NCERT solution of class 10 chapter magnetic effect of electric current
Page
no. 224
Question
1:
Why
does a compass needle get deflected when brought near a bar magnet?
Answer 1:
Magnetic
compass needle and bar magnet both have magnetic field. When they brought near
to each other, these magnetic fields interact each other giving deflection in
needle.
Page
no. 228
Question
1:
Draw
magnetic field lines around a bar magnet.
Answer
1:
Question
2:
List
the properties of magnetic lines of force.
Answer
2:
Properties
of magnetic lines of force (also known as magnetic field lines) are
listed
below:
Outside the magnet, the magnetic field lines are directed from N-pole of
magnet
towards S-pole. However, inside a magnet the field lines are
directed
from S-pole to N-pole. Thus magnetic field lines form a close
loop.
The magnetic field line at any point points in the direction of magnetic field
at
that point.
The relative strength of magnetic fields is given by degree of closeness of
the
field lines. The magnetic field is strong in the region where the field
lines
are crowded.
No two magnetic field lines can ever intersect with each other.
Question
3:
Why
don’t two magnetic lines of force intersect each other?
Answer
3:
No
two field-lines are found to cross each other. If they did, it would mean that
at
the point of intersection, the compass needle would point towards two
directions,
which is not possible.
Page
no. 229-230
Question
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.
Answer 1:
The
magnetic field lines have been shown in Figure given below. As per right-hand
rule, we find that inside the loop, the magnetic field lines are directed
perpendicular to the plane of paper in the inward direction. Outside the loop
magnetic field lines are directed out of the plane of paper.
Question 2:
The
magnetic field in a given region is uniform. Draw a diagram to represent it.
Answer 2:
The
uniform magnetic field is represented by parallel, equidistant lines of equal
length as shown in Figure
Question 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.
Answer 3:
(d)
is the same at all points.
Page
no. 231-232
Question
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
Answer 1:
(c)
Velocity and (d) Momentum
Question 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?
Answer 2:
(i)
If current in rod AB is increased, the displacement will also increase.
(ii)
If we use a stronger horse-shoe magnet then the displacement of rod AB will
increase.
(iii)
lf length of the rod is increased, force acting on it will increase and, hence,
displacement of the rod increases.
Question 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
Answer 3:
(d)
upward
In
accordance with Fleming's left-hand rule, the direction of magnetic field is
vertically upward.
Page
no. 233
Question
1:
State
Fleming’s left-hand rule.
Answer 1:
According
to Fleming’s left-hand rule, stretch the thumb, forefinger and middle finger of
your left hand such that they are mutually perpendicular. If the first finger
points in the direction of magnetic field and the second finger in the
direction of current, then the thumb will point in the direction of motion or
the force acting on the conductor.
Question 2:
What
is the principle of an electric motor?
Answer 2:
The
working principle of an electric motor is based on the magnetic effect of
current. A current-carrying loop experiences a force and rotates when placed in
a magnetic field. The direction of rotation of the loop is given by the
Fleming’s left-hand rule.
Question 3:
What
is the role of the split ring in an electric motor?
Answer 3:
The
split ring in the electric motor acts as a commutator. The commutator reverses
the direction of current flowing through the coil after each half rotation of
the coil. Due to this reversal of the current, the coil continues to rotate in
the same direction.
Page
no. 236
Question
1:
Explain
different ways to induce current in a coil.
Answer 1:
Different
ways to induce current in a coil are as given below:
If a magnetic field is changed around a coil then an induced current is set up
in the coil. It can be done by taking a bar magnet and bringing it closer to
the coil or taking it away from the coil.
If a coil is moved in a magnetic field, then again an induced current is set up
in the coil.
If a coil is rotated in a uniform magnetic field, it may also produce an
induced current in the coil.
Page
no. 237
Question
1:
State
the principle of an electric generator.
Answer 1:
An
electric generator is based on the principle of electromagnetic induction. When
a rectangular coil is rotated in a uniform magnetic field, an induced voltage
is generated between the ends of the coil.
Question 2:
Name
some sources of direct current.
Answer 2:
Some
sources of direct current are a cell, a battery and a D.C. generator.
Question 3:
Which
sources produce alternating current?
Answer 3:
A.C.
generator and invertors (used in house for emergency power supply) produces
alternating current.
Question 4:
Choose
the correct option.
A
rectangular coil of copper wires is rotated in a magnetic field. The direction
of the induced current changes once in each
(a)
two revolutions (b) one revolution
(c)
half revolution (d) one-fourth revolution
Answer 4:
(c).
