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The resistivity of a material is a measure of how much resistance a material provides.

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Resistance to motion

  • Resistance depends on the material of which the object is composed.
  • Different materials offer different resistances to the flow of charge.
  • Resistivity is an intrinsic property of a material, independent of its shape or size.
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  • The equation for resistivity is:
    • Resistivity = (resistance x area) ÷ length
    • ρ=RAL{\rho}=\frac{RA}{L}
  • Where ρ{\rho} is the resistivity, A is the area, R is the resistance and L is the length of the wire.

Applications of Thermistors

Thermistors are electrical components which change their resistance based on their temperature. This means we can use thermistors in devices where outputs depend on temperature variation.

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Types of thermistors

  • Thermistors come in two varieties: positive temperature coefficient and negative temperature coefficient thermistors.
  • You only need to focus on negative temperature coefficient thermistors (NTC).
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NTC thermistors

  • With negative temperature coefficient thermistors: the higher the temperature, the lower the resistance.
    • The resistance is then a proxy (something used to reflect the value of a factor in a calculation) for the temperature of the thermistor.

Determining the Resistivity of a Wire

We can perform the following experiment to determine the resistivity of a wire.

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  • First, determine the length of the wire (make sure the wire is straight) and the cross-sectional area using a micrometer screw gauge.
    • The assumption with the micrometer screw gauge is that the wire's cross-sectional area is circular.
  • Set up the wire with a flying lead that can be reattached at any arbitrary (random) point on the wire.
  • Make sure that the current used is low otherwise the wire will heat up and the results will inflate the resistivity of the material.
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  • Record the resistance for various lengths of wire.
    • The resistance can be determined by the ratio of observed potential difference to observed current.
  • Plot resistance (y-axis) against the length of the wire (x-axis).
  • Determine the gradient of the graph and multiply the gradient by the cross-sectional area to find the resistivity of the wire.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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