Know that e.m.f. is measured in volts (V)

Published by Patrick Mutisya · 14 days ago

Cambridge IGCSE Physics 0625 – 4.2.3 Electromotive Force and Potential Difference

4.2.3 Electromotive Force (e.m.f.) and Potential Difference

Learning Objective

Students should be able to state that electromotive force (e.m.f.) is measured in volts (V) and understand how this unit relates to other electrical quantities.

Key Definitions

  • Electromotive Force (e.m.f.): The energy supplied per unit charge by a source (such as a battery or generator) when no current is flowing. It is the maximum potential difference the source can provide.

  • Potential Difference (p.d.): The work done per unit charge as charge moves between two points in a circuit while a current is flowing. It is often called the voltage across a component.

Relationship Between e.m.f. and Potential Difference

When a circuit is open (no current), the measured voltage across the terminals of a source equals its e.m.f. Once the circuit is closed, the internal resistance of the source causes a drop, so the terminal p.d. is slightly less than the e.m.f.

Unit of e.m.f.

The SI unit of e.m.f. is the volt (V). By definition:

\$\$

1\ \text{V} = 1\ \frac{\text{joule}}{\text{coulomb}} = 1\ \frac{\text{J}}{\text{C}}

\$\$

Thus, a source with an e.m.f. of 1 V supplies 1 J of energy to each coulomb of charge that passes through it.

Comparing e.m.f. and Potential Difference

AspectElectromotive Force (e.m.f.)Potential Difference (p.d.)
When measuredOpen‑circuit (no current)Closed circuit (current flowing)
CauseEnergy supplied by sourceEnergy lost or gained as charge moves through components
Effect of internal resistanceNot affected (ideal condition)Reduced by \$I r{\text{int}}\$ where \$I\$ is current and \$r{\text{int}}\$ is internal resistance
Symbol\$\mathcal{E}\$\$V\$
UnitVolt (V)Volt (V)

Typical \cdot alues of e.m.f.

  1. AA alkaline cell: \$\mathcal{E} \approx 1.5\ \text{V}\$

  2. 9‑V battery: \$\mathcal{E} \approx 9\ \text{V}\$

  3. Car battery (lead‑acid): \$\mathcal{E} \approx 12\ \text{V}\$

  4. Household mains (UK): \$\mathcal{E} \approx 230\ \text{V}\$ (rms value)

Practical Measurement

To measure the e.m.f. of a source, use a voltmeter connected across the terminals while the circuit is open. The reading directly gives the e.m.f. in volts.

Suggested diagram: A simple circuit showing a battery (e.m.f. \$\mathcal{E}\$), internal resistance \$r_{\text{int}}\$, a load resistor \$R\$, and a voltmeter across the battery terminals.

Common Misconceptions

  • Thinking that e.m.f. is a force. It is a potential difference, not a mechanical force.

  • Assuming the terminal voltage always equals the e.m.f. – internal resistance causes a drop when current flows.

  • Confusing the unit “volt” with “ampere”. Voltage (V) measures energy per charge, while current (A) measures charge per time.

Summary

Electromotive force (e.m.f.) is the maximum potential difference a source can provide and is measured in volts (V). The relationship \$1\ \text{V}=1\ \text{J/C}\$ links the unit to fundamental energy and charge concepts. Understanding the distinction between e.m.f. and the potential difference across a component is essential for analysing real circuits.