Download physics - 3rd chapter- solution - e

File No.24/04/17/12/2014
VII CLASS - IIT/N.T.S.E FOUNDATION - OLYMPAID
ANDHRA PRADESH - TELANGANA
2014-2015 PROGRAM M E
PHYSICS - 3RD CHAPTER- SOLUTION
MAGNETISM
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1Sol. Iron is not the magnetic material ?
2Sol. Most powerful magnets are prepared by electrical method
3Sol. At a neutral point, the resultant magnetic field is zero
4Sol. Magnetic keepers are made of soft iron
5Sol. A metal bar can be identified as a magnet if an iron bar is attracted to it
6Sol. At a neutral point, the resultant magnetic field is zero.
7Sol. The strength of an electromagnet can be increased by increasing the current
8Sol. A freely suspended magnet will rest in north - south direction.
9Sol. Electromagnets are made of soft iron
10Sol. In the pase, humans made crude navigational eompass by using lodestone.
11Sol. The magnetic indicator plotted on a pin point arranged in a circular box is called compass needle
12Sol. Amagnet can induce magnetism in steel.
13Sol. When a steel bar is brought close to a bar magnet, there will be attract.
14Sol. Magnets are used in microphones, mariner’s compass and magntic resonance imageing
15Sol. Freely suspended hanging bar magnet always denotes north-South directions of earth
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16Sol. When the magnet comes to rest, the pole
facing- the
north direction is the south pole of the magnet.
17Sol. Pin is a magnetic substance.
18Sol. If the North Pole of a magnet is brought close to the South Pole of another magnet, the magnets attract
each other
19Sol. Magnetic length = 5/6 X geometric length
20Sol. In electrical method of magnetisation the specimen to be magnetised is placed inside along coil of insulated
copper wire
1Sol.
2Sol.
3Sol.
4Sol.
The magnetic length of bar magnetic 2  10–3 m then its Geometric length is  10–3 m 2.4
Length of the magnet is 80% of the genometric length
If the distance between the two edges of bar magnet is 6 cm. The magnet length is 6 cm.
The magnetic length of bar magnet is 15 cm, then it’s Gemomtric length is 0.18 m
5Sol. If the distance between the two edges of bar magnet is 6 cm. The the magnet length is 6 cm.
6Sol. The magnetic length of bar magnet is 15 cm. then it’s Geometric length is 0.18 m
7Sol. The magnetic length of a bar magnet is 10 cm. Its geometric length is 12 cm.
8Sol.
The north pole of eart’s magnet is near the geogrophical south.
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PHYSICS - CHAPTER SOLUTIONS - 3
File No.24/04/17/12/2014
VII CLASS - IIT/N.T.S.E FOUNDATION - OLYMPAID
9Sol. The lines of force due to the earth’s horizontal magnetic field are parallel and straight
10Sol. Magnetic lines of force exert magnetic force
11Sol. The magnetic lines force never intersect
12Sol. The horizontal component of earth’s magnetic field is always maximum at magnetic equator
13Sol. A neutral point in a magnetic field is a point at which the resultant magnetic induction is zero
14Sol. The horizontal component of earth’s magnetic field is zero at magnetic poles.
15Sol. The earth’s magnetic field always has a horizontal component except at the equator
16Sol. The best method for demagnetisation is sending alternating currents through the magnet
17Sol. The magnetic length of a bar magnet is 10 cm. Its geometric length is 12 cm
18Sol. Repulsion is the sure test of magnetism
19Sol. The magnetic line of force due to earth’s magnetic field start from south pole
20Sol. Magnetism in the middle of a bar magnet is zero
BRAIN TWISTERS
1Sol. An imaginary line joining the magnetic north and south pole of a bar magnet is its Magnetic axis, Axial line
2Sol. Magnet shown attractive property, directive property, induction and pole property.
3Sol. The strength of a magnetic field increases as the number of magnetic lines of force passing through a given
area increases, strength of the magnetic poles increases
4Sol. Electro magnets and the magnets made from soft iron are temporary magnets
The loss of magnetic property of a magnet is called demagnetisation
Strong magnets can be made by the electrical
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5Sol. The magnet have a general property(ies) if magnetic poles always exists in pairs, inside the magnet , the
magnetic lines of force travels from south to north
6Sol. A) Example of permanent magnet
B) Example of Temporary magnet
C) Example of Natural magnet
D) Use of Magnetic compass
Soft iron
Steel
Lodestone 3) Finding directions
Finding directions
7Sol. A) Magnet
Property of attraction
B) Compass
Property of direction Property of induction
C) Magnetisation
Property of induction
D) Unlike poles
Property of pole strength
8Sol. a) Single touch method
Method of magnetisation
b) Double thouch method
Method of magnetisation
c) Methods of demagnetisation
by heation, by rough handling
d) Divided touch mehtod
Method of magnetisation
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PHYSICS - CHAPTER SOLUTIONS - 3
File No.24/04/17/12/2014
VII CLASS - IIT/N.T.S.E FOUNDATION - OLYMPAID
9Sol. a) The rest position of a magnet is
directive property
b) Magnetic poles exist in pairs
Dipole
c) Iron is strongly attracted by magnet so it is
Ferromagnetic substance along N-S
when suspended
d) Gold is repelled by a magnet so it is
diamagnetic substance
10Sol. a) Magnetization
Double touch method, Single touch method
b) Temporary magnet
Electro magnet
c) Demagnetisation
Heating a magnet, Hammering on magnet
d) Electric bell
Electro magnet
11Sol. If th current appears to flow in the anticlockwise direction, then the end of the bar placed within the coil
takes south polarity electical method of magnetisation.
