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L_0836 | color | T_4249 | An opaque object is one that doesnt let light pass through it. Instead, it reflects or absorbs the light that strikes it. Many objects, such as the leaves pictured in the Figure 1.3, reflect just one or a few wavelengths of visible light and absorb the rest. The wavelengths that are reflected determine the color that an object appears to the human eye. For example, the leaves appear green because they reflect green light and absorb light of other wavelengths. A transparent or translucent material, such as window glass, transmits some or all of the light that strikes it. This means that the light passes through the material rather than being reflected by it. In this case, we see the material because of the transmitted light. Therefore, the wavelength of the transmitted light determines the color that the object appears. Look at the beautiful stained glass windows in the Figure 1.4. The different colors of glass transmit The color of light that strikes an object may also affect the color that the object appears. For example, if only blue light strikes green leaves, the blue light is absorbed and no light is reflected. Q: What color do you see if an object absorbs all of the light that strikes it? A: When all of the light is absorbed, none is reflected, so the object looks black. But black isnt a color of light. Black is the absence of light. | text | null |
L_0836 | color | T_4250 | The human eye can distinguish only red, green, and blue light. These three colors are called the primary colors of light. All other colors of light can be created by combining the primary colors. Look at the Venn diagram 1.5. Red and green light combine to form yellow light. Red and blue light combine to form magenta light, and blue and green light combine to form cyan light. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram, where all three primary colors of light combine. The result is white light. | text | null |
L_0836 | color | T_4251 | Many objects have color because they contain pigments. A pigment is a substance that colors materials by reflecting light of certain wavelengths and absorbing light of other wavelengths. A very common pigment is the dark green pigment called chlorophyll, which is found in plants. Chlorophyll absorbs all but green wavelengths of visible light. Pigments are also found in many manufactured products. They are used to color paints, inks, and dyes. Just three pigments, called primary pigments, can be combined to produce all other colors. The primary colors of pigments are the same as the secondary colors of light: cyan, magenta, and yellow. Q: A color printer needs just three colors of ink to print all of the colors that we can see. Which colors are they? A: The three colors of ink in a color printer are the three primary pigment colors: cyan, magenta, and yellow. These three colors can be combined in different ratios to produce all other colors, so they are the only colors needed for full-color printing. | text | null |
L_0837 | combining forces | T_4252 | You have probably heard of the famous equation E = mc2 . The "E" represent the amount of energy. The "m" represents mass. The "c" represent the speed of light. Writing a "c" is much easier than writing the actual speed of light. The speed of light is a really large number. The speed of light is about 300 million meters per second. Thats really, really fast. Light always travels at the same speed through space. In outer space, there is not any matter to get in its way. Think about riding your bicycle. When you ride on a hard surface, it is easy to pedal. You can go really fast. Imagine how your speed would change if you were riding through deep sand. You would find it hard to pedal. You would not be able to go as fast. The same is true for light. When there is no matter around, like in outer space, it can go fast. When matter gets in its way, it slows down. Light travels through some matter faster than through others. Table 1.1 gives the speed of light in six common materials. Material Air Water Glass Vegetable oil Alcohol Diamond Speed of Light (m/s) 299 million meters per second 231 million meters per second 200 million meters per second 150 million meters per second 140 million meters per second 125 million meters per second No matter how slow light travels, it still goes really, really fast. The important thing to remember is that it does travel. It is hard for us to imagine light taking time to cover a distance. Think about when you enter your science classroom. You step through the door. You tell your teacher, "Hello." You walk to your desk and sit down. It may take around 10 to 20 seconds to walk this distance. Imagine now your teacher turns the light off. She carries a small lamp over to the door you just entered. She asks you to watch carefully as she switches on the light. She flips the switch and you immediately see the light. The light just covered the same distance you just walked. Thats how fast light is. For us, it is hard to imagine that it moves. Now lets think about light traveling between the Sun and Earth. The Sun is 93 million miles away. What if we were able to turn off the Sun for just a second? How long would it take us to notice? Would we notice instantly like in the classroom? Remember, the Sun is a long way away. We wouldnt notice the change for a little over 8 minutes. That is because the Sun is a long way away. Even when moving as fast as light, it takes time to travel from the Sun to Earth. What do you think happens when it hits the air in our atmosphere? Air is made up of matter. When light travels through matter it slows down. How do scientists know it slows down? What evidence do scientists have? When sunlight hits Earths atmosphere it bends just a little. If sunlight goes through water droplets it bends even more. The bending of light through droplets of water is why we can see rainbows. It also explains why the straw in a glass of water appears to be broken. | text | null |
L_0837 | combining forces | T_4253 | When light passes from one medium (or type of matter) to another, it changes speed. You can actually see this happen. If light strikes a new substance at an angle, the light appears to bend. This is what explains the straw looking broken in the picture above. So, does light always bend as it travels into a new medium? If light travels straight into a new substance it is not bent. You may know this angle as perpendicular. The light still slows down, just does not appear to bend. Any angle other than perpendicular the light will bend as it slows down. The bending of light is called refraction. Figure 1.1 shows how refraction occurs. Notice that the angle of light changes again as it passes from the glass back to the air. In this case, the speed increases, and the ray of light resumes its initial direction. For a more detailed explanation of refraction, watch this video: Click image to the left or use the URL below. URL: | text | null |
L_0838 | combustion reactions | T_4254 | A combustion reaction occurs when a substance reacts quickly with oxygen (O2 ). For example, in the Figure usually referred to as fuel. The products of a complete combustion reaction include carbon dioxide (CO2 ) and water vapor (H2 O). The reaction typically gives off heat and light as well. The general equation for a complete combustion reaction is: Fuel + O2 CO2 + H2 O The burning of charcoal is a combustion reaction. | text | null |
L_0838 | combustion reactions | T_4255 | The fuel that burns in a combustion reaction contains compounds called hydrocarbons. Hydrocarbons are compounds that contain only carbon (C) and hydrogen (H). The charcoal pictured in the Figure 1.1 consists of hydrocarbons. So do fossil fuels such as natural gas. Natural gas is a fuel that is commonly used in home furnaces and gas stoves. The main component of natural gas is the hydrocarbon called methane (CH4 ). You can see a methane flame in the Figure 1.2. The combustion of methane is represented by the equation: CH4 + 2O2 CO2 + 2H2 O The combustion of methane gas heats a pot on a stove. Q: Sometimes the flame on a gas stove isnt just blue but has some yellow or orange in it. Why might this occur? A: If the flame isnt just blue, the methane isnt getting enough oxygen to burn completely, leaving some of the carbon unburned. The flame will also not be as hot as a completely blue flame for the same reason. | text | null |
L_0840 | compound machine | T_4258 | A compound machine is a machine that consists of more than one simple machine. Some compound machines consist of just two simple machines. You can read below about two examplesthe wheelbarrow and corkscrew. Other compound machines, such as bicycles, consist of many simple machines. Big compound machines such as cars may consist of hundreds or even thousands of simple machines. | text | null |
L_0840 | compound machine | T_4259 | Look at the wheelbarrow in the Figure 1.1. It is used to carry heavy objects. It consists of two simple machines: a lever and a wheel and axle. Effort is applied to the lever by picking up the handles of the wheelbarrow. The lever, in turn, applies upward force to the load. The force is increased by the lever, making the load easier to lift. Effort is applied to the wheel of the wheelbarrow by pushing it over the ground. The rolling wheel turns the axle and increases the force, making it easier to push the load. | text | null |
L_0840 | compound machine | T_4260 | The corkscrew in the Figure 1.2 is also a compound machine. It is used to pierce a cork and pull it out of the neck of a bottle. It consists of a screw and two levers. Turning the handle on top twists the screw down into the center of the cork. Then, pushing down on the two levers causes the screw to pull upward, bringing the cork with it. The levers increase the force and change its direction. | text | null |
