protein. Some eukaryotic species have just a few chromosomes, others have close to 100 or more. These chromosomes are protected within the nucleus. In addition to a nucleus, eukaryotic cells include other membrane-bound structures called organelles. Organelles allow eukaryotic cells to be more specialized than prokaryotic cells. Pictured below are the organelles of eukaryotic cells ( Figure 1.1), including the mitochondria, endoplasmic reticulum, and Golgi apparatus. These will be discussed in additional concepts. DNA (chromatin) is stored. Organelles give eukaryotic cells more functions than prokaryotic cells. | text | null |
L_0648 | prokaryotic and eukaryotic cells | T_3309 | Prokaryotic cells ( Figure 1.2) are usually smaller and simpler than eukaryotic cells. They do not have a nucleus or other membrane-bound organelles. In prokaryotic cells, the DNA, or genetic material, forms a single large circle that coils up on itself. The DNA is located in the main part of the cell. Nucleus DNA Membrane-Bound Organelles Examples Prokaryotic Cells No Single circular piece of DNA No Bacteria Eukaryotic Cells Yes Multiple chromosomes Yes Plants, animals, fungi | text | null |
L_0649 | protein synthesis and gene expression | T_3310 | A monomer is a molecule that can bind to other monomers to form a polymer. Amino acids are the monomers of a protein. The DNA sequence contains the instructions to place amino acids into a specific order. When the amino acid monomers are assembled in that specific order, proteins are made, a process called protein synthesis. In short, DNA contains the instructions to create proteins. But DNA does not directly make the proteins. Proteins are made on the ribosomes in the cytoplasm, and DNA (in an eukaryotic cell) is in the nucleus. So the cell uses an RNA intermediate to produce proteins. Each strand of DNA has many separate sequences that code for a specific protein. Insulin is an example of a protein made by your cells ( Figure 1.1). Units of DNA that contain code for the creation of a protein are called genes. | text | null |
L_0649 | protein synthesis and gene expression | T_3311 | There are about 22,000 genes in every human cell. Does every human cell have the same genes? Yes. Does every human cell make the same proteins? No. In a multicellular organism, such as us, cells have specific functions because they have different proteins. They have different proteins because different genes are expressed in different cell types (which is known as gene expression). Imagine that all of your genes are "turned off." Each cell type only "turns on" (or expresses) the genes that have the code for the proteins it needs to use. So different cell types "turn on" different genes, allowing different proteins to be made. This gives different cell types different functions. Once a gene is expressed, the protein product of that gene is usually made. For this reason, gene expression and protein synthesis are often considered the same process. | text | null |
L_0683 | sponges | T_3408 | Sponges ( Figure 1.1) are classified in the phylum Porifera, from the Latin words meaning "having pores." These pores allow the movement of water into the sponges sac-like bodies. Sponges must pump water through their bodies in order to eat. Because sponges are sessile, meaning they cannot move, they filter water to obtain their food. They are, therefore, known as filter feeders. Filter feeders must filter the water to separate out the organisms and nutrients they want to eat from those they do not. You might think that sponges dont look like animals at all. They dont have a head or legs. Internally, they do not have brains, stomachs, or other organs. This is because sponges evolved much earlier than other animals. In fact, sponges do not even have true tissues. Instead, their bodies are made up of specialized cells (cell-level organization) that do specific jobs. Other animals, including humans, have tissue-level organization because they have tissues with specific functions. Sponge cells perform a variety of bodily functions and appear to be more independent of each other than are the cells of other animals. For example, some cells control the flow of water, in and out of the sponge, by increasing or decreasing the size of the pores. The sponges often have tube-like bodies with many tiny pores. There are roughly 5,000 sponge species. Sponges are characterized by a feeding system unique among animals. As sponges dont have mouths, they must feed by some other method. Sponges have tiny pores in their outer walls through which water is drawn. Cells in the sponge walls filter food from the water as the water is pumped through the body and out other larger openings. The flow of water through the sponge is unidirectional, driven by the beating of flagella, which line the surface of chambers connected by a series of canals. Sponges reproduce by both asexual and sexual means. Sponges that reproduce asexually produce buds or, more often, structures called gemmules, which are packets of several cells of various types inside a protective covering. Freshwater sponges often produce gemmules prior to winter, which then develop into adult sponges beginning the following spring. Most sponges that reproduce sexually are hermaphroditic and produce eggs and sperm at different times. Sperm are frequently released into the water, where they are captured by sponges of the same species. The sperm are then transported to eggs, fertilization occurs and the zygotes develop into larvae. Some sponges release their larvae, where others retain them for some time. Once the larvae are in the water, they settle and develop into juvenile sponges. | text | null |
L_0708 | viruses | T_3484 | We have all heard of viruses. The flu, the common cold, and many other diseases are caused by viruses. But what is a virus? Do you think viruses are living? Which domain do they belong to? Bacteria? Archaea? Eukarya? | text | null |
L_0708 | viruses | T_3485 | The answer is actually no. A virus is essentially DNA or RNA surrounded by a coat of protein ( Figure 1.1). It is not made of a cell, and cannot maintain a stable internal environment ( homeostasis). Recall that a cell is the basic unit of living organisms. So if a virus is not made of at least one cell, can it be living? Viruses also cannot reproduce on their ownthey need to infect a host cell to reproduce. So a virus is very different from any of the organisms that fall into the three domains of life. Though viruses are not considered living, they share two important traits with living organisms. They have genetic material like all cells do (though they are not made of cells), and they can evolve. The genetic material of a virus can change (mutate), altering the traits of the virus. As the process of evolution has resulted in all life on the planet today, the classification of viruses has been controversial. It calls into question the very definition of life. | text | null |
