Things to know • deduce from Faraday’s experiments on • electromagnetic induction or other appropriate experiments: • (i) that a changing magnetic field can induce an • e.m.f. in a circuit • (ii) that the direction of the induced e.m.f. • opposes the change producing it • (iii) the factors affecting the magnitude of the • induced e.m.f. • (b) describe a simple form of a.c. generator (rotating • coil or rotating magnet) and the use of slip rings (where needed)
(c) sketch a graph of voltage output against time for a simple a.c. generator • (d) describe the structure and principle of operation of a simple iron-cored transformer as used for voltage transformations • (e) recall and apply the equations VP / Vs = NP / Ns and VPIP = VsIs to new situations or to solve related problems (for an ideal transformer) • (f) describe the energy loss in cables and deduce the advantages of high voltage transmission
Electromagnetic Induction Definition: Electromagnetic induction is the production of electricity using magnetism. • Need to know: Describe an experiment which shows that a changing magnetic field can induce an e.m.f. in a circuit
Hollow paper or plastic tube 0 -1 1 -2 2 A stationary magnet is near the coil N S SensitiveGalvanometer In this experiment, no battery is connected to the coil. Hence no e.m.f. is found in the coil.
Hollow paper or plastic tube 0 -1 1 -2 2 motion N S Induced I G When the magnet is moving towards the coil, an electric current is induced simultaneously.
Hollow paper or plastic tube 0 -1 1 Induced I more -2 2 G Faster motion N S When the magnet is moving faster, the induced current is more.
Hollow paper or plastic tube No current 0 -1 1 -2 2 G Not moving N S When the magnet is not moving, no current is induced even though the magnetic flux is linked with the coil closely.
Faraday’s Law of Electromagnetic Induction: The magnitude (how strong) of the induced emf (or induced current) is directly proportional to the rate of change of the magnetic flux linked with the coil or the rate at which the magnetic flux and wire are cutting each other. This means that when the magnetic field is not moving in relation to the coil, there will be NO induced emf at all.
N S 0 -1 1 -2 2 Self Test Question Not moving G There is plenty of magnetic flux linkage with the coil, but there is no motion. Is there any induced current in the coil now? Answer: _________ Please draw the needle of the galvanometer.
What law did you apply when you answer the question in the previous slide?
Moving constantly D D C C B B E E A A N S G 0 -1 1 -2 2 Self Test Question Deflection of G or emf induced
Constant speed moving D D C C B B E E A A N S G 0 -1 1 -2 2 Self Test Question: Sketch the graph as the magnet moves from A to E Deflection of G or emf induced Playing back of the graph `
By now, you have learned that the size or strength of the induced current (or induced e.m.f.) is determined by the speed of change of the magnetic flux linkage with the coil. There is still one more thing about electromagnetic induction you need to investigate. Look at the next slide.
When a current is induced in a coil, it has to flow in the certain direction. What factor determines the direction of the induced current?
Lenz’s Law of electromagnetic induction: The direction of the induced current is such that its own magnetic effect always opposes the change producing it. This law is actually related to the Law of Conservation of Energy. The coil needs to oppose something in order to obtain energy from it. The coil itself cannot CREATE energy!
Beware of a different way the coil can be wound: The paper tube can be taken away to test you
Can you spot the difference of winding? Note: The dotted parts are at the back. The solid lines are at the front.
Please mark one arrow on the left end of the coil and one arrow through the bulb to show how the induced current should flow: Motion N S Induced CURRENT
Please mark + or – signs at the points X and Y to show the induced e.m.f. : Motion N S X Y +
Please mark + or – signs at the points X and Y to show the presence of induced e.m.f. : Motion N S X Y + Induced emf This induced emf is still there as long as the magnet is moving, even though the circuit is broken and the induced current cannot flow.
N S G Coil is stationary
N S G Coil is in motion, approaching the magnet Induced current coil motion N Induced current
N S G Coil is in motion, approaching the magnet coil motion N Induced current
N S G Coil is approaching the magnet coil motion N Existing flux Induced current Motion of wire Can you see the Right Hand Rule ? This is also called the Dynamo Rule
N S G Coil is stationary again Existing flux No more induced current here
Fleming’s Right Hand Rule is also called the Dynamo Rule. thuMb -- the Motion of the wire First finger -- magnetic Flux (Field) seCond finger -- induced Current This is actually the result of Lenz’s Law So, sometimes you use the right hand rule instead of Lenz’s Law.
Straight wire moving vertically to magnetic field.Current is induced away from you The straight wire stops moving.No current is induced at all. This straight wire is not moving No current is induced This straight wire is moving along the magnetic fieldNo current is induced Wire stops moving No current is induced at all Wire moving vertically to the fluxCurrent is induced towards you
Weak induced current Strong induced current wire cutting flux obliquely wire cutting flux vertically No current is induced in the wire No current is induced in wire wire moving alongside flux wire moving alongside flux
N Magnetic flux S
B C N Motion S Motion A D
B Induced current Motion N Magnetic Flux S C A Motion D
B motion No induced current N S No pole is needed A C motion No induced current D
B N motion S A motion C D
C B Motion N S D A
C Induced current Motion N Magnetic Flux S B D A
C No induced current motion N Magnetic flux S D B No induced current A
C N motion S D motion B A
B C N Motion S Motion A D
B C N Red arrows represents magnetic flux S A D A D Induced current Time
B N S C A Red arrows represents magnetic flux D A D Induced current Time
B motion N S A C motion D Induced current Time A D
C B N S D A motion Flux Induced current Time A D
C motion N S D B A Induced current Time D A
B C N S A D A D Induced current Time
Induced emf 2.5V Time -2.5V
How would you demonstrate electromagnetic induction here? G The iron core