In a potential divider, which resistor has the larger p.d. across it?

the one with the larger resistance. The current is the same in both, and V = IR.

A thermistor and a fixed resistor form a potential divider, with the output across the thermistor. The temperature rises. What happens to the output?

It falls. The thermistor's resistance falls, so its share of the p.d. falls.

What is a potential divider used for?

to provide a fraction of the supply voltage. The output can never exceed the supply.

Potential dividers · Edexcel IAL Physics WPH revision

In plain words

Put two resistors in a row across a battery and they share the voltage between them, in proportion to their resistances. Tap off the voltage across one of them and you have a potential divider: a way of getting any voltage you like, or of turning a change in temperature or light into a change in voltage.

4 things to know

Two resistors in series share the supply p.d. in the ratio of their resistances.
V_out = V_in × R₂ ÷ (R₁ + R₂), where V_out is taken across R₂.
Replacing one resistor with a thermistor or an LDR makes V_out depend on temperature or light level: the basis of sensor circuits.
Along a uniform wire carrying a current, the potential falls evenly with distance, so the p.d. across any part of the wire is proportional to its length.

Worked example

A 12 V supply is connected across a 4.0 kΩ and an 8.0 kΩ resistor in series. Find the p.d. across the 8.0 kΩ resistor.

V_out = V_in × R₂ ÷ (R₁ + R₂).
= 12 × 8.0 ÷ (4.0 + 8.0).
= 8.0 V.

Tips and tricks

The bigger resistance takes the bigger share of the voltage.
In sensor questions, decide first which way the sensor's resistance changes, then which resistor the output is across.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Potential dividers: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

A 10 V supply is connected across 2 Ω and 8 Ω in series. What is the p.d. across the 8 Ω resistor?
- 2 V
- 5 V
- 8 V (the answer)
- 10 V
10 × 8 ÷ 10.
In a potential divider, which resistor has the larger p.d. across it?
- the one with the larger resistance (the answer)
- the one with the smaller resistance
- they are always equal
- the one nearest the positive terminal
The current is the same in both, and V = IR.
A thermistor and a fixed resistor form a potential divider, with the output across the thermistor. The temperature rises. What happens to the output?
- It rises.
- It falls. (the answer)
- It stays the same.
- It becomes equal to the supply.
The thermistor's resistance falls, so its share of the p.d. falls.
A 12 V supply is across R₁ = 3 kΩ and R₂ = 1 kΩ. What is the p.d. across R₂?
- 3 V (the answer)
- 4 V
- 9 V
- 12 V
12 × 1 ÷ 4.
What is a potential divider used for?
- to increase the supply voltage
- to provide a fraction of the supply voltage (the answer)
- to store charge
- to measure current
The output can never exceed the supply.

5 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 7 marks. Write your answers on paper, then check them.

Potential dividers

Edexcel International A Level Physics WPH · 7 marks · papermunch.org

Name ______________________________ Date ______________

A 9.0 V supply is connected across two equal resistors in series. Find the p.d. across each.[2]
Show answerHide answer
4.5 V.
I got this one
A 6.0 V supply is connected across a 2.0 kΩ resistor in series with an LDR, with the output taken across the LDR. In the dark the LDR has resistance 10 kΩ. Find the output p.d.[3]
Show answerHide answer
5.0 V. 6.0 × 10 ÷ 12.
I got this one
In the circuit above the light gets brighter. State and explain what happens to the output p.d.[2]
Show answerHide answer
It decreases. The resistance of the LDR falls, so it takes a smaller share of the supply p.d.
I got this one

Answers: Potential dividers

1. 4.5 V.
2. 5.0 V. 6.0 × 10 ÷ 12.
3. It decreases. The resistance of the LDR falls, so it takes a smaller share of the supply p.d.