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Physics NEET MCQ
Magnetism Mcq
Quiz 8
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Q.1
A magnet makes 10 oscillations per minute at a place where the horizontal component of earth's magnetic field (H) is 0.33 oersted. The time period of the magnet at a place where the value of H is 0.62 oersted will be
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a) 4.38 sec
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b) 0.38 sec
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c) 2.38 sec
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d) 8.38 sec
Explanation
Answer: (a)
Q.2
A magnet makes 10 oscillations per minute at one place and takes 5 seconds to complete one oscillations at another place. Compare the values of horizontal components of earth's field at two places
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a)25/36
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b) 36/25
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c)5/6
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d)6/5
Explanation
Answer: (a)
Q.3
The time period of vibration of two magnets is some position is 3 sec. When polarity of weaker magnet is reversed, the combination makes 12 oscillations per minute. compare the magnetic moments of to magnets
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a) 4
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b) 17/8
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c)8/17
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d)1/4
Explanation
Answer: (b)
Q.4
The magnetic susceptibility of paramagnetic substance is 3 ×10-It is placed in a magnetizing field of 4 × 103 amp/m. The intensity of magnetization will be ..
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a) 3 × 108 A/am
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b) 12 × 108 A/m
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c)12 A/m
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d)24 A/m
Explanation
USe formula χ=M/H here χ is magnetic susceptibility.H is magnetizing fields and M is intensity of magnetization Answer:(c)
Q.5
A magnetizing field of 2 × 103 amp/m produces a magnetic flux density of 8π Tesla in an iron rod. the relative permeability of the rod will be
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a) 102
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b) 100
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c) 104
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d) 101
Explanation
B be the magnetic flux density H be magnetizing field µ be the permeability of iron . Then µ=B/H If µr represents relative permeability Then µr=µ / µo ∴ µ=B/(µoH) Answer: (c)
Q.6
The correct curve between χ and 1/T for paramagnetic is
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a)
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b)
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c)
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d)
Explanation
For magnetic material χ ∝ 1/T thus straight line curveAnswer: (a)
Q.7
The mass of an iron rod is 80 gm and its magnetic moment is 10 A-mIf the density of iron is 8 gm/cc then the value of intensity of magnetization will be
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a) 106 A/m
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b) 104 A/m
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c)102 A/m
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d)10 A/m
Explanation
Volume of rod=80/8=10 cc 10 cc=10-5 m3 Intensity of magnetization=10/10-5=106 A/mAnswer: (a)
Q.8
A bar magnet is cut into two equal halves by a plane parallel to the magnetic axis, of the following physical quantities the one which remains unchanged is
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a) pole strength
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b) magnetic moment
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c)intensity of magnetization
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d)moment of inertia
Explanation
Answer:(c)
Q.9
Two isolated point pole of strength 30 A m and 60 A m are placed at a distance of 0.3 m. The force of repulsion
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a) 2 × 10-3 N
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b) 2 × 10-4 N
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c) 2 × 105 N
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d) 2 × 10-5 N
Explanation
Answer: (a)
Q.10
Magnetic lines of force are
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a)continuous
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b) discontinuous
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c)some times continuous and some times discontinuous
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d)nothing can be said
Explanation
Answer: (a)
Q.11
Unit of magnetic moment are
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a) JT
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b) JT-1
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c)Am2
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d)Am-2
Explanation
The unit A m2 is a correct SI unit for magnetic moment, though, unless the concept of “current in a coil” needs to be emphasized in a particular context, it is perhaps better to stick to N m T-1 or JT-1 Explanation: magnitude of the magnetic moment is defined as the maximum torque experienced by the magnet when placed in unit external magnetic field. The magnitude and direction of the torque is given by the equation τ = P × B SI unit of torque = N.m, torque is measured in Joules (J) SI unit of Magnetic field = T Answer: (b)
Q.12
Units of pole strength of magnet are
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a) A m-1
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b) A m2
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c) A m-2
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d)A m
Explanation
Answer:(d)
Q.13
A tiny loop of current behaves as magnetic dipole
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a) true
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b) falls
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c) may be true or falls
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d) nothing can be said
Explanation
Answer: (a)
Q.14
Magnetic field intensity due to a dipole various as dn where n=...
