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Chapter 17
MAGNATISM &
ELECTROSTATICS
GALVANOMETER
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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
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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
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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
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
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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
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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
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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 = (mew)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
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A solenoid is a long tightly wound cylindrical coil of wire.
• FIELD DUE TO SOLENOID
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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)
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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.Dl) for each element.
For element l1
EXPRESSION FOR FLUX DENSITY (B)
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(B.Delta l)1 = Bl1Cos(theta)
Since B is along the axis of l1. i.e. theta = 0
(B.Delta l1) = Bl1Cos0
(B.Delta l1) = Bl1(1)
(B.Delta l1) = Bl1
For element l2:
(B.Delta l)2 = Bl2Cosq
Since l2 is perpendicular to B i.e. theta = 90o
(B.Delta l)2 = Bl2Cos90o = Bl2 (0) = 0
For element l3:
(B.Delta l)3 = Bl3Cos (theta)
Since B is negligible i.e. B = 0 outside the coil.
(B.Delta l)3 = 0. l3Cos(theta)
EXPRESSION FOR FLUX DENSITY (B)
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(B.Delta l)3 = 0
For element l4:
(B.Delta l)4 = Bl4Cos(theta)
Since l4 is perpendicular to B i.e. q = 90o.
(B.Delta l)4 = Bl4Cos90o = Bl4(0) = 0.
Now, applying ampere’s circuital law
(B.Delta l)n = mewo (current enclosed)
B.l1 +0+0+0 = mewo (current enclosed)
B.l1 = mewo (current enclosed)law:
Now, if number of turns per unit length = n
And if current in each turn = I
Then current enclosed by the loop abcda = nIl1.
EXPRESSION FOR FLUX DENSITY (B)
• B.l1 = mewo (nIl1)
• B= mewonI
• 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
1. Electromagnet
2. Armature
3. Spring
4. Armature rod
5. Hammer
6. Gong
bell are :
CONSTRUCTION
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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
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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.