CIE AS Physics (9702) 2019-2021

Revision Notes

11.2.4 Resistivity

Resistivity

  • All materials have some resistance to the flow of charge
  • As free electrons move through a metal wire, they collide with ions which get in their way
  • As a result, they transfer some, or all, of their kinetic energy on collision, which causes electrical heating

Electrons and resistance, downloadable AS & A Level Physics revision notes

Free electrons collide with ions which resist their flow

 

  • Since current is the flow of charge, the ions resisting their flow causes resistance
  • Resistance depends on the length of the wire, the cross-sectional area through which the current is passing and the resistivity of the material

Resistivity equation, downloadable AS & A Level Physics revision notes

Electrical resistance equation

 

  • The resistivity equation shows that:
    • The longer the wire, the greater its resistance
    • The thicker the wire, the smaller its resistance

Factors affecting resistance, downloadable AS & A Level Physics revision notes

The length and width of the wire affect its resistance

 

  • Resistivity is a property that describes the extent to which a material opposes the flow of electric current through it
  • It is a property of the material, and is dependent on temperature
  • Resistivity is measured in Ω m

Resistivity of some materials at room temperature

Table of resistivity of materials at room temperature, downloadable AS & A Level Physics revision notes

    • The higher the resistivity of a material, the higher its resistance
    • This is why copper, with its relatively low resistivity at room temperature, is used for electrical wires — current flows through it very easily
    • Insulators have such a high resistivity that virtually no current will flow through them

 

Worked example

Worked example - resistivity (1), downloadable AS & A Level Physics revision notes

Worked example - resistivity (2), downloadable AS & A Level Physics revision notes

Exam Tip

  • You won’t need to memorise the value of the resistivity of any material, these will be given in the exam question.
  • Remember if the cross-sectional area is a circle e.g. in a wire, it is proportional to the diameter squared. This means if the diameter doubles, the area quadruples causing the resistance to drop by a quarter.

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