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a)2
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b) -2
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c)3
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d)-3
Explanation
Let the solenoid consists of n turns per unit length. Let its length be 2l and radius a. We can evaluate the axial field at a point P, at a distance r from the centre O of the solenoid. To do this, consider a circular element of thickness dx of the solenoid at a distance x from its centre. It consists of n d x turns. Let I be the current in the solenoid. The magnetic field on the axis of a circular current loop at point P due to the circular element is The magnitude of the total field is obtained by summing over all the elements — in other words by integrating from x = – l to x = + l . Thus, Consider the far axial field of the solenoid, i.e., r >> a and r >> l . Then the denominator is approximated by Note that the magnitude of the magnetic moment of the solenoid is, m = n (2l)I(πa2) = (total number of turns × current × cross-sectional area). Thus, Answer: (d)
Q.15
A short bar magnet place with axis at 30°, with uniform external magnetic field of 0.25 T experiences a torque of 4.5 × 10-2 N-m. Magnetic moment of the magnet is ...
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a) 0.36 J/T
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b) 0.72 J/T
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c)0.18 J/T
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d)zero
Explanation
Answer: (a)
Q.16
In hydrogen atom, an electron revolves with frequency of 6.8 × 109 megahertz in an orbit of diameter 1.06 Å. The equivalent magnetic moment is
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a) 7.9 × 10-24 Am2
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b) 9.7 × 10-24 Am2
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c)9.7 × 1024 Am2
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d)7.9 × 1024 Am2
Explanation
Current I=charge × frequency I=1.6×10-19 ×6.8 × 1015 I=10.88 ×10-4 amp Magnetic moment=IA Magnetic moment=10.88 ×10-4 × π × (0.59 × 10-10 Magnetic moment=9.7 × 10-24 Am2 Answer:(b)
Q.17
The dimensions of magnetic permeability are
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a) [MLT-2A-2]
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b) [ML2T-2A-2]
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c) [ML2T-2A-1]
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d) [M-1LT-2A-1]
Explanation
Answer: (a)
Q.18
A circular coil of radius 4cm having 20 turns carries a current of 3A. It is placed in a magnetic field of intensity 0.5 weber/mThe magnetic dipole moment of the coil is
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a)0.15 amp ×m2
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b) 0.3 amp × m2
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c)0.45 amp × m2
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d)0.6 amp × m2
Explanation
USe formula magnetic moment=NIA here A is cross-sectional area of coilAnswer: (b)
Q.19
The magnetic moment of a magnet is 0.1 amp × mIt is suspended in a magnetic field of intensity 3×10-4 weber/mThe couple acting upon it when deflected by 30° from the magnetic field is
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a) 1 × 100-5 Nm
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b) 1.5 × 100-5 Nm
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c)2 × 100-5 Nm
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d)2.5 × 100-5 Nm
Explanation
Answer: (b)
Q.20
In the un magnetized state, magnetic domains of a magnetic substance are oriented at
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a) 60°
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b) 90°
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c)Randomly
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d)zero
Explanation
Answer:(c)
Q.21
The time period of freely suspended magnet does not depend upon
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a) length of magnet
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b) pole strength of magnet
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c) horizontal component of earth's field
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d) length of the suspension
Explanation
Answer: (d)
Q.22
The magnetic induction B and the force F on a pole m are related by
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a)B=mF
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b) F=B/m
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c)F=Bm
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d)none of these
Explanation
Answer: (c)
Q.23
Calculate force exerted on a point N pole of 3200 A-m placed 10cm away from a point south pole of 30 A-m in air
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a) 1 N
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b) 1.28 N
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c)2 N
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d)3 N
Explanation
Answer: (b)
Q.24
Force acting on a magnetic pole of 7.5 ×10-2 A-m is 1.5 N. Magnetic field at the point is
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a) 20 Wb/m2
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b) 50 Wb/m2
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c)112.5 T
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d)2.0 T
Explanation
Answer:(a)
Q.25
Magnetic potential at a point distance d from a magnetic pole of strength m is
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a)
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b)
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c)
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d) None of these
Explanation
Answer: (a)
Q.26
S.I. unit of magnetic permeability is
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a)A-m
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b) Am2
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c)H-m
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d)H/m
Explanation
Answer: (d)
Q.27
Force between two magnetic poles depends on
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a) pole strength only
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b) distance only
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c)medium only
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d)all the three above
Explanation
Answer: (d)
Q.28
A bar magnet of magnetic moment M is placed in magnetic field of induction B. The torque exerted on it is [ GUJCET 2007]
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a) - B . M
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b)M × B
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c)- M × B
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d)M . B
Explanation
Answer:(b)
Q.29
The period of oscillation of a magnet is a vibration magnetometer is 2 seconds. the period of oscillation of magnet of moment of inertia four times that of first magnet is
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a)1 sec
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b) 4 sec
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c)8 sec
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d)0.5 scc
Explanation
Answer: (b)
Q.30
A point at which two or more magnetic fields cancel each other is called
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a) focal point
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b) inverse point
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c)neutral point
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d)none of the above
Explanation
Answer: (c)
0 h : 0 m : 1 s
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