Current, potential difference and resistance

Jonny Nelson explains resistance with a GCSE Physics practical experiment

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 ( \Omega)
curriculum-key-fact
One volt is the potential difference when one coulomb of charge transfers one joule of energy.

Example

A current of 2.0 A flows through a component with a resistance of 40 \Omega. Calculate the potential difference.

potential difference = current × resistance

= 2.0 × 40

= 80 V

Question

A current of 2.0 A flows through a component with a potential difference of 12 V. 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

Resistance in series and parallel circuits

Resistors in series

In a series circuit the total resistance across all of the components (the 'net resistance') increases as more components are added.

This circuit shows an example of resistors in series.

The two resistors have the same current flowing through them. The potential difference across them will be different if they have different resistances. The total resistance for this circuit is: R1 + R2.

Resistors in parallel

In a parallel circuit the net resistance decreases as more components are added, because there are more paths for the current to flow through.

This circuit shows an example of resistors in parallel.

The two resistors have the same potential difference across them. The current flowing through them will be different if they have different resistances.

The total current in the circuit is the sum of the currents through each branch. The total resistance for this circuit is calculated by dividing the potential difference of the cell by the total current for the circuit.