Unit 22aELECTROMAGNETIC INDUCTIONScientists discovered that current could be produced by moving a conductor in a magnetic field.

OBJECTIVES Deduce Faradays experiments on electromagnetic induction or other appropriate experiments: that a changing magnetic field can induce an e.m.f in a circuit, that the direction of the induced e.m.f. opposes the change the factors affecting the magnitude of the induced e.m.f.

OBJECTIVES Describe a simple form of a.c. generator (rotating coil or rotating magnet) and the use of slip rings (where needed). Sketch a graph of voltage output against time for a simple a.c. generator.

OBJECTIVES Describe the structure and principle of operation of a simple iron-cored transformer as used for voltage transformations. Recall the equations Vp/Vs = Np/Ns and VpIp = VsIs (ideal transformers). Apply the relationships between Vp, Vs, Np, Ns, Ip and Is to new situations or to solve related problems.

PRINCIPLES OF ELECTROMAGNETIC INDUCTIONLaws of Electromagnetic Induction

1. What is Electromagnetic Induction? Electromagnetic induction is the production of an induced e.m.f. in a conductor when the magnetic flux/field linked with it changes.

MICHAEL FARADAY (1791-1867) British physicist and chemist Best know for his discoveries of electromagnetic induction and of the laws of electrolysis

principles of electromagnetic induction electromagnetic inductionThe process by which induced current is produced when electricity is obtained from magnetism.centre-zero galvanometer

bar magnet

copper wire coil

Faradays Experiment When a magnet is moved into or out of the solenoid, the galvanometer deflects.

Faradays Experiment An e.m.f. is produced whenever the magnetic field inside the coil changes.Magnetic field

Faradays Experiment This method of producing electricity is called electromagnetic induction.

principles of electromagnetic inductionElectromagnetic inductionAn electromotive force (e.m.f.) and hence a current, is induced whenever the magnetic field inside a coil changes.

a changing magnetic field can induce an e.m.f. in a circuitmagnet moves into coilN S

magnet stationaryS

galvanometer deflects to right magnet moves out of coilN S

no deflection

coil moves toward magnet

N

S

galvanometer deflects to left

galvanometer deflects to right

2. Factors Affecting Magnitude of induced E.M.F A larger current is produced when: The magnet is moved at a faster speed in and out of the coil; A stronger magnet is used; The number of turns in the coil is increased.

Factors Affecting Magnitude of induced E.M.F Magnetic field lines would be concentrated through the coil.

A soft iron core can be inserted into the coil to produced a much larger current. No e.m.f. is produced if

The magnet is stationary outside or inside the coil There is no relative movement between the magnet and the coil

3. Faradays Law of Magnetic Induction The electromotive force (e.m.f.) generated in a conductor (usually copper) is directly proportional to the rate of change of magnetic field.

4. Lenzs Law

Lenzs Law The direction of the induced current is such that its magnetic effect always opposes the change producing it.

Lenzs Law Energy is conserved. (Conservation of energy) Mechanical (kinetic + potential) energy of the work done against opposing force of the moving magnet is converted into electrical energy.

direction of induced e.m.f.

principles of electromagnetic induction

The induced current always flows in a direction that opposes the change producing it. the direction of the induced e.m.f. opposes the change producing itcoil moves toward magnet B A N S A B N S

A

A

when the N-pole of the magnet is moved towards the coil, end A becomes a N-pole

when the N-pole of the magnet is moved away from the coil, end A becomes a S-pole

Quick Check How can an electromotive force be induced in a coil with the help of a magnet?

By moving a magnet towards or away from the coil. OR By moving the coil towards or away from the magnet.

Quick Check State the factors affecting the magnitude of the induced e.m.f. in a coil when a bar magnet is moved in and out of it. Speed of the magnet Strength of the magnet Number of turns of the coil/solenoid

Quick Check Why is it more difficult to move a magnet into a coil which has a larger number of turns?

A coil with a large number of turns can induce a larger electromotive force or larger amount of energy. Thus, by the principle of conservation of energy, more mechanical work needs to be done.

Quick Check Using the principle of conservation of energy, explain why an induced current must flow in such a direction so as to oppose the change producing it.

To induce a current and obtain electrical energy, mechanical work has to be done against the opposing force. Thus the kinetic energy is transformed into electrical energy in the presence of the magnetic energy.

THE ENDThank You!!