Electric Power Printer Friendly Version
Recall that the rate at which energy is converted from one form to another is power.

power = energy converted / time
power = (voltage x charge) / time
power = voltage x (charge / time)
power = voltage x current

The unit of power is the watt (or kilowatt = 1000 watts).

So in terms of units,

electric power (watts) = current (amperes) x voltage (volts)

Thus, 1 watt = 1 joule / 1 second = 1 ampere x 1 volt.

 What is the power when a voltage of 120 V drives a 2-A current through a device?

 What is the current when a 60-W lamp is connected to 120 V?

 How much current does a 100-W lamp draw when connected to 120 V?

 If part of an electric circuit dissipates energy at 6 W when it draws a current of 3 A, what voltage is impressed across it?

 The 100 watt bulb draws more current and must therefore have a thicker filament.

The equation power =  energyconverted / time  rearranged gives

energyconverted = power x time

 Explain the difference between a kilowatt and a kilowatt-hour.

 One deterrent to burglary is to leave your front porch light on all the time. If your fixture contains a 60-W bulb at 120 V, and your local power utility sells energy at 8 cents per kilowatt-hour, how much will it cost to leave the bulb on for the whole month?