L_0840 | compound machine | T_4260 | The corkscrew in the Figure 1.2 is also a compound machine. It is used to pierce a cork and pull it out of the neck of a bottle. It consists of a screw and two levers. Turning the handle on top twists the screw down into the center of the cork. Then, pushing down on the two levers causes the screw to pull upward, bringing the cork with it. The levers increase the force and change its direction. | text | null |
L_0840 | compound machine | T_4261 | Friction is a force that opposes motion between any surfaces that are touching. All machines have moving parts and friction, so they have to use some of the work that is applied to them to overcome friction. This makes all machines less than 100 percent efficient. Because compound machines have more moving parts than simple machines, they generally have more friction to overcome. As a result, compound machines tend to have lower efficiency than simple machines. When a compound machine consists of many simple machines, friction can become a serious problem, and it may produce a lot of heat. Lubricants such as oil or grease may be used to coat the moving parts of a machine so they slide over each other more easily. This is how friction is reduced in a car engine. | text | null |
L_0840 | compound machine | T_4262 | The mechanical advantage of a machine is the factor by which it changes the force applied to the machine. Many machines increase the force applied to them, and this is how they make work easier. Compound machines tend to have a greater mechanical advantage than simple machines. Thats because the mechanical advantage of a compound machine equals the product of the mechanical advantages of all its component simple machines. The greater the number of simple machines it contains, the greater its mechanical advantage tends to be. Q: Assume that the lever and the wheel and axle of a wheelbarrow each have a mechanical advantage of 2. What is the mechanical advantage of the wheelbarrow? A: The mechanical advantage of the wheelbarrow is the product of the mechanical advantage of the lever (2) and the mechanical advantage of the wheel and axle (2). Therefore, the wheelbarrow has a mechanical advantage of 4. | text | null |
L_0841 | compounds | T_4263 | A compound is a unique substance that forms when two or more elements combine chemically. For example, the compound carbon dioxide forms when one atom of carbon (grey in the model above) combines with two atoms of oxygen (red in the model). Another example of a compound is water. It forms when two hydrogen atoms combine with one oxygen atom. Click image to the left or use the URL below. URL: Q: How could a water molecule be represented? A: It could be represented by a model like the one for carbon dioxide in the opening image. You can see a sample Figure 1.1. A model of water. Two things are true of all compounds: A compound always has the same elements in the same proportions. For example, carbon dioxide always has two atoms of oxygen for each atom of carbon, and water always has two atoms of hydrogen for each atom of oxygen. A compound always has the same composition throughout. For example, all the carbon dioxide in the atmosphere and all the water in the ocean have these same proportions of elements. Q: How do you think the properties of compounds compare with the properties of the elements that form them? A: You might expect the properties of a compound to be similar to the properties of the elements that make up the compound. But you would be wrong. | text | null |
L_0841 | compounds | T_4264 | The properties of compounds are different from the properties of the elements that form themsometimes very different. Thats because elements in a compound combine and become an entirely different substance with its own unique properties. Do you put salt on your food? Table salt is the compound sodium chloride. It contains sodium and chlorine. As shown in the Figure 1.2, sodium is a solid that reacts explosively with water, and chlorine is a poisonous gas. But together in table salt, sodium and chlorine form a harmless unreactive compound that you can safely eat. Q: The compound sodium chloride is very different from the elements sodium and chlorine that combine to form it. What are some properties of sodium chloride? A: Sodium chloride is an odorless white solid that is harmless unless consumed in large quantities. In fact, it is a necessary component of the human diet. | text | null |
L_0841 | compounds | T_4265 | Compounds like sodium chloride form structures called crystals. A crystal is a rigid framework of many ions locked together in a repeating pattern. Ions are electrically charged forms of atoms. You can see a crystal of sodium chloride in the Figure 1.3. It is made up of many sodium and chloride ions. Sodium and chlorine combine to form sodium chloride, or table salt. A sodium chloride crystal consists of many sodium ions (blue) and chloride ions (green) arranged in a rigid framework. Click image to the left or use the URL below. URL: Compounds such as carbon dioxide and water form molecules instead of crystals. A molecule is the smallest particle of a compound that still has the compounds properties. It consists of two or more atoms bonded together. You saw models of carbon dioxide and water molecules above. | text | null |
L_0841 | compounds | T_4265 | Compounds like sodium chloride form structures called crystals. A crystal is a rigid framework of many ions locked together in a repeating pattern. Ions are electrically charged forms of atoms. You can see a crystal of sodium chloride in the Figure 1.3. It is made up of many sodium and chloride ions. Sodium and chlorine combine to form sodium chloride, or table salt. A sodium chloride crystal consists of many sodium ions (blue) and chloride ions (green) arranged in a rigid framework. Click image to the left or use the URL below. URL: Compounds such as carbon dioxide and water form molecules instead of crystals. A molecule is the smallest particle of a compound that still has the compounds properties. It consists of two or more atoms bonded together. You saw models of carbon dioxide and water molecules above. | text | null |
L_0843 | conservation of energy in chemical reactions | T_4269 | All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. Like the combustion reaction in a furnace, some chemical reactions require less energy to break bonds in reactants than is released when bonds form in products. These reactions, called exothermic reactions, release energy. In other chemical reactions, it takes more energy to break bonds in reactants than is released when bonds form in products. These reactions, called endothermic reactions, absorb energy. | text | null |
L_0843 | conservation of energy in chemical reactions | T_4270 | Whether a chemical reaction absorbs or releases energy, there is no overall change in the amount of energy during the reaction. Thats because energy cannot be created or destroyed. This is the law of conservation of energy. Energy may change form during a chemical reactionfor example, from chemical energy to heat energy when gas burns in a furnacebut the same amount of energy remains after the reaction as before. This is true of all chemical reactions. Q: If energy cant be destroyed during a chemical reaction, what happens to the energy that is absorbed in an endothermic reaction? A: The energy is stored in the bonds of the products as chemical energy. In an endothermic reaction, the products have more stored chemical energy than the reactants. This is represented by the graph on the left in the Figure 1.1. In an exothermic reaction, the opposite is true. The products have less stored chemical energy than the reactants. You can see this in the graph on the right in the Figure 1.1. Note: H represents the change in en- ergy. Q: What happens to the excess energy in the reactants of an exothermic reaction? A: The excess energy is generally released to the surroundings when the reaction occurs. In a home heating system, for example, the energy that is released during combustion in the furnace is used to heat the home. | text | null |
L_0845 | conservation of mass and energy in nuclear reactions | T_4273 | Einsteins equation is possibly the best-known equation of all time. Theres reason for that. The equation is incredibly important. It changed how scientists view energy and matter, which are two of the most basic concepts in all of science. The equation shows that energy and matter are two forms of the same thing. This new idea turned science upside down when Einstein introduced it in the early 1900s. Amazingly, the idea has withstood the test of time as more and more evidence has been gathered to support it. You can listen to an explanation of Einsteins equation at URL: https://youtu.be/hW7DW9NIO9M Q: What do the letters in Einsteins equation stand for? A: E stands for energy, m stands for mass, and c stands for the speed of light. The speed of light is 300,000 kilometers (186,000 miles) per second, so c2 is a very big number. Therefore, the amount of energy in even a small mass of matter is tremendous. Suppose, for example, that you have 1 gram of matter. Thats about the mass of a paperclip. Multiplying this mass by c2 would yield enough energy to power 3,600 homes for a year! | text | null |
L_0845 | conservation of mass and energy in nuclear reactions | T_4274 | Einsteins equation helps scientists understand what happens in nuclear reactions and why they produce so much energy. When the nucleus of a radioisotope undergoes fission or fusion in a nuclear reaction, it loses a tiny amount of mass. What happens to the lost mass? It isnt really lost at all. It is converted to energy. How much energy? E = mc2 . The change in mass is tiny, but it results in a great deal of energy. Q: In a nuclear reaction, mass decreases and energy increases. What about the laws of conservation of mass and conservation of energy? Are mass and energy not conserved in nuclear reactions? Do we need to throw out these laws when it comes to nuclear reactions? A: No, the laws still apply. However, its more correct to say that the sum of mass and energy is always conserved in a nuclear reaction. Mass changes to energy, but the total amount of mass and energy combined remains the same. | text | null |
