Magnetic
Effects of Electric Current
electric circuit, as shown in Fig. 13.1.
􀂄 Place a small compass near to this copper
wire. See the position
of its needle.
􀂄 Pass the current through the circuit by
inserting the key into
the plug.
􀂄 Observe the change in the position of the
1. Why does a compass
needle get deflected when brought near
a bar magnet?
The
compass needle is a small magnet. When the compass needle is brought close to a
bar magnet, the
magnetic
field lines of the compass needle interact with the magnetic field lines of bar
magnet which results in needle deflection.
1. Draw magnetic field
lines around a bar magnet.
Magnetic field lines of a bar magnet emerge from
the North Pole and terminate at the South Pole as
shown in the figure below.
2. List the properties of magnetic field lines.
The properties of magnetic field lines are as
follows:
1.Magnetic field lines do not
intersect with each other.
2.They emerge from the North
Pole and terminate at the South Pole.
3.Inside the magnet, the
direction of the field lines is from South Pole to North Pole.
3. Why don’t two magnetic field lines intersect each other?
If two magnetic field lines intersect then at
the point of intersection the compass needle shows two different direction
which is not possible hence they do not intersect with each other.
1. Consider a circular loop
of wire lying in the plane of the table. Let the current pass through the loop clockwise.
Apply the right-hand rule to find out the direction of the magnetic field
inside and outside the loop.
For the downward direction of the current, the
direction of the magnetic field will be as if emerging from the table outside
the loop and merging the table inside the loop. Similarly, for current flowing
in the upward direction, the direction of the magnetic field will as if they
are emerging from the table outside the loop and merging to the table inside
the loop as shown in the figure.
2. The magnetic field in a
given region is uniform. Draw a diagram to represent it.
3. Choose the correct
option.
The magnetic field inside a
long straight solenoid-carrying current
a. is zero.
b. decreases as we move
towards its end.
c. increases as we move
towards its end.
d. is the same at all
points.
d. is the same at all
points
The magnetic field inside a long straight
current carrying solenoid is uniform therefore it is same at all points.
1. Which of the following
property of a proton can change while it moves freely in a magnetic field? (There
may be more than one correct answer.)
a. Mass
b. Speed
c. Velocity
d. Momentum
Ans: (c) and (d)
When a proton enters the region of magnetic
field, it experiences magnetic force. Due to which the path of the proton
becomes circular. As a result, the velocity and the momentum change. Note speed
is directionless & velocity is vector quantity, magnetic force will cause
change in direction.
2. In Activity 13.7, how do
we think the displacement of rod AB will be affected if (i) current in rod AB is
increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the
rod AB is increased?
A current carrying conductor when placed in a
magnetic field experiences force. The magnitude of this force will increase
with the increase in the amount of current, length of conductor and the
strength of the magnetic field. Hence, the strength of the magnetic force
exerted on the rod AB and its displacement will increase if
(i) The current in rod AB is increased
(ii) Stronger horse shoe magnet is used
(iii) When the length of the rod AB increases
3. A positively-charged
particle (alpha-particle) projected towards west is deflected towards north by
a magnetic field. The direction of magnetic field is
a. towards south
b. towards east
c. downward
d. upward
The direction of the magnetic field can be
determined using the Fleming’s Left hand rule. According to the rule, if we
arrange our thumb, forefinger and the middle finger of the left hand right
perpendicular to each other, then the thumb points towards the direction of the
magnetic force, the middle finger the
direction of current and the forefinger the
direction of magnetic field. Since the direction of positively charged particle
is towards west, the direction of the current will also be towards the west.
The direction of the magnetic force is towards the north hence the direction of
magnetic field will be upward according to Fleming’s Left hand rule.
1. State Fleming’s left-hand rule.
Fleming’s Left hand rule states that if we
arrange our thumb, forefinger and middle finger of the left hand right angles
to each other, then the thumb points towards the direction of the magnetic
force, the forefinger points towards the direction of magnetic field and the middle
finger points towards the direction of current.