The two ends of the magnet where the attraction is minimum are called poles of the magnets.
Both Assertion and Reason are correct and Reason explains Assertion.
12Sol. In electric bells temporary magnets are used which help inrepeatedly making and breaking circuit.
Magents are used to separate magnetic substances from a mixture.
Both are correct but, Reason doesnot explain Assertion.
13Sol. The magnetic length is always lesser than Geometric length.
The magnetic length is always greater than Geometric length.
So, Assertion is correct and Reason is in correct.
14Sol. When a magenet is rought-handled (or) heated hammered (or) heated to red hot and then allowed to
cooled then it loses its magnetism.
Both Assertion and Reason are correct and Reason does not explain Assertion.
15Sol. When a given magnet is placed in contact with another similar magnet of same strength, such that their
similar poles are facing each other both the magnets get demagnetised.
A single bar magnet has a tendency to lose its magnetism by self induction.
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Both Assertion and Reason are correct and Reason explain Assertion.
16Sol. The property of attracting magnetic substances is called magnetism.
Magnetic poles never repel other poles. So, Assertion is correct and Reason is in correct.
17Sol. The specimen to be magnetised is placed and a permanent bar magnet is taken and its one pole is placed
on one end of the piece. The bar magnet is then drawn to the other end, keeping it in the inclined position
The permenent magnet is then lifted and the process is repeated several times this process of magnetisation
is called single touch method.
18Sol. The specimen to be magnetised is placed flat and Opposite poles of two strong bar magnets of equal
strength are placed together in the middle of the specimen. The ends of bar magnets are drawn towards
the opposite ends of the specimen, keeping the bar magnets inclined and the process is repeated several
times this process of magnetisation is called divide touch method.
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19Sol. The specimen to be magnetised is placed inside a long coil of insulated copper wire. A strong direct current
is passed through the coil for some time, then the specimen is magnetised this process of magnetisation is
called single touch method
20Sol. The relation between magnetic length and geometric length is Magnetic length=
x
 Geometric length ,then
6
the value of x is 5.
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PHYSICS - CHAPTER SOLUTIONS - 3
File No.24/04/17/12/2014
VII CLASS - IIT/N.T.S.E FOUNDATION - OLYMPAID
1Sol. Flux meter is used to measure magnetic field.
2Sol. A magnet is broken into three pieces in length. The strength of the new poles is the same as the strength of
the poles of original magnet
3Sol. Magnetic fields do not interact with stationary electric charge
4Sol. Magnetic field produced by electrons in atoms and molecules is due to their spin and orbital motion both
5Sol. A compass needle is placed at the magnetic pole of earth. It may stay in any direction
6Sol. Magnetic lines of force are continuous
7Sol. A magnetic field exerts no force on a charge at rest
8Sol. Force between two magnetic poles depends on pole strength only, distance only & medium only.
9Sol. Brass is not a magnetic substance.
10Sol. Can a system have magnetic moment even though its net charge is zero Yes
11Sol. A non magnetic material is that which is not effected even by strong magnetic fields
12Sol. A magnet can be completely demagnetised by applying a reverse field of appropriate strength
13Sol. The lines of force due to earth's horizontal magnetic field are parallel, straight and horizontal
14Sol. A point at which two or more magnetic fields cancel eachother is called neutral point
15Sol. When a magnet is suspended hrozontally at neutral point, its frequency of oscillation is zero.
16Sol. A magnetic needle suspended horizontally by an unspun silk fibre oscillates in the horizontal plane, because
of the restoring force originating mainly from the torsion of the silk fibre, the horizontal component of
earth's magnetic field and the force of gravity
17Sol. The most suitable metal for making electromagnets and trasformers cores is iron
18Sol. The most suitable metal for permanant magnets is steel.
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19Sol. A temporary magnet is made of soft iron
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20Sol. When a magnet marked N S is suspended in a horizontal position, so that it is free to rotate, the pole
marked N shall point towards south pole of earh's magnet
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PHYSICS - CHAPTER SOLUTIONS - 3