AQA GCSE Physics

Revision Notes

2.3.1 Electrical Power

Electrical Power

  • Power is defined as

The rate of energy transfer or the amount of energy transferred per second

  • The power of a device depends on:
    • The voltage of the device
    • The current of the device
  • The power of an electrical component (or appliance) is given by the equation:

Power equation, downloadable AS & A Level Physics revision notes

  • This equation can be rearranged with the help of a formula triangle:

PIV triangle (2), IGCSE & GCSE Physics revision notes

Power, current, voltage formula triangle

  • The unit of power is the watt (W), which is the same as a joule per second (J/s)

Worked Example

Calculate the potential difference through a 48 W electric motor with a current of 4 A.

Step 1: List the known quantities

    • Power, P = 48 W
    • Current, I = 4 A

Step 2: Write down the relevant equation

P = IV

Step 3: Rearrange for potential difference, V

Step 4: Substitute in the values

Exam Tip

Remember: Power is just energy per second.

Power & Resistance

  • The voltage across an electric device depends upon the current and resistance of that device
  • Using the equations P = IV with V = IR, power can be written in terms of resistance, R:

Power in terms of resistance, downloadable AS & A Level Physics revision notes

Worked Example

Calculate the power through a 20 Ω resistor with a current of 6 A through it.

Step 1: List the known quantities

    • Resistance, R = 20 Ω
    • Current, I = 6 A

Step 2: Write down the relevant equation 

P = I2R

Step 3: Substitute in the values

P = (6)2 × 20 = 720 W

Exam Tip

You can use the mnemonic “Twinkle Twinkle Little Star, Power equals I squared R” to remember whether to multiply or divide by resistance in the power equations

Which equation to use will depend on whether the value of current or voltage has been given in the question

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