Chapter 28: Electromagnetic Induction
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Changing flux ？ induced emf ？ induced current ？ magnetic field
Electromagnetic Induction occurs when an emf is induced in a coil due to a changing magnetic flux.
We have seen from the last two chapters that Electricity and Magnetism are inter-linked.
The English scientist Michael Faraday investigated this relationship. He found that if you moved a magnet in or out of a coil of wire, a voltage was generated (more properly called an emf
He also realised that the quicker you moved the magnet (or the coil), the greater was the emf generated.
This is now known as Faraday‟s Law of Electromagnetic Induction.
Demonstrating Faraday’s Law
1. Move the magnet in and out of the coil slowly and note a slight deflection. 2. Move the magnet quickly and note a greater deflection.
Later on it was found that the direction of the emf could also be predicted. This is known as Lenz‟s Law.
The two laws together are known as the laws of Electromagnetic Induction
The Laws of Electromagnetic Induction.
1. Faraday’s Law states that the size of the induced emf is proportional to the rate of change of flux. 2. Lenz’s Law states that the direction of the induced emf is always such as to oppose the change producing it.*
To calculate the size of the induced emf we need one more concept; Magnetic Flux. The symbol for magnetic flux is φ (pronounced “sigh”).
The unit of magnetic flux is the Weber (Wb)*
To introduce the idea of magnetic flux consider an area, A in a uniform magnetic field.
When the magnetic force lines are perpendicular to this area (see diagram) the total flux (φ) through the area is
defined as the product of B by A.
φ = BA
The magnetic flux, φ, can be visualised as the number of magnetic field lines passing through a given area. The number of magnetic field lines per unit area, i.e. B, is then referred to as the density of the magnetic flux or, more properly, the magnetic flux density.
Now we are in a position to calculate the induced emf:
Remember Faraday‟s Law: The size of the induced emf is proportional to the rate of change of flux.
In this case the proportional constant turns out to be 1 (remember where we came across this before? Hint: F = ma)
E = Induced emf = - (Final Flux –Initial Flux) / time taken
(the minus sign is a reference to Lenz‟s Law).
Finally, this formula assumes the coil has only one turn. If there are N turns, then the formula becomes
E = -N(Final Flux –Initial Flux) / Time Taken
Lenz’s Law states that the direction of the induced emf is always such as to oppose the change producing it.
We know that when a magnet and coil move relative to each other, an emf is induced. Now if the coil is a conductor the induced emf will drive a current around the coil. This current has a magnetic field associated with it.
The direction of this magnetic field will always be such as to oppose the change which caused it.
Demonstrating Lenz’s Law (i): Magnet, Plastic and Copper Tubes*
Copper pipe, plastic pipe, stopwatch, strong neodymium magnet, piece of neodymium, or MMiron, (same size as magnet). aProcedure agDrop the neodymium magnet down both tubes and compare the time taken for each for gneach. nePlCObservation et astopThe time taken for the magnet to fall down through the copper tube is much longer than t the time taken for the magnet to fall down the plastic tube. ic pe
The moving magnet induces an electric current in the copper. This current creates a pe pimagnetic field that exerts a force to oppose the motion of the magnet and hence slows it down. pe
Demonstrating Lenz’s Law (ii): Magnet and Aluminium Ring
ThrApparatus j ead ll Aluminium ring, magnet, thread, retort stand. sddf Procedure 1. Move one end of the bar magnet towards and into the ring. The ring moves away
from the magnet.
2. Hold the magnet in the ring and quickly pull it away. The ring follows the
When the magnet moves, the ring responds by moving in the same direction.