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If we can get magnetism out of electricity, why can

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... Generators Faraday s Law Faraday s Law of Induction Comment on Lenz s Law Direction of Induced Current ConcepTest: Lenz s Law Slide 19 Self ... – PowerPoint PPT presentation

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Title: If we can get magnetism out of electricity, why can


1
If we can get magnetism out of electricity, why
cant we get electricity from magnetism?
  • The answer..
  • Electromagnetic induction

2
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.

3
Experimental Observation of Induction
This effect can be quantified by Faradays Law
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 electricity magnetism
  • electric current magnetic field
  • magnetic field electric current

5
What does Faradays law say?
  • Faradays 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
Faradays Law of Induction
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

7
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8
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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 Earths 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

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
Faradays Law
  • How to change the flux?
  • Recall that flux is
  • Changing B or A or q will change the flux.

FB ? B A cosq
15
Faradays Law of Induction
rate of change of flux with time
induced emf
number of loops
  • Minus sign from Lenzs Law
  • Induced current produces a magnetic field which
    opposes the original change in flux

16
Comment on Lenzs 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
Direction of Induced Current
  • Bar magnet moves through coil
  • Current induced in coil
  • Reverse pole
  • Induced current changes sign
  • Coil moves past fixed bar magnet
  • Current induced in coil
  • Bar magnet stationary inside coil
  • No current induced in coil

18
ConcepTest Lenzs 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

This is a self-induced emf (also called back
emf)
L is the self-inductance units Henry (H)N2/R
PROPERTY OF A COIL DUE TO WHICH IT OPPOSES THE
CHANGE OF CURRENT OR FLUX THROUGH IT SELF
INDUCTANCE
21
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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 ? I1
  • rewrite as

M is the mutual inductance units Henry (H)
23
MUTUAL INDUCTANCE
  • Principle of operation of Transformer

24
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