CIE A Level Physics (9702) 2019-2021

Revision Notes

11.2.1 Resistance

Defining Resistance

  • Resistance is defined as the opposition to current
    • For a given potential difference: The higher the resistance the lower the current
  • Wires are often made from copper because copper has a low electrical resistance. This is also known as a good conductor
  • The resistance R of a conductor is defined as the ratio of the potential difference V across to the current I in it

Resistance equation, downloadable AS & A Level Physics revision notes

Resistance of a component is the ratio of the potential difference and current


  • Resistance is measured in Ohms (Ω)
  • An Ohm is defined as one volt per ampere
  • The resistance controls the size of the current in a circuit
    • A higher resistance means a smaller current
    • A lower resistance means a larger current
  • All electrical components, including wires, have some value of resistance

Calculating Resistance

Determining Resistance

  • To find the resistance of a component, we can set up a circuit like the one shown below


Resistance circuit, downloadable AS & A Level Physics revision notes

A circuit to determine the resistance of a component

  • The power supply should be set to a low voltage to avoid heating the component, typically 1-2 V
  • Measurements of the potential difference and current should then be taken from the voltmeter and ammeter respectively
  • Finally, these readings should be substituted into the resistance equation

Worked Example

A charge of 5.0 C passes through a resistor of resistance R Ω at a constant rate in 30 s.

If the potential difference across the resistor is 2.0 V, calculate the value of R.

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