Answer Section 4.1
1.A
Magnetic field lines will be from N to S as shown so the needle will point to the right.
(Diagram Labels: position of compass, bar magnet, N, S)
2. B
The reading increases because X is exerting more force. This is possible if it is being repelled by Y and being
pushed away (downwards). X and Y will repel each other if they are both magnets. Iron is a soft magnet and
magnets made of iron will attract, not repel.
3. B
1 and 2 will demagnetize it. For 3 to magnetize a magnet an alternating current instead of a direct current must
be used.
4. C
The steel magnetizes and forms South pole at X. B is wrong as copper is not magnetic and D is wrong since
opposite poles attract.
5. D
Magnetic induction occurs when magnetism is induced in a material due to the presence of another magnetic
material in its proximity. Since there is no potential difference induced it can't be electromagnetic or
electrostatic.
6. D
Like poles always repel each other and that is why option D is correct because it shows 2 North poles face to
face. The magnetism induced in iron will always result in an attractive force because induced magnetism always
causes a force of attraction. Cooper is non-magnetic so it won't experience anything near a magnet.
7. D
Both the coil and cone vibrate as they are connected to each other. Variations in an electric current cause
variations in the magnetic field produced by an electromagnet. This causes the coil and cone to move, which
creates pressure variations in the air and forms sound waves.
8. B
Magnetic lines always are from the North to the South pole. Magnetic field lines do not directly connect two
poles if there is repulsion between them and hence the poles cannot be the same because like poles always repel.
9. B
The rod M1 and M3 have a North pole at W since the induced pole is always opposite to the actual pole.
(Induced magnetism always causes a force of attraction). Since the rod M1 has a North pole at W and rod M3
has a North pole at X this means that the other end of rod M1 and M3 will be South poles. These South poles
will induce North poles in the iron ball Y.
10. B
End x of bar 1 is attracted to both the North and South poles and this means that it was not permanently
magnetized because induced magnetism always causes a force of attraction. End Y of bar 2 is attracted by the
North pole but repelled by the South pole of the magnet, this means that it must be magnetized itself because
only the like poles of two magnetized materials can repel each other. Since iron is a soft magnetic material it
must be bar 1 and since Steel is a hard magnetic material, it gets permanently magnetized and must be Bar 2.
11. C
Using Right hand rule with thumb in direction of current it can be seen that the direction of magnetic field lines
will be toward the left at the center.
12.D
The motor carries a greater current so to reduce resistance shorter and thicker wires are used. Resistance is
inversely proportional to area and directly proportional to length. Since the ignition switch carries smaller
current, it experiences lesser resistance and has longer, thinner wires.
13. B
Soft magnetic materials become demagnetized when the magnet is removed from them.
Option A is incorrect because since the paper clips are attracted to the South pole, they must have a North pole
induced in them at the top (Opposite poles attract). We cannot tell how many paper clips can be attached to the
magnet as we do not know about its magnetic strength.
14. B
Magnetic field lines always move outward from the North pole and since the poles are both North poles there
will be repulsion between them and a region between them where the magnetic field strength is zero. This
corresponds to diagram B.
15. A
When battery P is stopped the iron core demagnetized and the armature is no longer attracted to it hence it falls
completing the circuit for the bell which starts ringing.
16. ANS: C
Using Right hand rule with thumb in direction of current, the direction of magnetic field lines will be clockwise.
17. D
A soft iron core is used to concentrate magnetic field lines through it and hence strengthen the magnetic field. It
has nothing to do with electric current.
18. B
Magnetic field lines always are from the North to the South pole.
19. ANS: C
Magnetic field lines always are from the North to the South pole.
20. A
Using Right hand rule with thumb in direction of current it can be seen that the direction of magnetic field lines
will be clockwise. Since the electric current is increasing the strength of magnetic field around the wire will also
increase, this corresponds to an increase in magnetic flux density and the magnetic field lines will get closer
together.
21. D
Only repulsion can give correct information about the magnetization of a material. Only likes poles of two
magnetized material can repel each other.
22. C
Magnetic lines always are from the North to the South pole. At A and D, the magnetic field lines will be
pointing to the left and at B they will be pointing to the right.
23. C
Iron is a magnetic material and only magnetic materials can be attracted to other magnets.
24. A
The direction of magnetic field lines is defined to be the direction in which the north pole of a compass needle
points.
25. C
The core of an electromagnet should be a soft magnetic material. Such materials are easy to magnetize and
demagnetize.
26. A
When the current is increasing the magnetic field strength will also increase at every point.
27. B
End P of rod is attracted to both the North and South poles and this means that it was not permanently
magnetized because induced magnetism always causes a force of attraction. Iron is a soft magnetic material
while steel gets permanently magnetized. Since the end P is attracted to the North pole, it must have a South
pole induced in it. (Opposite poles attract).
28. A
Since the magnetized bar settles in a particular direction by aligning itself with the magnetic field of the Earth, it
means that any other bar of the SAME material will also be able to be magnetized and because induced
magnetism always causes a force of attraction it will experience an attractive force from both North and South
poles of the PQ bar.
29. D
Steel is a hard magnetic material it gets permanently magnetized.
30. C
Magnetic field lines always are from the North to the South pole. Magnetic field lines do not directly connect
two poles if there is repulsion between them and between two like poles there is a region where the magnetic
field strength is zero and hence the poles cannot be the same because likes poles always repel.