Power (OCR A Level Physics)

Revision Note

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Electrical Power

  • In mechanics, power P is defined as the rate of doing work
    • Potential difference is the work done per unit charge
    • Current is the rate of flow of charge

  • The electrical power dissipated (produced) by an electrical device is defined by the equation:

Power equation, downloadable AS & A Level Physics revision notes

  • The unit of power is Watt (W)

  • Using Ohm's Law V = IR to rearrange for either V or I and substituting into the power equation, means power can be written in terms of resistance R

Power in terms of resistance, downloadable AS & A Level Physics revision notes

  • This means for a given resistor if the current or voltage doubles the power will be four times as great

Worked example

Two lamps are connected in series to a 150 V power supply.WE - power question image, downloadable AS & A Level Physics revision notesWhich statement most accurately describes what happens?

A.     Both lamps light normally

B.     The 15 V lamp blows

C.     Only the 41 W lamp lights

D.     Both lamps light at less than their normal brightness

ANSWER:  A

Worked example - power (2), downloadable AS & A Level Physics revision notes

Examiner Tip

You can use the mnemonic “Twinkle Twinkle Little Star, Power equals I squared R” to remember whether to multiply or divide by resistance in the power equations.When doing calculations involving electrical power, remember the unit is Watts W, therefore, you should always make sure that the time is in seconds

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.