2. What is the principle of
an electric motor?
The working principle of electric motor is based
on the magnetic effect of current. A current carrying conductor when placed in
a magnetic field experiences force and rotates. The direction of the rotation
of the conductor can be determined by Fleming’s Left hand rule.
3. What is the role of
split ring in an electric motor?
Split ring plays the role of commutator in an
electric motor. The commutator reverses the direction of the current flowing
through the coil after each half rotation of the coil. Due to this reversal of
current, the coil continues to rotate in the same direction.
1. Explain different ways
to induce current in a coil.
Following are the different ways to induce
current in a coil:
If the coil is moved rapidly between the two
poles of horse shoe magnet, electric current is induced in the coil. When a
magnet is moved relative to the coil, an electric current is induced in the
coil.
1. State the principle of
an electric generator.
Electric generator works on the principle of
electromagnetic induction. In a generator, electricity is generated by rotating
a coil in the magnetic field.
2. Name some sources of
direct current.
DC generator and batteries are some sources of
direct current.
3. Which sources produce
alternating current?
Power plants and AC generators are some of the
sources that produce alternating current.
4. Choose the correct
option.
A rectangular coil of
copper wires is rotated in a magnetic field. The direction of the induced
currentchanges once in each
a. two revolutions
b. one revolution
c. half revolution
d. one-fourth revolution
c. half revolution
When a rectangular coil is rotated in magnetic
field, the direction of the induced current changes once in half revolution. As
result, the direction of the current in the coil remains the same.
1. Name two safety measures commonly
used in electric circuits and appliances.
The safety measured commonly used in electric
circuits are as follows:
(i) Fuse
Each circuit should be connected to a fuse because
a fuse prevents the flow of excessive current
through the circuit. When the current in the
circuit exceeds the maximum limit of the fuse
element, the fuse melts to stop the flow of
current protecting the appliance connected to circuit.
(ii) Earthing
Earthing protects the user from electric shocks.
Any leakage of current in an appliance is transferred to ground by earthing and
the people using the appliance is prevented from getting electrocuted.
2. An electric oven of 2 kW
power rating is operated in a domestic electric circuit (220 V) that has a current
rating of 5 A. What result do you expect? Explain.
The current drawn by the electric oven can be
calculated using the formula
P = V × I
I = P/V
Substituting the values, we get
I = 2000 W/220 V = 9.09 A
The current drawn by the electric oven is 9.09 A
which exceeds the safe limit of the circuit. This causes
the fuse to melt and break the circuit.
3. What precaution
should be taken to avoid the overloading of domestic electric circuits?
A few of the precautions to be taken to avoid
the overloading of domestic electric circuits are as follows:
1.Connecting too many devices
to a single socket should be avoided
2.Using too many appliances at
the same time should be avoided
3.Faulty appliances should
not be connected to the circuit
1. Which of the following
correctly describes the magnetic field near a long straight wire?
a. The field consists of
straight lines perpendicular to the wire.
b. The field consists of
straight lines parallel to the wire.
c. The field consists of
radial lines originating from the wire.
d. The field consists of
concentric circles centered on the wire.
d. The field consists of concentric circles
centered on the wire.
The magnetic field near a long straight wire are
concentric circles. Their centers lie on the wire.
2. The phenomenon of
electromagnetic induction is
a. the process of charging
a body.
b. the process of
generating magnetic field due to a current passing through a coil.
c. producing induced
current in a coil due to relative motion between a magnet and the coil.
d. the process of rotating
a coil of an electric motor.
c. producing induced current in a coil due to
relative motion between a magnet and the coil.
The phenomenon of inducing current in a coil due
to the relative motion between the coil and the magnet
Is known as electromagnetic induction.
3. The device used for producing
electric current is called a
a. generator
b. galvanometer
c. ammeter
d. motor
a. generator
The device used for producing electric current
is known as generator. Generator converts mechanical energy to electric energy.
