Resistance

There is a resistance to the flow of an electric current through most conductors.

The resistance in a wire increases as:

  • the length of the wire increases
  • the thickness of the wire decreases

An electric current flows when electrons move through a conductor, such as a metal wire. The moving electrons can collide with the ions in the metal. This makes it more difficult for the current to flow, and causes resistance.

The resistance of a long wire is greater than the resistance of a short wire because electrons collide with more ions as they pass through. The relationship between resistance and wire length is proportional.

A circuit with a cell and two lamps has high resistance and low current. A circuit with a cell and one lamp has low resistance and therefore higher current.Circuit with a cell, switch, lamp and ammeter connected in series

The resistance of a thin wire is greater than the resistance of a thick wire because a thin wire has fewer electrons to carry the current. The relationship between resistance and the area of the cross section of a wire is inversely proportional.

Cross section of wire. Thin wire shows there is not much space for electrons to move, resistance is high. Thick wire showing there is more space for electrons to move, resistance is low.Cross-sections of thin and thick wires

When resistance is increased in a circuit, for example by adding more electrical components, the current decreases as a result.