# Resistance

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

The resistance in a wire increases as:

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

An electric current flows when move through a conductor, such as a metal wire. The moving electrons can collide with the 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 .

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 .

Cross-sections of thin and thick wires

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