Energy & Power (Edexcel GCSE Physics)

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Energy & Power

  • Power is defined as the energy transferred per second
    • Power is measured in watts

  • Since power is energy transferred per second, the watt can also be defined as 1 joule per second

1 W = 1 J / s 

  • 1 kilowatt (1 kW) is equal to 1000 watts, or 1000 joules of energy transferred per second (1 kJ / s)

Calculating Energy Transfers

  • Power is defined as the energy transferred per unit time:

  • Power can also be defined as the work done per unit time:

  • Where:
    • E or W = The energy transferred, or work done, measured in joules (J)
    • t = time measured in seconds (s)
    • P = power measured in watts (W)

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

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

Work, power, time formula triangle

Worked example

Calculate the energy transferred if an iron of power 2000 W is used for 5 minutes.

Step 1: List the known values

    • Power, P = 2000 W
    • Time, t = 5 minutes = 5 × 60 = 300 s

Step 2: Write down the relevant equation 

Step 3: Rearrange for energy transferred, E

Energy transferred = power × time

E = Pt

Step 4: Substitute in the values

E = 2000 × 300 = 600 000 J

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

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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.