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If we can get magnetism out of electricity, why can’t we get electricity from magnetism? The answer……………….. Electromagnetic induction Transformers 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. 2 Experimental Observation of Induction This effect can be quantified by Faraday’s Law 3 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: electric electricity current magnetic field magnetism magnetic field electric current 4 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 5 Faraday’s Law of Induction B N t rate of change of flux with time induced emf number of loops The faster the change, the larger the induced emf The induced emf is a voltage 6 7 8 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. 9 Can we get emf induced in a motionless circuit? An induced emf produced in a motionless circuit is due to 1) 2) 3) 4) 5) a static (steady) magnetic field a changing magnetic field a strong magnetic field the Earth’s magnetic field a zero magnetic field 10 Induction in Stationary Circuit Switch closed (or opened) Current Steady No induced in coil B state current in coil A current induced in coil B A B 11 How does a magnetic field change? The B N t 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 12 Electric Generators Rotate a loop of wire in a uniform magnetic field: changing flux induced emf = B A cos = B A cos(t) changing B Rotation: = t 13 Faraday’s Law How to change the flux? Recall that flux is: B B A cos B or A or will change the flux. Changing 14 Faraday’s Law of Induction B N t rate of change of flux with time induced emf number of loops Minus sign from Lenz’s Law: Induced current produces a magnetic field which opposes the original change in flux 15 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!! 16 Direction of Induced Current Bar magnet moves through coil Current induced in coil S N N S S N N S v Reverse pole Induced current changes sign v Coil moves past fixed bar magnet Current induced in coil Bar magnet stationary inside coil No current induced in coil 17 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 18 SELF INDUCTANCE AND MUTUAL INDUCTANCE 19 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 This is a self-induced emf (also called “back” emf) N ddt L dIdt L is the self-inductance units = “Henry (H)”=N2/R induced emf PROPERTY OF A COIL DUE TO WHICH IT OPPOSES THE CHANGE OF CURRENT OR FLUX THROUGH IT SELF INDUCTANCE 20 21 Mutual Inductance Consider if two neighboring coils: current changes in coil #1, an emf is induced in coil #2 2 N d 1 dt B B I1 rewrite as: 2 M dI1 dt M is the “mutual inductance” units = Henry (H) 22 MUTUAL INDUCTANCE Principle of operation of Transformer 23 24