Current, resistance and potential difference

Potential difference

The current through a component depends on both the resistance of the component and the potential difference across the component.

curriculum-key-fact
Potential difference is a measure of how much energy is transferred to each Coulomb between two points in a circuit

Measuring potential difference

Circuit with a cell, switch and lamp. A voltmeter is connected in parallel with the lamp.

To measure the potential difference across a component, a voltmeter must be placed in parallel with that component in order to measure the difference in energy from one side of the component to the other. Potential difference is also known as voltage and is measured in volts (V).

Resistance

Jonny Nelson explains resistance with a GCSE Physics practical experiment

When a charge moves through a potential difference, electrical work is done and energy transferred. The potential difference can be calculated using the equation:

potential difference = current × resistance

\text{V} = \text{I} \times \text R

This is when:

  • potential difference ( \text{V}) is measured in volts (V)
  • current ( \text{I}) is measured in amps (A)
  • resistance ( \text{R}) is measured in ohms (Ω)

One volt is the potential difference when one joule of work is done moving one coulomb of charge.

curriculum-key-fact
Conductors have a low resistance. Insulators have large resistances.

Example

What is the potential difference if a current of 2 A flows through a resistance of 40 Ω?

\text{V} = \text{I} \times \text R

\text{V} = 2 × 40

\text{V} = 80 V

Question

What is the resistance of a component if 12 V causes a current of 2 A through it?

\text{V} = \text{I} \times \text R

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

\text{R} = \frac{12}{2}

\text{R} = 6 Ω