Ch. No. 5. All about Electromagnetism
Q1. (A) Choose the correct alternative and rewrite the following.
•The device used for producing current is called a ___________
•At the time of short circuit, the current in the circuit __________
3.Remains the same
4.Increases in steps
•The direction of the magnetic field around a straight conductor carrying
current is given by ____________
1.Right hand thumb rule
2.Fleming’s left hand rule
3.Fleming’s right hand rule
4.None of these.
(B) Say true or false. If false, write the correct
•Magnetic poles exist in pairs.
•Magnetic field increases as we go away from a
Ans. False:- Magnetic field decreases as we go away
from a magnet.
•Magnetic lines of force always cross each other.
Ans. False:- Magnetic lines of force never cross each
•Electric generator is used to generate current.
(C) Find the odd one out.
•Loudspeaker, microphone, electric motor, bar magnet.
Ans. Bar Magnet: - It is a magnet while the rest are
devices working on electromagnetism.
•Armature coil, brushes, commutator, direct current.
Ans. Direct current: - The rest are parts of a
•Fuse, insulating material, rubber shoes, generator.
Ans. Generator: - The rest are safety measures in
•Voltmeter, ammeter, galvanometer, thermometer.
Ans. Thermometer: - The rest are dives working on
•Magnetic field: - The region around magnet, in which the
force of attraction and repulsion exists, is called as ‘magnetic
•Electric motor: - A device which converts electrical energy
into mechanical energy is called an electric motor.
•Solenoid: - A coil of many turns of insulated copper wire
wrapped in the shape of cylinder is called a solenoid.
Q3. State the rules
•Fleming’s right hand rule
Ans. Stretch the thumb, forefinger and middle finger of the right
hand so that they are perpendicular to each other.
If the forefinger indicates the direction of the magnetic field and
the thumb shows the direction of the motion of conductor, then the
middle finger will show the direction of induced current.
The induced current is found to be the maximum when the
direction of motion of the coil is at right angles to the magnetic
•Fleming’s Left hand rule:
Ans. Stretch the forefinger, the central finger and the
thumb of you left hand mutually perpendicular to each
other. If the forefinger shows the direction of the field
and the central finger shows the direction of the
current, then the thumb will point towards the
direction of the motion of the conductor.
•Right hand thumb rule
Ans. Imagine that you are holding a current carrying
straight conductor in your right hand such that the
thumb points towards the direction current, then the
curled fingers around the conductor will give the
direction of the magnetic field. This is known as right
hand thumb rule.
Q.4. Distinguish between electric motor and electric generator.
1.Electric motor converts 1.Electric generator
electrical energy into
energy into electrical
2.It uses electricity.
3.It is based on the
2.It generates electricity.
principle that current
3.It is based on the
placed in a magnetic
field experiences a force. induction.
State the characteristics of magnetic lines of force.
Ans. The path along which the unit north pole moves in a magnetic field
is called magnetic lines of force or magnetic field lines.
The characteristics of magnetic lines of force are:
Magnetic lines of force are closed continuous curves. They start
from North Pole and ends on South Pole.
The tangent at any point on the magnetic lines of force gives the
direction of the magnetic field at that point.
No two magnetic lines of force can intersect each other.
Magnetic lines of force are crowded where the magnetic field is
strong and far from each other where the field is weak.
Q6. Give scientific reasons.
•Wires carrying electricity should not be
touched bare footed.
•Our body is a good conductor of
•If we touch the wire bare footed, a large
current may pass through our body.
•Due to this, we may receive severe electric
shock and sometimes even death.
•Therefore, wires, carrying electricity
should not be touched bare footed.
•We should not use many electrical
•Many of the electrical appliances that we use at
home have a high power rating. E.g. grinder,
A.C, Oven, washing-machine, etc.
•When we use these electrical appliances
simultaneously, it causes overloading, that is
flow of large amount of current in the circuit,
•This causes fire.
Hence, we should not use many electrical
•Alloys like alnico or nipermag are used in
•Alloys like alnico or nipermag are very hard and
are used in the production of permanent magnets.
•Permanent magnets of these alloys are used in
microphones, loudspeaker, ammeters, voltmeters,
•Alnico is an alloy of iron containing aluminium,
nickel and cobalt. Nipermag is an alloy of iron
containing nickel, aluminium and titanium.
•Hence, permanent magnets prepared from these
alloys are most suitable for use in industries.
