Physics

NTSE Foundation
Magnetism
(iv) Like poles of the magnets repel each other while
Magnetism
unlike poles attract each other.
A mineral was discovered in the town of magnesia
(v) Magnetic poles always exist in pair.
which was found to have a wondrous property. It
(vi) Repulsion is the surest5 test of magnetism. Since
could attract pieces of iron towards it. This mineral is
a magnet can attract magnetic substances and magnet
called magnetite. Further it was found that thin strips
also but it will always repel the magnet only.
of magnetite always align themselves in a particular
direction when suspended freely in air. It was found
(b) Types of Magnet:
that magnetite is mainly composed of oxides of iron
(i) Natural magnet:
(Fe3O4). Magnetite (Fe3O4) is the world’s first
A magnet which occurs naturally and is not made by
magnet. It is also called natural magnet.
any artificial means is called a natural magnet.
Eg. : Magnetite, which is an ore of iron [Fe3O4].
(a) Properties of a Bar Magnet:
(ii) Artificial magnet:
(i) It attracts small pieces of iron towards itself.
A substance to which properties of the natural magnet
are imparted by artificial means is called artificial
Activity:
magnet.
Spread out some iron fillings over a sheet of paper.
Eg. : The magnets made from iron, steel, cobalt and
Now, move a bar magnet in the fillings taking care
nictel.
that all parts of the magnet moves through iron
fillings and observe how the iron fillings are
Need of artificial magnet:
distributed all over the magnet.
Magnets are used in number of devices such as
telephone, loudspeaker, radio, television, etc. For
We will notice that most of the iron fillings cling near
making these devices, we need powerful magnets.
the ends of the magnet while there are a few iron
(i) Artificial magnets can be made very powerful,
fillings near the middle.
which is not possible in case of natural magnets.
(ii) Artificial magnets can be made in any desired
shape and size. This is not possible in case of natural
magnets which bread when shaped with cutting tools.

(c) Demagnetization of Magnet:

A magnet can be made to lose its magnetism, i.e. it
can be demagnetized, if the molecular magnets are
disturbed so that they no longer point in the same
direct5ion. This can be done by hammering the
magnet. Heating the magnet above a certain
temperature also demagnetizes it.

Types of Substances

(a) Magnetic Substances:

Those substances that are attracted by magnets are
called magnetic substances.
Eg. : Iron, cobalt, nickel, steel etc.
(b) Non-Magnetic Substances:
Repeat the experiment with a horseshoe magnet. This Those substances that are not attracted by magnets
activity explains the attractive property of magnets. are called non-magnetic substances.
Eg. : Plastic, rubber, glass etc.
(ii) the magnetic pull seems to come from two points
near the ends. These preferred regions of attraction Temporary and Permanent Magnets
are called the magnetic poles. We will find that all (a) Temporary magnets:
magnets have maximum attractive power at their The magnets which cannot retain their magnetism for
poles. Poles of a magnet remain slightly inside from a long time are called temporary magnets. The
the end points. temporary magnets are made from the soft iron.
Eg. : Electromagnet:
(iii) A freely suspended magnet always align itself (b) Permanent magnets:
along the north-south direction. The magnets which retain their magnetism for a very
The end of the magnet that points towards the north is long time are called permanent magnets. The
called the north pole (N-pole) end the other end of the permanent magnets are generally made from steel.
magnet pointing towards the south is called the south- More powerful permanent magnets are made from
pole (D-pole). ALMICO, an alloy of aluminium, nickel and cobalt
 or from ferrite. The ferrite made permanent magnets
are quite strong.

Magnetic Field and Magnetic Lines of Force

The region or space around a bar magnet where its

magnetic effect can be felt is called its magnetic field.

 S.I. unit of strength of magnetic field is Tesla (T).

