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Power & Resistance (WJEC GCSE Physics: Combined Science)

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Power & Resistance

Higher Tier

  • The power of an electrical device can also be found using:

P space equals space I squared R

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

  • This means for a given resistance, if the current is doubled the power will be four times as great

Worked example

A current of 6 A passes through a 20 Ω fixed resistor. 

Calculate the power of the resistor.

Answer: 

Step 1: List the known quantities

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

Step 2: Write out the relevant equation

P space equals space I squared R

Step 3: Substitute in the known values

P space equals space open parentheses 6 close parentheses squared space cross times space 20

P space equals space 720 space straight W

Examiner Tip

There are several different equations for calculating power. Always begin any calculation by writing out the known quantities with their symbols. Then you can choose an equation that contains the quantities that you know. 

For example:

  • If you are given resistance, R, and current, I, you would use P space equals space I squared R
  • If you are given current, I, and voltage, V, you would use P space equals space I V

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    Leander

    Author: Leander

    Expertise: Physics

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.