L_0708 | viruses | T_3486 | Viruses infect a variety of organisms, including plants, animals, and bacteria, injecting its genetic material into a cell of the host organism. Once inside the host cell, they use the cells own ATP (energy), ribosomes, enzymes, and other cellular parts to make copies of themselves. The host cell makes a copy of the viral DNA and produces viral proteins. These are then packaged into new viruses. So viruses cannot replicate or reproduce on their own; they rely on a host cell to make additional viruses. | text | null |
L_0708 | viruses | T_3487 | Viruses cause many human diseases. In addition to the flu and the common cold, viruses cause rabies, diarrheal diseases, AIDS, cold sores, and many other diseases ( Figure 1.2). Viral diseases range from mild to fatal. Cold sores are caused by a herpes virus. | text | null |
L_0714 | introduction to solutions | T_3508 | A solution forms when one substance dissolves in another. The substance that dissolves is called the solute. The substance it dissolves in is called the solvent. For example, ocean water is a solution in which the solute is salt and the solvent is water. In this example, a solid (salt) is dissolved in a liquid (water). However, matter in any state can be the solute or solvent in a solution. Solutions may be gases, liquids, or solids. In Table 10.1 and the video at the URL below, you can learn about solutions involving other states of matter. Solution Gas dissolved in gas Example: Earths atmosphere Gas dissolved in liquid Example: carbonated water Liquid dissolved in gas Example: moist air Solute oxygen (and other gases) Solvent nitrogen carbon dioxide water water air Solution Liquid dissolved in liquid Example: vinegar Solid dissolved in liquid Example: sweet tea Solid dissolved in solid Example: bronze Solute acetic acid Solvent water sugar tea copper tin When a solute dissolves in a solvent, it changes to the same state as the solvent. For example, when solid salt dissolves in liquid water, it becomes part of the liquid solution, salt water. If the solute and solvent are already in the same state, the substance present in greater quantity is considered to be the solvent. For example, nitrogen is the solvent in Earths atmosphere because it makes up 78 percent of air. | text | null |
L_0714 | introduction to solutions | T_3509 | When a solute dissolves, it separates into individual particles that spread evenly throughout the solvent. Exactly how this happens depends on the type of bonds the solute contains. Solutes with ionic bonds, such as table salt (NaCl), separate into individual ions (Na+ and Cl ). Solutes with covalent bonds, such as glucose (H6 C12 O6 ), separate into individual molecules. In either case, the individual ions or molecules spread apart and are surrounded by molecules of the solvent. This is illustrated in Figure 10.1 and in the videos at the URLs below. MEDIA Click image to the left or use the URL below. URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0714 | introduction to solutions | T_3510 | When you add sugar to a cold drink, you may stir it to help the sugar dissolve. If you dont stir, the sugar may eventually dissolve, but it will take much longer. Stirring is one of several factors that affect how fast a solute dissolves in a solvent. Temperature is another factor. A solid solute dissolves faster at a higher temperature. For example, sugar dissolves faster in hot tea than in ice tea. A third factor that affects the rate of dissolving is the surface area of the solute. For example, if you put granulated sugar in a glass of ice tea, it will dissolve more quickly than the same amount of sugar in a cube. Thats because granulated sugar has much more surface area than a cube of sugar. You can see videos of all three factors at these URLs: MEDIA Click image to the left or use the URL below. URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0714 | introduction to solutions | T_3511 | Water is a polar compound. This means it has positively and negatively charged ends. This is why it is so good at dissolving ionic compounds such as salt and polar covalent compounds such as sugar. Solutes that can dissolve in a given solvent, such as water, are said to be soluble in that solvent. So many solutes are soluble in water that water is called the universal solvent. However, there are substances that dont dissolve in water. Did you ever try to clean a paintbrush after painting with an oil-based paint? It doesnt work. Oil-based paint is nonpolar, so it doesnt dissolve in water. In other words, it is insoluble in water. Instead, a nonpolar solvent such as paint thinner must be used to dissolve nonpolar paint. You can see a video about soluble and insoluble solutes at this URL: (1:51). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0714 | introduction to solutions | T_3512 | When a solute dissolves in a solvent, it changes the physical properties of the solvent. Two properties that change when a solute is added are the freezing and boiling points. Generally, solutes lower the freezing point and raise the boiling point of solvents. You can see some examples of this in Figure below. To see why solutes change the freezing and boiling points of solvents, watch this video: (14:00). MEDIA Click image to the left or use the URL below. URL: In each of these examples, a solute changes the freezing and/or boiling points of a solvent. | text | null |
L_0715 | solubility and concentration | T_3513 | Solubility is the amount of solute that can dissolve in a given amount of solvent at a given temperature. Some solutes have greater solubility than others in a given solvent. For example, table sugar is much more soluble in water than is baking soda. You can dissolve much more sugar than baking soda in a given amount of water. Compare the solubility of these and other solutes in Figure 10.2. For a video about solubility, go to this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0715 | solubility and concentration | T_3514 | There is a limit on the amount of solute that can dissolve in a given solvent. Tanya found this out with her baking soda mixture. But even sugar, which is very soluble, has an upper limit. The maximum amount of table sugar that will dissolve in 1 L of water at 20C is about 2000 g. If you add more sugar than this, the extra sugar wont dissolve. A solution that contains as much solute as can dissolve at a given temperature is called a saturated solution. A solution that contains less solute than can dissolve at a given temperature is called an unsaturated solution. A solution of 2000 grams of sugar in 1 L of 20C water is saturated. Thats all the sugar the solution can hold. Any solution containing less than 2000 g of sugar is unsaturated. It can hold more sugar. To learn more about saturated and unsaturated solutions, watch the video at this URL: . You Try It! Problem: A solution contains 249 grams of Epsom salt in 1 L of water at 20C. Is the solution saturated or unsaturated? Problem: Give an example of an unsaturated solution of table salt in 1 L of 20C water. | text | null |