When a rectangular coil of copper wire is rotated in a magnetic field, the
direction of the induced current changes once in each half revolution.
Page
no. 238
Question
1:
Name
two safety measures commonly used in electric circuits and appliances.
Answer 1:
Two
safety measures are:
Use of earth wire and proper earthing.
Use of fuse (now a days fuse wire is replaced by MCB).
Question 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.
Answer 2:
Power
rating of electric oven P = 2 kW = 2000 W
Supply
voltage V = 220 V
So,
the current drawn by electric oven 𝐼=𝑃𝑉=2000
𝑊220 𝑉=9 𝐴
As
the current rating of domestic electric circuit is only 5 A and the oven draws
a current 9 A, which is more than the current rating, hence the circuit will be
damaged due to overheating/overloading.
Question 3:
What
precaution should be taken to avoid the overloading of domestic electric
circuits?
Answer 3:
The
precautions that should be taken to avoid the overloading of domestic circuits
are as follows:
Too many appliances
should not be connected to a single socket.
Too many appliances should not be used at the same time.
Faulty appliances should not be connected in the circuit.
Fuse should be connected in the circuit.
Exercise
Question
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 centred on the wire.
Answer 1:
(d)
The field consists of concentric circles centred on the wire.
On
applying right-hand thumb rule, we find the direction of magnetic field. The
field is in the form of concentric circles centred on the wire carrying
current.
Hence,
the option (d) is correct.
Question 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.
Answer 2:
(c)
producing induced current in a coil due to relative motion between a magnet and
the coil.
In
electromagnetic induction phenomenon an induced current begins to flow in a
coil whenever there is change in magnetic field in and around a coil.
Hence,
the option (c) is correct.
Question 3:
The
device used for producing electric current is called a
(a)
generator.
(b)
galvanometer.
(c)
ammeter.
(d)
motor
Answer 3:
(a)
generator.
An
A.C. generator is the device used for producing an electric current.
Hence,
the option (a) is correct.
Question 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
Answer 4:
(d)
AC generator has slip rings while the DC generator has a commutator
Question 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.
Answer 5:
(c)
increases heavily.
At
the time of short circuiting the live wire and the neutral wire come into
direct contact. As a result, the current in the circuit abruptly increases.
Hence,
the option (c) is correct.
Question 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 centre 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.
Answer 6:
(a)
An electric motor converts mechanical energy into electrical energy. (False)
(b)
An electric generator works on the principle of electromagnetic induction. (True)
(c)
The field at the centre of a long circular coil carrying current will be
parallel straight lines. (True)
(d)
A wire with a green insulation is usually the live wire of an electric supply.
(False)
Question 7:
List
three sources of magnetic fields.
Answer 7:
Three
methods of producing magnetic field are as follows:
Magnetic field can be produced by placing a permanent bar magnet or a
horse-shoe magnet at the place, where magnetic field is required.
Magnetic field is produced around a current-carrying straight conductor or a
current carrying circular coil.
A very good method to produce magnetic field is due to flow of current in a
solenoid.
Question 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.
answer
When
current is passed through a solenoid coil, magnetic field produced due to each
turn of solenoid coil is in the same direction. As a resu1t, the resultant
magnetic field become very strong and uniform. The field lines inside the
solenoid are in the form of parallel straight lines along the axis of solenoid.
Thus, the solenoid behaves like a bar magnet. One end of solenoid behaves as a magnetic
North pole while the other end behaves as the South Pole.
We
can determine the magnetic poles formed in a solenoid. The end of the
current-carrying solenoid, which attracts North Pole but repels South Pole of a
bar magnet, is behaving as south magnetic pole. The other end, which attracts
South Pole of a bar magnet but repels the North Pole, is behaving as north
magnetic pole. It is because like poles repel but unlike poles attract each
other.
Question 9:
When
is the force experienced by a current–carrying conductor placed in a magnetic
field largest?
Answer 9:
The
force experienced by a current-carrying conductor placed in a magnetic field is
largest when the current-carrying conductor is placed in a direction
perpendicular to that of magnetic field.
Question 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?
Answer 10:
An
electron beam moving horizontally from back wall towards the front wail is
equivalent to a current flowing in the opposite direction (i.e., from front
wall towards the back wall). The deflection of electron beam as seen by
observer is to his right side and is shown in Figure. On applying Fleming's
left-hand rule we find that the magnetic field is acting in vertically downward
direction.
Question 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?