L_0846 | conservation of mass in chemical reactions | T_4275 | A chemical reaction occurs when some substances change chemically to other substances. Chemical reactions are represented by chemical equations. Consider a simple chemical reaction, the burning of methane. In this reaction, methane (CH4 ) combines with oxygen (O2 ) in the air and produces carbon dioxide (CO2 ) and water vapor (H2 O). The reaction is represented by the following chemical equation: CH4 + 2O2 CO2 + 2H2 O This equation shows that one molecule of methane combines with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water vapor. All chemical equations must be balanced. This means that the same number of each type of atom must appear on both sides of the arrow. Q: Is the chemical equation for the burning of methane balanced? Count the atoms of each type on both sides of the arrow to find out. A: Yes, the equation is balanced. There is one carbon atom on both sides of the arrow. There are also four hydrogen atoms and four oxygen atoms on both sides of the arrow. | text | null |
L_0846 | conservation of mass in chemical reactions | T_4276 | Why must chemical equations be balanced? Its the law! Matter cannot be created or destroyed in chemical reactions. This is the law of conservation of mass. In every chemical reaction, the same mass of matter must end up in the products as started in the reactants. Balanced chemical equations show that mass is conserved in chemical reactions. | text | null |
L_0846 | conservation of mass in chemical reactions | T_4277 | How do scientists know that mass is always conserved in chemical reactions? Careful experiments in the 1700s by a French chemist named Antoine Lavoisier led to this conclusion. Lavoisier carefully measured the mass of reactants and products in many different chemical reactions. He carried out the reactions inside a sealed jar, like the one in the Figure 1.1. In every case, the total mass of the jar and its contents was the same after the reaction as it was before the reaction took place. This showed that matter was neither created nor destroyed in the reactions. Another outcome of Lavoisiers research was the discovery of oxygen. Click image to the left or use the URL below. URL: Q: Lavoisier carried out his experiments inside a sealed glass jar. Why was sealing the jar important for his results? What might his results have been if he hadnt sealed the jar? A: Sealing the jar was important so that any gases produced in the reactions were captured and could be measured. If he hadnt sealed the jar, gases might have escaped detection. Then his results would have shown that there was less mass after the reactions than before. In other words, he would not have been able to conclude that mass is conserved in chemical reactions. | text | null |
L_0847 | convection | T_4278 | Convection is the transfer of thermal energy by particles moving through a fluid (either a gas or a liquid). Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Convection is one of three ways that thermal energy can be transferred (the other ways are conduction and thermal radiation). Thermal energy is always transferred from matter with a higher temperature to matter with a lower temperature. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL: | text | null |
L_0847 | convection | T_4279 | The Figure 1.1 shows how convection occurs, using hot water in a pot as an example. When particles in one area of a fluid (in this case, the water at the bottom of the pot) gain thermal energy, they move more quickly, have more collisions, and spread farther apart. This decreases the density of the particles, so they rise up through the fluid. As they rise, they transfer their thermal energy to other particles of the fluid and cool off in the process. With less energy, the particles move more slowly, have fewer collisions, and move closer together. This increases their density, so they sink back down through the fluid. When they reach the bottom of the fluid, the cycle repeats. The result is a loop of moving particles called a convection current. | text | null |
L_0847 | convection | T_4280 | Convection currents transfer thermal energy through many fluids, not just hot water in a pot. For example, convection currents transfer thermal energy through molten rock below Earths surface, through water in the oceans, and through air in the atmosphere. Convection currents in the atmosphere create winds. You can see one way this happens in the Figure 1.2. The land heats up and cools off faster than the water because it has lower specific heat. Therefore, the land gets warmer during the day and cooler at night than the water does. During the day, warm air rises above the land and cool air from the water moves in to take its place. During the night, the opposite happens. Warm air rises above the water and cool air from the land moves out to take its place. Q: During the day, in which direction is thermal energy of the air transferred? In which direction is it transferred during the night? A: During the day, thermal energy is transferred from the air over the land to the air over the water. During the night, thermal energy is transferred in the opposite direction. | text | null |
L_0848 | cooling systems | T_4281 | A refrigerator is an example of a cooling system. Another example is an air conditioner. The purpose of any cooling system is to transfer thermal energy in order to keep things cool. A refrigerator, for example, transfers thermal energy from the cool air inside the refrigerator to the warm air in the kitchen. If youve ever noticed how warm the back of a running refrigerator gets, then you know that it releases a lot of thermal energy into the room. Q: Thermal energy always moves from a warmer area to a cooler area. How can thermal energy move from the cooler air inside a refrigerator to the warmer air in a room? A: The answer is work. | text | null |
L_0848 | cooling systems | T_4282 | A refrigerator must do work to reverse the normal direction of thermal energy flow. Work involves the use of force to move something, and doing work takes energy. In a refrigerator, the energy is usually provided by electricity. You can read in detail in the Figure 1.1 how a refrigerator does its work. | text | null |
L_0848 | cooling systems | T_4283 | The key to how a refrigerator or other cooling system works is the refrigerant. A refrigerant is a substance such as FreonTM that has a low boiling point and changes between liquid and gaseous states as it passes through the refrigerator. As a liquid, the refrigerant absorbs thermal energy from the cool air inside the refrigerator and changes to a gas. As a gas, it transfers thermal energy to the warm air outside the refrigerator and changes back to a liquid. Work is done by a refrigerator to move the refrigerant through the different components of the refrigerator. | text | null |
L_0849 | covalent bonding | T_4284 | A covalent bond is the force of attraction that holds together two atoms that share a pair of valence electrons. The shared electrons are attracted to the nuclei of both atoms. This forms a molecule consisting of two or more atoms. Covalent bonds form only between atoms of nonmetals. | text | null |
L_0849 | covalent bonding | T_4285 | The two atoms that are held together by a covalent bond may be atoms of the same element or different elements. When atoms of different elements form covalent bonds, a new substance, called a covalent compound, results. Water is an example of a covalent compound. A water molecule is modeled in the Figure 1.1. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. Q: How many valence electrons does the oxygen atom (O) share with each hydrogen atom (H)? How many covalent bonds hold the water molecule together? A: The oxygen atom shares one pair of valence electrons with each hydrogen atom. Each pair of shared electrons represents one covalent bond, so two covalent bonds hold the water molecule together. The diagram in the Figure 1.2 shows an example of covalent bonds between two atoms of the same element, in this case two atoms of oxygen. The diagram represents an oxygen molecule, so its not a new compound. Oxygen normally occurs in diatomic (two-atom) molecules. Several other elements also occur as diatomic molecules: hydrogen, nitrogen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). Q: How many electrons do these two oxygen atoms share? How many covalent bonds hold the oxygen molecule together? A: The two oxygen atoms share two pairs of electrons, so two covalent bonds hold the oxygen molecule together. | text | null |
L_0849 | covalent bonding | T_4285 | The two atoms that are held together by a covalent bond may be atoms of the same element or different elements. When atoms of different elements form covalent bonds, a new substance, called a covalent compound, results. Water is an example of a covalent compound. A water molecule is modeled in the Figure 1.1. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. Q: How many valence electrons does the oxygen atom (O) share with each hydrogen atom (H)? How many covalent bonds hold the water molecule together? A: The oxygen atom shares one pair of valence electrons with each hydrogen atom. Each pair of shared electrons represents one covalent bond, so two covalent bonds hold the water molecule together. The diagram in the Figure 1.2 shows an example of covalent bonds between two atoms of the same element, in this case two atoms of oxygen. The diagram represents an oxygen molecule, so its not a new compound. Oxygen normally occurs in diatomic (two-atom) molecules. Several other elements also occur as diatomic molecules: hydrogen, nitrogen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). Q: How many electrons do these two oxygen atoms share? How many covalent bonds hold the oxygen molecule together? A: The two oxygen atoms share two pairs of electrons, so two covalent bonds hold the oxygen molecule together. | text | null |
