Lenz's Law
Lenz's Law
- Faraday's law gives the e.m.f. magnitude. Lenz's law selects the direction that obeys energy conservation.
The induced effect opposes the change
Definition
2 marksWhat is meant by Lenz's law of electromagnetic induction?
Model answer: The direction of an induced e.m.f. is such that it produces effects that oppose the change that causes it.
The first mark is for the direction of the induced e.m.f. The second is for opposing the change that caused it (9702/42/O/N/24 Q1(b)(i)).
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- Push a north pole towards the coil. Its near face becomes north and repels the approaching magnet.
- Pull the north pole away. The near face becomes south and attracts the departing magnet.
- In both cases, an external force must do work. That energy becomes electrical energy and, if current flows, eventually thermal energy.
“Oppose the change” does not mean “always make an opposite magnetic field”. First decide whether the external flux is increasing or decreasing.
Choose the direction in three decisions
Stationary coil in a changing field:
- State how the external magnetic flux is changing.
- Choose the induced field that opposes that change.
- Use the right-hand grip rule to find current around the coil.
Moving conductor:
- Choose a magnetic force that opposes the conductor's motion.
- Use Fleming's left-hand rule with that force and the external field to find conventional current.
- If a circuit were completed, conventional current inside the e.m.f. source would move from negative to positive. This identifies the higher-potential end even when the conductor is open.
Your turndirection check
A north pole is pulled away from a coil. State the pole formed on the near face of the coil and explain its effect.
Show worked answer
The near face becomes a south pole. It attracts the departing north pole, so the magnetic effect opposes the separation.