Both electromotive force and potential difference are measured in volts. Explain why they are nevertheless different quantities.

Both are an energy per unit charge and so share the same unit, but they describe transfers in opposite directions: one is energy entering the electrical circuit from a source and the other is energy leaving it in a component.

Why is the terminal potential difference of a real cell less than its electromotive force when it is driving a current?

Some of the energy given to each coulomb by the cell is converted into internal energy inside the cell itself as the charge passes through its internal resistance, so less energy per coulomb is available to the external circuit.

Under what condition is the terminal potential difference of a cell equal to its electromotive force?

They are equal when the current is zero, or when the internal resistance is negligible, because in either case no energy is lost inside the source.

State the energy relationship that holds for a complete circuit loop containing a source and several components.

The sum of the electromotive forces around the loop equals the sum of the potential differences around it, because the electrical energy given to each coulomb by the sources must all be converted somewhere in the loop.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

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Current, Charge and Potential Difference

Comparing electromotive force and potential difference

3 things to know
Unit AS 1

Overview

Recall questions on the essential difference between the two, why both are measured in volts, where each is measured in a circuit, why terminal potential difference is less than the electromotive force and when the two are equal, and worked examples on a battery, a torch and a solar cell.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How does the electromotive force of a cell change as the cell is used, and why?

Key term

State the essential difference between electromotive force and potential difference.

Key term

Across which part of a circuit is an electromotive force measured, and across which part is a potential difference measured?

Questions from this subtopic

Each one with the model answer underneath.

1
State the essential difference between electromotive force and potential difference.
Model answer
Electromotive force is the electrical energy produced per unit charge by a source, whereas potential difference is the electrical energy converted into other forms per unit charge in a component.
2
Both electromotive force and potential difference are measured in volts. Explain why they are nevertheless different quantities.
Model answer
Both are an energy per unit charge and so share the same unit, but they describe transfers in opposite directions: one is energy entering the electrical circuit from a source and the other is energy leaving it in a component.
3
Across which part of a circuit is an electromotive force measured, and across which part is a potential difference measured?
Model answer
An electromotive force belongs to a source such as a cell, dynamo or solar cell, while a potential difference is measured across a component such as a resistor, lamp or motor.
4
Why is the terminal potential difference of a real cell less than its electromotive force when it is driving a current?
Model answer
Some of the energy given to each coulomb by the cell is converted into internal energy inside the cell itself as the charge passes through its internal resistance, so less energy per coulomb is available to the external circuit.
5
Under what condition is the terminal potential difference of a cell equal to its electromotive force?
Model answer
They are equal when the current is zero, or when the internal resistance is negligible, because in either case no energy is lost inside the source.
6
State the energy relationship that holds for a complete circuit loop containing a source and several components.
Model answer
The sum of the electromotive forces around the loop equals the sum of the potential differences around it, because the electrical energy given to each coulomb by the sources must all be converted somewhere in the loop.

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