EM InductionInduction 1 Basic definitions Electromagnetic induction : generation of electricity from magnetism Michael Faraday Next Slide Michael Faraday’s experiment in 1831 Lenz’s law : Diagram Example An induced current flows in a direction so as to oppose the change producing it. Photo

EM InductionInduction 2 Fleming’s right hand rule Motion of a straight wire in a magnetic field can produce an induced current. Fleming’s right hand rule Next Slide Diagram Moving-coil microphone Magnetic tape recording and playback Diagram Alternator Diagram Dynamo Diagram

EM InductionTransformer 1 Mutual Inductance Mutual inductance in two soft-iron C-cores Application : simple transformer (a.c. supply is used) Next Slide Diagram Example Diagram Practical transformer Photo Diagram Advantage in using transformer and a.c. Example

EM InductionTransformer 2 Electrical Power in HK Next Slide Arrangement : generation, transmission and distribution Discussion Diagram

END of EM Induction

EM Induction Click Back to Induction 1 Michael Faraday Back to

EM Induction Next Slide Induction 1 A coil is connected to a centre-zero galvanometer as shown in the following diagram. A bar magnet is pushed into the coil and left for a few seconds. Then it is removed from the coil. Flow of current in the coil would be indicated by the galvanometer. N S magnet coil galvanometer

EM Induction Next Slide Induction 1 When the magnet is pushed into a coil, current flows in the coil. This current is called induced current. N S direction of moving I : induced current NS Coil becomes a magnet

EM Induction Next Slide Induction 1 When the magnet is held motionless inside a coil, no current flows in the coil. N S The magnet remains at rest.

EM Induction Next Slide Induction 1 When the magnet is pulled out of the coil, current flows in the opposite direction. N S direction of moving I : induced current N S Coil becomes a magnet

EM Induction Click Back to Induction 1 Back to The experimental result is exactly the same except that the direction of the induced current is reversed when the polarity of the magnet is changed. Induced current flows if the coil moves instead of the magnet. Conclusion : an electromotive force (e.m.f.) is produced when there is relative motion between the coil and the magnet. Induced e.m.f. can be increased by : (a) increasing the speed of the relative motion, (b) increasing the number of turns in the coil, and (c) using a stronger magnet

EM Induction Next Slide Induction 1 Induced current changes the coil into a magnet such that a repulsive force is produced between the magnet and coil to oppose the motion of the magnet. N S direction of moving I : induced current NS Coil becomes a magnet

EM Induction Click Back to Induction 1 N S direction of moving I : induced current N S Coil becomes a magnet Induced current changes the coil into a magnet such that an attractive force is produced between the magnet and coil to oppose the motion of the magnet. Back to

EM Induction Next Slide Induction 2 Fleming’s right hand rule : The directions of current, magnetic and the motion of the conductor are represented by thumb, the first finger and the second finger respectively, if they are held perpendicular to each other. (IBM in short) Motion (M) Induced current (I) Magnetic field (B)

EM Induction Click Back to Induction 2 Application of Fleming’s right hand rule Back to SN motion induced current magnetic field

EM Induction Click Back to Induction 2 Moving-coil microphone Back to

EM Induction Click Back to Induction 2 Magnetic tape recording and playback Back to

EM Induction Next Slide Induction 2 In an alternator, a coil is made to rotate in a uniform magnetic field. As it rotates, induced current would be produced in the arms of the coil. N S A B m n p q

EM Induction Next Slide Induction 2 Direction of induced current is shown. NS A B

EM Induction Next Slide Induction 2 Current (mA) 01/4 1/2 3/4 1 Time (no. of revolutions) A BBA BA A BA B

EM Induction Next Slide Induction 2 At t = 0, side A and B of the coil are parallel to the magnetic field, no induced current is produced. From t = 0 to 1/4 no. of revolution, the induced e.m.f. increases from zero, reaching a maximum value at t = 1/4 no. of revolution, when the coil is horizontal. According to right-hand rule, the current flows from m  n  p  q N S A B m n p q

EM Induction Click Back to Induction 2 Back to From t = 1/2 to 3/4 no. of revolution, the current becomes reversed and flows in the direction q  p  n  m. The induced e.m.f. increases from zero to maximum again when t = 3/4 no. of revolution. From t = 1/4 to 1/2 no. of revolution, the induced e.m.f. decreases and the current still flows from m  n  p  q. When t = 1/2 no. of revolution, the induced e.m.f. becomes 0. N S A B m n p q

