Resistance (Cambridge (CIE) AS Physics): Revision Note

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

• Materials with low electrical resistance are good conductors

• The resistance of a conductor is calculated using the following equation, sometimes referred to as Ohm's law:

$R=\frac{V}{I}$

• Where:

  • $R$ = resistance, measured in Ω

  • $V$ = potential difference, measured in V

  • $I$ = current, measured in A

• One ohm is equivalent to one volt per ampere

  • $1\mathrm{\Omega }=1\mathrm{V}{\mathrm{A}}^{-1}$

• The resistance in a circuit determines the amount of charge that can flow (current)

  • 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

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

Determining resistance 

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 

• Finally, these readings should be used to calculate resistance