Resistivity and temperature
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 4 practice questions, 3 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Why power cables are thick
- Resistivity
Read the transcript of the teaching part
Last time
Before we start: last time, in Potential difference, the volt, the ohm and Ohm's law, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Why power cables are thick
The cable from a power station is thick, while the heating element in a toaster is a thin coil. Both are metal wires, but we want the cable to have a tiny resistance, and the element to have a large one. Length, thickness and material all matter, and resistivity pulls them together.
Resistivity
The resistance of a uniform wire is proportional to its length L, and inversely proportional to its cross-sectional area A. Combining them with a constant of proportionality for the material: R equals rho L over A. Rho is the resistivity of the material. Rearranged, rho equals R A over L, with units ohm metres. Resistivity is a property of the material. Resistance depends also on the shape of the sample.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Resistivity.
The whole chapter: Electricity, Electricity.
Chapter 9.4: Resistance and resistivity
- Resistivity and temperature 6 min
- Resistivity of wires and LDR circuits 6 min
- Resistivity, drift speed and charge carriers 7 min
Also for A Level Physics 9702
Which topics come up most in A Level Physics 9702 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.