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B.E. 1st YEAR (C)
Prepared by:- Biniwale Suraj
Laws of Magnetic Force
Direction of Magnetic Field
Magnetic Effect of Electric Current
Faraday’s Laws of Electromagnetic Induction
Electromagnetism is the key to the operation of great part
of electrical apparatus found in industry as well as home.
Thus, today magnetism has attained a place of a pride in
Without the aid of magnetism, it is impossible to operate
such a device as electrical generators, motors
Electricity and magnetism are different facets of
Without electromagnetism very few of our modern
devices are possible.
The origin of electricity is and magnetism sprang
from ancient men’s curiosity over the ability of two
materials, amber and lodestone to attract other
With the publication of James Clerk Maxwell's 1873
Treatise on Electricity and Magnetism in which the
interactions of positive and negative charges were
shown to be regulated by one force.
An electric current in a wire creates a circular
magnetic field around the wire, its direction
depending on that of the current.
In early 19th century faraday discover
Laws of magnetic force
Like poles repel each other and unlike poles attract each other.
The magnetic forces between two isolated magnetic poles placed in
a medium is directly proportional to the product of the pole strength and
inversely proportional to square of the distance between them.
F α m1 m2 Or F = K m1 m2
Where K is a constant whose value depends upon the surrounding
K = 1 Where μ0 = Absolute permeability
= Relative permeability
The magnets exert their influence in the surroundings,
this is called magnet field.
Magnetic field is a quantity that has both direction and
The direction of the magnetic field is taken to be the
direction in which a north pole of the compass needle
moves inside it.
Direction of Magnetic Field
(1) Right hand-palm rule :
“First make the thumb and fingers of the right hand
perpendicular to each other and put the thumb along the
wire in the direction of the current and fingers point
towards the point of observation”.
(2) Right hand-thumb rule :
“If the thumb is along the direction of current, wrapped fingers will
show the direction of circular magnetic field lines.”
(3) Fleming Left-hand rule :
“Hold out your left hand with forefinger, second finger and thumb at
right angle to one another. If the fore finger represents the direction of
the field and the second finger that of the current, then thumb gives the
direction of the force.”
(4) Right hand rule :
“Hold out the right hand with the first finger, second and
thumb at right angle to each other. If finger represent the
direction of the line of force, the thumb points in the
direction of motion or applied force, then second finger
point in the direction of the induced current.”
Magnetic Effect of Electric current
When a conductor carries a current, magnetic lines of
force are set around the length of the conductor.
Magnetic field produced by the current flowing in the
Magnetic lines of force in the form of concentric circle
around the conductor.
The direction of the line of force depends upon the
direction of the current
When the magnetic flux linking a conductor changes, an
e.m.f. is produced in the conductor. If the conductor
forms a closed circuit, a current will flow in it. This
phenomenon is known as Electromagnetic induction.
Faraday's Laws of Electromagnetic
Faraday's First Law:
“Any change in the magnetic field of a coil of wire will cause an
emf to be induced in the coil. This emf induced is called induced
emf and if the conductor circuit is closed, the current will also
circulate through the circuit and this current is called induced
current. Method to change magnetic field.”
(1)By moving a magnet towards or away from the coil
(2)By moving the coil into or out of the magnetic field.
(3)By changing the area of a coil placed in the magnetic field
(4)By rotating the coil relative to the magnet.
Faraday's Second Law:
“It states that the magnitude of emf induced in the coil is equal to
the rate of change of flux that linkages with the coil. The flux
linkage of the coil is the product of number of turns in the coil and
flux associated with the coil.”
Consider a magnet approaching towards a coil. Here we consider two
instants at time T1 and time T2. Flux linkage with the coil at time, T1 =
NΦ1 Wb Flux linkage with the coil at time, T2 = NΦ2 wb Change in flux
linkage = N(Φ2 - Φ1) Let this change in flux linkage be, Φ = Φ2 - Φ1 So,
the Change in flux linkage = NΦ Now the rate of change of flux linkage =
NΦ / t Take derivative on right hand side we will get The rate of change of
flux linkage = NdΦ/dt But according to Faraday's law of electromagnetic
induction, the rate of change of flux linkage is equal to induced emf.
Considering Lenz's Law.
