Download 7TH CLASSES PHYSICS DAILY PLAN

10TH CLASSES
PHYSICS
DAILY PLAN
discover this phenomenon and the relation between
magnetism and electricity.
SUBJECT: INDUCED EMF AND CURRENT
AIM: to understand the Lorentz force, Faraday’s
Law and the Lenz’s Law.
DURATION: 3 weeks
REAL LIFE:
PRESENTATION:
THE EMF INDUCED IN A CONDUCTOR MOVING IN
A UNIFORM MAGNETIC FIELD
Suppose that a conducting rod AB of length L slides on a
stationary -shaped conductor as shown in Fig 4.5 The rod

AB moves to the right with a velocity v , so that the velocity

vector is perpendicular to the magnetic field of density, B ,
that is directed into the page. The charge q on the conducting

rod AB moves inside the magnetic field with a velocity v .
The Lorentz force acts on the charges in the rod, and as a
result the charges are deflected. The end A of the rod is
charged positively and the end B of the rod is charged
negatively according to the right hand rule. Thus a potential
difference is established between the end of A and B of the
rod. The maximum potential difference between the ends A
and B of the rod is the induced EMF. The Lorentz force
does a work on the charges on the rod, and the work done per
unit charge equals the induced emf. The induction in the rod
enables us to consider the conductor moving in a magnetic
field as a source of electromotive force
Fig 4.1 Faraday’s experiment for electromagnetic induction
From Faraday’s experiment we can get the following
conclusion:
When the magnetic flux in an electric circuit changes an
induced electromotive force and consequently an induced
electric current is produced in the circuit. The direction of the
induced emf and the induced current when the flux increases
are opposite to directions of the emf and the current when the
flux decreases.
The value of the EMF induced in the circuit shown in Figure
4.5 can be expressed in a more general form.As the
conducting rod moves towards right with a speed v, each
second it sweeps out an area given by the following equation:
L. 
A
t
The Value of the Induced EMF can be calculated as it follows:
The charge moves from A to B in a direction perpendicular to
Since In 1831 faraday and Henry experimentally showed that
the emf induced in a conductor was directly proportional to
work done is given with the following equation:
Work = Force x Displacement
Work = Fmagnetic x Length of the rod
Since the rod moves perpendicularly on the flux  =90o
the average time rate of the change in magnetic flux, (

the magnetic flux density B and it travels a distance L. The
Induced
emf
=
Work
 Emf in
Charge


t
).
Since the product L. in equation 4.2 equals the area swept by
F .L
q
q..B.L
Emf
 .B.L
in
q
When the rod is moved with a velocity  such that it makes an
angle  with the lines of flux of density B, the emf induced
becomes: Emfin= B.L.. Sin
 Emf in 
INDUCED ELECTROMOTIVE FORCE AND INDUCED
CURRENT
The discovery of electromagnetic induction that will be
discussed in this chapter produces a large-scale electric
current, and produces a suitable way for transportation of
electric current to a far distance. Michael Faraday was able to
A
the conducting rod in a unit time, (
t
following relation
Since the changing in magnetic flux 
average time rate of change of the flux

t
=
B
), we can write the
 BA the
A
t
= B.L.
Since the above mentioned equation equals the EMFinduced

EMFinduced = -
t
Eq4.4 for each turn
The above-mentioned equation is known as Faraday’s Law of
electromagnetic induction, and it is valid for all electric
circuits through which the magnetic flux changes. The minus
(-) sign in this equation indicates that the direction of the
induced emf is opposite to the change in magnetic flux that
induces it (as it is obvious in Lenz’s Law and in law of
conservation of energy). The value of the emf induced in a
loop of wire containing N turns, on the other hand, is N times
grater than the emf produced in a single loop. Hence:
Emfin =
N

t
Eq4.6
Example: The magnetic flux density inside a coil contains
100 turns each with a cross-sectional area 10cm2 decreases at a
rate of 0.1T/sec. What is the emf induced in the coil?
Solution :
A= 10cm2 = 10.10-4 m2 = 10-3 m2
N= 100
B
t
 0.1 T / s
Emfin =
N

t
 NA
B
t
 100.10
3
.( 0.1)
Emfin = 0.01 V
DIRECTION OF INDUCED EMF-LENZ’S LAW
Faraday’s experiment showed that work must be done to
produce an induced current. Lenz was the firs to prove this
fact. He has determined the direction of the induced emf as it
follows: The induced current in a closed circuit produces a
magnetic field that opposes the change in the external
magnetic field that produces it.
Lenz’s law can be applied to the case described in Fig 4.7.
When the magnet is moved towards a conducting coil the
magnetic flux density of the external magnetic field inside the
coil increases. That is the number of the field lines linking
through the coil increases (Fig 4.7a). According to Faraday’s
law a current is induced in the circuit. The direction of the
induced magnetic field produced by the induced current is
opposite to that of the external magnetic field. Thus it tries
to reduce the increasing external magnetic field. When the
magnet is moved away from the coil (Fig 4.7b) the magnetic
flux density of the external magnetic field inside the coil
decreases. In this case the direction of the induced magnetic
field produced by the induced current is the same as that
of the external magnetic field. Thus it tries to strengthen
the decreasing external magnetic field.
Fig 4.7 a/b/c: Direction of induced current and induced
magnetic field (explanation of Lenz’s law)
Experiment
(Lenz’s
Law)
An aluminum ring is
slipped
into
the
protruding iron core of a
coil of 500 turns. When
an AC of 220V and
50Hz is passed through
the coil (Fig 4.8), the
ring is thrown quite
violently
toward
upwards and floats in
the mid-air due to the repulsion force between magnetic
field produced by the current induced in the ring and the
field produced by the current flowing through the coil.
Therefore, the ring is always repelled and floats in the air.
However the effect cannot be produced if the ring has a slot in
it.
HOMEWORK: 43-44-45-46-47-48-49-52
MULTIMEDIA:Serway3-AkademediaElectromagnetism
DEMONSTRATION: Lenz’s Law
EXPERIMENT:
TEACHER:
DIRECTOR: