P.1 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Reverse a motor Generators Check-point 3 Search coils Eddy currents Applications Check-point 4 5.2Generators and other applications of electromagnetic induction

P.2 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Reverse a motor Dismount a battery-powered handy fan. Reconnect the motor to the circuit shown: Turn the axis of motor rapidly. What happens? The bulb lights up, the buzzer sounds and the fan turns on. 5.5 Motor dynamo unit Video

P.3 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 5.2Generators and other applications of electromagnetic induction Generators / dynamos Examples:supplies mains electricity to homes, offices and factories from power stations provides current to bicycle lights 5.6 Shake-shake torch Video

P.4 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 5.2Generators and other applications of electromagnetic induction Generators / dynamos  Use electromagnetic induction  Based on the principle that current can be induced in a coil made to rotate in B-field  convert other forms of energy into electrical energy

P.5 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 1 Generators a Induced e.m.f and current in a rotating coil When the coil is rotated, AB moves upwards and CD downwards through the B-field.  E.m.f. is induced across AB and CD as if there were two cells in series. Combined induced e.m.f. drives a current to flow through circuit.

P.6 Book 4 Section 5.2 Generators and other applications of electromagnetic induction a Induced e.m.f and current in a rotating coil The coil is a quarter of a turn from horizontal position, passing through the vertical position: AB and CD are now moving parallel to the B-field momentarily.  no field lines are cut ∴ induced e.m.f. and current drop to zero.

P.7 Book 4 Section 5.2 Generators and other applications of electromagnetic induction a Induced e.m.f and current in a rotating coil An induced current flows but in an opposite direction. The coil has completed half a turn: ∵ AB and CD are now moving downwards and upwards respectively.

P.8 Book 4 Section 5.2 Generators and other applications of electromagnetic induction a Induced e.m.f and current in a rotating coil The coil has completed three quarters of turn:  Vertical again.  Induced current drops to zero. The coil completes one turn:  The situation repeats.  The current continue to flow back and forth.  Continuous supply of alternating current. 5.2 Induced current in a rotating coil Simulation

P.9 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 1 Generators b Simple a.c. generator ends of the coil fixed to two copper slip rings  rotate with the coil A simple a.c. generator / alternator: consists of a coil mounted on an axle between the poles of a magnet.

P.10 Book 4 Section 5.2 Generators and other applications of electromagnetic induction two carbon brushes press against the slip rings connecting them to the outside circuit. When the coil is rotated  alternating e.m.f. is induced  a.c. flows through the circuit b Simple a.c. generator

P.11 Book 4 Section 5.2 Generators and other applications of electromagnetic induction b Simple a.c. generator A graph showing how the current varies during one complete rotation of the coil: The current is the greatest when the coil is horizontal ( ∵ cutting through the field lines most rapidly) zero when the coil is vertical ( ∵ the rate of cutting field lines = 0) of the same frequency as the rotation of coil 5.3 Simple a.c. generator Simulation

P.12 Book 4 Section 5.2 Generators and other applications of electromagnetic induction b Simple a.c. generator Induced e.m.f. and current  when using a stronger magnet  the number of turns in the coil  the area of the coil (within B-field) winding the coil on a soft-iron core rotating the coil at a higher speed (i.e. higher frequency) Current of an a.c. generator Example 9

P.13 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Example 9 Current of an a.c. generator A graph showing how the alternating current of a simple a.c. generator varies with time: (a) How many revolutions are made by the coil within the first 0.04 s? 1 revolution

P.14 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Example 9 Current of an a.c. generator (b) Frequency of the a.c. produced = ? From (a), period T = 0.04 s. Frequency f = 1T1T = = 25 Hz

P.15 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Example 9 Current of an a.c. generator (c)When does the coil become (i) parallel (ii) perpendicular to the B-field? (i) At 0.01 s, 0.03 s, 0.05 s… (every 0.02 s later) ( ∵ The size of induced a.c. is max.) (ii) At 0, 0.02 s, 0.04 s, 0.06 s… (every 0.02 s later) ( ∵ The induced a.c. = 0.)