4. The essential difference
between an AC generator and a DC generator is that
a. AC generator has an
electromagnet while a DC generator has permanent magnet.
b. DC generator will
generate a higher voltage.
c. AC generator will
generate a higher voltage.
d. AC generator has slip
rings while the DC generator has a commutator.
d. AC generator has slip rings while the DC
generator has a commutator.
AC generators have two rings known as the slip
rings while DC generators have two half rings known as the commutator. This is
main difference between AC generator and DC generator.
5. At the time of short
circuit, the current in the circuit
a. reduces substantially.
b. does not change.
c. increases heavily.
d. vary continuously.
c. increases heavily
When two naked wires in the circuit come in
contact with each other, the amount of current flowing in the circuit increase
abruptly resulting in short circuit.
6. State whether the
following statements are true or false.
a. An electric motor
converts mechanical energy into electrical energy.
b. An electric generator
works on the principle of electromagnetic induction.
c. The field at the center
of a long circular coil carrying current will be parallel straight lines.
d. A wire with a green
insulation is usually the live wire of an electric supply.
a. False
An electric motor converts electrical energy
into mechanical energy.
b. True
An electric generator is a device that generates
electricity by rotating a coil in a magnetic field.
c. True
A long circular coil is a solenoid. The magnetic
field lines inside a solenoid are parallel straight lines.
d. False
Live wires have red insulation cover while the
earth wire has green insulation.
7. List two methods of
producing magnetic fields.
Following are the methods of producing magnetic
fields:
1.By using a permanent magnet
we can produce magnetic field and it can be visualized by
spreading iron fillings on a white paper and
keeping a magnet beneath the paper.
2.A current carrying straight
conductor produces magnetic field.
3.Different types of
conductors such as solenoid and circular loop can be used to see the presence
of magnetic field.
8. How does a solenoid
behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the
help of a bar magnet? Explain.
A solenoid is a long coil of circular loops of
insulated copper wire. The magnetic field produced around
the solenoid when the current is passed through
it is similar to the magnetic field produced around the bar
magnet when current is passed through it. The
figure shown below shows the arrangement of magnetic
fields produced around the solenoid when current
is passed through it.
When the north pole of the bar magnet is brought
close to the end connected to the negative terminal of
the battery, the solenoid repels the battery. As
like poles repel each other, we can infer that the end
connected to the negative terminal behaves as a
north pole while the end connected to the positive
terminal behaves as a south pole.
9. When is the force
experienced by a current–carrying conductor placed in a magnetic field largest?
When the direction of the current is perpendicular
to the direction of the magnetic field is when the force experienced is the
largest.
10. Imagine that you are
sitting in a chamber with your back to one wall. An electron beam, moving horizontally
from back wall towards the front wall, is deflected by a strong magnetic field
to your right side. What is the direction of magnetic field?
The direction of the magnetic field can be
determined using the Fleming’s Left hand rule. The direction of the magnetic
field will be perpendicular to the direction of current and the direction of
deflection, i.e., either upward or downward. The direction of the current is
from the front wall to the back wall because negatively charged electrons move
from the back wall to the front wall. The directed of the magnetic force is
rightward. Hence, using Fleming’s left hand rule it can be concluded that the
direction of the magnetic field inside the chamber is downward.
11. Draw a labelled diagram
of an electric motor. Explain its principle and working. What is the function
of a split ring in an electric motor?
An electric motor is a device that converts
electrical energy to mechanical energy. It works on the principle of magnetic
effect of current. The figure listed below shows a simple electric motor.
When current is made to flow through the coil
MNST by closing the switch, the coil starts to rotate in the anticlockwise
direction. This is due to the downward force acting on the length MN and
simultaneously an upward force acting along the length ST. As a result of which
the coil rotates in the anticlockwise
direction. Current in the length MN flows from M
to N and the magnetic fields act from left to right normal to the length MN.
According to Fleming’s Left Hand rule, a downward force acts along the length
MN. Similarly, the current along the length ST
flows from S to T and the magnetic field acts from left to right. Therefore, an
upward force acts along the length ST. These two forces together cause the coil
to rotate anti-clockwise. After half a rotation, the position of MN and ST
interchange. The half ring C come
in contact with brush B and the half ring D
comes in contact with rush C. Hence the direction of current in the coil MNST
gets reversed.
