Electricity & Power (Edexcel GCSE Physics)

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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 (potential difference) of the device
    • The current of the device

Examiner Tip

Remember: Power is just energy per second. Think of it this way will help you to remember the relationship between power and energy

Calculating Electrical Power

  • The power of an electrical component (or appliance) is given by the equation:

Power equation, downloadable AS & A Level Physics revision notes

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

  • Combining 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

  • Where:
    • P = power in watts (W)
    • I = current in amps (A)
    • V = voltage in volts (V)
    • R = resistance in ohms (Ω)

  • 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

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

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

Examiner 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 equationsWhich equation to use will depend on whether the value of current or voltage has been given in the questionYou can remember the unit of power by the phrase: “Watt is the unit of power?”

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.