ELECTROMAGNETIC INDUCTION – CHAPTER 23
1. Magnetic flux definition, Ф:
Product of magnetic flux density and perpendicular surface area.
SI unit: Weber (Wb) / (T m^2)
θ
B
B
Ф = magnetic flux, Wb
B = flux density, T
A = component area normal to the field, m²
θ = angle between a normal to the surface and the field
Weber
**Distinguish between magnetic flux density and magnetic flux.
2. Magnetic flux linkage
Magnetic flux linkage is defined as the product of the flux Ф and the number of turns N.
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3. Laws of Electromagnetic Induction
A moving magnet near a coil generates
a small current.
Demo : Coil, Galvanometer, magnets resistor and connecting cables.
 State the direction of current when the magnet enters the coil.
 Is there a current flow when the magnet is stationary in the coil?
 What can be observed when the magnet bar is pulled out from the coil?
 What are the changes if you were using two magnet bars?
 How does the observation changes if the coil is moved towards and away from the
magnet?
 Why do we need to fix a resistor in series with the galvanometer?
Outcome:
1.
2.
3.
Why there is emf induced?
Why the direction of induce emf changes?
What are the factors effecting the magnitude of induce emf?
Factors affecting magnitude of induced e.m.f. :
1. Magnetic flux density (B)
2. Cross sectional area of coil (A)
3. Number of turns of coil (N)
http://www.youtube.com/watch?v=bkSsgTQOXVI&NR=1
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Faraday’s law of electromagnetic induction:
Magnitude of induce e.m.f. in conductor is directly proportional to the rate of
change of magnetic flux.
E = N δФ
δt
Lenz’s law:
The direction of induced current / e.m.f. in a conductor is such that it opposes
the cause of it.
Combination of Faraday’s and Lenz’s law,
E = -N δФ
δt
Ф = BA cosθ
= BA cos ωt
θ = ωt
Therefore:
E = -N δ (BA cos ωt)
δt
= -BAN δ (cos ωt) = -BANω sin ωt
δt

How to determine the direction of the induced current in the conductor?
How energy is conserved in the above situation?
Which law is used to explain this?
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4. E.m.f induced in a rotating coil
The flux link in each turn is given by:
Ф = BA cos ωt
To calculate the induced e.m.f, E :
E = -N δ (BA cos ωt)
δt
(Faraday’s Law and Lenz’s Law)
= -BAN δ (cos ωt) = BANω sin ωt
δt
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Ф = BAN cos ωt = BAN
E = BANω sin ωt = 0
Ф = BAN cos ωt = -BAN
E = BANω sin ωt = 0
Ф = BAN cos 90o = 0
E = BANω sin ωt = BANω
Ф = BAN cos 270o = 0
E = BANω sin ωt = - BANω
5. APPLICATIONS of Induction
Back EMF in electric motors
You may have noticed that when something like a refrigerator or an air conditioner first
turns on in your house, the lights dim momentarily. This is because of the large current
required to get the motor inside these machines up to operating speed. When the motors
are turning, much less current is necessary to keep them turning.
One way to analyze this is to realize that a spinning motor also acts like a generator. A
motor has coils turning inside magnetic fields, and a coil turning inside a magnetic field
induces an emf. This emf, known as the back emf, acts against the applied voltage that's
causing the motor to spin in the first place, and reduces the current flowing through the
coils. At operating speed, enough current flows to overcome any losses due to friction
and to provide the necessary energy required for the motor to do work. This is generally
much less current than is required to get the motor spinning in the first place.
If the applied voltage is V, then the initial current flowing through a motor with coils of
resistance R is I = V / R. When the motor is spinning and generating a back emf, the
current is reduced:
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