A level Physics · Edexcel 9PH0 · Topic 3B

Resistance and resistivity

Key points

Resistance $R = \frac{V}{I}$ , measured in ohms (Ω). Ohm's law: current is proportional to p.d. for a metallic conductor at constant temperature.
$I$ – $V$ graphs: an ohmic resistor gives a straight line through the origin; a filament lamp curves because resistance rises as it heats up; a diode conducts only in one direction above about 0.6 V.
Resistivity is a property of the material: $R = \frac{\rho l}{A}$ , unit Ω m. Core practical: measure the resistance of different lengths of wire and use the gradient of $R$ against $l$ , with $A$ from micrometer readings of diameter.
In a metal, rising temperature makes lattice ions vibrate more, so resistance increases.
An NTC thermistor's resistance falls as temperature rises, and an LDR's falls as light intensity rises, because more charge carriers are released (larger $n$ ).
Power in a resistor: $P = I^2R = \frac{V^2}{R}$ .

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Resistance tells you how much p.d. is needed to drive a current through a component. These notes cover I–V characteristics, why resistance changes with temperature and light, resistivity and core practical 2.

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