Electrical power Revision Notes for AQA GCSE Physics Combined Science

Electrical power

What is electrical power?

Electrical power measures how fast electrical energy changes into other types of energy. It tells us the rate at which energy transfers happen each second.

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Power is fundamentally about the speed of energy conversion - it's not about the total amount of energy, but how quickly that energy is being transformed from one form to another.

Power is measured in watts (W). One watt means one joule of energy transfers every second.

How to calculate power

Power depends on different factors in electrical circuits:

The current flowing through the device (measured in amps, A)
The potential difference across the device (measured in volts, V)
The resistance of the device (measured in ohms, Ω)
The energy transferred and time taken

You need to learn three important equations for calculating power, and knowing which equation to use depends on what information you're given in a problem.

The three power equations

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Mastering these three equations is essential - you'll need to recognise which one to use based on the given information and be able to rearrange them to find unknown variables.

Equation 1: Power = Current × Potential difference

$P = IV$

Use this when you know the current and voltage

Equation 2: Power = Current² × Resistance

$P = I^2R$

Use this when you know the current and resistance
Remember: $I^2$ means current multiplied by itself

Equation 3: Power = Energy transferred ÷ Time taken

$P = \frac{E}{t}$

Use this when you know how much energy transfers over a certain time

Worked examples

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Worked Example 1: Using P = E ÷ t

A microwave transfers 300,000 J of electrical energy in 35 seconds.

Calculation:

$P = \frac{E}{t}$
$P = \frac{300000 \text{ J}}{35 \text{ s}} = 857 \text{ W}$

The microwave has a power rating of 857 W.

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Worked Example 2: Using P = I V

An electric oven connects to 230 V mains supply and draws 30 A current.

Calculation:

$P = IV$
$P = 30 \text{ A} \times 230 \text{ V} = 6900 \text{ W}$

The oven consumes 6900 W of electrical power.

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Worked Example 3: Using P = I² R

A 4 A current flows through a wire with 55 Ω resistance.

Calculation:

$P = I^2R$
$P = (4 \text{ A})^2 \times 55 \text{ Ω}$
$P = 16 \times 55 = 880 \text{ W}$

The power dissipated in the wire is 880 W.

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Key fact: An energy transfer of 1 joule per second equals a power of 1 watt. This fundamental relationship helps you understand the connection between energy and power.

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Worked Example 4: Rearranging equations

A device has 3400 W power rating and 120 Ω resistance. Find the current.

Using P = I² R, rearranged to find I:

$I = \sqrt{\frac{P}{R}}$
$I = \sqrt{\frac{3400 \text{ W}}{120 \text{ Ω}}} = 5.3 \text{ A}$

The current flowing through the device is 5.3 A.

Key tips

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Essential Problem-Solving Tips:

You might need to rearrange equations to make current, voltage, or resistance the subject
Always check your units match the equation you're using
Power equations work the same way as energy calculation triangles - cover up the quantity you want to find
Practice identifying which equation to use based on the given information

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Key Points to Remember:

Electrical power shows how fast energy changes from one type to another
Power is measured in watts (W) - one watt = one joule per second
Learn all three equations: $P = IV$ , $P = I^2R$ , and $P = \frac{E}{t}$
Choose the right equation based on what information you have
Practice rearranging equations to find different variables

Electrical power (AQA GCSE Physics Combined Science): Revision Notes