# 4.2.9 The Resistance of a Wire

### Resistance of a Wire: Basics

• As electrons pass through a wire, they collide with the metal ions in the wire

Electrons collide with ions, which resist their flow

• The ions get in the way of the electrons, resisting their flow
• If the wire is longer, each electron will collide with more ions and so there will be more resistance:
`The longer a wire, the greater its resistance`
• If the wire is thicker (greater diameter) there is more space for the electrons and so more electrons can flow:
`The thicker a wire, the smaller its resistance`

The length and width of the wire affect the resistance

Extended Only

### Resistance of a Wire

• The resistance of a wire is proportional to its length
• This means that if the length of a wire is doubled, its resistance will double
• The resistance of a wire is inversely proportional to its cross-sectional area
• This means that is the cross-sectional area of a wire is doubled, its resistance will halve

#### Exam Tip

Cross-sectional area is proportional to the diameter squared.

This means that if the diameter is doubled, the cross-sectional area will quadruple, causing the resistance to drop to a quarter.

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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