Voltage

You have learned that electric current is essentially the flow of electrons through a conductor, as illustrated by Figure 2.6.

In this example, a copper bar (conductor) was connected to a battery. By attracting the loosely held electrons of the atoms nearest the positive end, a flow of electrons was created by the battery.

If we remove the battery, or open the switch, the flow of electrons, and thus the electric current, stops.

The battery is a power source or a voltage source. Other examples of voltage sources are power supplies and the electrical generators used by your local power company.

The role of a voltage source is to create pressure on the electrons in a conductor to make them flow. This pressure is known by several terms: difference in potential, electromotive force (emf), and voltage.

Suppose we use an example of water pressure to demonstrate how electrical pressure works. In Figure 2.7 you see two water tanks. If we open the valve, water will flow from Tank A into Tank B. This is because gravity exerts a pressure on the water in Tank A.

Figure 2.7

How long will the water continue to flow from Tank A into Tank B? Until Tank B is completely full?

No. The water will stop flowing when the pressure on the water in Tank A is equal to the pressure on the water in Tank B.

In other words, water will continue to flow as long as there is a difference of pressure, or potential, between the tanks.

Now let's apply this principle to electricity. Electrons will flow through a conductor when a difference of potential exists between the positively charged end of the device and the negatively charged end of the device. This difference of potential, or electromotive force, or voltage, is produced by a power source.

Now let's return to the water tanks for a moment. As you look at Figure 2.8, try to answer this question: Does the water flow from Tank A into Tank B at a constant rate?

Figure 2.8

The correct answer is no! Rate of flow is directly related to pressure: the greater the pressure, the greater the rate of flow. Therefore, the rate of flow is at its greatest at the beginning, then it gradually drops off to zero when Tank A and Tank B are both half-full.

With electricity, the rate of electron flow is also directly related to pressure, or voltage. Current flow through an electric circuit is directly proportional to the difference in potential (or voltage, or emf) across the circuit. Increase the voltage, and you increase the current; decrease the voltage, and you decrease the current. Voltage is measured in units called volts.

On a practical level, if you replace a 1-volt battery with a 2-volt battery, the current flow doubles.