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CONTENTS OF CHAPTER # 17
Introduction
Magnetic Poles
Magnetic Force
Magnetic Field
Properties of Magnetic
Characteristics of Magnetic Lines of Forces
Magnetic Field of the Earth
Magnetic and Magnetic Materials
Methods of Making Magnets
Magnetic Effect of Current
Electromagnets and Their Applications
Force on a current carrying conductor in a Magnetic Field
Galvanometer
Ammeter
Voltmeter
Simple Electric Motor
FERROMAGNETIC SUBSTANCES
SOLENOID
ELECTRIC BELL
GALVANOMETER
GALVANOMETER
Galvanometer is an electromechanical instrument which is used for the detection of
electric currents
through a circuit. Being a sensitive instrument, Galvanometer can not
be used for the measurement of
heavy currents.
WORKING PRINCIPLE
Galvanometer works on the principle of conversion of electrical energy into mechanical
energy.
ESSENTIAL PARTS OF
GALVANOMETER
There are five essential parts of a Galvanometer.
1. A U-shaped permanent magnet with concave poles.
2. Flat rectangular coil of wire.
3. A soft iron cylinder.
4. A pointer or needle.
5. A scale.
CONSTRUCTION
The flat rectangular coil of thin enamel insulated wire of suitable number of turns
wound on an aluminum
frame is suspended between the poles of U-shaped magnet by
a thin strip. One end of the wire of coil is
soldered to connect to an external terminal.
The other end is soldered to a loose and soft spiral. A soft
iron cylinder is placed within
the frame of coil.
WORKING
When the current is passed through the coil it becomes a magnet. There is force of
attraction is setup
between the poles of magnet and coil. As a result a couple is
produced in the coil and it is deflected.
The current passes through the coil and the
angle of deflection has a direct relation with each other.
The deflection is measured by
a pointer attached to the coil.
AMMETER-VOLTMETER
AMMETER
Ammeter is an electrical measuring device, which is used to measure electric current
through the circuit.
CONNECTION OF AMMETER IN
CIRCUIT
An ammeter is always connected in series to a circuit.
SYMBOL
CONVERSION OF GALVANOMETER
INTO AMMETER
Since Galvanometer is a very sensitive instrument therefore it can’t measure heavy
currents. In order to
convert a Galvanometer into an Ammeter, a very low resistance
known as "shunt" resistance is
connected parallel to Galvanometer. Value of shunt is
so adjusted that most of the current passes
through the shunt. Fig . Rs shunt
resistance. In this way a Galvanometer is converted into Ammeter
and can measure
heavy currents without fully deflected.
VALUE OF SHUNT RESISTANCE
where
Rs = Shunt resistance
I = Current to be measured
Rg = Resistance of galvanometer
Ig = Current passing through
the galvanometer
VOLT METER
Voltmeter is an electrical measuring device, which is used to measure potential
difference between two
points in a circuit.
CONNECTION OF VOLTMETER IN
CIRCUIT
Voltmeter is always connected in parallel to a circuit.
SYMBOL
CONVERSION OF GALVANOMETER
INTO VOLTMETER
Since Galvanometer is a very sensitive instrument, therefore it can not measure high
potential
difference. In order to convert a Galvanometer into voltmeter, a very high
resistance known as "series
resistance" is connected in series to Galvanometer.
VALUE OF SERIES RESISTANCE
where
RX = series resistance
V = potential difference to be
measured
Rg = Resistance of galvanometer
Ig = Current passing through
the galvanometer
PROPERTIES OF MAGNET
PROPERTIES OF MAGNET
1. Magnets attract objects of iron, cobalt and nickel.
2. The force of attraction of a magnet is greater at its poles than in the middle.
3. Like poles of two magnets repel each other.
4. Opposite poles of two magnets attracts each other.
5. If a bar magnet is suspended by a thread and if it is free to rotate, its South Pole will
move towards
the North Pole of the earth and vice versa.
