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Electromotive Force (CIE A Level Physics)

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Electromotive force

  • When charge passes through a power supply such as a battery, it gains energy
  • The electromotive force (e.m.f) is the amount of energy transferred per coulomb of charge (C) when charge passes through a power supply
  • That energy drives the charge around the complete circuit and is transferred to the components
  • e.m.f is measured in Volts (V)

 

straight e. straight m. straight f space equals space fraction numerator energy space transferred space over denominator charge end fraction

  • e.m.f is a measure of the energy transferred into the circuit by the power source

  • E.m.f is also the potential difference across the cell when no current is flowing
  • E.m.f can be measured by connecting a high-resistance voltmeter around the terminals of the cell in an open circuit

Measuring e.m.f in a cell 

Measuring emf, downloadable AS & A Level Physics revision notes

E.m.f is measured using a voltmeter connected in parallel with the cell

EMF & Potential Difference

  • Potential difference is also the energy transferred per unit charge
  • But potential difference is a measure of the energy transferred out of the circuit via the components

 

potential space difference space equals space fraction numerator energy space transferred over denominator charge end fraction

  • When charge passes through a resistor, for example, energy is transferred from the charge (electron) to the resistor
    • The resistor therefore has a potential difference across it

Examiner Tip

Although voltage and potential difference are the same thing, make sure not to confuse them with e.m.f, which is slightly different!

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Ashika

Author: Ashika

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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.