Mammoth Memory

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 that energy is consumed by an item in an electrical circuit. The formula for calculating electrical power is:

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

38 Physics Formulas_1.jpg


Power = voltage x current                            P = VI

Power = 6 x 2 = 12 watts.

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