EM Induction Next Slide Induction 2 In a dynamo (d.c. generator), the half-rings of the commutator reverses the connection of the coil with the circuit. N S A B m n p q

EM Induction Next Slide Induction 2 Current (mA) 0 1/41/23/41 Time (no. of revolutions) A BBA BA A BA B

EM Induction Click Back to Induction 2 The induced e.m.f. and hence current, can be increased in alternator and dynamo by : (a) using a stronger magnet, (b) increasing the number of turns in the coil, (c) winding the coil on a soft-iron armature, (d) rotating the coil at a higher speed. Back to

EM Induction Next Slide Transformer 1 A ring -shape soft iron core is used for the wiring of the primary coil and the secondary coil are The primary coil is connected to a battery and a switch, the secondary coil is connected to a galvanometer. primary coil secondary coil soft-iron core

EM Induction Next Slide Transformer 1 When the switch is closed, the galvanometer gives momentary deflection.The rate of change in B-field is very great and hence current is induced in the secondary coil. primary coil secondary coil soft-iron core B-field due to primary coil B-field due to secondary coil

EM Induction Next Slide Transformer 1 When the switch is kept closed, the galvanometer gives no deflection. Although B-field exists, there is no change in B-field and hence no current is induced in the secondary coil. soft-iron core primary coil secondary coil B-field due to primary coil

EM Induction Next Slide Transformer 1 When the switch is opened, the galvanometer gives momentary deflection in the opposite direction as before.The rate of change in B-field is still great and hence current is induced in the secondary coil in opposite direction. primary coil secondary coil soft-iron core B-field due to primary coil B-field due to secondary coil

EM Induction Click Back to Transformer 1 This effect is called mutual inductance. We can transmit current even there is no direction connection between two circuits, provided that the current is not stable. Back to

EM Induction Next Slide Transformer 1 Now we connect an a.c. power supply to the primary coil so that an unstable current is maintained. A lamp is connected to the secondary coil. The lamp emits light since current is always induced in the secondary coil. This device is called transformer. primary coil secondary coil a.c. supply

EM Induction Next Slide Transformer 1 The relationship between the primary voltage and secondary voltage is given as : The ratio of the secondary voltage to the primary voltage is equal to the ratio of the no. of turns in the secondary coil to the primary coil. It means that the a.c. voltage can be easily changed to any desired value by using this kind of device. It is the advantage that using a.c. instead of d.c. in domestic circuit.

EM Induction Next Slide Transformer 1 If, it is a step-up transformer that increases the p.d. of the a.c. If, it is a step-down transformer that decreases the p.d. of the a.c. Step-down transformer’s symbol Step-up transformer’s symbol

EM Induction Click Back to Transformer 1 Transformer is also an energy transmission device. However, the energy transmission is not 100% efficient. Back to

EM Induction Next Slide Transformer 1 A transformer has 3000 turns in its primary coil and is used to operate a 12 V 24 W lamp from the 200 V a.c. mains as shown in the following diagram. Assume the lamp is operated at the correct rating. N P = V 24 W 200 V a.c. (a) Find the number of turns in the secondary coil. (b) Find the current flow in the secondary coil. (c) Find the current flow in the primary coil if the effeciency is equal to (i) 100% (ii) 50%

EM Induction Next Slide Transformer 1 (a) Applying the transformer equation, The secondary coil has 180 turns. (b) The correct rating of the lamp is 12 V and 24 W, The current rating of the lamp is 2 A and so the secondary current is 2 A.

EM Induction Click Back to Transformer 1 (c) (i) Power input = Power output The primary current is 0.12 A (ii) Power input = 2  Power output, The primary current is 0.24 A Back to

EM Induction Click Back to Transformer 1 A practical transformer is shown in the following photo. Soft iron core is used to trap the magnetic field. Resistance of coils Eddy currents in the core Magnetization and demagnetization of core Leakage of field lines Back to

EM Induction Next Slide Transformer 1 4 kW of power is supplied at the end of power cables of total resistance 5 . Calculate the power loss in the cables if power is transmitted (a) at 200 V, (b) at 4000 V. Solution : The power loss is greatly reduced by transmitting the power at high voltage.

EM Induction Click Back to Transformer 2 Back to power station step-down transformer network of power cables step-up transformer factoryhome 25 kV 400 kV overhead cables 11 kV220 V Power generation, transmission and distribution