 Its C.G.S. unit is Gauss (G). 1G = 10-4 T
 Magnetic material like iron fillings or a compass
placed within the magnetic field of a magnet will
experience a force.
 The direction of the force experienced by a freely
suspended compass needle placed in a magnetic field
are given by curves called magnetic lines of force. Electromagnet
 The direction of the magnetic lines of force is from
the north pole of the magnet to the South Pole. When an electric current flows through a coil of wire
 A magnetic compass needle may also be used to plot (solenoid). The coil behaves like a magnet.
the magnetic lines of force.  When this current carrying coil is brought near a
 The magnetic lines of force indicate the direction of suspended bar magnet, one side of the coil repels the
the magnetic force at each point. north pole of the magnet. The other side of the coil
 The lines of force of a magnet never intersect each attracts the north pole of the magnet.
other. This is because at each point in the magnetic
field of a magnet, the force of the magnet acts in only  A current carrying coil has both north and south poles
one unique direction. like a magnet. Such a magnet is called electromagnet.
 The direction of magnetic field at any point on the
lines of force is that of the tangent drawn at that Uses of Electromagnet:
point. (i) They are used in cranes to lift heavy loads of scrap
iron and iron sheets.
(ii) They are also used to separate magnetic
substances, like iron, nickel and cobalt from non-
magnetic substances like copper, zinc. Brass, plastic
and paper.
(iii) They are used to remove foreign bodies like iron
fillings from a patient’s body, particularly from the
eyes.
(iv) They are also used in electric bells, telegraphs,
telephones, speakers, audio and video tape recorders.

Application of Electromagnet:

Electric – bell:
When the switch of an electric bell is pressed, the
circuit is completed and current flows through the
Earth’s Magnetism coils of the electromagnet. It gets magnetized and
attracts the iron strip towards itself. This brings the
Sir William Gilbert was the first to put forward the
hammer in contact with the gong and the sound of the
idea that the earth behaves like a huge magnet. The
bell is heard. Due to the displacement of the iron strip
magnetic field of the earth is weak but can cause a
from its original position, it loses contact with the
freely suspended magnet to always point to the N-S
contact screw due to which the circuit of the
direction.
electromagnet breaks. Current stops flowing and the
 Towards the geographic north pole of the earth, lies electromagnet loses its magnetism. Hence, the iron
the magnetic south pole of earth. Similarly, towards strip comes back to its original position, which once
the geographic south pole of the earth, lies magnetic again completes the circuit. The intire process is
north pole of the earth. repeated as long as the switch is kept pressed. The
 Human beings cannot feel the effect of the earth’s hammer would strike the gong again and the again
magnetic field, but birds and animals can. Earth’s and the ringing of the bell would be heard.
magnetic field, also helps birds to find their way
when they migrate from one place to another.
3
 generator avoid sparkings and increases the
efficiency.
(iv) The AC voltage can be easily varied with the
help of a transformer which is a device for changing
alternating voltages. AC voltage can be easily stepped
up or down as per requirement.
(v) AC can be easily converted into DC (if needed)
by means of a rectifier.

 Disadvantages of AC:
(i) It is more dangerous to work with AC than DC.
(ii) Several chemical processes and effects such as
hydrolysis, electrolysis, electroplating, electrorefining
etc., are not at all possible with AC.
(iii) AC passes only through the outer layers of the
conductor, unlike DC which passes through whole
bulk of the conductor. Hence, several fine insulated
wires (and not a single thick wire) are required for
transmission of AC.
Electro magnetic Induction
Types of Magnetic Substances
An electric current can be produced in a wire or in a
coil by simply moving a magnet in and out of it. No (a) Ferromagnetic Substances:
battery or voltage source is needed. The same effect The substances which can be magnetized to greater
is observed if a part of wire loop is moved through extent are called ferromagnetic substances. A
the magnetic field of a magnet. ferromagnetic substance sets parallel to the direction
of magnetic field it suspended freely.
Electric Generators
 Iron, cobalt and nickel are the examples of
ferromagnetic substances.
Electric generator is a device for converting
mechanical energy into electrical energy. It is also (b) Paramagnetic Substances:
called dynamo. Generator is one of the most The substances which are feebly magnetized are
important application of the phenomenon of called paramagnetic substances. A paramagnetic
electromagnetic induction. There are two types of substance sets perpendicular to the direction of
generators i.e. AC generator and DC generator. magnetic field if suspended freely.
 The device which produces alternating current is
 Manganese, platinum and chromium are examples
known as an AC generator or alternator. An of paramagnetic substances.
alternating current (AC) is defined as a current which (c) Diamagnetic Substances:
continuously changes in amnitude and periodically The substances which are repelled by magnets are
reverses in direction. called diamagnetic substances. A diamagnetic
substance wets perpendicular to the direction of
 It is a modification of an AC generator that produces magnetic field if suspended freely.
a direct current (DC). A current of constant
magnitude and flowing in the same direction is called  Antimony, bismuth, silver are examples of
direct current. diamagnetic substances.