L_0715 | solubility and concentration | T_3515 | Certain factors can change the solubility of a solute. Temperature is one such factor. How temperature affects solubility depends on the state of the solute, as you can see in Figure 10.3. If a solute is a solid or liquid, increasing the temperature increases its solubility. For example, more sugar can dissolve in hot tea than in iced tea. If a solute is a gas, increasing the temperature decreases its solubility. For example, less carbon dioxide can dissolve in warm ocean water than in cold ocean water. The solubility of gases is also affected by pressure. Pressure is the amount of force pushing against a given area. Increasing the pressure on a gas increases its solubility. Did you ever open a can of soda and notice how it fizzes out of the can? Soda contains carbon dioxide. Opening the can reduces the pressure on the gas so it is less soluble. As a result, some of the carbon dioxide comes out of solution and rushes into the air. Do you wonder why temperature and pressure affect solubility in these ways? If so, watch the video at the URL below. It explains why. | text | null |
L_0715 | solubility and concentration | T_3515 | Certain factors can change the solubility of a solute. Temperature is one such factor. How temperature affects solubility depends on the state of the solute, as you can see in Figure 10.3. If a solute is a solid or liquid, increasing the temperature increases its solubility. For example, more sugar can dissolve in hot tea than in iced tea. If a solute is a gas, increasing the temperature decreases its solubility. For example, less carbon dioxide can dissolve in warm ocean water than in cold ocean water. The solubility of gases is also affected by pressure. Pressure is the amount of force pushing against a given area. Increasing the pressure on a gas increases its solubility. Did you ever open a can of soda and notice how it fizzes out of the can? Soda contains carbon dioxide. Opening the can reduces the pressure on the gas so it is less soluble. As a result, some of the carbon dioxide comes out of solution and rushes into the air. Do you wonder why temperature and pressure affect solubility in these ways? If so, watch the video at the URL below. It explains why. | text | null |
L_0715 | solubility and concentration | T_3516 | The concentration of a solution is the amount of solute in a given amount of solution. A solution with little dissolved solute has a low concentration. It is called a dilute solution. A solution with a lot of dissolved solute has a high concentration. It is called a concentrated solution. Concentration is often expressed as a percent. You can calculate the concentration of a solution using this formula: Concentration = Mass (or Volume) of Solute 100% Mass (or Volume) of Solution For example, if a 100 g solution of salt water contains 3 g of salt, then its concentration is: Concentration = 3g 100% = 3% 100 g For some problems that are more challenging, go to these URLs: MEDIA Click image to the left or use the URL below. URL: MEDIA Click image to the left or use the URL below. URL: You Try It! Problem: A 1 L container of juice drink, called brand A, contains 250 mL of juice. The rest of the drink is water. How concentrated is brand A juice drink? Problem: A 600 mL container of another juice drink, called brand B, contains 200 mL of juice. Which brand of juice drink is more concentrated, brand A or brand B? | text | null |
L_0757 | electric charge | T_3848 | Electric charge is a physical property of particles or objects that causes them to attract or repel each other without touching. All electric charge is based on the protons and electrons in atoms. A proton has a positive electric charge, and an electron has a negative electric charge (see Figure 23.2). When it comes to electric charges, opposites attract. In other words, positive and negative particles are attracted to each other. Like charges, on the other hand, repel each other, so two positive or two negative charges push apart from each other. The force of attraction or repulsion between charged particles is called electric force. It is illustrated in Figure 23.3. The strength of electric force depends on the amount of electric charge and the distance between the charged particles. The larger the charge or the closer together the charges are, the greater is the electric force. | text | null |
L_0757 | electric charge | T_3848 | Electric charge is a physical property of particles or objects that causes them to attract or repel each other without touching. All electric charge is based on the protons and electrons in atoms. A proton has a positive electric charge, and an electron has a negative electric charge (see Figure 23.2). When it comes to electric charges, opposites attract. In other words, positive and negative particles are attracted to each other. Like charges, on the other hand, repel each other, so two positive or two negative charges push apart from each other. The force of attraction or repulsion between charged particles is called electric force. It is illustrated in Figure 23.3. The strength of electric force depends on the amount of electric charge and the distance between the charged particles. The larger the charge or the closer together the charges are, the greater is the electric force. | text | null |
L_0757 | electric charge | T_3849 | Electric force is exerted over a distance, so charged particles do not have to be in contact in order to exert force over each other. Thats because each charged particle is surrounded by an electric field. An electric field is a space around a charged particle where the particle exerts electric force on other particles. Electric fields surrounding positively and negatively charged particles are illustrated in Figure 23.4 and at the URL below. When charged particles exert force on each other, their electric fields interact. This is also illustrated in Figure 23.4. | text | null |
L_0757 | electric charge | T_3850 | Atoms are neutral in electric charge because they have the same number of electrons as protons. However, atoms may transfer electrons and become charged ions, as illustrated in Figure 23.5. Positively charged ions, or cations, form when atoms give up electrons. Negatively charged ions, or anions, form when atoms gain electrons. Like the formation of ions, the formation of charged matter in general depends on the transfer of electrons either between two materials or within a material. Three ways this can occur are friction, conduction, and polarization. In all cases, the total charge remains the same. Electrons move, but they arent destroyed. This is the law of conservation of charge. | text | null |
