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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 9

Topic 4.3.3 · Action and use of circuit components

Resistance and p.d. at constant current

For a constant current, the p.d. across a conductor increases as its resistance increases (because V = IR).

The potential divider

A potential divider is two (or more) resistors in series across a supply. The supply p.d. is shared in proportion to the resistances.

For two resistors: R1 / R2 = V1 / V2

A potential divider is made from 2.0 kΩ and 4.0 kΩ in series across 12 V. The output is taken across the 4.0 kΩ resistor. Calculate the output p.d.

Worked example: potential divider output is 8.0 volts across the 4.0 kilohm resistor
Vout = (4.0 / 6.0) × 12 = 8.0 V.

The output is the p.d. across one of the resistors. A variable resistor, LDR or NTC thermistor in the divider makes the output change with setting, light or temperature.

Potential divider: two resistors in series across a supply, output taken from the junction, with V1/V2 = R1/R2.
Diagram 1: Potential divider: two resistors in series across a supply, output taken from the junction, with V1/V2 = R1/R2.
Potential divider using an LDR (or NTC thermistor) and a fixed resistor, showing the output p.d. changing as light (or temperature) changes.
Diagram 2: Potential divider using an LDR (or NTC thermistor) and a fixed resistor, showing the output p.d. changing as light (or temperature) changes.

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