# Unit 1 Section 3 Resistivity and Superconductors

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## What is Resistivity?

p = (R x A) / L

• p = resistivity in ohm-metres
• R = resistance in ohms
• A = cross-sectional area in m^2
• L = length in m

Length. the longer the wire, the more difficult it is to make a current flow through it. The resistance is proportional to the length of the wire.

Area. The wider the wire, the easier it will be for electrons to pass along it.

Resistivity. This is a measure of how much a particular material resists current flow. It depends on the structure of the material as well as on environmental factors such as temperature and light intensity. It is a property of the material. The lower the resistivity of a material, the better it is at conducting electricity.

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## Superconductors

All materials have some resistivity, meaning that whenever electricity flows through them, they heat up and some electrical energy is wasted as heat.

The resistivity of many materials can be lowered by cooling them down. If cooled to below a 'transition temperature' their resistivity disappears entirely and they become a superconductor. Without any resistance, none of the electrical energy is wasted as heat.

Most metal conductors have transition temperatures below 10 K. Such low temperatures are tricky and expensive. However some metal oxides have been developed to superconduct at 140 K which is much easier.

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## Uses of Superconductors

Power cables that transmit electricity without any loss of power.

Really strong electromagnets that have lots of applications, e.g. in medicine and Maglev trains.

Electronic circuits that work really fast, because their's no resistance to slow the current down.

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## Glossary

Resistivity: The resistance of a 1m length of material with a 1m^2 cross-sectional area. It is measured in ohm-metres.

Superconductor: A material that has zero resistivity when cooled below a transition temperature.

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