Resistance (Cambridge International AS Physics)

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

  • $1Ω=1VA^{-1}$

• 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

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.

Answer:

Step 1: Recall the Resistance equation

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

Step 2: Calculate the current from the charge and time

$Q=It$

• Rearrange for I

$I=\frac{Q}{t}$

$I=\frac{5.0}{30}=0.167A=0.17A(2s.f.)$

Step 3: Substitute values into the resistance equation

$R=\frac{2.0}{0.17}$

$R=12.0Ω(2s.f.)$