If we can get magnetism out of electricity, why can’t we get electricity from magnetism? TThe answer……………….. EElectromagnetic induction
2 This is how first experiment by Faraday was done He only got a deflection of the galvanometer when the switch is opened or closed Steady current does not make induced emf. Transformers
3 This effect can be quantified by Faraday’s Law Experimental Observation of Induction
4 Electromagnetic Induction Faraday discovered that a changing magnetic flux leads to a voltage in a wire loop Induced voltage (emf) causes a current to flow !! Symmetry: electricitymagnetism electric current magnetic field magnetic fieldelectric current
5 What does Faraday’s law say? Faraday’s law says that a) an emf is induced in a loop when it moves through an electric field b) the induced emf produces a current whose magnetic field opposes the original change c) the induced emf is proportional to the rate of change of magnetic flux
6 Faraday’s Law of Induction induced emf number of loops rate of change of flux with time The faster the change, the larger the induced emf The induced emf is a voltage
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9 TYPES OF INDUCED EMF Statically induced emf Conductor remains stationary and flux linked with it is changed (the current which creates the flux changes i.e increases or decreases) TYPES Self induced Mutually induced Dynamically induced emf Field is stationary and conductors cut across it Either the coil or the magnet moves.
10 Can we get emf induced in a motionless circuit? An induced emf produced in a motionless circuit is due to 1) a static (steady) magnetic field 2) a changing magnetic field 3) a strong magnetic field 4) the Earth’s magnetic field 5) a zero magnetic field
11 Induction in Stationary Circuit Switch closed (or opened) Current induced in coil B Steady state current in coil A No current induced in coil B A B
12 How does a magnetic field change? The field can itself be changing in nature Either the magnet itself should move or the conductor should move with respect to each other Hence there should be a relative motion between magnet and the conductor
13 Electric Generators Rotate a loop of wire in a uniform magnetic field: changing changing flux induced emf B = B A cos = B A cos( t) Rotation: = t
14 Faraday’s Law How to change the flux? Recall that flux is: Changing B or A or will change the flux. B B A cos
15 Faraday’s Law of Induction induced emf number of loops rate of change of flux with time Minus sign from Lenz’s Law: Induced current produces a magnetic field which opposes the original change in flux
16 Comment on Lenz’s Law Why does the induced current oppose the change in flux? Consider the alternative If the induced current reinforced the change, then the change would get bigger, which would then induce a larger current, and then the change would get even bigger, and so on... This leads to a clear violation of conservation of energy!!
17 Bar magnet moves through coil Current induced in coil v S N Reverse pole Induced current changes sign v N S Coil moves past fixed bar magnet Current induced in coil S N Bar magnet stationary inside coil No current induced in coil N S Direction of Induced Current
18 ConcepTest: Lenz’s Law If a N pole moves towards the loop from above the page, in what direction is the induced current? (a) clockwise (b) counter-clockwise (c) no induced current
19 SELF INDUCTANCE AND MUTUAL INDUCTANCE
20 Self - Inductance Consider a single isolated coil: Current (red) starts to flow clockwise due to the battery But the buildup of current leads to changing flux in loop Induced emf (green) opposes the change L is the self-inductance units = “Henry (H)”=N 2 /R induced emf This is a self-induced emf (also called “back” emf) PROPERTY OF A COIL DUE TO WHICH IT OPPOSES THE CHANGE OF CURRENT OR FLUX THROUGH IT SELF INDUCTANCE
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22 Mutual Inductance Consider two neighboring coils: if current changes in coil #1, an emf is induced in coil #2 B B I 1 rewrite as: M is the “mutual inductance” units = Henry (H)
23 MUTUAL INDUCTANCE Principle of operation of Transformer
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