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# Resistivity for OCR A-level Physics

This page covers the following topics:

1. Resistivity

2. Temperature and resistance

3. Thermistors

4. Superconductors

Resistivity is the electrical property of materials which characterises their ability to conduct electricity and is a material constant. It can be used to calculate the resistance of a material, based on its length and its cross-sectional area. Resistivity is measured in Ohm meters (Ωm) and is dependent on temperature.

The vibrations of the ions that a metal conductor is made up of will increase as the temperature increases. Therefore, as electrons flow through the conductor, there will be more collisions between them and the ions. This means that the resistance of the conductor has increased with temperature. Typically, the temperature of a metal conductor will increase as the current in it increases.

A thermistor is a non-ohmic conductor whose resistance changes with temperature. Thermistors have a negative temperature coefficient, meaning that the resistance of thermistors decreases as the temperature increases. This is because as the temperature increases, more electrons become free to carry the current and therefore the resistance decreases. Some applications of thermistors are in ovens, fire alarms and thermometers.

Below a critical temperature, the resistance of a material suddenly reduces to zero. Such a material is called a superconductor. Useful applications of superconductors are in circuits where large currents are needed. Some examples of superconducting materials are mercury and lead.

# 1

Define resistivity.

Resistivity is the electrical property of materials which characterises their ability to conduct electricity.

# 2

What happens to the resistance of a thermistor as the temperature decreases?

The thermistor has a negative temperature coefficient, therefore as the temperature decreases, the resistance of the thermistor increases.

# 3

State what is meant by a superconductor.

A superconductor is a material whose resistance will suddenly decrease to 0 below a critical temperature.

# 4

Define what is meant by the critical temperature of a superconductor.

The critical temperature is the temperature at which the resistance of a superconductor becomes zero.

# 5

Explain how increasing the temperature of a thermistor will affect its resistance.

The thermistor has a negative temperature coefficient, therefore as the temperature increases, the resistance of the thermistor decreases.

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