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Electromagnetic induction
When a conducting wire is placed in a
magnetic field, there will be a certain
amount of magnetic flux (
φ
B
) linked
with it which is a measure of the
number of lines of force crossing the
conductor at right angles.
When the conductor is made to move across the magnetic
filed, the magnetic flux linked with the conductor changes.
http://micro.magnet.fsu.edu/electro
mag/java/faraday2/
This change in magnetic flux linked with the conductor
induces an emf in the conductor.
The magnitude of the induced emf can be increased by
increasing the speed of motion of the conductor.
Reversing the direction of motion will reverse the
direction of induced current.
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View Full Document If a rectangular coil of N turns is rotated in a magnetic field
produced by a bar magnet as shown in the figure on the right,
free charges (electrons) in the coil of wire will experience a force
and will move causing a current to flow in the coil.
For one complete rotation of the coil,
the direction of the induced current will
reverse.
If a magnetic field B crosses at right
angles to an area A, the magnetic
flux
φ
B
crossing the area A is
B
= B A
If B makes an angle
θ
with the normal to
the surface, the expression for the flux
becomes
B
= B A Cos
θ
A coil of N turns of area A placed in a magnetic field B will
have an amount of flux linked with it given by
B
= NB A Cos
θ
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View Full Document Example 1
A 40 cm long solenoid has 600 turns and a radius of 2.5 cm. If it
carries a current of 7.5 A, what is the magnetic flux through it?
The number of turns per unit length
n = 600 turns/0.4 m = 1500 turns per
meter, I = 7.5 A, area of the coil A =
π
r
2
=
π
(0.025m)
2
= 0.00196 m
2
The filed inside the solenoid is
B =
μ
o
n I = 4
π
× 10
7
TmA
1
×
1500 turns per m × 7.5 A = 0.01414
T
Magnetic flux through the solenoid
is
φ
B
= NBA
= 600 × 0.014 T × 0.00196 m
2
=
0.0166 Wb
Faraday’s Law of Electromagnetic Induction
Faraday’s investigation on electromagnetic induction showed that
the magnitude of the induced emf (E) is
equal to the rate of change of magnetic flux.
B
d
E
dt
φ
=
The direction of the induced emf is given by
Lenz’s law
The induced emf causes a current to flow in the conductor. This
current will produce a magnetic filed which will oppose the
changing magnetic field that is the cause of the induction.
In other words,
the direction of the induced emf is such as to
oppose the cause of induction
.
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View Full Document Combining Lenz’s law with Faraday’s law we can write
B
d
E
dt
φ
= 
If a coil of N turns is subjected to a changing
magnetic flux, the induced emf will be
B
d
E
N
t
= 
dt
When the north pole of a
magnet is moved towards a
ring, the flux linked with the
ring conductor increases.
This induces a current in the
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This note was uploaded on 05/01/2011 for the course PHY 2049 taught by Professor George during the Spring '11 term at Edison State College.
 Spring '11
 George
 Physics, Force

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