PHYSICS [5 Mark Question Answer]

Magnetic declination: The angle through which freely suspended magnetic needle is inclined to the geographic axis is known as magnetic declination.
OR
The angle between the geographic meridian and magnetic meridian it is given place is called declination.
Magnetic dip: The angle between the horizontal axis passing through freely suspended magnet and the direction of earth's magnetic field is called magnetic dip.
(a) Magnetic meridian: The vertical plane containing the magnetic axis of a freely suspended magnet at rest, under the action of magnetic intensity of earth is called magnetic meridian.
Geographic meridian: The vertical plane which contains geographic north and south pole of earth at a given place is called geographic meridian.
(b) The angle of dip is maximum i. e. 90° at the magnetic poles. The angle of dip is minimum i.e. 0° at the magnetic equator.

(a) Magnetic keeper : A magnetic keeper is a ferromagnetic bar made from soft iron or steel, which is placed across the poles of a permanent magnet. Magnetic keepers are used to preserve the strength of the magnet by completing the magnetic circuit.
(b) Ewing suggested the molecular theory of magnetism as follows:
1. Each molecule of a magnetic substance, whether it is magnetised or unmagnetised, is an independent magnet.
2. In a magnetised substance, the molecules are arranged in an order so as to produce an external effect. In this order, all the north poles of the molecules of the magnetised substances point to one direction and all their south poles point to a direction opposite to that to which their north poles points.

3.In an unmagnetised substance, the molecules are not arranged in any order, so they neutralise the magnetic forces of each other.

(a) Each molecule of a magnetic substance is an independent magnet. But in an unmagnetised iron piece, molecular/atoms (tiny magnets) are not arranged in any order and hence they neturalise the magnetic forces of each other As a result, any unmagnetised iron piece can not behave as a magnet.
(b)
1. When a magnet is cut into two equal parts as shown in figure (i), then pole strength of each piece remains same as that of original magnet.
2. When a magnet is cut into two equal parts as shown in figure (ii), then pole strength of each pole is half as that of the pole strength of original magnet.

(a)Bar magnet placed in magnetic meridian with its north pole pointing geographic north.

(b)Bar magnet placed in magnetic meridian with its north pole pointing geographic south.

Bar magnets are often stored in pair as shown in the figure. E and F being pieces of metal.

Magnetism is a property of molecules of a magnet.
Experiment-1 : Take a bar magnet and break it into as small parts as possible. We shall find that each small part retain the original polarity i.e. even the smallest part of the magnet has its own north and south pole and can attract the magnetic substances. It shows that, if it were possible to break a magnet into its molecules then each molecule would retain the property of a magnet.
Experiment-2 : Take a soft iron bar. Place it in a magnetic field. We shall find that there is small increase in its length on getting magnetised.
A soft iron bar becomes magnetised and get lengthened only if each molecule of soft iron bar behave as a magnet. Due to setting of molecular magnets along straight chains, there is slight increase in length of soft iron bar. So we can say that magnetism is a property of molecules of a magnet.

1. A magnet can be damagnetised by any of the following methods:
Electrical Method : An insulated copper coil is wound around a card board tube and inside it is placed a permanent bar magnet. The coil is placed in East-West direction and its ends are connected to a step-down transformer. The alternating current is switched on for one minute, then the bar magnet gets demagnetised.