L_0757 | electric charge | T_3851 | Did you ever rub an inflated balloon against your hair? You can see what happens in Figure 23.6. Friction between the rubber of the balloon and the babys hair results in electrons from the hair "rubbing off" onto the balloon. Thats because rubber attracts electrons more strongly than hair does. After the transfer of electrons, the balloon becomes negatively charged and the hair becomes positively charged. As a result, the individual hairs repel each other and the balloon and the hair attract each other. Electrons are transferred in this way whenever there is friction between materials that differ in their ability to give up or accept electrons. | text | null |
L_0757 | electric charge | T_3852 | Another way electrons may be transferred is through conduction. This occurs when there is direct contact between materials that differ in their ability to give up or accept electrons. For example, wool tends to give up electrons and rubber tends to accept them. Therefore, when you walk across a wool carpet in rubber-soled shoes, electrons transfer from the carpet to your shoes. You become negatively charged, while the carpet becomes positively charged. Another example of conduction is pictured in Figure 23.7. The device this girl is touching is called a van de Graaff generator. The dome on top is negatively charged. When the girl places her hand on the dome, electrons are transferred to her, so she becomes negatively charged as well. Even the hairs on her head become negatively charged. As a result, individual hairs repel each other, causing them to stand on end. You can see a video demonstration of a van de Graff generator at this URL: . | text | null |
L_0757 | electric charge | T_3853 | Polarization is the movement of electrons within a neutral object due to the electric field of a nearby charged object. It occurs without direct contact between the two objects. You can see how it happens in Figure 23.8. When the negatively charged plastic rod in the figure is placed close to the neutral metal plate, electrons in the plate are repelled by the positive charges in the rod. The electrons move away from the rod, causing one side of the plate to become positively charged and the other side to become negatively charged. Polarization may also occur after you walk across a wool carpet in rubber-soled shoes and become negatively charged. If you reach out to touch a metal doorknob, electrons in the neutral metal will be repelled and move away from your hand before you even touch the knob. In this way, one end of the doorknob becomes positively charged and the other end becomes negatively charged. | text | null |
L_0757 | electric charge | T_3854 | Polarization leads to the buildup of electric charges on objects. This buildup of charges is known as static electricity. Once an object becomes charged, it is likely to remain charged until another object touches it or at least comes very close to it. Thats because electric charge cannot travel easily through air, especially if the air is dry. Consider again the example of your hand and the metal doorknob. When your negatively charged hand gets very close to the positively charged doorknob, the air between your hand and the knob may become electrically charged. If that happens, it allows electrons to suddenly flow from your hand to the knob. This is the electric shock you feel when you reach for the knob. You may even see a spark as the electrons jump from your hand to the metal. This sudden flow of electrons is called static discharge. Another example of static discharge, on a much larger scale, is lightning. You can see how it occurs in Figure 23.9. At the URL below, you can watch a slow-motion lightning strike. Be sure to wait for the real-time lightning strike at the very end of the video. | text | null |
L_0758 | electric current | T_3855 | Electric current is a continuous flow of electric charges. Current is measured as the amount of charge that flows past a given point in a certain amount of time. The SI unit for electric current is the ampere (A), or amp. Electric current may flow in just one direction, or it may keep reversing direction. When current flows in just one direction, it is called direct current (DC). The current that flows through a battery-powered flashlight is direct current. When current keeps reversing direction, it is called alternating current (AC). The current that runs through the wires in your home is alternating current. Graphs of both types of current are shown in Figure 23.10. You can watch an animation of both types at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0758 | electric current | T_3856 | Why do charges flow in an electric current? The answer has to do with electric potential energy. Potential energy is stored energy that an object has due to its position or shape. An electric charge has potential energy because of its position in an electric field. For example, when two negative charges are close together, they have potential energy because they repel each other and have the potential to push apart. If the charges move apart, their potential energy decreases. Electric charges always move spontaneously from a position where they have higher potential energy to a position where their potential energy is lower. This is similar to water falling over a dam from an area of higher to lower potential energy due to gravity. In general, for an electric charge to move from one position to another, there must be a difference in electric potential energy between the two positions. The difference in electric potential energy is called potential difference, or voltage. Voltage is measured in an SI unit called the volt (V). For example, the terminals of the car battery in Figure 23.11 have a potential difference of 12 volts. This difference in voltage results in a spontaneous flow of charges, or electric current. | text | null |
L_0758 | electric current | T_3857 | Batteries like the one in Figure 23.11 are one of several possible sources of voltage needed to produce electric current. Sources of voltage include generators, chemical cells, and solar cells. Generators change the kinetic energy of a spinning turbine to electrical energy in a process called electromag- netic induction. You can read about generators and how they work in the chapter "Electromagnetism." Chemical and solar cells are devices that change chemical or light energy to electrical energy. You can read about both types of cells and how they work below. | text | null |
L_0758 | electric current | T_3858 | Chemical cells are found in batteries. They produce voltage by means of chemical reactions. A chemical cell has two electrodes, which are strips made of different materials, such as zinc and carbon (see Figure 23.12). The electrodes are suspended in an electrolyte. An electrolyte is a substance containing free ions that can carry electric current. The electrolyte may be either a paste, in which case the cell is called a dry cell, or a liquid, in which case the cell is called a wet cell. Flashlight batteries contain dry cells. Car batteries contain wet cells. Animations at the URL below show how batteries work. Both dry and wet cells work the same basic way. The electrodes react chemically with the electrolyte, causing one electrode to give up electrons and the other electrode to accept electrons. In the case of zinc and carbon electrodes, the zinc electrode attracts electrons and becomes negatively charged, while the carbon electrode gives up electrons and becomes positively charged. Electrons flow through the electrolyte from the negative to positive electrode. If wires are used to connect the two electrodes at their terminal ends, electric current will flow through the wires and can be used to power a light bulb or other electric device. | text | null |
