Resistance is the measure of opposition to the flow of current (how difficult it is for a current to flow through a component). Components and wires in a circuit reduce the flow of current through them.

  • Resistance is measured in ohms (Ω)

The resistance of an object is also dependent on its material. Some materials have a high resistance and other materials have a low resistance.

Insulators and Conductors


A high resistance means it is difficult for a current to flow through.

Materials with a high resistance are called insulators. For example, plastic. This is why the coating of wires tends to be made out of plastic, this allows us to be able to handle wires safely. This also reduces the amount of energy lost by heat increasing its efficiency.


A low resistance means it is easy for charge to flow through

Materials with a low resistance are called conductors. Metals have low resistance and thus are conductors. This is why they tend to be used in wires.

Identifying insulators and conductors

If you place a material into a circuit, you can determine if it is a conductor or an insulator. If the lamp lights up then the object is a conductor, if it does not light up then the object is an insulator.

Calculating Resistance

The equation that links resistance, potential difference and current is:

Adding more components increases the resistance in a circuit.

In order to measure the resistance of a component, we need to measure the potential difference. We can do this using a voltmeter.

We would also need to measure the current, which we can do using an ammeter.

When calculating the resistance of a lamp in a circuit, you take the measurements from both the voltmeter and the ammeter, then put them both into the equation.


There is a series circuit in the diagram above, which includes a lamp, a resistor, an ammeter in the circuit loop and the voltmeter parallel to the lamp.

Let’s say the reading on the ammeter is 10 amps (A) and the reading on the voltmeter is 5 volts (V). We can then calculate the resistance using the resistance equation:

Resistance (Ω) = Potential difference (V) ÷ Current (A)

Now let’s put the values in the equation.

Resistance = 5 V ÷ 10 A

So, resistance = 0.5 Ω

We also know that potential difference is directly proportional to current. This means that if you plot potential difference against current, you would get a straight line.