ASPhysicsCIERevision Notes9. Electricity9.2 Potential Difference & PowerElectrical Power

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First teaching 2023

First exams 2025

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Electrical Power (CIE AS Physics)

Revision Note

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Physics

Calculating Electrical Power

  • In “Work, Energy and Power”, Power P was defined as the rate of doing work
    • Potential difference is the work done per unit charge
    • Current is the rate of flow of charge
  • So, the power dissipated (produced) by an electrical device is defined as:

P = IV

    • P = power (W)
    • I = current (A)
    • V = potential difference/voltage (V)
  • Using V = IR to rearrange for either V or I and substituting into the power equation means we also write power in terms of resistance R 

bold italic P bold space bold equals bold italic I to the power of bold 2 bold italic R

bold italic P bold space bold equals bold V to the power of bold 2 over bold R

    • R = resistance (Ω)
  • This means for a given resistance for example, 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

Step 1: Calculate current needed for both lamps to operate

P = IV

Step 2: Rearrange for I

bold italic I bold space bold equals bold P over bold V

Step 3: Substitute in values

For the 41W lamp:

 bold italic I bold space bold equals fraction numerator bold 41 bold W over denominator bold 135 bold V end fraction bold equals bold 0 bold. bold 3 bold italic A

For the 4.5W lamp:

 bold italic I bold equals fraction numerator bold 4 bold. bold 5 bold W over denominator bold 15 bold V end fraction bold equals bold 0 bold. bold 3 bold italic A

Step 4:

For both to operate at their normal brightness, a current of 0.3A is required. Since the lamps are connected in series, the same current would flow through both.

Step 5:

The lamps will light at their normal brightness - option A

Exam 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 equationsWhich equation to use will depend on whether the value of current or voltage has been given in the question

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    Katie M

    Author: Katie M

    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.