CIE A Level Physics (9702) 2019-2021

Revision Notes

12.1.2 Electromotive Force

Electromotive Force

  • When charge passes through a power supply such as a battery, it gains electrical energy
  • The electromotive force (e.m.f) is the amount of chemical energy converted to electrical energy per coulomb of charge (C) when charge passes through a power supply
  • e.m.f is measured in Volts (V)

 

emf definition, downloadable AS & A Level Physics revision notes

Definition of e.m.f with regards to energy transfer

  • 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 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

  • The difference between potential difference and e.m.f is the type of energy transfer per unit charge

 

Potential difference definition, downloadable AS & A Level Physics revision notes

Definition of potential difference with regards to energy transfer

  • When charge passes through a resistor, for example, its electrical energy is converted to heat in the resistor
    • The resistor therefore has a potential difference across it
  • Potential difference describes the loss of energy from charges; ie. when electrical energy is transferred to other forms of energy in a component
  • e.m.f. describes the transfer of energy from the power supply to electrical charges within the circuit

 

Exam 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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