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Magnetism Mcq
Quiz 4
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Q.1
Vibration magnetometer work on the principle of [ MPPMT 199]
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a) Torque acting on the bar magnet
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b)Force acting on the bar magnet
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c)Both the force and the torque acting on the bar magnet
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d)None of the above
Explanation
Answer: (a)
Q.2
A magnetic needle suspended by a silk-thread is vibrating in the earth's magnetic field. If the temperature of the needle is increased by 500°C, then [ MNR 1995]
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a) the time periode decreases
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b) the time period remains unchanged
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c)the time period increases
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d)the needle stops vibrating
Explanation
Needle will loose its magnetic property Answer:(d)
Q.3
A bar magnet is placed North-south with its North pole due North. The point of zero magnetic field will be in which direction from the centre of the magnet? [ PMT 1995]
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a) North and South
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b) East and West
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c) North-East and South-West
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d) North-West and South-East
Explanation
Answer: (b)
Q.4
A bar magnet of magnetic moment M is kept in uniform magnetic field of strength b making angle θ with its direction. The torque acting on it is [ MPPMT 1998]
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a)MB
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b) MBcosθ
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c)MB( 1- cosθ)
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d)MBsinθ
Explanation
Answer: (d)
Q.5
If a piece of metal was through to be magnet, which one of the following observations would offer conclusive evidence? [ CET 1994]
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a) it attracts a known magnet
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b) it repels a known magnet
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c)non of the above
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d)it attracts a steel screw driver
Explanation
Answer: (b)
Q.6
The magnetic lines of force inside a bar magnet [ AIEEE 2003]
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a) are from north-pole to south-pole of the magnet
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b) do not exist
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c)depend upon the area of cross section of the bar magnet
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d)are from south-pole to north-pole of the magnet
Explanation
Answer:(d)
Q.7
A tangent galvanometer shows no deflection when current is passed through it. But when the current is reversed, it gives deflection of 180°, then the plane of the coil is
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a) in the magnetic meridian
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b) normal to the magnetic meridian
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ac)at an angle of 45° with the magnetic meridian
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d)at an angle of 60° with the magnetic meridian
Explanation
Answer:(b)
Q.8
The length of a magnet is larger compared to its width and breadth. the time period of its oscillation in a vibration magnetometer is 2s. The magnet is cut along its length into three equal parts and these parts are placed on each other with their like poles together. the time period of this combination will be [ AIEEE 2004]
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a) 2√3 s
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b) (2/3) s
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c) 2 s
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d) 2 / √3 s
Explanation
When magnet is cut in three parts its length becomes l/3 amd mass becomes m/3 moment of inertia I' When three parts are joined pole strength becomes three times the earlier thus Magnetic moment M'=3m(2l/3)=MTime period T'= Answer: (b)
Q.9
The material suitable for making electromagnets should have [ AIEEE 2004]
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a)high retentivity and law coercivity
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b) low retentivity and low coercivity
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c)high retentivity and high coercivity
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d)low retentivity and high coercivity
Explanation
Electromagnet should be amenable to magnetization and demagnetizationAnswer: (b)
Q.10
A magnetic needle is kept in a non-uniform magnetic field it experiences [ AIEEE 2005]
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a) neither a force nor torque
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b) torque but not a force
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c)a force but not a torque
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d)a force and a torque
Explanation
Answer: (d)
Q.11
Needles N1, N2 and N3 are made of a ferromagnetic, a paramagnetic and a diamagnetic substance respectively. A magnet when brought close to them will [ AIEEE 2006]
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a) attract N1 and N2 strongly but repel N3
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b) attract N1 strongly, N2 weakly but repel N3 weakly
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c)attract N1 strongly, but repel N2 and N3 weakly
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d)attract all three of them
Explanation
Answer:(b)
Q.12
Relative permitivity and permeability of material εr and µr respectively. Which of the following values of these quantities are allowed for a diamagnetic material? [ AIEEE 2008]
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a) εr=0.5 and µr=1.5
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b) εr=1.5 µr=0.5
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c) εr=0.5 and µr=0.5
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d) εr=1.5 and µr=1.5
Explanation
For diamagnetic material, the value of µr is less than one. For any material, the value of εr is always greater than 1. Answer: (b)
Q.13
Two bar magnets of the same mass, same length and breadth but having magnetic moments M and 2M are joined togather pole to pole and suspended by a string. The time period of assembly in a magnetic field of strength H is 3 seconds. If now the polarity of one of the magnets is reversed and the combination is again made to oscillate in the same field, the time oscillation is :
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a)√3 secs
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b) 3√3 secs
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c)3 secs
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d)6 secs
Explanation
Let K1 and K2 be the moment of inertia of magnets For fist case For second case Dividing and then solving equation for T we get T=3√3Answer: (b)
Q.14
The earth's magnetic field may be considered to be due to a short magnet placed at the centre of the earth and oriented along the magnetic south-north direction. the ratio of the magnitude of the magnetic field on the earth's surface of magnetic equator to that at the magnetic poles is
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a) 1 : 2
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b) 2 : 1
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c)1 : 4
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d)4 : 1
Explanation
magnetic field at equator is given by magnetic field at pole is given by By taking ratio we get option (a)Answer: (a)
Q.15
The radius of curvature of the path of a charged particle in a uniform magnetic field is directly proportional to [ MNR 1995]
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a) the charge on the particle
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b) the momentum of the particle
