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Electrical power

Electrical power is the rate or speed at which electrical energy is transferred in a circuit. It can also be thought of as the rate at which energy is consumed by an item in an electrical circuit. The formula for calculating electrical power is:

To calculate electrical power you need to measure voltage multiplied by currentElectrical power memory aid

Electrical power turned me into the village idiot.


Electrical power is measured in watts. One watt of electrical power is one joule per second.

Example 1

A lamp is rated at 12V, 6W. How many joules of energy are transferred by the lamp in 5 minutes? And what is the current passing through the lamp when it is connected to a 12V supply?


6 watts = 6 joules per second

5 minutes = 60 x 5 = 300 seconds

So the energy transferred = 300 x 6 = 1800 Joules

Power = voltage x current                            P = VI

Divide both sides by voltage to make current the subject,

\frac{Power}{voltage}=\frac{\strike{voltage}\times current}{\strike{voltage}}

So current I = P/V        6/12 = 0.5 Amps

Example 2

The circuit below shows where an ammeter and voltmeter are placed. If the ammeter reads 2A and the voltmeter reads 6V, what is the power of the lamp?

Example of electrical power shown in a circuit


Power = voltage x current                            P = VI

Power = 6 x 2 = 12 watts.

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