Electrical Power (P) – the rate at which electrical energy is transferred.
This is exactly the same as mechanical power (the rate at which work is done), here we are just being more specific about what work is done and looking at how to calculate electrical power using electrical quantities such as current.
Electrical power is the rate at which electrical energy is transferred to some other form of energy (such as light energy).
Electrical power can be found using the equation below:
Where I is the current measured in amps (A) and V is the potential difference (or voltage) measured in volts (V).
As with mechanical power the SI unit for electric power is the watt:
Watt – one watt is equal to one joule per second.
Power and Resistance
Using the equation for resistance (V=IR) we can substitute out either the current or potential difference to give us another two ways of calculating electrical power.
Substituting out the potential difference gives us this equation:
Where R is the resistance measured in ohms (Ω).
Substituting out the current gives us this equation:
Work Done
As power is the rate at which work is done we can calculate how much work is done (how much energy is transferred) by multiplying the electrical power by the time.
Any of the above equations multiplied by time (in seconds) will give us the work done in that period of time.
For example, if a 60W light bulb was used for 2s we can calculate the amount of work done (energy transferred) by simply multiplying 60 by 2 to get 120J.
In equation form this can be shown as:
Where W is the work done measured in joules (J), P is the power measured in watts (W) and t is the time measured in seconds (s).
Worked Examples
Example 1
A kettle operating with a voltage of 230V has a power rating of 1800W. Calculate the current flowing through the kettle.
Example 2
Calculate the resistance of a 70W fan, operating at a potential difference of 240V, and the amount of energy transferred in 5 minutes.
Example 3
100C of charge pass through a 20Ω resistor in 50s. Calculate the power.
This question requires knowledge of charge and current.
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