Internal Resistance – What It Is, How We Measure It, and Why It Matters

In school physics we often treat power supplies and batteries as if they provide a perfect, constant voltage. In reality, every battery and power supply has something hidden inside it called internal resistance.

Understanding this idea is crucial for GCSE and A-Level Physics, because it explains why the voltage of a battery drops when we draw current from it.

What is Internal Resistance?

Inside every cell or battery there are materials that resist the movement of charge. This resistance is caused by:

The electrolyte in the cell
The electrodes and internal wiring
Chemical reactions occurring inside the battery

This means that some of the electrical energy supplied by the battery is lost as heat inside the battery itself.

Because of this, the voltage available to the external circuit is less than the battery’s emf.

The relationship is given by the well-known equation:

V = 𝓔 − Ir

Where:

V = terminal potential difference across the circuit
𝓔 (emf) = electromotive force of the battery
I = current
r = internal resistance

As the current increases, the voltage lost inside the battery (Ir) increases.

Why Does Voltage Drop When You Use a Battery?

If you connect a battery to a device:

Current flows through the external circuit.
The same current must also pass through the battery’s internal resistance.
Energy is lost inside the battery as heat.

So the voltage you measure across the terminals becomes:

Terminal voltage = emf − voltage lost internally

This is why a battery that reads 1.5 V when unused might drop to 1.3 V when powering a motor or lamp.

Measuring Internal Resistance in the Lab

One of the most useful school experiments measures internal resistance directly.

Equipment needed

Power supply or cell
Voltmeter
Ammeter
Variable resistor (or rheostat)
Switch
Connecting wires

Method

Connect the circuit with the ammeter in series and voltmeter across the cell.
Use the variable resistor to change the current in the circuit.
Record several pairs of readings of current (I) and terminal voltage (V).

Typical results look something like this:

Current (A)	Voltage (V)
0.10	1.48
0.20	1.46
0.30	1.43
0.40	1.39

As the current increases, the voltage falls.

Using a Graph to Find Internal Resistance

Plot a graph of:

Terminal Voltage (V) vs Current (I)

You obtain a straight line with a negative gradient.

From the equation:

V = 𝓔 − Ir

The y-intercept = emf (𝓔)
The gradient = −r

So:

Internal resistance = − gradient of the graph

This is a beautiful example of physics linking theory, experiment, and graph analysis.

Why Internal Resistance Is So Important

Internal resistance affects many real-world technologies:

1. Batteries in phones and laptops
As batteries age, internal resistance increases. This is why older batteries discharge quickly.

2. Electric vehicles
Battery efficiency depends heavily on keeping internal resistance low.

3. High-current devices
Things like motors or heating elements draw large currents, which can cause large voltage drops.

4. Power losses
Energy lost inside a battery becomes heat, reducing efficiency.

A Simple Demonstration for Students

A good classroom demonstration is to:

Measure the open-circuit voltage of a battery.
Connect a small motor or lamp.
Measure the voltage again.

Students immediately see the voltage drop under load, making internal resistance very real.

Using sensors such as PASCO current and voltage probes, you can even capture the data live and plot the graph instantly.

Final Thought

Internal resistance reminds us that real electrical systems are never perfect.

Every battery, from a small AA cell to the massive batteries in electric cars, loses some energy internally. Understanding this helps physicists and engineers design more efficient power systems.

And it also explains why your phone battery seems to struggle when it gets old!

17 March 2026