Power (Edexcel GCSE Physics)

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Definition of Power

  • Machines, such as car engines, transfer energy from one energy store to another constantly over a period of time

  • The rate of this energy transfer, or the rate of work done, is called power

  • Time is an important consideration when it comes to power
  • Two cars transfer the same amount of energy, or do the same amount of work to accelerate over a distance
  • If one car has more power, it will transfer that energy, or do that work, in a shorter amount of time

Power cars, IGCSE & GCSE Physics revision notes

Two cars accelerate to the same final speed, but the one with the most power will reach that speed sooner

 

  • Two electric motors:
    • lift the same weight
    • by the same height
    • but one motor lifts it faster than the other

  • The motor that lifts the weight faster has more power

Electric Motors Power, downloadable AS & A Level Physics revision notes

Two motors with different powers

  • Power ratings are given to appliances to show the amount of energy transferred per unit time
  • Common power ratings are shown in the table below:

Power Ratings Table

Calculating Power

  • Power is defined as

energy transferred per unit time

  • And 

work done = energy transferred

  • Therefore, power is also

work done per unit time

  • Power can be calculated using the following equation:

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

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

  • 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 oven of power 2500 W is used for 50 minutes.

 

Step 1: List the known values

    • Power, P = 2500 W
    • Time, t = 50 minutes = 50 × 60 = 3000 s

Step 2: Write down the power equation 

P space equals space E over t

Step 3: Rearrange to make energy the subject 

E space equals space P t

Step 4: Substitute in the known values

E space equals space 2500 space cross times space 3000

E space equals space 7 space 500 space 000 space straight 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.

The Watt

  • The watt is the unit of power
  • 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)

Examiner Tip

One way to remember this unit is it remember the saying “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.