L_0849 | covalent bonding | T_4286 | Covalent bonds form because they give atoms a more stable arrangement of electrons. Look at the oxygen atoms in the Figure 1.2. Alone, each oxygen atom has six valence electrons. By sharing two pairs of valence electrons, each oxygen atom has a total of eight valence electrons. This fills its outer energy level, giving it the most stable arrangement of electrons. The shared electrons are attracted to both oxygen nuclei, and this force of attraction holds the two atoms together in the oxygen molecule. | text | null |
L_0850 | crystalline carbon | T_4287 | Graphite is one of three forms of crystalline, or crystal-forming, carbon. Carbon also exists in an amorphous, or shapeless, form in substances such as coal and charcoal. Different forms of the same element are called allotropes. Besides graphite, the other allotropes of crystalline carbon are diamond and fullerenes. All three forms exist as crystals rather than molecules. In a crystal, many atoms are bonded together in a repeating pattern that may contains thousands of atoms. The arrangement of atoms in the crystal differs for each form of carbon and explains why the different forms have different properties. Click image to the left or use the URL below. URL: Q: How do you think the properties of diamond might differ from the properties of graphite? A: Diamond is clear whereas graphite is black. Diamond is also very hard, so it doesnt break easily. Graphite, in contrast, is soft and breaks very easily. | text | null |
L_0850 | crystalline carbon | T_4288 | Diamond is a form of carbon in which each carbon atom is covalently bonded to four other carbon atoms. This forms a strong, rigid, three-dimensional structure (see Figure 1.1). Diamond is the hardest natural substance, and no other natural substance can scratch it. This property makes diamonds useful for cutting and grinding tools as well as for rings and other jewelry (see Figure 1.2). | text | null |
L_0850 | crystalline carbon | T_4288 | Diamond is a form of carbon in which each carbon atom is covalently bonded to four other carbon atoms. This forms a strong, rigid, three-dimensional structure (see Figure 1.1). Diamond is the hardest natural substance, and no other natural substance can scratch it. This property makes diamonds useful for cutting and grinding tools as well as for rings and other jewelry (see Figure 1.2). | text | null |
L_0850 | crystalline carbon | T_4289 | Graphite is a form of crystalline carbon in which each carbon atom is covalently bonded to three other carbon atoms. The carbon atoms are arranged in layers, with strong bonds within each layer but only weak bonds between layers (see Figure 1.3). The weak bonds between layers allow the layers to slide over one another, so graphite is relatively soft and slippery. This makes it useful as a lubricant. Q: Why do graphites properties make it useful for pencil leads? A: Being slippery, graphite slides easily over paper when you write. Being soft, it rubs off on the paper, allowing you to leave marks. Graphites softness also allows you to sharpen it easily. (Imagine trying to sharpen a diamond!) | text | null |
L_0850 | crystalline carbon | T_4290 | A fullerene (also called a Bucky ball) is a form of carbon in which carbon atoms are arranged in a hollow sphere resembling a soccer ball (see Figure 1.4). Each sphere contains 60 carbon atoms, and each carbon atom is bonded to three others by single covalent bonds. The bonds are relatively weak, so fullerenes can dissolve and form solutions. Fullerenes were first discovered in 1985 and have been found in soot and meteorites. Possible commercial uses of fullerenes are under investigation. Fullerene Crystal | text | null |
L_0851 | daltons atomic theory | T_4291 | Around 1800, the English chemist John Dalton brought back Democritus ancient idea of the atom. You can see a picture of Dalton 1.1. Dalton grew up in a working-class family. As an adult, he made a living by teaching and just did research in his spare time. Nonetheless, from his research he developed one of the most important theories in all of science. Based on his research results, he was able to demonstrate that atoms actually do exist, something that Democritus had only guessed. | text | null |
L_0851 | daltons atomic theory | T_4292 | Dalton did many experiments that provided evidence for the existence of atoms. For example: He investigated pressure and other properties of gases, from which he inferred that gases must consist of tiny, individual particles that are in constant, random motion. He researched the properties of compounds, which are substances that consist of more than one element. He showed that a given compound is always comprised of the same elements in the same whole-number ratio and that different compounds consist of different elements or ratios. This can happen, Dalton reasoned, only if elements are made of separate, discrete particles that cannot be subdivided. | text | null |
L_0851 | daltons atomic theory | T_4293 | From his research, Dalton developed a theory about atoms. Daltons atomic theory consists of three basic ideas: All substances are made of atoms. Atoms are the smallest particles of matter. They cannot be divided into smaller particles, created, or destroyed. All atoms of the same element are alike and have the same mass. Atoms of different elements are different and have different masses. Atoms join together to form compounds, and a given compound always consists of the same kinds of atoms in the same proportions. Daltons atomic theory was accepted by many scientists almost immediately. Most of it is still accepted today. However, scientists now know that atoms are not the smallest particles of matter. Atoms consist of several types of smaller particles, including protons, neutrons, and electrons. | text | null |
L_0851 | daltons atomic theory | T_4294 | Because Dalton thought atoms were the smallest particles of matter, he envisioned them as solid, hard spheres, like billiard (pool) balls, so he used wooden balls to model them. Three of his model atoms are pictured in the Figure and used to model compounds. Q: When scientists discovered smaller particles inside the atom, they realized that Daltons atomic models were too simple. How do modern atomic models differ from Daltons models? A: Modern atomic models, like the one pictured at the top of this article, usually represent subatomic particles, including electrons, protons, and neutrons. | text | null |
L_0852 | dangers and uses of radiation | T_4295 | A low level of radiation occurs naturally in the environment. This is called background radiation. One source of background radiation is rocks, which may contain small amounts of radioactive elements such as uranium. Another source is cosmic rays. These are charged particles that arrive on Earth from outer space. Background radiation is generally considered to be safe for living things. | text | null |
L_0852 | dangers and uses of radiation | T_4296 | Long-term or high-dose exposure to radiation can harm both living and nonliving things. Radiation knocks electrons out of atoms and changes them to ions. It also breaks bonds in DNA and other compounds in living things. One source of radiation that is especially dangerous to people is radon. Radon is a radioactive gas that forms in rocks underground. It can seep into basements and get trapped inside buildings. Then it may build up and become harmful to people who breathe it. Long-term exposure to radon can cause lung cancer. Exposure to higher levels of radiation can be very dangerous, even if the exposure is short-term. A single large dose of radiation can burn the skin and cause radiation sickness. Symptoms of this illness include extreme fatigue, destruction of blood cells, and loss of hair. Nonliving things can also be damaged by radiation. For example, high levels of radiation can weaken metals by removing electrons. This is a problem in nuclear power plants and space vehicles because they are exposed to very high levels of radiation. Q: Can you tell when you are being exposed to radiation? For example, can you sense radon in the air? A: Radiation cant be detected with the senses. This adds to its danger. However, there are other ways to detect it. | text | null |
L_0852 | dangers and uses of radiation | T_4297 | You generally cant see, smell, taste, hear, or feel radiation. Fortunately, there are devices such as Geiger counters that can detect radiation. A Geiger counter, like the one pictured in the Figure 1.1, contains atoms of a gas that is ionized if it encounters radiation. When this happens, the gas atoms change to ions that can carry an electric current. The current causes the Geiger counter to click. The faster the clicks occur, the higher the level of radiation. | text | null |
L_0852 | dangers and uses of radiation | T_4298 | Despite its dangers, radioactivity has several uses. For example, it can be used to determine the ages of ancient rocks and fossils. It can also be used as a source of power to generate electricity. Radioactivity can even be used to diagnose and treat diseases, including cancer. Cancer cells grow rapidly and take up a lot of glucose for energy. Glucose containing radioactive elements can be given to patients. Cancer cells take up more of the glucose than normal cells do and give off radiation. The radiation can be detected with special machines like the one in the Figure 1.2. The radiation may also kill cancer cells. | text | null |
L_0853 | decomposition reactions | T_4299 | A decomposition reaction occurs when one reactant breaks down into two or more products. It can be represented by the general equation: AB A + B In this equation, AB represents the reactant that begins the reaction, and A and B represent the products of the reaction. The arrow shows the direction in which the reaction occurs. Q: What is the chemical equation for the decomposition of hydrogen peroxide (H2 O2 ) to water (H2 O) and oxygen (O2 )? A: The equation for this decomposition reaction is: 2 H2 O2 2 H2 O + O2 | text | null |
L_0853 | decomposition reactions | T_4300 | Two more examples of decomposition reactions are described below. Carbonic acid (H2 CO3 ) is an ingredient in soft drinks. A decomposition reaction takes place when carbonic acid breaks down to produce water (H2 O) and carbon dioxide (CO2 ). This occurs when you open a can of soft drink and some of the carbon dioxide fizzes out. The equation for this reaction is: H2 CO3 H2 O + CO2 Another decomposition reaction occurs when water (H2 O) breaks down to produce hydrogen (H2 ) and oxygen (O2 ) gases (see Figure 1.1). This happens when an electric current passes through the water, as illustrated below. The equation for this reaction is: 2 H2 O 2 H2 + O2 Decomposition of water. Q: What ratio of hydrogen molecules (H2 ) to oxygen molecules (O2 ) is produced in the decomposition of water? A: Two hydrogen molecules per oxygen molecule are produced because water (H2 O) has a ratio of two hydrogen atoms to one oxygen atom. | text | null |
L_0854 | democrituss idea of the atom | T_4301 | Democritus lived in Greece from about 460 to 370 B.C.E. Like many other ancient Greek philosophers, he spent a lot of time wondering about the natural world. Democritus wondered, for example, what would happen if you cut a chunk of mattersuch as a piece of cheese into smaller and smaller pieces. He thought that a point would be reached at which the cheese could not be cut into still smaller pieces. He called these pieces atomos, which means uncuttable in Greek. This is where the modern term atom comes from. | text | null |
L_0854 | democrituss idea of the atom | T_4302 | Democritus idea of the atom has been called the best guess in antiquity. Thats because it was correct in many ways, yet it was based on pure speculation. It really was just a guess. Heres what Democritus thought about the atom: How many times could you cut this piece of cheese in half? How small would the smallest pieces be? All matter consists of atoms, which cannot be further subdivided into smaller particles. Atoms are extremely smalltoo small to see. Atoms are solid particles that are indestructible. Atoms are separated from one another by emptiness, or void. Q: How are Democrituss ideas about atoms similar to modern ideas about atoms? A: Modern ideas agree that all matter is made up of extremely small building blocks called atoms. Q: How are Democrituss ideas different from modern ideas? A: Although atoms are extremely small, it is now possible to see them with very powerful microscopes. Atoms also arent the solid, uncuttable particles Democritus thought. Instead, they consist of several kinds of smaller, simpler particles as well as a lot of empty space. In addition, atoms arent really indestructible because they can be changed to other forms of matter or energy. | text | null |
L_0854 | democrituss idea of the atom | T_4303 | Did you ever notice dust motes moving in still air where a beam of sunlight passes through it? You can see an example in the forest scene in the Figure 1.2. This sort of observation gave Democritus the idea that atoms are in constant, random motion. If this were true, Democritus thought, then atoms must always be bumping into each other. When they do, he surmised, they either bounce apart or stick together to form clumps of atoms. Eventually, the clumps could grow big enough to be visible matter. Q: Which modern theory of matter is similar to Democritus ideas about the motion of atoms? A: The modern kinetic theory of matter is remarkably similar to Democritus ideas about the motion of atoms. According to this theory, atoms of matter are in constant random motion. This motion is greater in gases than in liquids, and it is greater in liquids than in solids. But even in solids, atoms are constantly vibrating in place. | text | null |
L_0854 | democrituss idea of the atom | T_4304 | Democritus thought that different kinds of matter vary because of the size, shape, and arrangement of their atoms. For example, he suggested that sweet substances are made of smooth atoms and bitter substances are made of sharp atoms. He speculated that atoms of liquids are slippery, which allows them to slide over each other and liquids to flow. Atoms of solids, in contrast, stick together, so they cannot move apart. Differences in the weight of matter, he argued, could be explained by the closeness of atoms. Atoms of lighter matter, he thought, were more spread out and separated by more empty space. Q: Democritus thought that different kinds of atoms make up different types of matter. How is this similar to modern ideas about atoms? A: The modern view is that atoms of different elements differ in their numbers of protons and electrons and this gives them different physical and chemical properties. Dust motes dance in a beam of sunlight. | text | null |
L_0854 | democrituss idea of the atom | T_4305 | Democritus was an important philosopher, but he was less influential than another Greek philosopher named Aristo- tle, who lived about 100 years after Democritus. Aristotle rejected Democritus idea of the atom. In fact, Aristotle thought the idea was ridiculous. Unfortunately, Aristotles opinion was accepted for more than 2000 years, and Democritus idea was more or less forgotten. However, the idea of the atom was revived around 1800 by the English scientist John Dalton. Dalton developed an entire theory about the atom, much of which is still accepted today. He based his theory on experimental evidence, not on lucky guesses. | text | null |
L_0857 | descriptive statistics | T_4310 | The girls in the picture above make up a small samplethere are only four of them. In scientific investigations, samples may include hundreds or even thousands of people or other objects of study. Especially when samples are very large, its important to be able to summarize their overall characteristics with a few numbers. Thats where descriptive statistics come in. Descriptive statistics are measures that show the central tendency, or center, of a sample or the variation in a sample. | text | null |
L_0857 | descriptive statistics | T_4311 | The central tendency of a sample can be represented by the mean, median, or mode. The mean is the average value. It is calculated by adding the individual measurements and dividing the sum by the total number of measurements. The median is the middle value. To find the median, rank all the measurements from smallest to largest and then find the measurement that is in the middle. The mode is the most common value. It is the value that occurs most often. Q: A sample of five children have the following heights: 60 cm, 58 cm, 54 cm, 62 cm, and 58 cm. What are the mean, median, and mode of this sample? A: The mean is (60 cm + 58 cm + 54 cm + 62 cm + 58 cm) 5 = 58 cm. The median and mode are both 58 cm as well. The mean, median, and mode are not always the same, as they are for this sample. In fact, sometimes these three statistics are very different from one another for the same sample. | text | null |
L_0857 | descriptive statistics | T_4312 | Many samples have a lot of variation in measurements. Variation can be described with a statistic called the range. The range is the total spread of values in a sample. It is calculated by subtracting the smallest value from the largest value. Q: What is the range of heights in the sample of children in the previous question? A: The range is 62 cm - 54 cm = 8 cm. | text | null |
L_0859 | direction | T_4315 | Direction can be described in relative terms, such as up, down, in, out, left, right, forward, backward, or sideways. Direction can also be described with the cardinal directions: north, south, east, or west. On maps, cardinal directions are indicated with a compass rose. You can see one in the bottom left corner of the map in the Figure 1.1. You can use the compass rose to find directions on the map. For example, to go to the school from Jordans house, you would travel from east to west. If you wanted to go on to the post office, you would change direction at the school and then travel from south to north. | text | null |
L_0859 | direction | T_4316 | Look again at the map in the Figure 1.1. The distance from Jordans house to the post office is 3 km. But if Jordan told a friend how to reach the post office from his house, he couldnt just say go 3 kilometers. The friend might end up at the park instead of the post office. Jordan would have to include direction as well as distance. He could say, go west for 2 kilometers and then go north for 1 kilometer. | text | null |
L_0859 | direction | T_4317 | When both distance and direction are considered, motion can be represented by a vector. A vector is a measurement that has both size and direction. It may be represented by an arrow. If you are representing motion with an arrow, the length of the arrow represents distance, and the way the arrow points represents direction. The red arrows on the map in the Figure 1.1 are vectors for Jordans route from his house to the school and from the school to the post office. Q: How would you draw arrows to represent the distances and directions from the post office to the park on the map in the Figure 1.1? A: The vectors would look like this: | text | null |
L_0861 | distance | T_4322 | Distance is the length of the route between two points. The distance of a race, for example, is the length of the track between the starting and finishing lines. In a 100-meter sprint, that distance is 100 meters. | text | null |
L_0861 | distance | T_4323 | The SI unit for distance is the meter (m). Short distances may be measured in centimeters (cm), and long distances may be measured in kilometers (km). For example, you might measure the distance from the bottom to the top of a sheet of paper in centimeters and the distance from your house to your school in kilometers. | text | null |
L_0861 | distance | T_4324 | Maps can often be used to measure distance. The map in the Figure 1.1 shows the route from Jordans house to his school. You can use the scale at the bottom of the map to measure the distance between these two points. Q: What is the distance from Jordans house to his school? A: The distance is 2 kilometers. | text | null |
