Principles Of Electromagnetic Induction & Lenz’s Law



Faraday’s Law Of Electromagnetic Induction

Faraday’s Law of Electromagnetic induction is the process in which an electromotive force (emf) is induced in a closed circuit due to changes in the magnetic field around the circuit.

Lenz’s Law

Lenz’s law states that the direction of the induced e.m.f. and hence the induced current in a closed circuit is always such as to oppose the change in magnetic flux producing it.

The induced e.m.f. is always present even when the conductor is not in a complete circuit. BUT there will only be an induced current flowing only if the set-up is in a complete circuit.

Examples Of Lenz’s Law

A bar magnet with the North pole closer to a coil of wires is moved towards the coil. The current induced will be such that a North pole is created on the side of the coil that the magnet is approaching. This “created” north pole repels the approaching N pole of the magnet.

If such a magnet as above is moved away from the coil, the current induced in the coil will be such that a South pole is created on the side the North pole of the magnet is leaving. This “created” south pole attracts the North pole of the magnet.

(A Level) The negative sign in the equation $\epsilon = \, Р\, \frac{d \Phi}{dt}$ indicates that the induced e.m.f. and the change in flux have opposite signs. The polarity of the induced e.m.f. is such that it tends to produce a current that creates a magnetic flux to oppose the change in magnetic flux through the area enclosed by the loop.

Animation Showing Lenz’s Law In Action & Explanation

copper-pipe-magnet

The GIF above shows a demonstration of Lenz’s Law, which is a concept in electromagnetism. Lenz’s Law states that the direction of an induced electromotive force (EMF) and hence the induced current in a closed circuit is such that it opposes the change in magnetic flux that produced it.

In the animation, someone is dropping a magnet through a non-magnetic, conductive tube, presumably made of copper. As the magnet falls through the tube, it creates a changing magnetic flux through the cross-section of the tube. According to Faraday’s Law of electromagnetic induction, a changing magnetic flux through a circuit induces an EMF in the circuit.

Lenz’s Law then tells us the direction of the induced EMF: it must oppose the change in magnetic flux. Since the magnet is falling due to gravity, the change in magnetic flux is downwards. Thus, the induced EMF generates a current that creates its own magnetic field opposing the magnet’s motion (i.e., it generates a magnetic field that pushes upwards against the falling magnet).

This opposition to the magnet’s motion causes the magnet to fall much more slowly through the tube than it would if it were falling through air or a non-conductive tube. This is because the induced magnetic field in the copper acts like a magnetic “cushion” that resists the magnet’s acceleration due to gravity.

Faraday’s Experiments Showing Lenz’s Law

Faraday experiments
Faraday’s experiments

From the figure above:

  • No. 1: The north pole of a magnet is moved towards the coil. By Lenz’ law, the coil will generate an e.m.f. such that a north pole is induced on the right side of the coil to oppose the change. (Why north pole? To “repel” away the incoming north pole) From the right hand grip rule, the current flow is as shown in the diagram.
  • No. 2: The north pole is moved away from the coil. By Lenz’ law, the coil will generate an e.m.f. such that a south pole is induced on the right side of the coil to oppose the change.
  • No. 3: The south pole is moved away from the coil. (Line of reasoning similar to above. Drop a comment if you have problems.)
  • No. 4: The south pole is moved towards the coil. (Line of reasoning similar to above. Drop a comment if you have problems.)

From this, we can conclude that emf is induced whenever the magnetic field lines are “cut” by the coil. (A more proper terminology will be the change of magnetic flux in the coil induces an emf in the coil)

You might say that there are electrical energy generated from thin air as shown in the experiments. But there are no free lunches in the world (universe in this case). Mechanical energy (from pushing/pulling of the magnet) is converted into electrical energy. This is how cycling a bike with a dynamo converts your mechanical energy into electrical energy.

Fleming’s Right Hand Rule – Direction Of Induced Current

flemings_right_rule

Using Fleming’s right hand rule, you can predict the direction of the induced current with the knowledge of  direction of magnetic field and force.


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