A level Physics · Edexcel 9PH0 · Topic 3D

Potential dividers

Key points

A potential divider is two (or more) resistors in series across a supply; the p.d. is shared in the ratio of their resistances.
Output equation: $V_{out} = \frac{R_2}{R_1 + R_2} \times V_{in}$ , where $V_{out}$ is measured across $R_2$ .
Replacing one resistor with a thermistor or LDR makes a sensor circuit: the output changes with temperature or light level.
Example: if the thermistor is $R_1$ and it warms up, its resistance falls, so a larger share of the p.d. appears across $R_2$ .
A potentiometer (variable resistor with a sliding contact) gives a continuously variable output from 0 V up to the supply p.d.
Connecting a load across the output puts it in parallel with $R_2$ , which lowers that part's resistance and so reduces $V_{out}$ , unless the load resistance is much larger.
Explain answers by linking the resistance change to the share of p.d.

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A potential divider shares a supply p.d. between resistors in series, giving an output that can be set or made to respond to the surroundings. It is the basis of sensor circuits and of the thermostat in core practical 12.

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