Resistance

The wires and the other components in a circuit reduces the flow of charge through them. This is called resistance.

The unit of resistance is the ohm, and it has the symbol Ω (an uppercase Greek letter omega). For example, a 2 Ω component has a greater resistance than a 1 Ω component, and will reduce the flow of charge through it more effectively.

Adding components

The resistance increases when you add more components in series. For example, the resistance of two lamps is greater than the resistance of one lamp, so less current will flow through them.

Two series circuits. The first has a cell, a closed switch, a bright lamp and an ammeter reading 0.4 A . The second has a cell, closed switch, two dimmer lamps and an ammeter reading 0.2 A.The more lamps, the greater the resistance and the lower the current

Calculating resistance

To find the resistance of a component, you need to measure:

  • the potential difference across it
  • the current flowing through it

The resistance is the ratio of potential difference to current. We use this equation to calculate resistance:

curriculum-key-fact
resistance = potential difference ÷ current

For example, 3 A flows through a 240 V lamp. What is the resistance of the lamp?

resistance = 240 ÷ 3 = 80 Ω

If you plot a graph of current against potential difference for a wire, you get a straight line.

Graph of potential difference (V) against current (A), showing a straight line running from origin to top right (positive)The gradient of the line is equal to the resistance of the wire