•A magnetic crane is used to load and
transport scrap iron.
•It is not feasible to create a permanent magnet
of a big size required to load and transport scrap
•It is also not possible to store such a big
•Hence, we create a powerful electromagnet by
passing a current through a big iron disc.
Therefore, a magnetic crane is used to load and
transport scrap iron.
. Explain the construction, working and
uses of Electric motor.
Ans. A device which converts electrical
energy into mechanical energy is called an
Principle: - Electric motor works on the
principle that a current carrying conductor
placed in a magnetic field experiences a
•Armature coil: - A large number of turns of insulated copper
wire wound on iron core in rectangular shape forms an
armature coil ABCD as shown in the above figure.
•Strong magnet: -The armature coil is placed in between two
pole pieces (N and S) of a strong magnet. This provides a
strong magnetic field.
•Split ring commutator: - It consists of two halves (R1 and
R2) of a metallic ring. The ends of the armature coil are
connected to these rings. Commutator reverses the direction of
current in the armature coil.
•Brushes: - Two carbon brushes B1 and B2, used to press the
•Battery: - The battery supplies the current (D.C.) to the
Working: •When current is passed through the coil ABCD, arms AB and CD
experience force. According to Fleming’s left hand rule the force
experienced by arm AB is in the downward direction and arm CD in the
•Both these forces are equal and opposite. This force rotates the coil in
clockwise direction until the coil is vertical.
•At this position, the contact between commutator and brushes break. So
the supply to the coil is cut off. Thus no force acts on the coil.
•But the coil does not stop due to inertia. It goes on rotating until the
commutator again comes in contact with the brushes B1 and B2.
•Again the current starts passing through coil and the arm AB rotates
through 900, 1800, 2700 and 3600. Now the force acting on arm AB is
downward and CD is upward. Again this force moves the coil in
•Thus, the coil rotates with the help of electrical energy. The coil of DC
motor continues to rotate in the same direction.
Uses of DC motor:
•These motors are used in domestic
refrigerators and washing machines.
•These motors are used in electric fans, hair
dryers, record players, tape recorders and
•These motors are used in electric cars,
rolling mills, electric cranes, electric lifts
and electric trains.
An electric device which converts mechanical energy
into electric energy is called an electric generator.
Principle: Electric generator works on the principle
of electromagnetic induction. When the coil of electric
generator rotates in a magnetic field, the magnetic
field induces a current in this coil. This induced
current then flows into the circuit connected to the
Types of electric generators:
A generator which converts
mechanical energy into
electric energy in the form of
alternating current is called
The main components of AC generator are as
shown in figure.
1. Armature 2. Strong magnets 3. Slip rings
and 4. Brushes.
Armature, strong magnets and brushes are the
same as used for electric motors.
The two ends of the armature coil are connected
to two brass slip rings R1 and R2. These rings
rotate along with the armature coil.
a.When the armature coil ABCD rotates in the
magnetic field provided by the strong magnets, it cuts
the magnetic lines of forces.
b.Thus the changing magnetic field produces induced
current in the coil. The direction of induced current is
determined by the Flemings right hand rule.
c.The current flows out through the brush B1 in one
direction in the first half of the revolution and through
the brush B2 in the next half revolution in the reverse
direction, this process is repeated.
d.Therefore, the induced current produced is of
alternating nature. Such a current is called as
A generator which converts
mechanical energy into
electrical energy in the form
of direct current is called as
The main components of D.C. generator
1. Armature coil 2. Strong magnets 3.
Split rings or commutator 4. Brushes 5.
The components such as armature coil,
strong magnet, brushes are the same which
are used for AC generators. Slip rings or
commutator is the same as used in electric
Working of DC generator:
1.When the coil of DC generator rotates in the
magnetic field, potential difference is produced in
the coil. This gives rise to the flow of current. This
is shown by glowing of the bulb.
2.In D.C. generator, the flow of current in the
circuit is in the same direction as long as the coil
rotates in the magnetic field.
3.This is because one brush is always in contact
with the arm of the armature moving up and other
brush is in contact with the arm of the armature
moving downward in the magnetic field.
Observe the figure. If the current in the coil A is changed, will some
current be induced in the coil B ? Explain.
•If the current in coil A is changed, then some current will be induced in
•As the current in coil A changes, the magnetic field related to it also
•Due to the changing magnetic field, current is induced in coil B.
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