Experiment: Lenz’s law


Lenz’s Law states that the current induced in a metal object will oppose the change when it experiences a change in flux, such as a moving magnetic field. As seen the formula below contains a negative value, since the induced emf is negative.

The emf that is induced is due to eddy currents which are created in the magnetic material experiencing the change in flux.

We will go into the theory of this in greater depth in a following lesson.

Lenz’s law states that the direction of an induced current in a conductor will be such that it will oppose the change in the magnetic field that caused it. In other words, the induced current will generate its own magnetic field that is opposite to the change in the original magnetic field. This is known as Faraday’s law of electromagnetic induction. This law is also one of the principle behind the operation of generators and motors.

An analogy to explain Lenz’s law would be a person pushing a shopping cart. If the person is pushing the cart in one direction and someone else suddenly comes and starts pushing the cart in the opposite direction, the person will naturally push back against the second person with the same force in order to keep the cart moving in the original direction. Similarly, an induced current in a conductor will generate its own magnetic field in the opposite direction of the changing magnetic field that caused it, in order to oppose and counteract the change.

Lenz’s law is simply a result of the conservation of energy. Energy goes into pushing the electrons around in circles to create eddy currents. These Eddy currents have an associated magnetic field. This magnetic field acts to oppose the falling magnet, taking kinetic energy away from the magnet, hence it falls slower.


Question 1. [Not an exam style question]

Knowing an emf must be induced in order for the demonstration above to work, predict whether a plastic tube will make the object fall faster, slower or no effect on a falling magnet compared to the magnet falling through a copper tube.

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