 NOTE: the word ‘generator’ is a misnomer. i.e. a

Magnetic Field due to straight Current Carrying Wire
wrong name has been given to the device, because
nothing is generated by the device. It is just a When current flows in the wire a magnetic field
converter which converts one form (mechanical around the wire is created whose direction depends
energy) of energy into another (electrical energy). on the direction of current and intensity depends on
the amount of the current
 Advantages of AC over DC: o I
More than 90% of electric power generated in the Magnetic field strength, B 
world, is in the form of alternating current and power 2r
generated in the form of DC is less than 10%. In India µo = Permeability of vacuum (µo = 10-7 Tm/A)
AC changes its direction after every 1/100 of second I = Current (flowing in conductor) and
i.e. the frequency of AC is 50 Hz. The advantages of R = Distance from the conductor (where magnetic
AC over DC are a follows: field is measured).
(i) AC can be transmitted to distant places with very
small loss in AC power. (ii) Direction of magnetic field:
(ii) AC generator is cheaper than DC generator. The direction of magnetic field (lines of force)
(iii) AC generators are strong and do not require produced due to flow of current can be known by the
much attention. The absence of commutators in AC following rules:

4
 Magnetic Field due to a Solenoid Carrying Current
(A) Maxwell’s cork screw rule:
Imaging a right handed cork screw lying with its axis If a conducting wire is wounded in the form of a
coincides with the current carrying wire it is now cylindrical coil whose diameter is less in comparison
rotated such that it advances in the direction of the to the length, then this coil is called a solenoid (it
current, the direction in which the screw rotates gives looks like a helical spring).
the direction of the magnetic lines of force. The magnetic field lines in a solenoid, through which
current is passed, are as shown in figure.

(B) Right hand thumb rule: The magnetic field, thus produced, is very much
The direction of the magnetic field at a point P due to similar to that of a bar magnet and one end of the coil
a straight wire can be found by a slight variation in
acts like a magnetic north pole, while the other acts
the right hand thumb rule. If we stretch the thumb of
the right hand along the current and curl our fingers like a south pole.
to pass through the point P, the direction of the  The lines of force inside the solenoid are nearly
fingers at P gives the direction of the magnetic field straight and parallel to the axis of the solenoid.
there.
 A strong magnetic field can be obtained by increasing
the current strength.

 The magnetic field is increased if the number of turns

in the solenoid of given length is increased.

 The magnetic field is also increased if a soft iron core

is kept along the axis of the solenoid.
(C) Ampere’s swimming rule:
Imagine a man swimming along the wire in the  Thus a current carrying solenoid behaves like a bar
direction of current (such that the current enters at his magnet with fixed polarities at its ends.
feet and leaves him at his head) facing towards a
magnetic needle kept undemeath the wire, then the
magnetic field produced is such that the north pole of
the needle will be deflected towards his left hand.

The strength of magnetic field produced by a current

carrying solenoid depends upon :

(i) The number of turns per unit length in the

solenoid:
Larger the current passed through solenoid, greater
will be the magnetic field produced.

(ii) The strength of current in the solenoid:

Larger the current passed through solenoid, stronger
will be the magnetic field produced.

5
 5. The S.I. unit of magnetic field intensity is :
(iii) The nature of “core material” used in making (A) Weber (B) Tesla
solenoid: (C) Oersted (D) none of these
The use of soft iron rod as core in a solenoid produces
the strongest magnet. 6. C.G.S. unit of magnetic field intensity is :
(A) Tesla (B) Gauss
Magnetic field inside the solenoid is : (C) Weber (D) none of these
B = µo n I
[Here n is number of turns per unit length] 7. Alnico is a material which is used to make :
At the ends of the solenoid the magnetic field: (A) temporary magnet (B) permanent magnet
1 (C) both (A) and (B) (D) none of these
Bend   o nI
2 8. The chemical formula of magnetite (natural magnet)
The polarity of the faces of the coil depends on the is:
direction of current and is determined by the clock (A) Fe2O3 (B) Fe3O4
rule. Looking at the face of the coil, if the current (C) ReO (D) ReO2
around that face is in an anticlockwise direction, the
face has north polarity, while if the current at that 9. The south pole of the earth’s magnetic field points to
face is in the clockwise direction, the face has south the:
polarity. This can be tested by using a compass (A) geographic North (B) geographic South
needle. (C) both (A) and (B) are correct
(D) none of these

10. When current is flowing in straight conductor, the

associated magnetic lines of force are :
\\ (A) straight (B) elliptical
(C) circular (D) parabolic

