FHSST Physics/Electricity/Power in Electric Circuits

Power in electric circuits

In addition to voltage and current, there is another measure of free electron activity in a circuit: power. First, we need to understand just what power is before we analyze it in any circuits.

Power is a measure of how much work can be performed in a given amount of time. Work is generally defined in terms of the lifting of a weight against the pull of gravity. The heavier the weight and/or the higher it is lifted, the more work has been done. Power is a measure of how rapidly a standard amount of work is done.

In electric circuits, power is a function of both voltage and current.

$P=IE$

In this case, however, power (P) is exactly equal to current (I) multiplied by voltage (E), rather than merely being proportional to IE. When using this formula, the unit of measurement for power is the watt, abbreviated with the letter "W."

It must be understood that neither voltage nor current by themselves constitute power. Rather, power is the combination of both voltage and current in a circuit. Remember that voltage is the specific work (or potential energy) per unit charge, while current is the rate at which electric charges move through a conductor. Voltage (specific work) is analogous to the work done in lifting a weight against the pull of gravity. Current (rate) is analogous to the speed at which that weight is lifted. Together as a product (multiplication), voltage (work) and current (rate) constitute power.

A circuit with high voltage and low current may be dissipating the same amount of power as a circuit with low voltage and high current. Neither the amount of voltage alone nor the amount of current alone indicates the amount of power in an electric circuit.

In an open circuit, where voltage is present between the terminals of the source and there is zero current, there is zero power dissipated, no matter how great that voltage may be. Since $P=IE$ and $I=0$ and anything multiplied by zero is zero, the power dissipated in any open circuit must be zero.

Heat as a by-product

Usually heat is produced as a "side-effect" of using an electrical appliance such as a TV set or a computer. Heat is one of the forms of power and there must be a way to get rid of that heat or else the temperature may keep increasing, possibly leading to a breakdown of the apparatus concerned, or even, in an extreme case, to a fire. Make sure there is always enough circulation of air, for examples, in a TV set or a computer; never block the special openings provided in the case of the TV set, or of the computer, etc.; these openings are there on purpose to get rid of the heat.

Often the "rating" of an electrical or electronic item depends on its maximum permitted temperature. A resistor is rated at one watt, for example. Then it will get hot, but not too hot, if its voltage times its current equal no more than one Watt; but that is only true if enough air can get to it to cool it down. Some people will, for example, never "load" a resistor that is rated at a given amount of Watts with more than half that rated power.

The name plate at the rear of the TV set or computer, for example, usually indicates the total amount of Watts that is used, including the part that is converted into heat.