L_0862 | doppler effect | T_4325 | The Doppler effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener. (It can also occur when the sound source is stationary and the listener is moving.) The Figure 1.1 shows how the Doppler effect occurs. The sound waves from the police car siren travel outward in all directions. Because the car is racing forward (to the left), the sound waves get bunched up in front of the car and spread out behind it. Sound waves that are closer together have a higher frequency, and sound waves that are farther apart have a lower frequency. The frequency of sound waves, in turn, determines the pitch of the sound. Sound waves with a higher frequency produce sound with a higher pitch, and sound waves with a lower frequency produce sound with a lower pitch. | text | null |
L_0862 | doppler effect | T_4326 | As the car approaches listener A, the sound waves get closer together, increasing their frequency. This listener hears the pitch of the siren get higher. As the car speeds away from listener B, the sound waves get farther apart, decreasing their frequency. This listener hears the pitch of the siren get lower. Q: What will the siren sound like to listener A after the police car passes him? A: The siren will suddenly get lower in pitch because the sound waves will be much more spread out and have a lower frequency. | text | null |
L_0863 | earth as a magnet | T_4327 | Imagine a huge bar magnet passing through Earths axis, as in the Figure 1.1. This is a good representation of Earth as a magnet. Like a bar magnet, Earth has north and south magnetic poles. A magnetic pole is the north or south end of a magnet, where the magnet exerts the most force. | text | null |
L_0863 | earth as a magnet | T_4328 | Although the needle of a compass always points north, it doesnt point to Earths north geographic pole. Find the north geographic pole in the Figure 1.2. As you can see, it is located at 90 north latitude. Where does a compass Q: The north end of a compass needle points toward Earths north magnetic pole. The like poles of two magnets repel each other, and the opposite poles attract. So why doesnt the north end of a compass needle point to Earths south magnetic pole instead? A: The answer may surprise you. The compass needle actually does point to the south pole of magnet Earth. However, it is called the north magnetic pole because it is close to the north geographic pole. This naming convention was adopted a long time ago to avoid confusion. | text | null |
L_0863 | earth as a magnet | T_4328 | Although the needle of a compass always points north, it doesnt point to Earths north geographic pole. Find the north geographic pole in the Figure 1.2. As you can see, it is located at 90 north latitude. Where does a compass Q: The north end of a compass needle points toward Earths north magnetic pole. The like poles of two magnets repel each other, and the opposite poles attract. So why doesnt the north end of a compass needle point to Earths south magnetic pole instead? A: The answer may surprise you. The compass needle actually does point to the south pole of magnet Earth. However, it is called the north magnetic pole because it is close to the north geographic pole. This naming convention was adopted a long time ago to avoid confusion. | text | null |
L_0863 | earth as a magnet | T_4329 | Like all magnets, Earth has a magnetic field. Earths magnetic field is called the magnetosphere. You can see a model of the magnetosphere in the Figure 1.3. It is a huge region that extends outward from Earth in all directions. Earth exerts magnetic force over the entire field, but the force is strongest at the poles, where lines of force converge. Click image to the left or use the URL below. URL: | text | null |
L_0864 | efficiency | T_4330 | A dolly is a machine because it changes a force to make work easier. What is work? In physics, work is defined as the use of force to move an object over a distance. It is represented by the equation: Work = Force x Distance All machines make work easier, but they dont increase the amount of work that is done. You can never get more work out of a machine than you put into it. In fact, a machine always does less work on an object than the user does on the machine. Thats because a machine must use some of the work put into it to overcome friction. Friction is the force that opposes motion between any surfaces that are touching. All machines involve motion, so they all have friction. How much work is needed to overcome friction in a machine depends on the machines efficiency. | text | null |
L_0864 | efficiency | T_4331 | Efficiency is the percent of work put into a machine by the user (input work) that becomes work done by the machine (output work). The output work is always less than the input work because some of the input work is used to overcome friction. Therefore, efficiency is always less than 100 percent. The closer to 100 percent a machines efficiency is, the better it is at reducing friction. Look at the ramp in the Figure 1.1. A ramp is a type of simple machine called an inclined plane. It is easier to push the heavy piece of furniture up the ramp to the truck than to lift it straight up off the ground, but pushing the furniture over the surface of the ramp creates a lot of friction. Some of the force applied to moving the furniture must be used to overcome the friction with the ramp. Q: Why would it be more efficient to use a dolly to roll the furniture up the ramp? A: There would be less friction to overcome if you used a dolly because of the wheels. So the efficiency of the ramp would be greater with the dolly. | text | null |
L_0864 | efficiency | T_4332 | Efficiency can be calculated with the equation: Output work Efficiency = Input work 100% Consider a machine that puts out 6000 joules of work. To produce that much work from the machine requires the user to put in 8000 joules of work. To find the efficiency of the machine, substitute these values into the equation for efficiency: 6000 J 100% = 75% 8000 J Q: Rani puts 7500 joules of work into pushing a box up a ramp, but only 6700 joules of work actually go into moving the box. The rest of the work overcomes friction between the box and the ramp. What is the efficiency of the ramp? A: The efficiency of the ramp is: 6700 J 100% = 90% 7500 J | text | null |
L_0865 | einsteins concept of gravity | T_4333 | In the late 1600s, Isaac Newton introduced his law of gravity, which identifies gravity as a force of attraction between all objects with mass in the universe. The law also states that the strength of gravity between two objects depends on their mass and distance apart. Newtons law of gravity was accepted for more than two centuries. It can predict the motion of most objects and was even used by NASA to land astronauts on the moon. Its still used for most practical purposes. However, Newtons law doesnt explain why gravity occurs. It only describes how gravity seems to affect objects. There are also some cases in which Newtons law doesnt even describe what happens. Q: Newton expressed his ideas about gravity as a law. A law in science is a description of what always occurs in nature. For example, according to Newtons law, objects on Earth always fall down, not up. What is needed to explain gravity? A: A theory is needed to explain gravity. In science, a theory is a broad explanation that is supported by a great deal of evidence. | text | null |
L_0865 | einsteins concept of gravity | T_4334 | In the early 1900s, Albert Einstein came up with a theory of gravity that actually explains gravity rather than simply describing its effects. Einstein showed mathematically that gravity is not really a force that of attraction between all objects with mass, as Newton thought. Instead, Einstein showed that gravity is a result of the warping, or curving, of space and time, which made up the same space-time fabric. These ideas about space-time and gravity became known as Einsteins theory of general relativity. | text | null |
L_0865 | einsteins concept of gravity | T_4335 | Einstein derived his theory using mathematics. However, you can get a good grasp of it with the help of a simple visual analogy. Imagine a bowling ball pressing down on a trampoline. The surface of the trampoline would curve downward instead of being flat. Now imagine placing a lighter ball at the edge of the trampoline. What will happen? It will roll down toward the bowling ball. This apparent attraction to the bowling ball occurs because the trampoline curves downward, not because the two balls are actually attracted to one another by an invisible force called gravity. Einstein theorized that the sun and other very massive bodies affect space and time around them in a way that is similar to the effect of the bowling ball on the trampoline. The more massive a body is, the more it causes space-time to curve. This idea is represented by the Figure 1.1. According to Einstein, objects move toward one another because of the curves in space-time, not because they are pulling on each other with a force of attraction. Einsteins theory is supported by evidence and widely accepted today, although Newtons law is still used for many calculations. | text | null |
L_0866 | elastic force | T_4336 | Something that is elastic can return to its original shape after being stretched or compressed. This property is called elasticity. As you stretch or compress an elastic material like a bungee cord, it resists the change in shape. It exerts a counter force in the opposite direction. This force is called elastic force. The farther the material is stretched or compressed, the greater the elastic force becomes. As soon as the stretching or compressing force is released, elastic force causes the material to spring back to its original shape. Click image to the left or use the URL below. URL: Q: What force stretches the bungee cord after the jumper jumps? When does the bungee cord snap back to its original shape? A: After the bungee jumper jumps, he accelerates toward the ground due to gravity. His weight stretches the bungee cord. As the bungee cord stretches, it exerts elastic force upward against the jumper, which slows his descent and brings him to a momentary stop. Then the bungee cord springs back to its original shape, and the jumper bounces upward. | text | null |