L_0758 | electric current | T_3859 | Solar cells convert the energy in sunlight to electrical energy. They contain a material such as silicon that absorbs light energy and gives off electrons. The electrons flow and create electric current. Figure 23.13 and the animation at the URL below show how a solar cell uses light energy to produce electric current and power a light bulb. Many calculators and other devices are also powered by solar cells. | text | null |
L_0758 | electric current | T_3859 | Solar cells convert the energy in sunlight to electrical energy. They contain a material such as silicon that absorbs light energy and gives off electrons. The electrons flow and create electric current. Figure 23.13 and the animation at the URL below show how a solar cell uses light energy to produce electric current and power a light bulb. Many calculators and other devices are also powered by solar cells. | text | null |
L_0758 | electric current | T_3860 | Electric current cannot travel through empty space. It needs a material through which to travel. However, when current travels through a material, the flowing electrons collide with particles of the material, and this creates resistance. | text | null |
L_0758 | electric current | T_3861 | Resistance is opposition to the flow of electric charges that occurs when electric current travels through matter. The SI unit of resistance is the ohm (named for the scientist Georg Ohm, whom you can read about below). Resistance is caused by electrons in a current bumping into electrons and ions in the matter through which the current is flowing. Resistance is similar to the friction that resists the movement of one surface as it slides over another. Resistance reduces the amount of current that can travel through the material because some of the electrical energy is converted to other forms of energy. For example, when electric current flows through the tungsten wire inside an incandescent light bulb, the tungsten resists the flow of electric charge, and some of the electrical energy is converted to light and thermal energy. | text | null |
L_0758 | electric current | T_3862 | Some materials resist the flow of electric current more or less than other materials do. Materials that have low resistance to electric current are called electric conductors. Many metalsincluding copper, aluminum, and steelare good conductors of electricity. Water that has even a tiny amount of impurities in it is an electric conductor as well. Materials that have high resistance to electric current are called electric insulators. Wood, rubber, and plastic are examples of electric insulators. Dry air is also an electric insulator. You probably know that electric wires are made of metal and coated with rubber or plastic (see Figure 23.14). Now you know why. Metals are good electric conductors, so they offer little resistance and allow most of the current to pass through. Rubber and plastic are good insulators, so they offer a lot of resistance and allow little current to pass through. When more than one material is available for electric current to flow through, the current always travels through the material with the least resistance. Thats why all the current passes through a metal wire and none flows through its rubber or plastic coating. | text | null |
L_0758 | electric current | T_3863 | For a given material, three properties of the material determine how resistant it is to electric current: length, width, and temperature. Consider an electric wire like one of the wires in Figure 23.14. A longer wire has more resistance. Current must travel farther, so there are more chances for it to collide with particles of wire. A wider wire has less resistance. A given amount of current has more room to flow through a wider wire. A cooler wire has less resistance than a warmer wire. Cooler particles have less kinetic energy, so they move more slowly. Current is less likely to collide with slowly moving particles. Materials called superconductors have virtually no resistance when they are cooled to extremely low temperatures. | text | null |
L_0758 | electric current | T_3864 | Voltage, or a difference in electric potential energy, is needed for electric current to flow. As you might have guessed, greater voltage results in more current. Resistance, on the other hand, opposes the flow of electric current, so greater resistance results in less current. These relationships between current, voltage, and resistance were first demonstrated by a German scientist named Georg Ohm in the early 1800s, so they are referred to as Ohms law. Ohms law can be represented by the following equation. Current (amps) = Voltage (volts) Resistance (ohms) | text | null |
L_0758 | electric current | T_3865 | You may have a better understanding of Ohms law if you compare current flowing through a wire from a battery to water flowing through a garden hose from a tap. Increasing voltage is like opening the tap wider. When the tap is opened wider, more water flows through the hose. This is like an increase in current. Stepping on the hose makes it harder for the water to pass through. This is like increasing resistance, which causes less current to flow through a material. Still not sure about the relationship among voltage, current, and resistance? Watch the video at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0758 | electric current | T_3866 | You can use the equation for current (above) to calculate the amount of current flowing through a material when voltage and resistance are known. Consider an electric wire that is connected to a 12-volt battery. If the wire has a resistance of 3 ohms, how much current is flowing through the wire? Current = 12 volts = 4 amps 3 ohms You Try It! Problem: A 120-volt voltage source is connected to a wire with 20 ohms of resistance. How much current flows through the wire? | text | null |
L_0759 | electric circuits | T_3867 | A closed loop through which current can flow is called an electric circuit. In homes in the U.S., most electric circuits have a voltage of 120 volts. The amount of current (amps) a circuit carries depends on the number and power of electrical devices connected to the circuit. But home circuits generally have a safe upper limit of about 20 or 30 amps. | text | null |