Reason : When the current rapidly changes the direction in the insulated copper coil, the polarity set up in the coil also rapidly changes. This in turn acts inductively on the bar magnet, whose molecular magnets rapidly try to align themselves with the changing magnetic polarity. Thus, molecular magnets form closed chains and hence, the bar magnet gets demagnetised.
2. By Rough Handling: When a magnet is rough-handled (i.e. it is allowed to drop repeatedly) or hammered, it gets completely demagnetised.
Reason: In a bar magnet the molecular magnets are arranged in straight line chains. On hammering or rough handling, they gain kinetic energy and vibrate rapidly about their mean positions. In doing so they form closed magnetic chains and hence, the magnetism is lost.
3. By Heating: When a magnet is heated to red hot temperature and then allowed to cool, it loses its magnetism. Reason: Due to heat energy, the kinetic energy of the molecules increases. Thus, from straight line molecular chains, they form closed molecular chains and hence, the magnetism is lost.
4. By Induction : When a given magnet is placed in contact with another similar magnet (i.e., the other magnet should be of same strength), such that their similar poles are facing each others then both the magnets get demagnetised in a couple of days.
Reason: It is because both the magnets will induce opposite polarity in each other. In doing so the molecular magnets in each magnet form closed molecular chains and hence, they get demagnetised.
5. By Self-Induction : A single bar magnet has a tendency to lose its magnetism. Reason : In a bar magnet the molecular magnets (dipoles) are arranged in straight line chains. However, two or more parallel chains have their north and south poles facing each other. Thus, dipoles act inductively on each other and hence, turn to form closed molecular chains. Thus, single bar magnet gets demagnetised.

(a) Ewing suggested the molecular theory of magnetism as , follows:
1. Each molecule of a magnetic substance, whether it is magnetised or unmagnetised, is an independent magnet.
2. In a magnetised substance, the molecules are arranged in an order so as to produce an external effect. In this order, all the north poles of the molecules of the magnetised substances point to one direction and all their south poles point to a direction opposite to that to which their north poles points.

3.In an unmagnetised substance, the molecules are not arranged in any order, so they neutralise the magnetic forces of each other.

4.The molecular theory of magnetism was a considerable step forward but later there came an electrical explanation for the magnetism of atoms.Atoms consist of negatively charged particles (electrons) which revolve around the positively changed nucleus. Electrical current loops are formed in an atom due to the circulation of these electrons. Each current loop behaves a magnetic dipole and hence produce magnetic field. Also electrons are also spinning like tops and this adds further magnetism to the atom.

(a) Iron is a magnetic substance and hence its each atom behaves as a tiny magnet.When iron piece is placed in a coil carrying direct current, then all the north poles of all the atoms of iron will align themselves in one direction and all the south poles of all the atoms of iron align themselves in a direction opposite to that to which their north poles point. As a result, iron piece gets magnetised.
(b) Bar magnets lose their magnetism when heated strongly. Due to heat energy, the kinetic energy of the molecules of a barg magnet increases. Thus from straight line molecular chains, they form closed molecular chains and hence, magnetism is lost.
(c) Steel makes better permanent magnet than soft iron because on magnetising steel, steel retain their magnetic behaviour for longer time even after the removal of source which is magnetising the steel.
While the soft iron retains the properties of magnetism only so long as the current is passing through the coil i.e. as long as the source which is magnetising the soft iron is present.
(d) In magnets, external fields like earth's magnetic field can randomize the domains. Perhaps stray fields caused by flowing currents in near by electric circuits can also disturb i the alignment of domains lying inside a magnet. Given enough time, such magnets may find their domains randomly oriented and hence their net magnetisation may get lost. A keeper for magnets is just a strong permanent magnet that keeps all the domains pointing the same way and realign those that may have gone stray and hence magnet, can retain its magnetism for a long time.

Magnetic field: The space surrounding a magnet within which the magnet has its influence is called magnetic field.
Magnetic meridian: The vertical plane containing the magnetic axis of a free suspended magnet at rest, under the action of magnetic intensity of earth is called magnetic meridian. Geographical meridian : The vertical plane which contains geographical north and south poles of earth at a given place is called geographical meridian.
Decimation: The phenomenon due to which the earth's geographical meridian is inclined to earth's magnetic meridian is called declination.
Magnetic equator : An imaginary line right bisecting the effective length of bar magnet is called magnetic equator.

(a) Magnetic substance : Those substances which are affected by the magnetic field are known as magnetic substances.
For example : Iron, nickel, cobalt etc. are the magnetic substances.
Non-magnetic substances : Those substance which are not affected by the magnetic field are known as nonmagnetic substances.
For example : Paper, glass, wood etc.

Nature of bar	Action of compass needle
	North Pole	South Pole
Non-magnetic	No action	No action
Soft iron rod	Attracted	Attracted
North pole of a bar magnet	repelled	attracted
South pole of a bar magnet	Attracted	repelled