Answer 11:
Principle
A
current-carrying conductor, when placed in a magnetic field, experiences a
force. If the direction of the field and that of the current are mutually
perpendicular then force acting on the conductor will be perpendicular to both
and will be given by Fleming's left-hand rule. Due to this force the conductor
begins to move.
Working
Current
in the coil ABCD enters from the source battery through conducting brush X and
flows back to the battery through brush Y. The current in arm AB of the coil
flows from A to B. In arm CD it flows from C to D, that is, opposite to the
direction of current through arm AB. We find that the force acting on arm AB
pushes it downwards while the force acting on arm CD pushes it upwards. Thus
the coil and the axle O, mounted free to turn about an axis, rotate anti-clockwise.
At half rotation, Q makes contact with the brush X and P with brush Y.
Therefore the current in the coil gets reversed and flows along the path DCBA.
The reversal of current also reverses the direction of force acting on the two
arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now
pushed up and the arm CD previously pushed up is now pushed down. Therefore the
coil and the axle rotate half a turn more in the same direction. The reversing
of the current is repeated at each half rotation, giving rise to a continuous
rotation of the coil and to the axle.
Question 12:
Name
some devices in which electric motors are used.
Answer 12:
Electric
motors are used in all such devices where we want to convert electrical energy
into mechanical energy so as to drive that machine. In our houses, electric
motors are being fitted in electric fans, coolers, air conditioners, mixer
grinders, washing machines, refrigerators, juicers, computers etc. In
factories, motors are used in almost all machines.
Question 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?
Answer 13:
A
current induces in a solenoid if a bar magnet is moved relative to it. This is
the principle of electromagnetic induction.
(i)
When a bar magnet is pushed into a coil of insulated copper wire, a current is
induced momentarily in the coil. As a result, the needle of the galvanometer
deflects momentarily in a particular direction.
(ii)
When the bar magnet is withdrawn from inside the coil of the insulated copper
wire, a current is again induced momentarily in the coil in the opposite
direction. As a result, the needle of the galvanometer deflects momentarily in
the opposite direction.
(iii)
When a bar magnet is held stationary inside the coil, no current will be
induced in the coil. Hence, galvanometer will show no deflection.
Question 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.
Answer 14:
Yes,
a current is induced in the coil B.
When
the current in the coil A is changed, the magnetic field associated with it
also changes. As coil B is placed close to A, hence magnetic field lines around
this coil also change. Due to change in magnetic field lines associated with
coil B, an induced current is also induced in it.
Question 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.
Answer 15:
(i)
Maxwell’s right hand thumb rule
(ii)
Fleming’s left hand rule
(iii)
Fleming’s right hand rule
Question 16:
Explain
the underlying principle and working of an electric generator by drawing a
labelled diagram. What is the function of brushes?
Answer 16:
Principle
An
electric generator works on the principle of electromagnetic induction
phenomenon. According to it, whenever a coil is rotated between the poles of a
magnet, an induced current is set up in the coil, whose direction is given by
Fleming's right-hand rule.
Working
When
the axle attached to the two rings is rotated such that the arm AB moves up
(and the arm CD moves down) in the magnetic field produced by the permanent
magnet. The coil ABCD is rotated clockwise in the arrangement. By applying
Fleming’s right-hand rule, the induced currents are set up in these arms along
the directions AB and CD. Thus an induced current flows in the direction ABCD.
If there are larger numbers of turns in the coil, the current generated in each
turn adds up to give a large current through the coil. This means that the
current in the external circuit flows from B2 to B1. After half a rotation, arm
CD starts moving up and AB moving down. As a result, the directions of the
induced currents in both the arms change, giving rise to the net induced
current in the direction DCBA. The current in the external circuit now flows
from B1 to B2. Thus after every half rotation the polarity of the current in
the respective arms changes. Such a current, which changes direction after
equal intervals of time, is called an alternating current (abbreviated as AC). This
device is called an AC generator.
Question 17:
When
does an electric short circuit occur?
Answer 17:
If
either the insulation of wires used in an electrical circuit is damaged or
there is a fault in the appliance, live wire and neutral wire may come in
direct contact. As a result, the current in the circuit abruptly rises and
short-circuiting occurs.
Question 18:
What
is the function of an earth wire? Why is it necessary to earth metallic
appliances?
Answer 18:
The
metallic body of electric appliances is connected to the earth by means of
earth wire so that any leakage of electric current is transferred to the
ground. This prevents any electric shock to the user. That is why earthing of
the electrical appliances is necessary.
