EMFT PPt

2. CONTENTS
Faraday’s Life
Magnetic Flux
Experiments of Faraday
Lenz law
Motional EMF
Eddy Current
Self Inductance
Mutual Inductance
Applications of Faraday Law
Generator
Induction Stove
Electric Guitar

3. History of Faraday(1791 - 1867)
• Born to a poor family in Newington,
• England
• Diligent and intelligent
• He became an errand boy for a
• bookbinding shop and he read every
• book that he bound
• He became interested in the concept of force,or
specifically force
• Because of his early reading and experiments with
the idea of force,he was able to make important
discoveries in electricity later in life

4. • Wrote to Sir Humphry Davy
• Chemist at the Royal
Institution
• Begged for a job and sent
along with a bound volume
of notes, which he had taken
at Davy’s lectures
• Impressed by the boy’s zeal,
Davy made Faraday his
laboratory assistant in 1813
Big Change in Faraday’s
life

5. Big change (continued)
 Since then (21 year-old), drank in knowledge from
Davy
 Finished his second apprenticeship in 1820
 Great accomplishment for a man who was almost
completely self-educated originally

7. Magnetic Flux
 Magnetic flux represents how closely
together magnetic field lines are packed.
A
B


8.  Magnetic flux is higher for stronger magnetic
fields. Strong magnet=more lines.
 In the easiest case, with a constant magnetic field
B, and a flat surface of area A, the magnetic flux
is
Magnetic flux (Contd…)
Φm = A.B
Φm = AB cos θ
Units: 1 weber = 1Wb = 1Tm2

9. GRAPHICAL INTERPRETATION OF
MAGNETIC FLUX
The magnetic flux
is
proportional
to the number of
field lines
that pass
through a surface.

10. MAGNETIC FLUX
An electric current arises in a closed wire loop when it is
moved through a magnetic field. An arbitrary shaped
surface can be placed through the loop and flux calculated.
C
dS
Φ = ∫ B.dS
area

11. Magnetic Flux in a non-uniform field
 Divide the loop
into many small
pieces
The flux is the
sum of all these:
Φm =∫B.dA

13. Almost 200 years ago, around 1831, Faraday
looked for evidence that a magnetic field
would induce an electric current with this
apparatus:
FARADAY’S EXPERIMENT

14. He found no evidence when the magnet was
stationary, but did see a deflection in the
galvanometer when the magnet was moved
away or towards the coil
FARADAY’S EXPERIMENT(CONTD)

15. FARADAY’S
EXPERIMENT(CONTD)
He then changed the:
 no. of turns of the coil,
 the relative speed of the magnet and coil
 the direction of relative motion of the coil and magnet
and observed the change in the deflections shown by the
galvanometer.

16. With the help of his experiment, Faraday drew four important
Conclusions, which provided the basis of his law:
1.The galvanometer showed deflection whenever there was
relative motion between the magnet and the coil.
2.The deflection was more when the relative motion was faster
and less when the relative motion was slower.
3.The direction of the deflection changed if the polarity of the
magnet was changed
4.The deflection in galvanometer changes with the change in
the number of turns of coil-more the number of turns, greater
the deflection.
IMPORTANT CONCLUSIONS

17. Faraday’s Law of Induction
N S
i
v
i
i/t
S
EMF
Ɛ = - FB /t
The emf induced
around a loop
equals the rate of change
of the flux through that loop
Moving the magnet changes
the flux FB
Changing the current changes
the flux FB
Faraday Law: changing the flux induces
an emf.

18. Faraday’s Law of Induction
The induced e.m.f. in a wire loop is
proportional to the rate of change of
magnetic flux through the loop.
Magnetic flux:
Unit of magnetic flux: WEBER ( Wb )
1 Wb = 1 T·m2

19. Faraday’s law of induction:
[1 loop only]
[ For N loops]
Faraday’s Law of Induction (contd…)

20. This drawing shows the variables in the
flux equation:
VARIABLES OF THE FLUX EQUATION

21. Magnetic flux will change if the area of
the loop changes:
CHANGING THE VARIABLE A

22. Magnetic flux will change if the angle between
the loop and the field changes:
CHANGING THE VARIABLE ‘θ’

23. The magnetic flux is analogous to the electric
flux – it is proportional to the total number of
lines passing through the loop.
CHANGING THE VARIABLE ‘θ’

24. The -ve sign gives
the direction of
The induced EMF
Faraday’s Law of Induction(contd…)
A current produced by an induced EMF moves in a direction
so that the magnetic field it produces tends to restore the
changed field.

25. Faraday’s law gives the magnitude and direction of the induced
emf, and therefore the direction of any induced current.
Lenz’s law is a simple way to get the directions straight, with less
effort.
Lenz’s Law:
The induced emf is directed so that any induced current flow
will oppose the change in magnetic flux (which causes the
induced emf).
This is easier to use than to say ...
Decreasing magnetic flux  emf creates additional
magnetic field
Increasing flux  emf creates opposed magnetic field
Lenz’s law

26. Most Important Point of Faraday’s Law: A changing
magnetic field produces or creates an electric field.
Two types of electric fields. One is created by
charge and the other is created by a changing
magnetic field.

