Electromagnetic Induction – Learning Outcomes  Define magnetic flux.  Solve problems about magnetic flux.  State Faraday’s Law.  HL: Solve problems using Faraday’s Law.  State Lenz’s Law.  Demonstrate the principles and laws of electromagnetic induction.  Discuss electromagnetic induction in generators.  Solve problems about converting between mechanical and electrical energy. 1

Electromagnetic Induction  Just as a current generates a magnetic field, a changing magnetic field can induce an e.m.f. in a conductor (usually we consider a coil).  This effect is called electromagnetic induction. 2

Magnetic Flux 3

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Faraday’s Law 5

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To Demonstrate Faraday’s Law 1.Connect a coil of wire to a galvanometer. 2.Move a bar magnet slowly towards the coil. 3.Note a small deflection in the galvanometer. 4.Move the bar magnet quickly towards the coil. 5.Note a larger deflection in the galvanometer. 6.The size of the current (and thus, emf) increases as the speed of the magnet (and thus rate of change of flux) increases. 7

Lenz’s Law  Lenz’s Law states that the direction of an induced current is always such as to oppose the change producing it.  e.g. if a north pole is brought near a solenoid, the solenoid will produce a current such that its field has a north pole repulsing the incoming one.  e.g. if a north pole is moved away from a solenoid, the solenoid will produce a current such that its field has a south pole attracting the north pole. 8

Lenz’s Law  e.g. If the bar magnet is not moving, in which direction will the current in the solenoid flow? 9

Lenz’s Law  e.g. If the bar magnet is brought towards the solenoid, in which direction will the current flow? 10

Lenz’s Law  e.g. If the bar magnet is moved away from the solenoid, in which direction will the current flow? 11

To Demonstrate Lenz’s Law 1.Suspend a ring of aluminium from a length of string. 2.Move one pole of a bar magnet quickly towards the ring. 3.Note that the ring is repelled. 4.Move the magnet quickly away from the ring. 5.Note that the ring is pulled along with the magnet. 6.When the ring is repelled, the induced current creates a pole the same as the approaching one on that side. 7.When the ring is attracted, the induced current creates a pole opposite the leaving one on that side. 12

To Demonstrate Lenz’s Law 13

Generators  An electric generator is a device that converts mechanical energy to electrical energy.  Generators use the principles of electromagnetic induction to perform this conversion.  In most cases, chemical energy (coal, oil, biomass, nuclear) is burned for heat, producing steam which drives a turbine. Wind, hydroelectric, tidal harvest kinetic energy to drive the turbine.  Turbines consist of a coil which can rotate in a magnetic field, inducing an emf. 14

Generators  Generators appear in:  Electricity power stations – as described earlier.  Car alternators – convert rotation in the engine to electricity to supply the car’s electrical system and charge the battery.  Bicycle dynamos harvest the bicycle’s kinetic energy to operate the lights. 15

Generators 16  Hint: Conservation of energy tells us that the work done by the magnet is equal to the heat produced in the coil.

Generators 17