Electric power is the rate of conversion of electrical energy. Electrical energy is the energy given up by the moving charges in the circuit, but is has no physical embodiment of its own. For an electrical circuit to be useful to us, the final result of the charge movement must be something which excites one of our senses.
The electrical energy must be converted into sound, heat, light, or motion (smell and taste don’t get to play electrical games) The rate at which these conversions takes place is a measure of how much energy is being used in a an amount of time.
The unit of electrical power is the same as any other power, watts, which is equal to Joules / sec. A 60 W light bulb converts 60 J of electrical energy into light energy per second. A 1500 watt hair dryer converts 1500 J of electrical energy into heat and motion per second.
Watt’s law states the relationship between electrical power, current, and voltage;
P = I V
For a given voltage, devices which allow more current (lower resistance) will possess higher power ratings. This means that a 45 w light bulb has a higher resistance than a 100 w light bulb (assuming they are operated at the same voltage).
By working with Ohm’s law and Watt’s law you can determine the voltage, current, resistance, and power at any point in an electrical circuit. Now that’s power.
EX. What is the current in a 60 w light bulb if its plugged into the 120 V wall outlet?
I = P / V = 60 w / 120 v = 0.5 A
EX. What is the resistance of an amplifier which has a power of 2.5 w when operated by four
dry cells (6.0 v)?
R = V2 / P = (6.0)2 v / 2.5 w = 14.4 W
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