P.16 Book 4 Section 5.2 Generators and other applications of electromagnetic induction (d)If it is rotated twice as fast, what happens to (i) frequency and (ii) max. current? Example 9 Current of an a.c. generator Sketch the I –t graph. (i) Frequency is doubled, i.e. 50 Hz. (ii) Max current produced  ( ∵ the coil cuts the magnetic field lines at a higher rate.)

P.17 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 1 Generators c Simple d.c. generator Replace slip rings with a commutator then a.c. generator  d.c. generator When the coil passes through the vertical, the commutator reverses the connections of coil.  current in the outside circuit always flows in the same direction.

P.18 Book 4 Section 5.2 Generators and other applications of electromagnetic induction c Simple d.c. generator 5.4 Simple d.c. generator How the current varies during one complete rotation of the coil: Simulation The current varies in size but its direction does not change.  direct current

P.19 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 1 Generators d Practical generators i Bicycle dynamos Bicycle dynamo has a fixed coil a rotating permanent magnet  avoids the circuit from making connection via a moving contact Rotation of magnet produces an induced a.c. to light up the lamp.

P.20 Book 4 Section 5.2 Generators and other applications of electromagnetic induction d Practical generators ii Alternators The practical a.c. generator generally has a rotating electromagnet and fixed coils. Huge alternators in power stations:  generates electricity  usually driven by steam turbine  powered by fossil fuel or a nuclear reactor It is also called an alternator.

P.21 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Check-point 3 – Q1 What are the turning frequencies of the coils of the a.c. generator and the d.c. generator?

P.22 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Check-point 3 – Q1 The current from an a.c. generator changes sign for every _______ revolution(s). Therefore, the coil of the a.c. generator takes ____ s to complete one revolution. half 0.1 By, the rotation frequency of the coil of the a.c. generator is ______ Hz. f = 1T1T 10

P.23 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Check-point 3 – Q1 The current from a d.c. generator has two peaks in _______ revolution(s). Therefore, the coil of the d.c. generator takes _____ s to complete one revolution. one By, the turning frequency of the coil of the d.c. generator is ______ Hz. f = 1T1T

P.24 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 2 Search coils Search coil: makes use of electromagnetic induction to measure varying B-fields consists of a circular coil with about 5000 turns and an area of about 10 –4 m 2 (1 cm 2 ) When the search coil is inside a changing magnetic field B  the coil, an e.m.f. will be induced across the ends.

P.25 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Area of the coil is constant  magnitude of the induced e.m.f.  rate of change of the magnetic field B 2 Search coils Measuring magnetic field using a search coil  = –N   t = –NA BtBt If a B-field (varies with time) is measured by a search coil, the induced e.m.f. will also vary accordingly. Expt 5d

P.26 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Experiment 5d Measuring magnetic field using a search coil 1.Set up a search coil probe and connect it to a CRO. 2.Connect a long straight wire to a signal generator. Measure the B-field at different distances from the wire and with different currents through the wire.

P.27 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Experiment 5d Measuring magnetic field using a search coil 3.Repeat step 2 by replacing the long straight wire with a slinky solenoid. Compress it and observe the change in B-field.

P.28 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Experiment 5d Measuring magnetic field using a search coil 5.7 Expt 5d - Measuring magnetic field using a search coil 4.Repeat step 2 by replacing the long straight wire with rectangular solenoids of different areas and no. of coils per unit length. Observe the change in B-field. Video

P.29 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 3 Eddy currents Eddy current Expt 5e

P.30 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 1.Put a coil of 1100 turns round the bottom of a retort stand. Insert an aluminium ring on top of the coil. Connect the coil to d.c. power supply. 2.Switch on the power supply and observe what happens to the aluminium ring. Experiment 5e Eddy current

P.31 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 3.Replace aluminium ring with another one with a slit. Repeat step 2. 4.Replace the d.c. power supply with an a.c. power supply. Repeat steps 1–3. Experiment 5e Eddy current

P.32 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Experiment 5e Eddy current 5.8 Expt 5e - Eddy current 5.Suspend an aluminium plate. Allow it to swing freely. Place it between a pair of magnets. 6.Pull the plate to one side and release it. Observe what happens. Repeat with aluminium plate with slots. Video

P.33 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 3 Eddy currents When a piece of metal moves in a B-field, or when it is placed in a changing B-field, induced e.m.f. will be produced.  Induced currents (eddy currents) circulates within the body of metal. By Lenz’s law, induced current flows such that the B-field it produces opposes the change which started it. ∴ The ring jumps and the plate brakes in Expt 5e.

