In a water system, water can flow as long as pressure is applied to one end of a pipe and the other end of the pipe is open. The greater the pressure in a water system, the greater the quantity of water that will flow. Similarly, in an electrical system, electrons will flow as long as electric pressure is applied to the system. Voltage, potential difference, and electromotive force are all terms used to describe electric pressure.
Recall that the law of electric charges states that unlike charges attract. Consequently, there is a pull, or force, of attraction between two dissimilarly charged objects. We call this pull of attraction a field of force.
Another way of looking at this is to picture excess electrons (the negative charge) as straining to reach the point where there are not enough electrons (the positive charge). If the two charges are connected by a conductor, the excess electrons will flow to the point where there are not enough electrons. But if the two charges are separated by an insulator, which prevents the flow of electrons, the excess electrons cannot move. Hence, an excess of electrons will pile up at one end of the insulator, with a corresponding lack, or deficiency, of electrons at the other end.
As long as the electrons cannot flow, the field of force between the two dissimilarly charged ends of the insulator increases. The resulting strain between the two ends is called the electric pressure. This pressure can become quite great. After a certain limit is reached, the insulator can no longer hold back the excess electrons, as discussed in the previous section. Hence, the electrons will rush across the insulator to the other end.
Electric pressure that causes electrons to flow is called voltage. Voltage is the difference in electric potential (or electric charge) between two points. The volt (V) is the amount of pressure required to force 1 ampere (A, the unit of measurement for current flow) through a resistance of 1 ohm (Ω, the unit of measurement for resistance; Ω is the Greek letter omega). In the industry, voltage is almost always measured in the range of the common volt. In other areas, the voltage may be measured on a smaller scale of a millivolt (mV), or one-thousandth of a volt. For larger measurements of the volt, the kilovolt (kV), equal to 1000 volts, is used.
1 millivolt = 0.001 volt
1 kilovolt = 1000 volts
To maintain electric pressure, we must have some way to move electrons in the same manner that water pressure moves water. In an electric circuit, this can be maintained by a battery, as shown in Figure 1, or by a generator or alternator, as shown in Figure 2. The battery forces electrons to flow to the positive electrode and causes electric pressure. A generator causes electric pressure by transferring electrons from one place to another.
Electromotive force can be produced in several ways. The most popular method of producing an electromotive force is by using an alternating current generator. The alternating current generator is supplied with power from another source. Then a wire loop is rotated through the magnetic field created by the voltage being applied, and an electromotive force is produced through the wire loop. We will discuss these ideas in more detail in succeeding sections.