Chapter Eleven : Electromagnetic Induction
Definitions
and Basic concepts:
1- electromagnetic induction: It is a phenomenon in which
an induceelectromotive force and also an induced current are gnerated
in the coil on plunging a magnet into or withdrawing a magnet out of
a coil.
2- The presence of a soft iron core inside a coil
concentrates the lines of magnetic flux that link with the coil. This
in turn increases the induced emf and also the induced current.
3-
Faraday's laws for the induced emf: induced emf generated in a coil
by electromagnetic induction is proportional to the time rate by
which the conductor cuts the lines of magnetic flux and is also
proportional to the number of turns of the coil.
4- Lenz's
rule: The direction of the induced current generated by induction is
such that to oppose the change in magnetic flux producing it.
5-
Fleming right hand rule: place the thumb, the pointer and the middle
finger (with the rest of the fingers) of the right hand mutually at
right angles. If the pointer points in the direction of the magnetic
field and the thumb in the direction of motion then the middle finger
(with the rest of the fingers) will point in the direction of the
induced current.
6- Mutual induction: It is thelectromagnetic
interaction between two coils kept close to each other (or one inside
the other). An electric current with time varying intensity passing
in one coil (primary coil) will produce in the second one (secondary
coil) an induced current in a direction such that to oppose the
variabtions of the current intensity in the primary coil.
7-
Self induction: It is the electromagnetic effect induced in the same
coil when the intensity of the current increases or decreases. This
effect acts to resist such a change in the intensity of current.
8-
Coefficient of self induction: It is measured numerically by the
"emf" generated by induction in the coil when the intensity
of the current passing through it changes at a rate of 1 A/s.
9-
The unit of measuring the self induction (Henry): Is the self
induction of a coil in which an emf of 1V is induced when a current
passes through it which changes at a rate of 1 A/s
10-
The self induction of a coil depends on:
a) its geometry
b) its number of turns.
c) the spacing
between its turns d) the magnetic
permeability of its core.
11- The Dynamo (AC generator): It is
advice used to convert the mechanical energy to electric energy (AC
current and voltage) when its coil rotates in a magnetic field.
12-
The simple dynamo (AC generator) consists of:
a) Field magnet
(strong magnet). b) a coil of insulated copper
wire suspended between the two poles of the magnet.
b) Two
meltallic rings in contact whith two graphite brushes connected to an
external cirrcut.
13- Acommutator: (cylinder split into number
of insulated segments) is used to obtain a DC current and voltage (DC
generator)
14- The alternating current: It is current which
changes periodically its intensity and direction with time according
to a sinusoidal curve.
15- The electric transformer: It is an
electric device used to step up or step downn AC emf through mutual
electromagnetic induction.
16- The efficiency of the
transformer: It is the ratio between the out put electric energy
given in the secondary and that available to the primary.
17-
The electric motor: It is an electric device used to convert the
electric energy to mechanical energy.
Basic
Laws::
1- The induced emf generated in a coil of N turns as a result
of time variation of magnetic flux m
linked with the coil in an interval of time is given by the relation:
2-
The negative sign indicate that the direction of the induced emf is
such that to opposite the cause producing it.
3- The emf
induced in a secondary coil due to thtime variation in the lines of
magnetic flux resulting from a primary coil linking with the
secondary coil in the time interval t
is given by the relation:
4-
The emf induced by self induction as a result of the current I
passing through the coil in a time t
is given by the relation:
where
L is the coefficient of self induction of coil .
5- The emf
induced in a straight wire of length
moving
with a uniform velocity
v
making an angle
with the magnetic field of magnetic flux density "B" is
given by the relation:
6-
The emf induced in the dynamo is given by the relation:
when
N is the number of turns, B the magnetic flux density, A is the area
of the face of the coil,
is the angle confined between v and b and
is the angular velocity given by the relation:
7-
The emf is maximum when
= 90o
( sin 90o=
1)
8- The relation between the effective value of current Ieff
and its value Imax
is given by :
Ieff=
0.707 Imax.
9- The relation between the two emf in the two coils of the
transofrmer is :
.
10- The relation between the two currents in the two coils of
the trnasformer is:
11-
The efficiency of the trnsformer is given by the relation:
