Talha's Physics Academy
Electromagnetism - Faraday's Laws & Lenz's Law
Faraday's Laws of Electromagnetic Induction
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First Law:
An electromotive force (emf) is induced in a coil whenever the magnetic flux linked with the coil changes. The induced emf lasts only as long as the change in magnetic flux is in progress and becomes zero as soon as the flux through the coil becomes constant.
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Second Law:
The magnitude of the induced emf ($\mathcal{E}$) is directly proportional to the product of the number of turns ($N$) in the coil and the time rate of change of magnetic flux ($\Delta \Phi / \Delta t$).
It is expressed mathematically as:
$\mathcal{E} = -N \frac{\Delta \Phi}{\Delta t}$Note: The negative sign indicates the direction of the induced emf and is explained by Lenz's Law.
Lenz's Law
"The induced current always flows in such a direction as to oppose the cause that gives rise to it."
Figure: Interaction between a bar magnet and a coil showing magnetic opposition according to Lenz's Law.
Explanation via Examples
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Case (a): Magnet Moving Towards the Coil
When the North ($N$) pole of a bar magnet is brought towards the face of a coil, that face of the coil becomes a North pole by the induction of current in an anticlockwise direction. This magnetic repulsion opposes the forward motion of the magnet.
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Case (b): Magnet Moving Away from the Coil
When the North ($N$) pole of a bar magnet is moved away from the face of a coil, that face becomes a South pole by the induction of current in a clockwise direction. This magnetic attraction opposes the backward motion of the magnet.
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