ent (ac). Our discussion starts with the traditional laws of direct current. Later we briefly discuss some basic principles of alternating currents. A.2.1 Electromotive Force (EMF) and Voltage We require a source of "electric pressure," called electromotive force (emf), to establish a flow of electrical current. The standard analogy is to use water and its flow through a water circulating system. This is illustrated in Figure A.1. The figure shows the water pressure source as a pump, two different-diameter pipes, and some sort of mechanism to measure water flow. Flow can be measured in several ways such as liters or gallons per second or minute. The pump creates a difference in pressure between points X and y. Some will say that the pump sets up a "head of pressure." This will cause water to flow from point X at the output of the pump up the large-diameter pipe, across the small diameter pipe, and down through another large-diameter pipe in which we installed a flow pressure gauge. The water returns to the low pressure side of the pump, shown as y in the drawing. The amount of water that will flow (i.e., analogy to amperage) depends on the difference in pressure between points X and y and the size of the small pipe. The difference in pressure is analogous to electromotive force. Figure A.2 is an electrical representation of the circulating water system shown in Figure A.1. The battery (see Section 2.3.1) supplies the electrical pressure or emf. It causes the electricity to flow from the high potential side of the battery, labeled X, to its low potential side, labeled y in Figure A.2. The amount of current that will flow will depend upon the emf and the nature of the resistor, shown between points X and y. The difference of water pressure may be measured in units such as "difference in head in feet." The emf of the electric circuit, on the other hand, is measured in terms of a unit called a volt. Another important term is electric potential. In Figure A.2 we can say that the electric potential at the positive term