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c)the intensity of the particle
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d)the energy of the particle
Explanation
Answer:(b)
Q.16
The angle of dip at a place is 60°. A magnetic needle oscillates in a horizontal plane at this place with period T. The same needle will oscillate in a vertical plane coinciding with the magnetic meridian with a period
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a) T
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b) 2T
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c) T/2
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d) T/√3
Explanation
From the formula for periodic time Answer: (d)
Q.17
A dip needle lies initially in the magnetic meridian when it shows an angle of dip θ at a place. The dip circle is rotated through an angle x in the horizontal plane and then it shows an angle of dip θ'. Then ( tanθ' / tanθ) is
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a)1 /cos x
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b) 1/ sin x
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c)1 /tan x
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d)cos x
Explanation
from figure tanθ' / tanθ=H/H '=H/Hcos x=1/cos xAnswer: (a)
Q.18
Which of the following is diamagnetic
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a) copper sulphate
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b) liquid oxygen
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c)water
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d)air
Explanation
Answer: (c)
Q.19
A magnetic needle is kept in a uniform magnetic field. It experiences
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a) a force and torque
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b) a force but not torque
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c)torque but not force
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d)neither force nor torque
Explanation
Answer:(c)
Q.20
Soft iron is used to manufacture electromagnets because their
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a) magnetic saturation limit is high while retentivity and coercive force are small
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b) area of hysteresis loop is small
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c) retentivity is small but coercive force is high
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d) area of hysteresis loop is large
Explanation
Answer: (b)
Q.21
The magnetic field of short magnet at a distance of 1 m on the axial line is 1 oersted. At a distance of 2m on the same axis, the intensity in oersted is
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a)0.5
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b) 0.25
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c)0.125
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d)0.75
Explanation
B ∝ 1/r3Answer: (c)
Q.22
Two magnets are held together in a stirp up and allowed to oscillate in the earth's field when their like poles are pointing in the same direction, they make 12 vibrations per minute. When unlike poles are pointing in the same direction, they make 4 vibrations per minute. Then the ratio of their magnetic moments are
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a) 3 : 1
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b) √3 : 1
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c)9 : 1
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d)10 : 8
Explanation
For the formula for periodic time Answer: (d)
Q.23
A small compass needle is placed at the centre of the circular coil in a tangent galvanometer. This is done to ensure that
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a) the compass needle does not produce a strong magnetic field
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b) the galvanometer does not have a large size
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c)the moment of inertia is large
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d)the compass needle moves in the region of uniform magnetic field
Explanation
Answer:(d)
Q.24
What happens to the force between two magnetic poles when their pole strengths and distance between them are both doubled
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a) no change
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b) force increases to four times the previous value
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c) force decreases to half the previous value
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d) force increases to two times the previous value
Explanation
Answer: (a)
Q.25
A uniform magnetic needle is suspended from its centre by a thread. Its upper end is now loaded with a mass of 50 mg when the needle becomes horizontal. If the strength of each pole is 98.1 ab-amp × cm and g is 981 cm/sec2, then the vertical componenet of the earth's magnetic induction is
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a)0.50 gauss
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b) 0.25 gauss
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c)0.05 gauss
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d)0.005 gauss
Explanation
At equilibrium, algebric sum of moment of forece about centreO must be zeromgl- MBl - MBl=0B=mg/2M B=(50 × 10-3 × 981) / (2×98.1)B=0.25 gaussAnswer: (b)
Q.26
The magnetic field due to a short magnetic dipole of magnetic moment M at a point on the axis of the dipole and at a distance d from it is given by
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a) M/d3
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b) M/d2
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c)2M/d3
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d)2M/d2
Explanation
Answer: (c)
Q.27
A magnetic needle vibrates in a vertical plane parallel to the magnetic meridian, about a horizontal axis passing through its centre. Its frequency is n. If the plane of oscillation is turned about a vertical axis by 90°, the frequency of its oscillation in vertical plane will be
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a) n
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b) zero
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c)less than n
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d)more than n
Explanation
In any plane other than magnetic meridian the dip needdle will dip more than true dipNow earths Horizontal componant of magnetic field will be Hcos θ From the formula for periodic time Value of cosθ decreases ∴ n decreases Answer:(c)
Q.28
If a bar magnetic moment 80 units be cut into two halves of equal lengths, the magnetic moment of the each half will be
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a) 80 units
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b) 40 units
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c) 160 units
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d) 20 units
Explanation
Answer: (b)
Q.29
A magnet 20cms long with its poles concentrated at its ends, is placed vertically with its ends, is placed vertically with its north pole on the table. At a point due to 20cms south of the pole, a neutral point is obtained. IF H horizontal component of earths magnetic field H=0.3 gauss, then the pole strength of the magnet is approximately
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a)1.85 amp × metre
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b) 18.5ab-amp × cm
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c)185 amp × metre
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d)185 ab-amp × cm
Explanation
Filed at P due to S pole B2=m/ (20√2)2 Component of field B2 along B1 but opposite to it=B2 cos45°=m/(20√2)2 × √2 . ∴ For P to be neutral point Answer: (d)
Q.30
The magnet can be completely demagnetized by [ CET 1994]
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a) breaking the magnet in small pieces
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b) heating it slightly
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c)dropping it into ice cold water
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d)a reverse field of appropriate strength
Explanation
Answer: (d)
0 h : 0 m : 1 s
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