11. A magnetic field can be produced by:

(a) (b) (A) a moving charge
(B) A changing electric field
(C) Both (A) and (B) (D) None of these
EXERCISE
1. The magnetic lines of force: 12. Which one of these is temporary magnet?
(A) intersect at the neutral point (A) Bar magnet (B) Electromagnet
(B) intersect near north and south poles. (C) U shaped magnet (D) Horse-shoe magnet
(C) cannot intersect at all
(D) depend upon the position of the magnet
13. A generator or dynamo works on the principle of:
2. In an electric bell when iron strip displaces from its (A) magnetic effect of electric current
original position, then : (B) electromagnetic induction
(A) current will stop flowing (C) chemical effect of electric current
(B) electromagnet looses its magnetism (D) heating effect of electric current
(C) both A and B
(D) none of these
14. The magnetic field lines due to a bar magnet are
3. Magnetic field is a : correctly shown in :
(A) vector quantity
(B) scalar quantity
(C) scalar as well as vector quantity
(A) (B)
(D) neither vector nor scalar

4. Magnetic lines of force determines:

(A) the shape of the magnetic field
(B) only the direction the magnetic field
(C) only the relative strength of the magnetic field
(C) (D)
(D) both the direction and the relative strength of the
magnetic field

6
15. Magnetic effect of current was discovered by: (D) none of these
(A) Faraday (B) Oersted
(C) Ampere (D) Bohr 24. If a bar magnet is cut lengthwise into 3 parts, the total
number of poles will be :
16. In an electric motor, conversion takes place of : (A) 2 (B) 6
(A) Chemical energy into electrical energy (C) 3 (D) 4
(B) Electrical energy into mechanical energy
(C) Electrical energy into light 25. A soft iron bar is introduced inside a current carrying
(D) electrical energy into chemical energy solenoid. The magnetic field inside the solenoid:
(A) will become zero (B) will decrease
17. The vertical plane which passes through the magnetic (C) will increase (D) will remain unaffected
axis of a freely suspended magnet is :
(A) Magnetic meridian 26. For making a strong electromagnet, the material of
(B) Geographical meridian the core should be:
(C) North meridian (A) soft iron (B) steel
(D) South meridian (C) brass (D) laminated steel strips

18. The similar magnets of steel are …………than the 27. The permanent magnet are kept with soft iron pieces
magnets of soft iron. at ends an keepers :
(A) to magnetise the soft iron pieces
(A) stronger (B) of equal strength (B) to increase the strength of the magnetic
(C) weaker (D) none of these (C) to avoid self demagnetization
(D) for physical safety of the magnets
19. the magnetism in a magnet is mainly due to :
(A) The orbital motion of the electrons 28. Value of tesla in gauss is :
(A) 103 (B) 106
(B) The spin motion of the electrons (C) 104 (D) 102
(C) The nuclear charge
(D) None of these 29. The earth’s magnetic field is maximum:
(A) at poles of the earth
20. A magnet can be demagnetized by : (B) at centre of the earth
(C) at outer surface of the earth
(A) Hammering the magnet (D) none of these
(B) By heating the magnet
(C) Without use of keepers 30. The magnetic south pole of the earth is near to:
(D) All of these (A) geographical north pole
(B) geographical south pole
(C) east
21. The effective length of the magnet is : (D) west
(A) the complete length of the magnet
(B) the distance between the two poles of the magnet 31. What happens when a bar-magnet is broken into two
(C) the half of the length of the magnet pieces?
(D) the square of the length of the magnet (A) The magnetism is destroyed
(B) Each piece becomes a magnet
(C) One piece becomes a north-pole and the other
22. Magnetic field lines outside the magnet start : becomes a south-pole
(A) from N-poles (D) One piece remains a magnet and the other
(B) from S-poles becomes an ordinary bar
(C) from current-carrying wires
32. Which of the following statements is true?
(D) none of these (A) An electromagnet does not attract a piece of iron.
(B) An electric current flowing in a circuit deflects a
23. A transformer used to reduce the alternating voltage magnetic needle
is : (C) An electric bell has a permanent magnet
(A) Step-up transformer (D) An electromagnet can be used to separate plastic
bags from a garbage heap.
(B) Step-down transformer
(C) both (A) and (B)
7
 ANSWER – KEY

MAGNETISM
Q. 1 2 3 4 5 6 7 8 9 10
A. C C A D B B B B A C
Q. 11 12 13 14 15 16 17 18 19 20
A. C B B D B B A C B D
Q. 21 22 23 24 25 26 27 28 29 30
A. B A B B C A C C A A
Q. 31 32
A. B B