27. This image shows another way the magnetic flux can
change:
EMF Induced in a Moving Conductor

28. The induced current
is in a direction that
tends to slow the
moving bar – it will
take an external
force to keep it
moving.
EMF Induced in a Moving Conductor

29. What happens when the
conducting horizontal bar
falls?
EMF Induced in a Moving Conductor
1.As the bar falls, the flux of B
changes , so a current is induced,
and the bulb lights.
2.The current on the horizontal bar
is in a magnetic field , so it
experiences a force , opposite to
gravity

30.  When current is induced in a bulk piece of conductor,
the induced current often flows in small circles that are
strongest at the surface and penetrate a short distance
into the material. These current flow patterns are thought
to resemble eddies in a stream, which are the tornado
looking swirls of the water that we sometimes see.
Because of this presumed resemblance, the electrical
currents were named eddy currents.
 These circulating eddies of current have inductance and
thus induce magnetic fields. These fields can cause
repulsive, attractive and heating effects.
Eddy Current

31. Eddy Current

32. Eddy currents
Eddy currents are undesirable
as they heat up the conductor
and dissipate electrical energy.
Hence we always try to
minimise the eddy
currents by laminating the
metals or by cutting slots in
the metal piece as shown.

33. The effect in which a changing current in a circuit induces
and emf in the same circuit is referred to as self induction.
Self inductance

34. t
I
L




E
self
inductance
SI Unit of self inductance: (Henry)
H
1
A
s
1V 

Self inductance

35. The changing current in the
primary coil creates a changing
magnetic flux through the
secondary coil, which leads to
an
induced emf in the secondary
coil.
The effect in called
mutual induction.
Mutual induction
t
N

F


E

36. t
I
M




p
s
E
Emf due to mutual induction
mutual inductance
SI Unit of mutual inductance: (Henry)
H
1
A
s
1V 

Mutual induction

37. APPLICATIONS OF
FARADAY’S LAW

38. Faraday’s law gives rise to countless technological
applications. The law has far-reaching consequences
that have revolutionized the living of mankind after its
discovery. Faraday’s discovery of electromagnetic
induction has numerous industrial, technological,
medical and other applications. Some of them are
briefed as follows:

40. Electric Generators
A generator transforms mechanical energy into
electrical energy. This is an ac generator:
The axle is rotated
by an external force
such as falling water
or steam. The
brushes are in
constant electrical
contact with the slip
rings.

41. When the coil is rotated with a constant angular speed ω, the magnetic
field vector B and the area vector A of the coil at any instant t is θ=wt
(assuming θ=0° at t=0). As a result the flux Φ changes. The flux at any
time t is given as
Φ = Bacosθ = BAcosωt
From Faradays law, the induced EMF for the rotating coil of N turns is the
E = -N dΦ/dt = -NBA d(cosωt)/dt
Thus the instantaneous value of EMF is
Derivation of the emf induced
Electric Generators

42. Electric Generators(contd.)
Variation of EMF with rotation of the coil

43. Inductionstove
The pan on the stove is heated by eddy currents
produced by induction.

44. An ac current in a coil in the
stove top produces a changing
magnetic field at the bottom of
a metal pan.
The changing magnetic field
gives rise to a current in the
bottom of the pan.
Because the pan has resistance, the current heats the pan.
If the coil in the stove has low resistance it doesn’t get hot
but the pan does.
An insulator won’t heat up on an induction stove.
Induction stove

45. Induction Stove
In induction cooking container’s size, shape and
positioning is very important to achieve maximum
output
Only Ferromagnetic materials like enameled steel,
cast iron and stainless steel designed for induction can
be used for induction cooking.

47. TRANSFORMER
A transformer is basically two coils of wire wrapped around each
other, or wrapped around an iron core.
“ A transformer is a device for increasing or decreasing an
ac voltage.”
When an ac voltage is applied to the primary coil, it induces an ac
voltage in the secondary coil.

48. The ac voltage in the primary coil causes a magnetic flux change given by
.
B
P P
Δφ
= N
Δt
V
The changing flux (which is efficiently “carried” in the transformer core)
induces an ac voltage in the secondary coil given by
.
B
S S
Δφ
= N
Δt
V
Dividing the two equations gives the transformer equation
.
S S
P P
N
=
N
V
V
A “step up” transformer increases the output voltage in the secondary coil; a
“step down” transformer reduces it.

49. For a step-up transformer, NS > NP and VS > VP (the voltage is
stepped up).
For a step-down transformer, NS < NP and VS < VP (the voltage is
stepped down).
.
S P
P S
N
=
N
I
I
A transformer that steps up the voltage simultaneously and
steps down the current, and a transformer that steps down the
voltage and steps up the current.
Energy must be conserved; therefore, in the absence of losses, the
ratio of the currents must be the inverse of the ratio of turns.
Power in = power out,
Vp.Ip = Vs.Is,

50. In, practice real transformer are less than 100% efficient.
Resistive losses in the coils which can be reduced by making
their cross section large and by using high purity copper.
Eddy currents losses in the core which can be reduced by
laminating the core .Laminations reduce the area of circuits in the
core, and so reduce the emf and current flowing in the core and
energy is lost.
Hysteresis losses in the core. The magnetisation and
demagnetisation curves for magnetic material are often a little
different and this means that the energy required to magnetise a
core(while current is increasing) .The difference in energy is lost
as heat in the core.
Geometric design as well as material of the core may be
optimised to ensure that the magnetic flux in each coil of the
secondary is nearly the same as that in each coil of the primary.
LOSSES IN TRANSFORMER

51. Electric guitar

52. A magnetic pickup consists of
a permanent magnet with a core of
material such as alnico or ceramic,
wrapped with a coil of several
thousand turns of fine
enameled copper wire.
The pickup is most often
mounted on the body of the
instrument.
The permanent magnet
magnetizes the steel strings above it
ELECTRIC GUITAR (CONTD…)

53. The output of the amplifier is sent to the loudspeaker, which produces
the sound wave we hear.
when the string vibrates at some frequency, its magnetized segment
produces a changing flux through the coil. The changing flux induces an
emf in the coil that is fed to an amplifier.
The output of the amplifier is sent to the loudspeaker, which produces
the sound wave we hear.
ELECTRIC GUITAR (CONTD…)