L_0866 | elastic force | T_4337 | Elastic force can be very useful and not just for bungee jumping. In fact, you probably use elastic force every day. A few common uses of elastic force are shown in the Figure 1.1. Do you use elastic force in any of these ways? Q: How does the resistance band work? How does it use elastic force? A: When you pull on the band, it stretches but doesnt break. The resistance you feel when you pull on it is elastic force. The farther you stretch the band, the greater the resistance is. The resistance of the band to stretching is what gives your muscles a workout. After you stop pulling on the band, it returns to its original shape, ready for the next stretch. Springs like the spring toy pictured in the Figure 1.2 also have elastic force when they are stretched or compressed. Q: Can you think of other uses of springs? A: Bedsprings provide springy support beneath a mattress. The spring in a door closer pulls the door shut. The spring in a retractable ballpoint pen retracts the point of the pen. The spring in a pogo stick bounces the rider up off the ground. | text | null |
L_0866 | elastic force | T_4337 | Elastic force can be very useful and not just for bungee jumping. In fact, you probably use elastic force every day. A few common uses of elastic force are shown in the Figure 1.1. Do you use elastic force in any of these ways? Q: How does the resistance band work? How does it use elastic force? A: When you pull on the band, it stretches but doesnt break. The resistance you feel when you pull on it is elastic force. The farther you stretch the band, the greater the resistance is. The resistance of the band to stretching is what gives your muscles a workout. After you stop pulling on the band, it returns to its original shape, ready for the next stretch. Springs like the spring toy pictured in the Figure 1.2 also have elastic force when they are stretched or compressed. Q: Can you think of other uses of springs? A: Bedsprings provide springy support beneath a mattress. The spring in a door closer pulls the door shut. The spring in a retractable ballpoint pen retracts the point of the pen. The spring in a pogo stick bounces the rider up off the ground. | text | null |
L_0881 | electromagnetic spectrum | T_4379 | Electromagnetic radiation is energy that travels in waves across space as well as through matter. Most of the electromagnetic radiation on Earth comes from the sun. Like other waves, electromagnetic waves are characterized by certain wavelengths and wave frequencies. Wavelength is the distance between two corresponding points on adjacent waves. Wave frequency is the number of waves that pass a fixed point in a given amount of time. Electromagnetic waves with shorter wavelengths have higher frequencies and more energy. | text | null |
L_0881 | electromagnetic spectrum | T_4380 | Visible light and infrared light are just a small part of the full range of electromagnetic radiation, which is called the electromagnetic spectrum. You can see the waves of the electromagnetic spectrum in the Figure 1.1. At the top of the diagram, the wavelengths of the waves are given. Also included are objects that are about the same size as the corresponding wavelengths. The frequencies and energy levels of the waves are shown at the bottom of the diagram. Some sources of the waves are also given. On the left side of the electromagnetic spectrum diagram are radio waves and microwaves. Radio waves have the longest wavelengths and lowest frequencies of all electromagnetic waves. They also have the least amount of energy. On the right side of the diagram are X rays and gamma rays. They have the shortest wavelengths and highest frequencies of all electromagnetic waves. They also have the most energy. Between these two extremes are waves that are commonly called light. Light includes infrared light, visible light, and ultraviolet light. The wavelengths, frequencies, and energy levels of light fall in between those of radio waves on the left and X rays and gamma rays on the right. Q: Which type of light has the longest wavelengths? A: Infrared light has the longest wavelengths. Q: What sources of infrared light are shown in the diagram? A: The sources in the diagram are people and light bulbs, but all living things and most other objects give off infrared light. | text | null |
L_0882 | electromagnetic waves | T_4381 | Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. Like other waves, electro- magnetic waves transfer energy from one place to another. The transfer of energy by electromagnetic waves is called electromagnetic radiation. Electromagnetic waves can transfer energy through matter or across empty space. Click image to the left or use the URL below. URL: Q: How do microwaves transfer energy inside a microwave oven? A: They transfer energy through the air inside the oven to the food. | text | null |
L_0882 | electromagnetic waves | T_4382 | A familiar example may help you understand the vibrating electric and magnetic fields that make up electromagnetic waves. Consider a bar magnet, like the one in the Figure 1.1. The magnet exerts magnetic force over an area all around it. This area is called a magnetic field. The field lines in the diagram represent the direction and location of the magnetic force. Because of the field surrounding a magnet, it can exert force on objects without touching them. They just have to be within its magnetic field. Q: How could you demonstrate that a magnet can exert force on objects without touching them? A: You could put small objects containing iron, such as paper clips, near a magnet and show that they move toward the magnet. An electric field is similar to a magnetic field. It is an area of electrical force surrounding a positively or negatively charged particle. You can see electric fields in the following Figure 1.2. Like a magnetic field, an electric field can exert force on objects over a distance without actually touching them. | text | null |
L_0882 | electromagnetic waves | T_4383 | An electromagnetic wave begins when an electrically charged particle vibrates. The Figure 1.3 shows how this happens. A vibrating charged particle causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field. The two types of vibrating fields combine to create an electromagnetic wave. | text | null |
L_0882 | electromagnetic waves | T_4383 | An electromagnetic wave begins when an electrically charged particle vibrates. The Figure 1.3 shows how this happens. A vibrating charged particle causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field. The two types of vibrating fields combine to create an electromagnetic wave. | text | null |
L_0882 | electromagnetic waves | T_4384 | As you can see in the Figure 1.3, the electric and magnetic fields that make up an electromagnetic wave are perpendicular (at right angles) to each other. Both fields are also perpendicular to the direction that the wave travels. Therefore, an electromagnetic wave is a transverse wave. However, unlike a mechanical transverse wave, which can only travel through matter, an electromagnetic transverse wave can travel through empty space. When waves travel through matter, they lose some energy to the matter as they pass through it. But when waves travel through space, no energy is lost. Therefore, electromagnetic waves dont get weaker as they travel. However, the energy is diluted as it travels farther from its source because it spreads out over an ever-larger area. | text | null |
L_0882 | electromagnetic waves | T_4385 | When electromagnetic waves strike matter, they may interact with it in the same ways that mechanical waves interact with matter. Electromagnetic waves may: reflect, or bounce back from a surface; refract, or bend when entering a new medium; diffract, or spread out around obstacles. Electromagnetic waves may also be absorbed by matter and converted to other forms of energy. Microwaves are a familiar example. When microwaves strike food in a microwave oven, they are absorbed and converted to thermal energy, which heats the food. | text | null |
L_0882 | electromagnetic waves | T_4386 | The most important source of electromagnetic waves on Earth is the sun. Electromagnetic waves travel from the sun to Earth across space and provide virtually all the energy that supports life on our planet. Many other sources of electromagnetic waves depend on technology. Radio waves, microwaves, and X rays are examples. We use these electromagnetic waves for communications, cooking, medicine, and many other purposes. | text | null |
L_0884 | electron cloud atomic model | T_4390 | Up until about 1920, scientists accepted Niels Bohrs model of the atom. In this model, negative electrons circle the positive nucleus at fixed distances from the nucleus, called energy levels. You can see the model in Figure 1.1 for an atom of the element nitrogen. Bohrs model is useful for understanding properties of elements and their chemical interactions. However, it doesnt explain certain behaviors of electrons, except for those in the simplest atom, the hydrogen atom. | text | null |
L_0884 | electron cloud atomic model | T_4391 | In the mid-1920s, an Austrian scientist named Erwin Schrdinger thought that the problem with Bohrs model was restricting the electrons to specific orbits. He wondered if electrons might behave like light, which scientists already knew had properties of both particles and waves. Schrdinger speculated that electrons might also travel in waves. Q: How do you pin down the location of an electron in a wave? A: You cant specify the exact location of an electron. However, Schrdinger showed that you can at least determine where an electron is most likely to be. Schrdinger developed an equation that could be used to calculate the chances of an electron being in any given place around the nucleus. Based on his calculations, he identified regions around the nucleus where electrons are most likely to be. He called these regions orbitals. As you can see in the Figure 1.2, orbitals may be shaped like spheres, dumbbells, or rings. In each case, the nucleus of the atom is at the center of the orbital. | text | null |
