OCR A Level Physics

Revision Notes

4.4.4 E.m.f & Potential Difference

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Potential Difference (p.d.)

  • A cell makes one end of the circuit positive and the other negative
    • This sets up a potential difference (p.d) across the circuit

  • The potential difference across a component in a circuit is defined as

The energy transferred per unit charge flowing from one point to another

  • The energy transfer is from electrical energy into other forms
  • Potential difference is measured in volts (V). This is the same as a Joule per coulomb (J C-1)
    • If a bulb has a voltage of 3 V, every coulomb of charge passing through the bulb will lose 3 J of energy

  • The potential difference of a power supply connected in series is always shared between all the components in the circuit

Potential difference in a circuit, downloadable AS & A Level Physics revision notes

The potential difference is the voltage across each component in a circuit

 
  • Potential difference or voltage is measured using a voltmeter
  • A voltmeter is always set up in parallel to the component you are measuring the voltage for

Voltmeter in a circuit, downloadable AS & A Level Physics revision notes

Potential difference can be measured by connecting a voltmeter in parallel between two points in a circuit

E.m.f & 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

  • 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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