Power (Cambridge O Level Physics)

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Dan MG

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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

Vehicles with Different Powers

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

Two Motors with Different Powers

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

Power is the rate of transfer of energy, or rate of work done. The higher power motor will do the same work in a shorter time.

  • 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 Rating Table

Appliance or Object Power rating / W
Torch 1
Electric lightbulb 100 = 1 × 102
Electric cooker 10 000 = 1 × 104
Railway engine 1 000 000 = 1 × 106
Saturn V space rocket 100 000 000 = 1 × 108
Very large power station 10 000 000 000 = 1 × 1010
World demand for power 10 000 000 000 000 = 1 × 1013
A star similar to the Sun 1 00 000 000 000 000 000 000 000 000 = 1 × 1026

Calculating Power

  • Since power is defined as

The rate of doing work

  • And work is

Work done = energy transferred

 

  • Then, power can be expressed in equation form as

P = Wt{"language":"en","fontFamily":"Times New Roman","fontSize":"18"} or P=Et{"language":"en","fontFamily":"Times New Roman","fontSize":"18"}

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

  • Note that these two equations may be written slightly differently, but are representing the same thing - a transfer of energy over time
  • This equation can be rearranged with the help of a formula triangle:

Formula Triangle for Power

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

Work, power, time formula triangle

Worked example

Calculate the energy transferred when an iron with a power rating of 2000 W is used for 5 minutes.

 Answer:

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 

P = Et{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}

Step 3: Rearrange for energy transferred, ΔE

E = Pt{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}

Step 4: Substitute in the known values

E = 2000 × 300{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}

E = 600 000 J{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}

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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Dan MG

Author: Dan MG

Expertise: Physics

Dan graduated with a First-class Masters degree in Physics at Durham University, specialising in cell membrane biophysics. After being awarded an Institute of Physics Teacher Training Scholarship, Dan taught physics in secondary schools in the North of England before moving to SME. Here, he carries on his passion for writing enjoyable physics questions and helping young people to love physics.