Unit-3
Unit-3
Electromagnetism
Q1)The amount of flux present in around magnetic bar was measured at 0.013 weber. If the material has a diameter of 14cm, calculate the flux density.
Sol: Area=
r2
Diameter=2r
r=14/2=7cm=0.07m
Area=3.14 x 0.072=0.0154m2
Flux Density B==
A=0.013/0.0154=0.843 Tesla
Q2) Calculate the radius of the material having flux density of 0.5 T and flux present around the magnetic bar is 0.02T.
Sol: Flux Density B=
A
A=
B=0.02/0.5=0.04 m2
Area=
r2
r=0.113m
Q3) A wire 2.5m long is bent into a square and into a circle. If the current flowing through the wire is 100 A, find the magnetising force at the centre of the square and the centre of the circle?
Sol: Value of h at centre of square will be H=
= 
=144 AT/m
Value of H at the centre of circle is H=I/2
r
=125.6AT/m
Q4) Calculate the magnetising force and flux density at a distance of 4cm from a long straight circular conductor carrying a current of 250A and placed in air?
Sol: H=I/2
r
=250/2
x 0.04=994.71 AT/m
B=μoH
=4
x10-7x994.71=1.25x10-3 Wb/m2
Q5). Find 
Q6). Find 
Q7) List the difference between Electrical and magnetic circuits?
Sol: 
Q8) Derive the expression for energy in linear magnetic circuit?
Sol: Magnetic field can be of permanent magnet or electro-magnet. Both magnetic fields store some energy. Permanent magnet always creates the magnetic flux and it does not vary upon the other external factors. But electromagnet creates its variable magnetic fields based on how much current it carries. The dimension of this electro-magnet is responsible to create the strength the magnetic field and hence the energy stored in this electromagnet.
First we consider the magnetic field is due to electromagnet i.e. a coil of several no. Turns. This coil or inductor is carrying current I when it is connected across a battery or voltage source through a switch.
Suppose battery voltage is V volts, value of inductor is L Henry, and current I will flow at steady state.
When the switch is ON, a current will flow from zero to its steady value. But due to self induction a induced voltage appears which is
this E always in the opposite direction of the rate of change of current.
Now here the energy or work done due to this current passing through this inductor is U.
As the current starts from its zero value and flowing against the induced emf E, the energy will grow up gradually from zero value to U.
dU = W.dt, where W is the small power and W = – E.I
So, the energy stored in the inductor is given by
Now integrate the energy from 0 to its final value.
Again,
as per dimension of the coil, where N is the number of turns of the coil, A is the effective cross-sectional area of the coil and l is the effective length of the coil.
Again,
Where, H is the magnetizing force, N is the number of turns of the coil and l is the effective length of the coil.
Now putting expression of L and I in equation of U, we get new expression i.e.
So, the stored energy in a electromagnetic field i.e. a conductor can be calculated from its dimension and flux density.
Q9) Define the terms Magnetic Field Strength, Reluctance, Permeance and Permeability?
Sol: Flux intensity/Magnetic Field Strength
Is is also known as magnetic field strength. It is numerically equal to the force experienced by the N-pole of one weber placed at that point. It is denoted by H.
H= 
A/m or N/Wb
Reluctance
The obstruction offered by a magnetic circuit to the magnetic flux is known as reluctance.
S=
l – the length of the conductor
μo – permeability of vacuum which is equal to 4π Χ10-7 Henry/metre.
μr – relative permeability of the material.
A – cross-section area of the conductor.
Its SI unit is AT / Wb (ampere-turns / Weber)
Permanence
Every medium is supposed to have two permittivities.
a) Absolute permitivitty
b) Relative permittivity 
r
To measure 
r vaccum or free space is chosen. It has an absolute permittivity of 8.854 x 10-12 F/m. For any other medium
= 
0 
r
Permeability
The phenomenon of magnetism and electromagnetism depends on certain property of material called as permeability.Every medium is supposed to have two permeabilities.
a) Absolute permeability 
b) Relative permeability 
0
To measure 
r vacuum or free space is chosen. It has an absolute permeability of 4
x 10-7 H/m. For any other medium
= 
0 
r
Q10) Explain Flemings Left and Right-Hand Rule?
Sol: Fleming’s Left-hand rule
When a current carrying conductor is placed inside a magnetic field, a force acts on the conductor in a direction perpendicular to both the directions of the current and the magnetic field. If the forefinger represents the direction of the field and the second finger represents that of the current, then thumb gives the direction of the force.
Fleming’s Right-hand rule
From Faraday’s Law when a conductor moves inside a magnetic field an induced current is always there. If this conductor is forcefully moved inside the magnetic field, then relation between applied force direction, magnetic field and current can be seen This relation is determined by Fleming’s right-hand Rule.
Hold out the right hand with the first finger, second finger and thumb at the right angle to each other. If forefinger represents the direction of the line of force, the thumb points in the direction of motion or applied force, then second finger points in the direction of the induced current.
