Talha's Physics Academy
Electromagnetic Induction, Faraday's Law & Lenz's Law
Electromagnetic Induction
"The phenomenon in which an electromotive force (EMF) is set up in a coil placed in a magnetic field whenever the magnetic flux linked with it changes is called electromagnetic induction."
If the coil forms part of a closed circuit, the induced EMF causes a current to flow through the circuit, which is referred to as an induced current.
Methods of Changing Magnetic Flux
The magnetic flux through a circuit can be changed in several ways:
- By changing the relative position or orientation of the coil with respect to a magnetic field or current-carrying solenoid.
- By changing the current in a neighboring coil (mutual induction) or by changing the current in the coil itself (self-induction).
- By moving a straight conductor through a magnetic field so that it cuts magnetic lines of force.
Faraday’s Laws of Electromagnetic Induction
- First Law: An EMF is induced in a circuit whenever the magnetic flux linked with it changes. The induced EMF lasts only as long as the change in magnetic flux continues and becomes zero as soon as the flux becomes constant.
- 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 ($\Delta \Phi / \Delta t$).
Expressed mathematically:
$$\mathcal{E} = -N \frac{\Delta \Phi}{\Delta t}$$
Note: The negative sign indicates the direction of the induced EMF and is dictated by Lenz's Law, ensuring conservation of energy.
Lenz’s Law
"The induced current always flows in such a direction as to oppose the cause that gives rise to it."
Explanation with Examples
- Approaching Magnet (North Pole): When the North pole of a bar magnet is brought towards the face of a coil, that face acquires North polarity by induction (current flows counter-clockwise) to repel and oppose the forward motion of the magnet.
- Receding Magnet (North Pole): When the North pole of a bar magnet is moved away from the face of a coil, that face acquires South polarity by induction (current flows clockwise) to attract and oppose the backward motion of the magnet.
Fig: Magnet moving towards/away from a coil illustrating induced current direction according to Lenz's Law.
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