L_0759 | electric circuits | T_3868 | All electric circuits have at least two parts: a voltage source and a conductor. The voltage source of the circuit in Figure 23.16 is a battery. In a home circuit, the source of voltage is an electric power plant, which may supply electric current to many homes and businesses in a community or even to many communities. The conductor in most circuits consists of one or more wires. The conductor must form a closed loop from the source of voltage and back again. In Figure 23.16, the wires are connected to both terminals of the battery, so they form a closed loop. The circuit in Figure 23.16 also has two other parts: a light bulb and a switch. Most circuits have devices such as light bulbs that convert electric energy to other forms of energy. In the case of a light bulb, electricity is converted to light and thermal energy. Many circuits have switches to control the flow of current through the circuit. When the switch is turned on, the circuit is closed and current can flow through it. When the switch is turned off, the circuit is open and current cannot flow through it. | text | null |
L_0759 | electric circuits | T_3869 | When a contractor builds a new home, she uses a set of plans called blueprints that show her how to build the house. The blueprints include circuit diagrams that show how the wiring and other electrical components are to be installed in order to supply current to appliances, lights, and other electrical devices in the home. You can see an example of a very simple circuit diagram in Figure 23.17. Different parts of the circuit are represented by standard symbols, as defined in the figure. An ammeter measures the flow of current through the circuit, and a voltmeter measures the voltage. A resistor is any device that converts some of the electricity to other forms of energy. It could be a light bulb, doorbell, or similar device. | text | null |
L_0759 | electric circuits | T_3870 | There are two basic types of electric circuits, called series and parallel circuits. They differ in the number of loops through which current can flow. You can see an example of each type of circuit in Figure 23.18. A series circuit has only one loop through which current can flow. If the circuit is interrupted at any point in the loop, no current can flow through the circuit and no devices in the circuit will work. In the series circuit in Figure 23.18, if one light bulb burns out the other light bulb will not work because it wont receive any current. Series circuits are commonly used in flashlights. You can see an animation of a series circuit at this URL: http://regentsprep.org/regents/physics/phys03/bsercir/default.htm . A parallel circuit has two (or more) loops through which current can flow. If the circuit is interrupted in one of the loops, current can still flow through the other loop(s). For example, if one light bulb burns out in the parallel circuit in Figure 23.18, the other light bulb will still work because current can by-pass the burned-out bulb. The wiring in a house consists of parallel circuits. You can see an animation of a parallel circuit at this URL: http://regentsprep.org/regents/physics/phys03/bsercir/default.htm . | text | null |
L_0759 | electric circuits | T_3871 | We use electricity for many purposes. Devices such as lights, stoves, and stereos all use electricity and convert it to energy in other forms. However, devices may vary in how quickly they change electricity to other forms of energy. | text | null |
L_0759 | electric circuits | T_3872 | The rate at which a device changes electric current to another form of energy is called electric power. The SI unit of powerincluding electric poweris the watt. A watt equals 1 joule of energy per second. High wattages are often expressed in kilowatts, where 1 kilowatt equals 1000 watts. The power of an electric device, such as a microwave, can be calculated if you know the current and voltage of the circuit. This equation shows how power, current, and voltage are related: Power (watts) = Current (amps) Voltage (volts) Consider a microwave that is plugged into a home circuit. Assume the microwave is the only device connected to the circuit. If the voltage of the circuit is 120 volts and it carries 10 amps of current, then the power of the microwave is: Power = 120 volts 10 amps = 1200 watts, or 1.2 kilowatts You Try It! Problem: A hair dryer is connected to a 120-volt circuit that carries 12 amps of current. What is the power of the hair dryer in kilowatts? | text | null |
L_0759 | electric circuits | T_3873 | Did you ever wonder how much electrical energy it takes to use an appliance such as a microwave or hair dryer? Electrical energy use depends on the power of the appliance and how long it is used. It can be represented by the equation: Electrical Energy = Power Time 1 Suppose you use a 1.2-kilowatt microwave for 5 minutes ( 12 hour). Then the energy used would be: Electrical Energy = 1.2 kilowatts 1 hour = 0.1 kilowatt-hours 12 Electrical energy use is typically expressed in kilowatt-hours, as in this example. How much energy is this? One kilowatt-hour equals 3.6 million joules of energy. Therefore, the 0.1 kilowatt-hours used by the microwave equals 0.36 million joules of energy. You Try It! Problem: A family watches television for an average of 2 hours per day. The television has 0.12 kilowatts of power. How much electrical energy does the family use watching television each day? | text | null |
L_0759 | electric circuits | T_3874 | Electricity is dangerous. Contact with electric current can cause severe burns and even death. Electricity can also cause serious fires. A common cause of electric hazards and fires is a short circuit. | text | null |
L_0759 | electric circuits | T_3875 | An electric cord contains two wires. One wire carries current from the outlet to the appliance or other electric device, and one wire carries current back to the outlet. Did you ever see an old appliance with a damaged cord, like the one in Figure 23.19? A damaged electric cord can cause a severe shock if it allows current to pass from the cord to a person who touches it. A damaged cord can also cause a short circuit. A short circuit occurs when electric current follows a shorter path than the intended loop of the circuit. For example, if the two wires in a damaged cord come into contact with each other, current flows from one wire to the other and bypasses the appliance. This may cause the wires to overheat and start a fire. | text | null |
