Energy Transfer (OCR A Level Physics)

Revision Note

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Energy Transfer in Circuits

  • E.m.f is defined as the energy transferred by the power supply per unit charge
  • This energy transformed is also equal to the work done by the moving charge
  • This is defined by the equation:

W = εQ

  • Where:
    • W = work done / energy transferred (J)
    • ε = e.m.f (V)
    • Q = charge (C)

  • The potential difference is the energy transferred to the electrical component per unit charge
  • This means the equation can also be written as:

W = VQ

  • Where:
    • V = potential difference (V)

  • These equations show that

1 V = 1 J C-1

Worked example

An electric kettle requires 0.4 MJ to be supplied to boil a cup of water. The e.m.f. of the mains supply is 230 V.Calculate the charge supplied.

Step 1: List the known quantities

    • Energy transferred (work done), W = 0.4 MJ = 0.4 × 106 J
    • E.m.f, ε = 230 V

Step 2: Write the relevant equation

W = εQ

Step 3: Rearrange for charge, Q

Q = W ÷ ε

Step 4: Substitute in the values

Q = (0.4 × 106) ÷ 230 = 1739.13 = 1700 C

Energy Transfer for Charged Particles

  • A dynamo or battery transfer energy to each charge carrier, which are electrons in metals, as they pass through
    • Each electron has a charge of e

  • Since the energy transferred (work done) W is defined as

W = QV

  • Then:

W = eV

  • Where the charge Q is now replaced with the charge of the electron e
  • 1 eV is 1 electronvolt, which is defined as:

A unit of energy equal to the work done by an electron accelerated through a potential difference of 1 volt

  • When a potential difference is applied across a conductor, the electrons are accelerated and gain kinetic energy

 

Electronvolt, downloadable AS & A Level Physics revision notes

Electron beam accelerated through a potential difference of 1 V

  • This kinetic energy gained is equal to an electronvolt:

eV = ½ mv2

  • Where:
    • e = elementary charge (C)
    • V = potential difference (V)
    • m = mass of the electron (kg)
    • v = speed of the electrons (m s-1)

  • To convert between eV and J:
    • eV → J: multiply by 1.6 × 10-19
    • J → eV: divide by 1.6 × 10-19

Worked example

Calculate the speed of an electron that is accelerated through a potential difference of 5 V.

Examiner Tip

Remember to clearly state the difference between lower case v meaning velocity and upper case V meaning the potential difference in your exam, otherwise, you may define or substitute the wrong value!

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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.