Current, potential difference and resistance

The current in a circuit depends on the potential difference and the resistance. It is easy to think of each of these in the following way:

  • the current is the flow of electric charges
  • the potential difference provides the 'push'
  • the resistance restricts the flow of charges

Resistance

Electric wires are made of metal, which have electrical resistance. The atoms in a solid metal are arranged in a regular lattice structure. The outer electrons from each atom are free to move through the structure, forming a current. However, they may collide with atoms or meet defects in the lattice. This reduces the number of electrons flowing, which reduces the current.

Conductors have a low resistance and insulators have a high resistance.

Calculating potential difference

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

potential difference = current × resistance

This is when:

  • potential difference is measured in volts (V)
  • current is measured in amps (A)
  • resistance is measured in ohms (Ω)
curriculum-key-fact
One volt is the potential difference when one coulomb of charge transfers one joule of energy.

Example

There is a current of 2.0 A in a component with a resistance of 40 Ω. Calculate the potential difference across the component.

potential difference = current × resistance

= 2.0 × 40

= 80 V

Question

There is a current of 2.0 A in a component with a potential difference of 12 V across the component. Calculate the resistance of the component.

Rearrange the equation to find resistance and then substitute in the known values.

potential difference = current × resistance

resistance = \frac{potential~difference}{current}

resistance = \frac{12}{2.0}

= 6~\Omega