L_0759 | electric circuits | T_3876 | Because electricity can be so dangerous, safety features are built into electric circuits and devices. They include three-prong plugs, circuit breakers, and GFCI outlets. Each feature is described and illustrated in Table 23.1. You can learn more about electric safety features in the home by watching the video at this URL: Electric Safety Feature Three-Prong Plug Circuit Breaker Description A three-prong plug is generally used on metal appli- ances. The two flat prongs carry current to and from the appliance. The round prong is for safety. It connects with a wire inside the outlet that goes down into the ground. If any stray current leaks from the circuit or if there is a short circuit, the ground wire carries the current into the ground, which harmlessly absorbs it. A circuit breaker is a switch that automatically opens a circuit if too much current flows through it. This could happen if too many electric devices are plugged into the circuit or if there is an electric short. Once the problem is resolved, the circuit breaker can be switched back on to close the circuit. Circuit breakers are generally found in a breaker box that controls all the circuits in a building. Electric Safety Feature GFCI Outlet Description GFCI stands for ground-fault circuit interrupter. GFCI outlets are typically found in bathrooms and kitchens where the use of water poses a risk of shock (because water is a good electric conductor). A GFCI outlet contains a device that monitors the amounts of current leaving and returning to the outlet. If less current is returning than leaving, this means that current is escaping. When this occurs, a tiny circuit breaker in the outlet opens the circuit. The breaker can be reset by pushing a button on the outlet cover. | text | null |
L_0759 | electric circuits | T_3877 | Even with electric safety features, electricity is still dangerous if it is not used safely. Follow the safety rules below to reduce the risk of injury or fire from electricity. Never mix electricity and water. Dont turn on or plug in electric lights or appliances when your hands are wet, you are standing in water, or you are in the shower or bathtub. The current could flow through the waterand youbecause water is a very good electric conductor. Never overload circuits. Avoid plugging too many devices into one outlet or extension cord. The more devices that are plugged in, the more current the circuit carries. Too much current can overheat a circuit and start a fire. Never use devices with damaged cords or plugs. They can cause shocks, shorts, and fires. Never put anything except plugs into electric outlets. Plugging in other objects is likely to cause a serious shock that could be fatal. Never go near fallen electric lines. They could be carrying a lot of current. Report fallen lines to the electric company as soon as possible. | text | null |
L_0760 | electronics | T_3878 | Did you ever make a secret code? One way to make a code is to represent each letter of the alphabet by a different number. Then you can send a coded message by writing words as strings of digits. This is similar to how information is encoded using an electric current. The voltage of the current is changed rapidly and repeatedly to encode a message, called an electronic signal. There are two different types of electronic signals: analog signals and digital signals. Both are illustrated in Figure 23.20. A digital signal consists of pulses of voltage, created by repeatedly switching the current off and on. This type of signal encodes information as a string of 0s (current off) and 1s (current on). This is called a binary ("two-digit") code. DVDs, for example, encode sounds and pictures as digital signals. An analog signal consists of continuously changing voltage in a circuit. For example, microphones encode sounds as analog signals. | text | null |
L_0760 | electronics | T_3879 | Electronic components are the parts used in electronic devices such as computers. The components transmit and change electric current. They are made of materials called semiconductors. | text | null |
L_0760 | electronics | T_3880 | A semiconductor is a solid crystalusually consisting mainly of siliconthat can conduct current better than an electric insulator but not as well as an electric conductor. Very small amounts of other elements, such as boron or phosphorus, are added to the silicon so it can conduct current. A semiconductor is illustrated in Figure 23.21. There are two different types of semiconductors: n-type and p-type. An n-type semiconductor consists of silicon and an element such as phosphorus that gives the silicon crystal extra electrons. An n-type semiconductor is like the negative terminal in a chemical cell. A p-type semiconductor consists of silicon and an element such as boron that gives the silicon positively charged holes where electrons are missing. A p-type semiconductor is like the positive terminal in a chemical cell. | text | null |
L_0760 | electronics | T_3881 | Electronic components contain many semiconductors. Types of components include diodes, transistors, and inte- grated circuits. Each type is described in Table 23.2. Electronic Component Diode Transistor Integrated Circuit (Microchip) Description A diode consists of a p-type and an n-type semicon- ductor placed side by side. When a diode is connected by leads to a source of voltage, electrons flow from the n-type to the p-type semiconductor. This is the only direction that electrons can flow in a diode. This makes a diode useful for changing alternating current to direct current. A transistor consists of three semiconductors, either p- n-p or n-p-n. Current cant flow through a transistor unless a small amount of current is applied to the center semiconductor (through the base). Then a much larger current can flow through the transistor from end to end (from collector to emitter). This means that a transmitter can be used as a switch, with pulses of a small current turning a larger current on and off. A transistor can also be used to increase the amount of current flowing through a circuit. You can learn more about transistors and how they work at this URL: http An integrated circuitalso called a microchipis a tiny, flat piece of silicon that consists of layers of elec- tronic components such as transistors. An integrated circuit as small as a fingernail can contain millions of electronic components. Current flows extremely rapidly in an integrated circuit because it doesnt have far to travel. You can learn how microprocessors are made at this URL: | text | null |
