2. If we can get magnetism out of electricity, why can’t we get electricity from magnetism? TThe answer……………….. EElectromagnetic induction

3. 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

4. 3 This effect can be quantified by Faraday’s Law Experimental Observation of Induction

5. 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

6. 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

7. 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

8. 7

9. 8

10. 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.

11. 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

12. 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

13. 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

14. 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

15. 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 

16. 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

17. 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!!

18. 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

19. 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

20. 19  SELF INDUCTANCE AND MUTUAL INDUCTANCE

21. 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

22. 21

23. 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)

24. 23 MUTUAL INDUCTANCE  Principle of operation of Transformer

25. 24