Electrical Resistivity (Edexcel International A Level Physics)

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Electrical 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 cause 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

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

Resistivity of some materials at room temperature

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

 

Worked example

Two electrically-conducting cylinders made from copper and aluminium respectively.

Their dimensions are shown below.

WE - resistivity question image, downloadable AS & A Level Physics revision notes

Copper resistivity = 1.7 × 10-8 Ω m

Aluminium resistivity = 2.6 × 10-8 Ω m

Which cylinder is the better conductor?

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

Exam Tip

You don’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, then area is proportional to the radius or diameter squared. This means if the diameter doubles, the area will quadruple. This causes the resistance to drop by a quarter. Considering how changing one property affects the value of the others is a common exam question.

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Joanna

Author: Joanna

Joanna obtained her undergraduate degree in Natural Sciences from Cambridge University and completed her MSc in Education at Loughborough University. After a decade of teaching and leading the physics department in a high-performing academic school, Joanna now mentors new teachers and is currently studying part-time for her PhD at Leicester University. Her passions are helping students and learning about cool physics, so creating brilliant resources to help with exam preparation is her dream job!

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