Define the internal resistance of a source of electromotive force.

The internal resistance of a source is the resistance of the source itself to the current it drives, so it is the resistance of the chemicals, electrodes or windings through which the charge must pass inside the source.

Show how V = E − Ir follows from conservation of energy in a circuit of external resistance R.

Each coulomb receives energy E from the source and gives up IR in the external circuit and Ir inside the source, so E = IR + Ir; since the terminal potential difference is V = IR, this rearranges to V = E − Ir.

A cell of electromotive force 6.0 V and internal resistance 0.50 Ω drives a current of 2.0 A. Calculate the terminal potential difference.

The terminal potential difference is V = 6.0 − (2.0 × 0.50) = 5.0 V, and adding the lost volts of 1.0 V back to 5.0 V returns the 6.0 V electromotive force as a check.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

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Internal Resistance and Potential Dividers

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Internal Resistance and Potential Dividers

Internal resistance and terminal potential difference

3 things to know
Unit AS 1

Overview

Internal resistance of a source of electromotive force, lost volts of a cell and terminal potential difference of a cell on open circuit.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the equation relating terminal potential difference to electromotive force and define every symbol with its unit.

Key term

Why does every real source of electromotive force have some internal resistance?

Key term

What is meant by the lost volts of a cell?

Questions from this subtopic

Each one with the model answer underneath.

1
Define the internal resistance of a source of electromotive force.
Model answer
The internal resistance of a source is the resistance of the source itself to the current it drives, so it is the resistance of the chemicals, electrodes or windings through which the charge must pass inside the source.
2
Why does every real source of electromotive force have some internal resistance?
Model answer
Charge has to travel through the material of the source itself, which is never a perfect conductor, so energy is dissipated inside the source as well as in the external circuit.
3
State the equation relating terminal potential difference to electromotive force and define every symbol with its unit.
Model answer
The equation is V = E − Ir, where V is the terminal potential difference in volts, E the electromotive force in volts, I the current in amperes and r the internal resistance in ohms.
4
What is meant by the lost volts of a cell?
Model answer
The lost volts are the quantity Ir, the potential difference across the internal resistance, which represents the energy per coulomb converted to internal energy inside the source rather than delivered to the external circuit.
5
Show how V = E − Ir follows from conservation of energy in a circuit of external resistance R.
Model answer
Each coulomb receives energy E from the source and gives up IR in the external circuit and Ir inside the source, so E = IR + Ir; since the terminal potential difference is V = IR, this rearranges to V = E − Ir.
6
A cell of electromotive force 6.0 V and internal resistance 0.50 Ω drives a current of 2.0 A. Calculate the terminal potential difference.
Model answer
The terminal potential difference is V = 6.0 − (2.0 × 0.50) = 5.0 V, and adding the lost volts of 1.0 V back to 5.0 V returns the 6.0 V electromotive force as a check.

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