Syllabus Edition

First teaching 2023

First exams 2025

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Power (CIE A Level Physics)

Revision Note

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Defining power

  • Power is defined as:

The rate of energy transfer

  • Since work done is equal to the energy transferred, power can also be defined as:

 Work done per unit time

  • Power can be calculated using the following equations:

P space equals fraction numerator space E over denominator t end fraction space equals fraction numerator space W over denominator t end fraction

  • Where:
    • P = power in watts (W)
    • E = energy in joules (J)
    • W = work done in joules (J) or newton metres (N m)
    • t = time in seconds (s)

 

  • You may be familiar with power ratings on household appliances, such as 60 W or 1000 W
  • These power ratings tell you the amount of energy transferred electrically from the mains supply per second

Solving Problems Involving Power

  • Since power describes the energy transferred or the work done per unit time, power calculations commonly include energy or work calculations, which can also include efficiency

Worked example

A car engine exerts a force of 500 N for 1.0 km over 200 s.

What is the average power developed by the engine?

Answer: 

Step 1: List the known quantities and convert to SI units

  • Force, F = 500 N
  • Displacement, s = 1.0×103 m
  • Time, t = 200 s

Step 2: Substitute the work done equation into the power equation

P space equals fraction numerator space W over denominator t end fraction

W space equals space F s

P space equals fraction numerator space F s over denominator t end fraction

Step 3: Substitute the known values to calculate

P space equals space fraction numerator 500 space cross times space open parentheses 1.0 cross times 10 cubed close parentheses over denominator 200 end fraction

P space equals space 2500 space straight W

Examiner Tip

Think of power as “energy per second”. Thinking of it this way will help you to remember the relationship between power and energy: “Watt is the unit of power?”

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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.