Lenz’s law :
“The direction of current induced in a conductor by a
changing magnetic field due to Faraday's law of
induction will be such that it will create a field that
opposes the change that produced it.”
Lenz's law is shown by the negative sign in Faraday's law of
When a moving charge cuts through the flux lines of a
magnetic field, it experiences a force given by
F = qv×B. To see what occurs, consider Fig. where
the crosses represent a flux field B directed away from
the reader. When conducting bar ab is moving to the
right with velocity v, then every charge within the bar
is moving to the right, cutting past flux lines, with a
velocity v. The consequence of this is that a force of
F = qv×B is exerted on every charge within the bar
where the direction of the vector F is along the bar,
directed from b to a. This force amounts to an emf
within the bar, tending to produce a current from b to
a. If the bar is part of a circuit, or connected to a
galvanometer as in Fig. 1 above, that emf will cause a
current to flow. If the bar is not part of a circuit, then
what will happen is that the free electrons in the bar
will all move toward end b, making end b negative and
end a positive.
Emf induced in a moving conductor. The emf induced in a straight
conductor of length l moving with velocity v perpendicular to a
magnetic field B is
1) E = Blv
where B, l and v are mutually perpendicular. The emf is in volts
when B is in webers/m2, l is in meters, and v is in m/sec.
If the velocity vector v makes an angle θ with the direction of the
magnetic field, 1) becomes
2) E = Blv sin θ
Magnetic hysteresis is an important phenomenon and refers to the
irreversibility of the magnetisation and demagnetisation process. When a
material shows a degree of irreversibility it is known as hysteretic. We will
now explore the physics behind ferromagnetic hysteresis.
When a demagnetised ferromagnetic material is placed in an applied
magnetic field the domain that has a direction closest to that of the applied
field grows at the expense of the other domains. Such growth occurs by
motion of the domain walls. Initially domain wall motion is reversible, and
if the applied field is removed the magnetisation will return to the initial
demagnetised state. In this region the magnetisation curve is reversible and
therefore does not show hysteresis.
The crystal will contain imperfections, which the domain boundaries
encounter during their movement. These imperfections have an associated
magnetostatic energy. When a domain wall intersects the crystal
imperfection this magnetostatic energy can be eliminated as closure
domains form. This pins the domain wall to the imperfection, as it is a local
The applied magnetic field provides the energy to allow the domain wall to
move past the crystal imperfection, but the domains of closure cling to the
imperfection forming spike-like domains that stretch as the domain wall
moves further away. Eventually these spike domains snap off and the
domain wall can move freely. As the spike domains snap off there is a
discontinuous jump in the boundary leading to a sharp change in the
magnetic flux, which can be detected by winding a coil around the specimen
connected to a speaker. In doing so, crackling noises are heard
corresponding to the spike domains breaking away from the domain walls.
This phenomenon is known as the Barkhausen effect.
Eventually all the domain walls will have been eliminated leaving a single
domain with its magnetic dipole moment pointing along the easy axis
closest to the direction of the applied magnetic field. Further increase in
magnetisation can occur by this domain rotating away from the easy
direction to an orientation parallel to that of the externally applied field. The
magnetisation of the material at this stage is called the saturation
magnetisation (see Figure J) . The ease of this final rotation is dependent on
the magnetocrystalline energy of the material; some materials require a
large field to reach this saturation magnetisation.
If the external applied field is removed the
single domain will rotate back to the easy
direction in the crystal. A demagnetising
field will be set up due to the single domain,
and this field initiates the formation of
reverse magnetic domains as these will
lower the magnetostatic energy of the
sample by reducing the demagnetising field.
However the demagnetising field is not
strong enough for the domain walls to be
able to grow past crystal defects so they can
never fully reverse back to their original
positions when there is no external applied
field. This results in the hysteresis curve as
some magnetisation will remain when there
is no external applied field. This
magnetisation is called the remanent
magnetisation, Br. The field required to
reduce the magnetisation of the sample to
zero is called the coercive field Hc. And the
saturation magnetisation Bs is the
magnetisation when all the domains are
aligned parallel to the external field. These
are shown on the schematic below:
Electrical Technology Volume 1
(B.L. Theraja, A.K. Theraja)
A. Beiser(1987) Concepts of modern physics (4th ed.)
University of Cambridge
(TLP library ferromagnetic materials-hysteresis)