exists but current does not flow because the circuit is open, no work is

done. This is similar to the spring under tension that produced no motion.

When voltage causes electrons to move, work is done. The instantaneous rate

at which work is done is called the electric power rate, and is measured in

watts.

A total amount of work may be done in different lengths of time.

For

example, a given number of electrons may be moved from one point to another

in one second or in one hour, depending on the rate at which they are moved.

In both cases, the total work done is the same. However, when the work is

done in a short time, the wattage, or instantaneous power rate, is greater

than when the same amount of work is done over a longer period of time.

As stated, the basic unit of power is the watt. Power, in watts, is equal

to the voltage across a circuit multiplied by current through the circuit.

This represents the rate at which work is being done at any given instant.

The symbol P indicates electrical power. Thus, the basic power formula is P

= E x I, where E is volts and I is current in the circuit. The amount of

power changes when either voltage or current, or both voltage and current,

are caused to change.

In practice, the only factors that can be changed are voltage and

resistance.

In explaining the different forms that formulas may take,

current is sometimes presented as a quantity that is changed. Remember, if

current is changed it is because either voltage or resistance has been

changed.

Figure 16 (on the following page) shows a basic circuit using a source of

power that can be varied from 0 to 8 volts, and a graph that indicates the

relationship between voltage and power.

The resistance of this

circuit is 2 Ohms; this value does not change.

Voltage (E) is increased

(by increasing the voltage source), in steps of 1

volt, from 0 volts to 8

volts. By applying Ohm's law, the current (I) is

determined for each step

of voltage. For instance, when E is 1 volt, the

current is: