# Electromagnetic induction

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• Created by: emma
• Created on: 16-05-13 13:48

## Faraday's law and Lenz's law

• The induced EMF in a circuit is equal in size to the rate of change of magnetic flux linkage through the circuit
• an induced EMF acts in such a direction as to oppose the change that produces it.
• faraday's law can be investigated by dropping a magnet through a coil of wire connected to a voltage data logger.
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## dropping a magnet through a coil

Graph produced:

• positive region - flux through coil increases as magnet approaches
• negative region - flux decreases as magnet exits the coil
• maximum emf when approaching is lower than maximum emf when leaving as it accelerates as it falls ( induced emf is rate of change of flux linkage and rate of change of flux through the coil is greater as magnet leaves)
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## Lenz's law

• drop magnet down copper tube
• it falls more slowly than a non-magnetic piece of metal of similar dimensions
• friction force should be indetical for the two falling objects
• falling magnet induces emf in each copper circlet
• small current flows around the tube
• current causes magnetic field which interact with the falling magnet
• depending on direction of field, it will repel or attract magnet
• if it attracted it it would end up with more energy than the GPE it originaly had.
• this is not possible (law of conservation of energy)
• it must act in such a direction as to repel the magnet
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## Transformers

• pair of coils linked by soft iron core
• ac in primary coil produces changing magnetic field
• this is carried to secondary coil by iron core
• changing magnetic field through secondary coil induces emf in coil

Ratio of the voltages between primary and secondary coil is the same as the ration of the number of turns.

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## how can the amount of EMF induced in a coil be cha

• changing the angle between the coil and the field: The more aligned the coil is to the field, the fewer lines it cuts through and the smaller the induced e.m.f
• changing the number if turns:  the grater the number of turns, the more points in each coil cut each flux line so the e.m.f induced is greater
• changing the area of the coil: the larger the area, the more flux lines are cut through and the greater the induced e.m.f
• changing the magnetic field strength: the greater the flux density, the greater the number of flux lines per unit area , so the coil  cuts more flux and more emf is induced
• changing the angular speed of the coil: increasing the rate of rotation of a coil increases the number of flux lines cut by the coil in a given time, increasing the induced e.m.f
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## Dynamos

• coil rotates
• flux linkage = 0 when coil is parallel with field
• flux linkage = maximum when coil is perpendicular to the field
• alternating emf generated
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