Electromagnetic Induction

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Electromagnetic Induction

Electromagnetic induction is the process through which an induced e.m.f is produced in a conductor due to a changing magnetic field. Faraday's solenoid experiment: a moving magnet being inserted into a solenoid can induce an e.m.f. in the solenoid. The magnitude of the induced e.m.f. can be increased by increasing the number of turns in the solenoid, strength of the magnet, or speed at which the magnet moves with respect to the solenoid. Faraday's Law of electromagnetic induction states that the magnitude of the induced e.m.f. in a circuit is directly proportional to the rate of change of magnetic flux in the circuit. When a permanent magnet is inserted into a solenoid, the galvanometer needle deflects in one direction whereas when it is withdrawn, the needle deflects in the opposite direction. However, the needle is not deflected when the magnet is stationary in the solenoid.

Electromagnetic Inductionby: Devina

The Simple A.C. GeneratorA rectangular coil of wire is mounted on an axle. By turning the handle, the coil rotates between the poles of two permanent magnets. As the coil rotates in the magnetic field, the change in magnetic flux creates and induced e.m.f. and hence an induced current in the coil. The slip rings are always in contact with the carbon crushes as the axle rotates. The slip rings ensure that the induced current in the coil is transferred tot he external circuit. The induced current powers the electrical load connected to the external circuit, so the lamp lights up.

Simple a.c. generator transforms mechanical energy into electrical energy, generating electricity. As the coil rotates, the rate at which it cuts the magnetic field changes with time. This induces an alternating e.m.f. which drives an alternating induced current in the external circuit. The slip rings are in continuous contact with the carbon brushes, ensuring that alternating induced current in the coil is transferred to the external circuit.Fleming's Right Hand Rule to predict the direction of hte induced current.

Cathode-Ray OscilloscopeIt works by deflecting a beam of electrons, which is emitted by the elctro gun, in an electric field. The voltage to be studied is applied across the Y-plates to vary the vertical position of the elctron beam. The trace on the fluorescent screen is similar to a voltage-time graph where the y-axis gives the voltage and the x-axis gives the time.Uses of C.R.O. :- Measuring voltage. The voltage to be measured is applied to the Y-plates via the Y-input terminals with the time-base switched off. When the voltage is applied, an electric field is set up between the plates. The deflection of the electron beam by the elctric fiels is proprotional to the voltage applied. The gain of the Y-input determines the sensitivity of the oscilloscope.- Displaying voltage waveforms. It is done by using the C.R.O. with the time-base switched on and selecting a suitable frequency for the time-base. The number of complete cycles with the time-base on is given by the ratio: fy/fx where fy is the frequency of the a.c. input and fx is the frequency of the time-base.- Measuring short intervals of time. E.g. the time taken for sound to travel a short distance. A pulse of sound is produced by a microphone and it is shownas the first pulse on the C.R.O. The pulse bounces off the wall and is received by the microphone which is the second pulse on the C.R.O. The distance between the two signals is a measure of time taken for the sound to travel from the microphone to the wall and back again.

Power in primary coil = power in secondary coilVp Ip = Vs IsIp: current in primary coil, in A. Is: current in secondary coil, in A.Efficiency = output power / input power x 100 %I = P out / VP loss = I^2R = (P out / V)^2 R

Induced e.m.f. is maximum when the coil is parallel to the magnetic fields as sides AB and CD are cutting through the magnetic field lines at the greatest rate. Induced e.m.f. is zero when the coil is perpendicular to the magnetic fields as sides AB and CD are moving parallel to the magnetic field and not cutting through the magnetic field lines. The induced e.m.f. of an a.c generator can be increased by increasing the number of turns on the coil, the frequency of rotation of the coil, using stronger permanent magnets and winding the coil on a soft iron core to strengthen the mangetic field lines of force through the coil.

As the N pole of the magnet bar enters the solenoid, there is a change in the numbers of magnetic field lines linking the solenoid. By Faraday's Law, the change of the magnetic flux in the solenoid results in an induced e.m.f. in the circuit. This e.m.f. drives an induced current through the closed circuit. The induced current produces a galvanometer needle deflection. By Len'z Law, the induced current creates an N pole at the right end of the solenoid. to oppose the incoming N pole. Thus, the galvanometer needle is deflected momentarily to one side. At the instant when the bar magnet travels past the mid-length point of the solenoid, there is no change in the magnetic flux in the solenoid. There is no induced e.m.f. and hence no induced current to cause the galvanometer needle to be deflected. As the S pole of the bar magnet exits the solenoid, there is again a change in the magnetic flux in the solenoid. By Faraday's Law, this produces an induced e.m.f. and hence an induced current. By Len'z Law, the induced current creates an Npole at the left end of the solenoid to oppose the outgoing S pole. Thus, the galvanometer needle is deflected momentarily to the other side.

Faraday's Iron RIng Experiment: When the switch is closed or open, the compass needle deflected momentarily but it stops when the switch was left closed or open. The magnetic field increases form zero to maximum when the current is first switched on and drops to zero when the current is switched off. The sudden increase or decrease of the magnetic field in the soft iron ring induces an e.m.f. in coil B which drives an induced current to flow in coil B.

Lenz's Law states that the direction of the induced e.m.f. and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it.

Vs/Vp = Ns/NpVs= secondary (output) voltage, in V.Vp= primary (input) voltage, in V.Ns= number of turns in secondary coil.Np= number of turns in primary coil.

Step-up transformer: the number of turns in the secondary coil is greater than that in the primary coil. Thus, its output voltage is higher than input voltage.For step-down transformer, it is vice versa. The ouput voltage is lower than the input voltage.

TransformerIt is a device that changes a high alternating voltage at low current to a low alternating voltage at high current and vice versa. It is used for electrical power transmission from power stations to households and factories and regulating voltages for proper operation of electrical appliances e.g. the television and CD player. A closed-core transformer consists of two coils of wire, the primary and secondary coils. These coils are wound around a laminated soft iron core which consists of thin sheets of laminated soft iron. Soft iron is used as it is easily magnetised and demagnetised. This ensures better magnetic flux linkage between the two coils.How it works: The primary coil is connected to an alternating voltage. A varying magnetic field is set up in the laminated soft iron core. An e.m.f. is induced in the secondary coil. This voltage is called the output voltage. Since the circuit is closed, a current is also induced in the coil.Electrical energy is transeferred from the primary coil to the secondary coil in a transformer.

Causes of power loss include heat loss due to the resistance of the coils, leakage of magnetic field lines between the primary and secondary coil, heat loss due to eddy current induced in the iron core and hysteresis loss caused by the flipping of magnetic dipoles in the iron core due to the a.c. The solution is to have very thick cables to keep resistance low or to reduce the current in the cables by using a step-up transformer. This will cause greater voltage to flow and smaller power loss. Higher voltage and thick cables, however, would raise the cost.


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