P.34 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 3 Eddy currents Eddy current follows low resistance path  may be large even if induced e.m.f. is small. Eddy currents can produce heating and magnetic effects  can be useful but sometimes undesirable Jumping ring and metal plate pendulum Example 10

P.35 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Example 10 Jumping ring and metal plate pendulum Larry carries out Expt 5e. (a) He first carries out steps 1–2. (i)What will happen to the aluminium ring when he just switches on the power supply?

P.36 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Example 10 Jumping ring and metal plate pendulum Eddy current opposes the increase of B-field, i.e. opposite the current direction  produces another B-field  the ring is repelled upwards (a) (i) Switch on the power   of B-field in the coil  eddy current inside the ring

P.37 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Example 10 Jumping ring and metal plate pendulum (a) (ii) What will happen to the ring when the power supply has been switched on for a while? The B-field becomes steady.  No eddy current and therefore no opposing B-field  The aluminium ring drops back

P.38 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Example 10 Jumping ring and metal plate pendulum (b) Larry then carries out steps 5–6. The aluminium plate with no slots is pulled to one side and released. Explain what will happen to the plate.

P.39 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Example 10 Jumping ring and metal plate pendulum When the plate is moving inside the constant B-field, eddy current is produced and flows to oppose the motion of the plate. The motion of the plate will cease quickly.  creates a braking force for the plate  motion of the plate ceases quickly

P.40 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 4 Applications a Moving-coil microphones A moving-coil microphone consists of a small coil attached to a diaphragm: The coil can move up or down inside a cylindrical magnet.

P.41 Book 4 Section 5.2 Generators and other applications of electromagnetic induction a Moving-coil microphones The induced a.c. varies with frequency and amplitude of sound waves, and can be amplified and converted back to sound using a loudspeaker. Speak into the microphone  the diaphragm vibrates  the coil moves up and down between the poles of magnet  a.c. induced in the coil

P.42 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 4 Applications b Induction cookers The Induction cooker uses coils of wires with high frequency a.c.  fast changing B-field The pot gets hot due to the heating effect of currents and cooks the food. 5.9 Induction cooker  induces large currents in metal cooking pot Video

P.43 Book 4 Section 5.2 Generators and other applications of electromagnetic induction 4 Applications c Hard disks Each platter is coated with a thin layer of fine grains of magnetic material  like tiny magnets The hard disk consists of several rotating disks called platters. The read-write head (1 head per platter face) is tiny soft-iron core with a coil wound round it.

P.44 Book 4 Section 5.2 Generators and other applications of electromagnetic induction c Hard disks ‘Write’ Information:  convert information into a.c. signals  a magnetic ‘copy’ of the information  magnetizes and rearranges the grains on platter in a certain pattern

P.45 Book 4 Section 5.2 Generators and other applications of electromagnetic induction c Hard disks ‘Read’ Information:  B-field of grains induces a certain a.c. in the head  a.c. signals are converted back to information  hard disks can be found in video recorders, camcorders, etc.

P.46 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Check-point 4 – Q1 A 4 plates (same dimensions) are pivoted freely, pulled sideways to the same height and then released at the same instant. Which of them would come to rest first? BCD

P.47 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Check-point 4 – Q2 An aluminium disc is placed above an induction cooker and the cooker is then switched on. What would happen? (a) The disc would get hot. ( Yes / No ) (b) The disc would be repelled upwards ( Yes / No ) and then drops back.( Yes / No )

P.48 Book 4 Section 5.2 Generators and other applications of electromagnetic induction Check-point 4 – Q3 We usually use a search coil to measure ( constant / varying ) magnetic field and use a hall probe to measure ( constant / varying ) magnetic field.

P.49 Book 4 Section 5.2 Generators and other applications of electromagnetic induction The End