Potential Dividers

Objectives

Lesson outcomes

Explain the principle of a potential divider circuit.
Use the potential divider principle to calculate or predict output voltage.
Describe how a potentiometer can compare potential differences using a null method.
Explain how thermistors and LDRs produce temperature- or light-dependent output voltages.

Syllabus

CIE 9702 syllabus points

4 linked

10.3.1 understand the principle of a potential divider circuit
10.3.2 recall and use the principle of the potentiometer as a means of comparing potential differences
10.3.3 understand the use of a galvanometer in null methods
10.3.4 explain the use of thermistors and light-dependent resistors in potential dividers to provide a potential difference that is dependent on temperature and light intensity

Lesson Notes

Student guidance and lesson notes

Overview

Potential dividers use series resistors to share a supply voltage. This lesson shows how the output voltage depends on resistance values, then applies the same principle to potentiometers and sensor circuits using thermistors and LDRs.

What You Need to Know

In a potential divider, two or more components are connected in series across a supply.
The supply voltage is shared between the components in proportion to their resistances.
The output voltage is usually measured across one part of the divider, so changing one resistance changes the output.
For two fixed resistors, use the fraction of total resistance across the output section to find the output voltage.
A potentiometer is a variable potential divider that can be used to compare potential differences.
In a null method, a galvanometer shows zero current when the compared potential differences are balanced.
Replacing one resistor with an LDR or thermistor makes the output voltage depend on light intensity or temperature.

How to Work Through It

Start by reviewing series circuits: the same current flows through both resistors, and p.d.s add to the supply voltage.
Calculate output voltage for fixed resistor dividers by identifying exactly which resistor or section the output is taken across.
Use potentiometer examples to connect slider position with changing output p.d.
Predict sensor behaviour by deciding first whether the LDR or thermistor resistance increases or decreases, then tracing the effect on output voltage.

Check Your Understanding

Why must the resistors in a basic potential divider be in series?
If the output is taken across the lower resistor, what happens when that resistor becomes a larger fraction of the total resistance?
What does a zero reading on the galvanometer mean in a null method?
How does an LDR’s resistance change when light intensity increases?

Common Mistakes

Taking the output voltage across the wrong component.
Treating a potential divider as if the resistors were in parallel.
Memorising one sensor outcome without checking where the output is measured.
Thinking a null method means there is no potential difference anywhere in the circuit, rather than no current through the galvanometer branch.

Next Steps

Practise sketching the circuit before calculating the output voltage.
Use resistance, sensor behaviour, and Kirchhoff’s laws together when explaining divider circuits.