CHARACTERISTICS OF
MAGNETIC LINES OF FORCE
1. Magnetic lines of force start from the North Pole and end at the South Pole.
2. They are continuos through the body of magnet
3. Magnetic lines of force can pass through iron more easily than air.
4. Two magnetic lines of force can not intersect each other.
5. They tend to contract longitudinally.
6. They tend to expand laterally.
FERROMAGNETIC
SUBSTANCES
Substances that behave like a magnet in the presence of a magnetic field are known as
Ferromagnetic Substances.
EXAMPLES: Iron, cobalt and nickel are ferromagnetic substances.
SOLENOID
Solenoid is a coil of wire. Solenoid is a coil wound on a cylindrical frame of iron or any
material when an
electric current passes through the Solenoid, a magnetic field is
produced around it. It has suitable numbers of turns of wire.
Magnetic field of solenoids is given by
B = onI
Magnetic field inside the solenoid is very strong and uniform but it is very weak outside
the solenoid.
MAGNETIC FIELD OF INDUCTION DUE TO SOLENOID
SOLENOID
A solenoid is a long tightly wound cylindrical coil of wire.
FIELD DUE TO
SOLENOID
When a current is passed through a solenoid the magnetic field is produced, which is
strong and
uniform inside, while it is negligibly weak outside.
EXPRESSION FOR FLUX
DENSITY (B)
To calculate B consider a rectangular path a, b, c and d. The path consist of for length
elements l1,
l2, l3 and l4. Let us calculate the product (B.l) for each element.
For element l1
(B.l)1 = Bl1Cos
Since B is along the axis of l1. i.e. 
=0
(B.l1) = Bl1Cos0
(B.l1) = Bl1(1)
(B.l1) = Bl1
For element l2:
(B.l)2 = Bl2Cos
Since l2 is perpendicular to B i.e.
 = 90o
(B.l)2 = Bl2Cos90o = Bl2 (0) = 0
For element l3:
(B.l)3 = Bl3Cos
Since B is negligible i.e. B = 0 outside the coil.
(B.l)3 = 0. l3Cos
(B.l)3 = 0
For element l4:
(B.l)4 = Bl4Cos
Since l4 is perpendicular to B i.e.  = 90o.
(B.l)4 = Bl4Cos90o = Bl4(0) = 0.
Now, applying ampere’s circuital law:
(B.l)n = o (current enclosed)
B.l1 +0+0+0 = o (current enclosed)
B.l1 = o (current enclosed)
Now, if number of turns per unit length = n
And if current in each turn = I
Then current enclosed by the loop abcda = nIl 1.
B.l1 = o (nIl1)
B= onI
This is the expression for flux density of solenoid. It shows,
B is uniform with in a long solenoid.
The direction of B is along the axis of solenoid.
ELECTRIC BELL
MAIN COMPONENT
OF
ELECTRIC BELL
Important parts of an
electric
bell are :
1. Electromagnet
2. Armature
3. Spring
4. Armature rod
5. Hammer
6. Gong
CONSTRUCTION
One end of armature winding is connected to terminal T1 and the other to a spring,
which is mounted on
a soft iron strip. A rod is attached to the armature and the free end
of the rod carries a small hammer,
which strikes a bell. A very light spring is attached to
a screw, which is joined to terminal T2.
WORKING OF
ELECTRIC BELL
The electric circuit is completed through a battery and push switch button connected to
the terminal T1
and T2. When the push button is pressed the electric circuit is completed
and the armature is attracted
towards the electromagnet as a result, the small spring
gets detached from the screw due to which the
electric circuit is broken and the
electromagnet is demagnetized. Hence, the attraction disappears and
the armature is
brought back by the spring to its original position. Contact of the spring with the screw is
now remade, which completes the electric circuit. The action is repeated over and over
again
consequently. The armature vibrates and hammer attached to it strikes the gong
and the bell rings and
sound is produced.