L_0760 | electronics | T_3882 | Many of the devices you commonly use are electronic. Electronic devices include computers, mobile phones, TV remotes, DVD and CD players, game systems, MP3 players, and digital cameras. All of these devices use electric current to encode, analyze, or transmit information. Consider the computer as an example of an electronic device. A computer contains microchips with millions of tiny electronic components. Information is encoded as 0s and 1s and transmitted as electrical pulses. One digit (either 0 or 1) is called a bit, which stands for "binary digit." Each group of eight digits is called a byte. A gigabyte is a billion bytes thats 8 billion 0s and 1s! Because a computers circuits are so tiny and close together, the computer can be very fast and capable of many complex tasks while remaining small. The parts of a computer that transmit, process, or store digital signals are pictured in Figure 23.22 and described below. They include the CPU, hard drive, ROM, and RAM. The motherboard ties all these parts of the computer together. The CPU, or central processing unit, carries out program instructions. You can learn more about CPUs and how they work by watching the video at this URL: . The hard drive is a magnetic disc that provides long-term storage for programs and data. ROM (read-only memory) is a microchip that provides permanent storage. It stores important information such as start-up instructions. This memory remains even after the computer is turned off. RAM (random-access memory) is a microchip that temporarily stores programs and data that are currently being used. Anything stored in RAM is lost when the computer is turned off. The motherboard is connected to the CPU, hard drive, ROM, and RAM. It allows all these parts of the computer to receive power and communicate with one another. | text | null |
L_0763 | electricity and magnetism | T_3897 | In 1820, a physicist in Denmark, named Hans Christian Oersted, discovered how electric currents and magnetic fields are related. However, it was just a lucky accident. Oersted, who is pictured in Figure 25.1, was presenting a demonstration to his students. Ironically, he was trying to show that electricity and magnetism are not related. He placed a wire with electric current flowing through it next to a magnet, and nothing happened. After class, a student held the wire near the magnet again, but in a different direction. To Oersteds surprise, the pointer of the magnet swung toward the wire so it was no longer pointing to Earths magnetic north pole. Oersted was intrigued. He turned off the current in the wire to see what would happen to the magnet. The pointer swung back to its original position, pointing north again. Oersted had discovered that an electric current creates a magnetic field. The magnetic field created by the current was strong enough to attract the pointer of the nearby compass. Oersted wanted to learn more about the magnetic field created by a current, so he placed a magnet at different locations around a wire with current flowing through it. You can see some of his results in Figure 25.2. They show that the magnetic field created by a current has field lines that circle around the wire. You can learn more about Oersteds investigations of current and magnetism at the URL below. MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0763 | electricity and magnetism | T_3897 | In 1820, a physicist in Denmark, named Hans Christian Oersted, discovered how electric currents and magnetic fields are related. However, it was just a lucky accident. Oersted, who is pictured in Figure 25.1, was presenting a demonstration to his students. Ironically, he was trying to show that electricity and magnetism are not related. He placed a wire with electric current flowing through it next to a magnet, and nothing happened. After class, a student held the wire near the magnet again, but in a different direction. To Oersteds surprise, the pointer of the magnet swung toward the wire so it was no longer pointing to Earths magnetic north pole. Oersted was intrigued. He turned off the current in the wire to see what would happen to the magnet. The pointer swung back to its original position, pointing north again. Oersted had discovered that an electric current creates a magnetic field. The magnetic field created by the current was strong enough to attract the pointer of the nearby compass. Oersted wanted to learn more about the magnetic field created by a current, so he placed a magnet at different locations around a wire with current flowing through it. You can see some of his results in Figure 25.2. They show that the magnetic field created by a current has field lines that circle around the wire. You can learn more about Oersteds investigations of current and magnetism at the URL below. MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0763 | electricity and magnetism | T_3898 | The magnetic field created by a current flowing through a wire actually surrounds the wire in concentric circles. This magnetic field is stronger if more current is flowing through the wire. The direction of the magnetic field also depends on the direction that the current is flowing through the wire. A simple rule, called the right hand rule, makes it easy to find the direction of the magnetic field if the direction of the current is known. The right hand rule is illustrated in Figure 25.3. When the thumb of the right hand is pointing in the same direction as the current, the fingers of the right hand curl around the wire in the direction of the magnetic field. You can see the right hand rule in action at this URL: . | text | null |
L_0764 | using electromagnetism | T_3899 | A solenoid is a coil of wire with electric current flowing through it, giving it a magnetic field (see Figure 25.5). Recall that current flowing through a straight wire produces a weak electromagnetic field that circles around the wire. Current flowing through a coil of wire, in contrast, produces a magnetic field that has north and south poles like a bar magnet. The magnetic field around a coiled wire is also stronger than the magnetic field around a straight wire because each turn of the wire has its own magnetic field. Adding more turns increases the strength of the field, as does increasing the amount of current flowing through the coil. You can see an actual solenoid with a compass showing its magnetic north pole at this URL: . | text | null |
L_0764 | using electromagnetism | T_3900 | Solenoids are the basis of electromagnets. An electromagnet is a solenoid wrapped around a bar of iron or other ferromagnetic material (see Figure 25.6). The electromagnetic field of the solenoid magnetizes the iron bar by aligning its magnetic domains. The combined magnetic force of the magnetized iron bar and the wire coil makes an electromagnet very strong. In fact, electromagnets are the strongest magnets made. Some of them are strong enough to lift a train. The maglev train described earlier, in the lesson "Electricity and Magnetism," contains permanent magnets. Strong electromagnets in the track repel the train magnets, causing the train to levitate above the track. Like a solenoid, an electromagnet is stronger if there are more turns in the coil or more current is flowing through it. A bigger bar or one made of material that is easier to magnetize also increases an electromagnets strength. You can see how to make a simple electromagnet at this URL: (4:57). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0764 | using electromagnetism | T_3901 | Many common electric devices contain electromagnets. Some examples include hair dryers, fans, CD players, telephones, and doorbells. Most electric devices that have moving parts contain electric motors. You can read below how doorbells and electric motors use electromagnets. | text | null |
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