12. Name some devices in
which electric motors are used.
A few devices in which electric motors are used
are:
1.Electric fans
2.Water pumps
3.Mixers
4.Washing machines
13. A coil of insulated
copper wire is connected to a galvanometer. What will happen if a bar magnet is
(i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held
stationary inside the coil?
(i) When
a bar magnet is pushed into the coil, current is induced in the coil
momentarily as a result the
galvanometer deflects in a particular direction
momentarily.
(ii) When the bar magnet is withdrawn from
inside the coil, current is induced momentarily but in the
opposite direction and the galvanometer deflects
in the opposite direction momentarily.
(iii) When the bar magnet is held stationary
inside the coil, no current will be induced as a result there
will be no deflection in the galvanometer.
14. Two circular coils A
and B are placed closed to each other. If the current in the coil A is changed,
will some current be induced in the coil B? Give reason.
When the current in coil A changes, the magnetic
field associated with it also changes. As a result the magnetic field around
coil B undergoes change. The change in the magnetic field of coil B induces current
in it.
15. State the rule to
determine the direction of a (i) magnetic field produced around a straight conductor-carrying
current, (ii) force experienced by a current-carrying straight conductor placed
in a magnetic field which is perpendicular to it, and (iii) current induced in
a coil due to its rotation in a
magnetic field.
(i) The
rule used to determine the direction of the magnetic field produced around a
straight conductor carrying
current is the Maxwell’s right hand thumb rule.
(ii) The rule used to determine the force
experienced by a current-carrying straight conductor placed in a
magnetic field which is perpendicular to it is
the Fleming’s left hand rule.
(iii) The rule used to determine the current
induced in a coil due to its rotation in a magnetic field is the
Fleming’s right-hand rule.
16. Explain the underlying
principle and working of an electric generator by drawing a labelled diagram.
What is the function of brushes?
The electric generator coverts the mechanical
energy into the electrical energy. The working principle of the electric
generator is the electromagnetic induction. It generates electricity by
rotating a coil in the magnetic field. The figure below shows the construction
of a simple AC generator.
In the diagram,
A and B are brushes,
C and D are slip rings
X is the axle
G is the galvanometer
When the axle X is rotated clockwise, MN moves
upwards while ST moves downward. The movement of MN and ST in the magnetic
field results in the production of electric current due to electromagnetic induction.
MN moves upwards and the magnetic fields act from left to right. Therefore,
according to
Fleming’s right hand rule, the direction of the
induced current will be from M to N along the length MN.
Similarly, the direction of the induced current
will be from S to T along the length ST. The direction of the current in the
coil is MNST. Hence, galvanometer shows a deflection in a particular direction.
After half a rotation, length MN starts moving
downwards while the length ST starts moving upwards. Now, the direction of the
induced current reverses to TSNM. Since the direction of the induced current reverses
every half rotation, the current induced is known as alternating current.
Function of Brushes
Brushes are kept pressed on to two slip rings
separately. Outer ends of brushes are connected to the
galvanometer. Thus, brushes help in
transferring current from coil to the external circuit.
17. When does an electric short circuit
occur?
Listed below are two instances of when a
short-circuit can occur:
1) When too many appliances are connected to a
single socket or when high power rating appliances are connected to a light
circuit, the resistance of the circuit becomes low as a result the current
flowing through the circuit becomes very high. This condition results in a
short-circuit.
2) When live wires whose insulation have worn
off come in contact with each other, the current flowing in the circuit
increases abruptly which results in a short circuit.
18. What is the function of
an earth wire? Why is it necessary to earth metallic appliances?
The metallic body of electric appliances is
earthed by means of earth wire. Any leakage of electric wire is transferred to
the ground by means of earth wire. This prevents the user of the electric
appliance from getting electric shocks. This is the reason why it is important
for the metallic appliances to be earthed
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