Extended · Practice questions · Potential dividers

Sharing the supply.

Six original Cambridge-style questions on potential dividers: the divider equation, sharing voltage by resistance, scaling both resistors, and sensing circuits with an LDR or thermistor.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

How to handle divider questions

It is the ratio of resistances.

Equation: V_out = V × R₂ ÷ (R₁ + R₂), output across R₂.
Shares add: the p.d.s across R₁ and R₂ add up to the supply.
Sensing: an LDR or thermistor changes one resistance, so the output tracks light or temperature.

Recall

[2 marks]

State what a potential divider is, and write the equation for the output voltage taken across the lower resistor R₂.

Two (or more) resistors in series across a supply, with an output taken across one of them. ✓
V_out = V × R₂ ÷ (R₁ + R₂). ✓

Calculation

[2 marks]

A 12 V supply is connected across a 6 Ω resistor and a 6 Ω resistor in series. Calculate the output voltage across one of them.

V_out = 12 × 6 ÷ (6 + 6)

V_out = 6.0 V

equal resistors share the voltage equally

Calculation

[3 marks]

A 9.0 V supply is connected across R₁ = 200 Ω and R₂ = 100 Ω in series. Calculate the output voltage across R₂, and the p.d. across R₁.

V_out = 9.0 × 100 ÷ (200 + 100) = 9.0 × 100 ÷ 300

V_out = 3.0 V

p.d. across R₁ = 9.0 − 3.0 = 6.0 V. ✓

Analysis

[2 marks]

In a divider, both resistors are doubled in value. State what happens to the output voltage, and explain.

The output voltage is unchanged. ✓
The output depends on the ratio R₂ ÷ (R₁ + R₂), which is the same when both are doubled. ✓

Application

[3 marks]

An LDR is used as the lower resistor R₂ in a divider, with the output taken across the LDR. Describe how the output voltage changes as it gets dark, and explain in terms of resistance.

As it gets dark, the output voltage rises. ✓
Darkness increases the LDR resistance. ✓
So R₂ takes a larger share of the supply, raising V_out. ✓

Application

[2 marks]

A thermistor is used in a potential divider to help switch on a cooling fan when a room gets hot. State how the thermistor's resistance changes as the room warms, and hence how the output it controls changes.

As the room warms, the thermistor resistance falls. ✓
So its share of the supply falls, while the p.d. across the other resistor rises, and that rising output can switch the fan on. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.