L_0884 | electron cloud atomic model | T_4391 | In the mid-1920s, an Austrian scientist named Erwin Schrdinger thought that the problem with Bohrs model was restricting the electrons to specific orbits. He wondered if electrons might behave like light, which scientists already knew had properties of both particles and waves. Schrdinger speculated that electrons might also travel in waves. Q: How do you pin down the location of an electron in a wave? A: You cant specify the exact location of an electron. However, Schrdinger showed that you can at least determine where an electron is most likely to be. Schrdinger developed an equation that could be used to calculate the chances of an electron being in any given place around the nucleus. Based on his calculations, he identified regions around the nucleus where electrons are most likely to be. He called these regions orbitals. As you can see in the Figure 1.2, orbitals may be shaped like spheres, dumbbells, or rings. In each case, the nucleus of the atom is at the center of the orbital. | text | null |
L_0884 | electron cloud atomic model | T_4392 | Schrdingers work on orbitals is the basis of the modern model of the atom, which scientists call the quantum mechanical model. The modern model is also commonly called the electron cloud model. Thats because each orbital around the nucleus of the atom resembles a fuzzy cloud around the nucleus, like the ones shown in the Figure 1.3 for a helium atom. The densest area of the cloud is where the electrons have the greatest chances of being. Q: In the model pictured in the Figure 1.3, where are the two helium electrons most likely to be? A: The two electrons are most likely to be inside the sphere closest to the nucleus where the cloud is darkest. | text | null |
L_0888 | electrons | T_4403 | Electrons are one of three main types of particles that make up atoms. The other two types are protons and neutrons. Unlike protons and neutrons, which consist of smaller, simpler particles, electrons are fundamental particles that do not consist of smaller particles. They are a type of fundamental particles called leptons. All leptons have an electric charge of -1 or 0. Click image to the left or use the URL below. URL: | text | null |
L_0888 | electrons | T_4404 | Electrons are extremely small. The mass of an electron is only about 1/2000 the mass of a proton or neutron, so electrons contribute virtually nothing to the total mass of an atom. Electrons have an electric charge of -1, which is equal but opposite to the charge of proton, which is +1. All atoms have the same number of electrons as protons, so the positive and negative charges cancel out, making atoms electrically neutral. | text | null |
L_0888 | electrons | T_4405 | Unlike protons and neutrons, which are located inside the nucleus at the center of the atom, electrons are found outside the nucleus. Because opposite electric charges attract each other, negative electrons are attracted to the positive nucleus. This force of attraction keeps electrons constantly moving through the otherwise empty space around the nucleus. The Figure shown 1.1 is a common way to represent the structure of an atom. It shows the electron as a particle orbiting the nucleus, similar to the way that planets orbit the sun. | text | null |
L_0888 | electrons | T_4406 | The atomic model above is useful for some purposes, but its too simple when it comes to the location of electrons. In reality, its impossible to say what path an electron will follow. Instead, its only possible to describe the chances of finding an electron in a certain region around the nucleus. The region where an electron is most likely to be is called an orbital. Each orbital can have at most two electrons. Some orbitals, called S orbitals, are shaped like spheres, with the nucleus in the center. An S orbital is pictured in Figure 1.2. Where the dots are denser, the chance of finding an electron is greater. Also pictured in Figure 1.2 is a P orbital. P orbitals are shaped like dumbbells, with the nucleus in the pinched part of the dumbbell. Click image to the left or use the URL below. URL: Q: How many electrons can there be in each type of orbital shown above? A: There can be a maximum of two electrons in any orbital, regardless of its shape. Q: Where is the nucleus in each orbital? A: The nucleus is at the center of each orbital. It is in the middle of the sphere in the S orbital and in the pinched part of the P orbital. | text | null |
L_0888 | electrons | T_4407 | Electrons are located at fixed distances from the nucleus, called energy levels. You can see the first three energy levels in the Figure 1.3. The diagram also shows the maximum possible number of electrons at each energy level. Electrons at lower energy levels, which are closer to the nucleus, have less energy. At the lowest energy level, which has the least energy, there is just one orbital, so this energy level has a maximum of two electrons. Only when a lower energy level is full are electrons added to the next higher energy level. Electrons at higher energy levels, which are farther from the nucleus, have more energy. They also have more orbitals and greater possible numbers of electrons. Electrons at the outermost energy level of an atom are called valence electrons. They determine many of the properties of an element. Thats because these electrons are involved in chemical reactions with other atoms. Atoms may share or transfer valence electrons. Shared electrons bind atoms together to form chemical compounds. Q: If an atom has 12 electrons, how will they be distributed in energy levels? A: The atom will have two electrons at the first energy level, eight at the second energy level, and the remaining two at the third energy level. Q: Sometimes, an electron jumps from one energy level to another. How do you think this happens? A: To change energy levels, an electron must either gain or lose energy. Thats because electrons at higher energy levels have more energy than electrons at lower energy levels. | text | null |
L_0889 | elements | T_4408 | A pure substance is called an element. An element is a pure substance because it cannot be separated into any other substances. Currently, 92 different elements are known to exist in nature, although additional elements have been formed in labs. All matter consists of one or more of these elements. Some elements are very common; others are relatively rare. The most common element in the universe is hydrogen, which is part of Earths atmosphere and a component of water. The most common element in Earths atmosphere is nitrogen, and the most common element in Earths crust is oxygen. Click image to the left or use the URL below. URL: | text | null |
L_0889 | elements | T_4409 | Each element has a unique set of properties that is different from the set of properties of any other element. For example, the element iron is a solid that is attracted by a magnet and can be made into a magnet, like the compass needle shown in the Figure 1.1. The element neon, on the other hand, is a gas that gives off a red glow when electricity flows through it. The lighted sign in the Figure 1.2 contains neon. The needle of this compass is made of the element iron. Q: Do you know properties of any other elements? For example, what do you know about helium? A: Helium is a gas that has a lower density than air. Thats why helium balloons have to be weighted down so they wont float away. Q: Living things, like all matter, are made of elements. Do you know which element is most common in living things? A: Carbon is the most common element in living things. It has the unique property of being able to combine with many other elements as well as with itself. This allows carbon to form a huge number of different substances. | text | null |
L_0889 | elements | T_4410 | For thousands of years, people have wondered about the substances that make up matter. About 2500 years ago, the Greek philosopher Aristotle argued that all matter is made up of just four elements, which he identified as earth, air, water, and fire. He thought that different substances vary in their properties because they contain different proportions of these four elements. Aristotle had the right idea, but he was wrong about which substances are elements. Nonetheless, his four elements were accepted until just a few hundred years ago. Then scientists started discovering many of the elements with which we are familiar today. Eventually they discovered dozens of different elements. | text | null |
L_0889 | elements | T_4411 | The smallest particle of an element that still has the properties of that element is the atom. Atoms actually consist of smaller particles, including protons and electrons, but these smaller particles are the same for all elements. All the atoms of an element are like one another, and are different from the atoms of all other elements. For example, the atoms of each element have a unique number of protons. Consider carbon as an example. Carbon atoms have six protons. They also have six electrons. All carbon atoms are the same whether they are found in a lump of coal or a teaspoon of table sugar (Figure 1.3). On the other hand, carbon atoms are different from the atoms of hydrogen, which are also found in coal and sugar. Each hydrogen atom has just one proton and one electron. Carbon is the main element in coal (left). Carbon is also a major component of sugar (right). Q: Why do you think coal and sugar are so different from one another when carbon is a major component of each A: Coal and sugar differ from one another because they contain different proportions of carbon and other elements. For example, coal is about 85 percent carbon, whereas table sugar is about 42 percent carbon. Both coal and sugar also contain the elements hydrogen and oxygen but in different proportions. In addition, coal contains the elements nitrogen and sulfur. | text | null |
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