Potential Dividers Lesson | GCSE Physics OCR 21st Century Higher Triple

Potential Dividers

This lesson covers:

What a potential divider is
How variable resistors can be used to provide a variable potential difference
How potentiometers can be used to provide a variable potential difference

Potential dividers

In its most basic form, a potential divider is a circuit with two resistors in series with a power supply such as a cell.

The diagram below shows a simple potential divider circuit.

Diagram of a potential divider circuit with two 20 ohm resistors in series and a 10 volt power supply showing 5 volts across each resistor.

When the resistors are identical, the potential difference is split evenly across each resistor.

If there are 2 identical resistors, the potential difference across each resistor will be half the supply voltage.

If there are 3 identical resistors, the potential difference across each resistor will be a third of the supply voltage.

Worked example: Potential divider calculation

The diagram below shows a potential divider circuit where the resistors are not identical.

Diagram of a potential divider circuit with 10 ohm and 20 ohm resistors and a 9V battery.

If the resistors are not identical, the potential difference across each will split based on the ratio of their resistances.

This is how to calculate the voltage across each resistor:

Variable resistors

Variable resistors can be used in a potential divider to provide a variable potential difference.

Circuit diagram showing a potential divider with a 10V battery, a 50 ohm resistor, a variable resistor, and a voltmeter.

When the variable resistor is set to 0 Ω

A variable resistor can vary it's resistance from 0 Ω to its maximum rated value.

When the variable resistor is set to 0 Ω:

There is no potential difference across variable resistor.
The voltmeter reading will be equal to the supply potential difference.

Increasing the resistance of the variable resistor

As the resistance of the variable resistor increases:

As the resistance of the variable resistor increases, the potential difference across it increases.
The voltmeter reading will decrease.

Potentiometers

A potentiometer uses the same concept as a potential divider, with a single variable resistor taking the place of the two fixed resistors.

The variable resistor allows the ratio of resistances R₁ and R₂ to be adjusted by moving a slider.

Circuit diagram showing a potential divider with a variable resistor R2.

When the potentiometer wiper is all the way to the left, R₁ is zero and R₂ is maximum.

The voltmeter is in parallel with R₁, so the voltmeter reading is zero.

Circuit diagram showing a potentiometer with a 10V power supply and resistors R1 and R2, with a 5V output.

When the potentiometer wiper is between the two ends, a potential divider is formed..

Here the wiper is half way across and so R₁and R₂ are the same.

As R₁ represents 50% of the total resistance, the voltmeter reading is 50% of the supply voltage.

Circuit diagram showing a potentiometer with a variable resistor labeled R2 and a 10V power supply.

When the wiper is at the other end R₁ is maximum and R₂ is zero.

Now R₁ represents the total resistance and so the entire supply voltage is dropped across it.

The voltmeter reads the voltage across R₁ and so is the same as the supply voltage.

Worked example: Potentiometer

The diagram below shows a potentiometer being used to provide a variable output voltage.

The output voltage of the potential divider can be calculated using:

Output voltage = total circuit resistanceR1× supply voltage

Diagram showing a potentiometer used to provide a variable output voltage with resistors R1 and R2 and a voltmeter.

This is how to calculate the voltage shown on the voltmeter:

Circuit diagram showing a potential divider with a 9V battery, a 40 ohm resistor, a 60 ohm resistor, and voltmeters V1 and V2.

Which of the following shows the correct readings for the voltmeter readings?

V₁ = 4.5 V and V₂ = 4.5 V

V₁ = 9 V and V₂ = 0 V

V₁= 3.6 V and V₂ = 5.4 V

V₁ = 3 V and V₂ = 6 V

What happens to the voltmeter reading as the variable resistor increases in resistance?

voltmeter reading decreases

voltmeter reading increases

voltmeter reading stays the same

Circuit diagram showing a 10V battery, a 50 ohm resistor, a variable resistor, and a voltmeter.

The resistance of the variable resistor is 75 Ω. Which of the following represents the voltmeter reading?

7 V

10 V

4 V

6 V

Which of the following best describes a potential divider?

A system that separates charges

A circuit that distributes current evenly

A device that divides electrical energy

A component that splits potential difference

Which of the following is a common application of potentiometers?

Controlling the speed of a motor

Adjusting screen brightness on a smartphone

Monitoring air pressure in a weather station

Measuring heart rate in a fitness tracker