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Physics MCQs for NEET — Practice Questions with Answers

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A domain in a ferro magnetic substance is in the form of a cube of side length $ 1 \mu m$. If it contains $ 8 \times 10^{10} $ atoms and each atomic dipole has a dipole moment of $ 9 \times 10^{-24} A.m^2$ then magnetization of the domain is ............. A.m-1.

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Explanation

$ Volume of the domain = ( 1 \times 10^{-6 })^3 m^3 = 10^{-18} m^3 $ $ \therefore New dipole moment mnet = Nm $ $ = 8 \times 10^{10} \times 9 \times 10^{-24} $ $ = 72 \times 10^{-14} A.m^2 $ $ \therefore Magnetization M = { m_{net} \over vol} $ $ = { 72 \times 10^{-14} \over 10^{-18 }}{A.m^2 \over m^3 } $ $ = 72 \times 10^{+4} $ $ = 7.2 \times 10^5 Amp.meter^{-1} $

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The magnetic susceptibility is negative for

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Explanation

Diamagentic materials

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When 2 Amp current is passed tarough a tangent galvanometer. It gives a deflection of $ 30 ^ \circ $ . For $ 60^ \circ $ deflaction, the current must be

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Explanation

$ I \;\alpha\; tan \phi $ $ { I_1 \over I_2 } = { tan \phi1 \over tan \phi 2 } $ $ { 2 \over I_2 } = { tan 30 ^\circ \over tan 60 ^\circ } $ $ I_2 = 6Amp $

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The time period of a freely suspended magnet is a 4 seconds. If it is broken in length into two equal parts and one part is suspended in the same way, then its time period will be

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Explanation

$ \sqrt [2 \pi] { I \over MB_H} = 4 sec $ $ where I = { 1 \over 12 } ML^2 $ when magnet is cut into two equal havies, then new magnet moment M' = M/2 moment of inertia $ I' = 1/12 M' L'2 $ $ = { 1 \over 12 } \left({ M \over 2} \right) \left( { L \over 2 } \right)^2 $ $ = {1 \over 8 } . {1 \over 12} ML^2 $ $ I' = { I \over 8 } $ $ \therefore new time period T' = \sqrt [2 \pi] { I' \over M'BH} $ $ = \sqrt [ 2 \pi ] { { I \over 8 } \over \left( M \over 2 \right) BH} $ $ = {1 \over 2 } \sqrt [ 2 \pi ] { I \over MH } $ $ = { T \over 2 } $ $ = { 4 \over 2 } = 2 sec $

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A thin magnetic needle oscillates in a horizontal plane with a period T. It is broken into nequal parts. The time period of each part will

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Explanation

Moment of inertio $ I = { mass \times (length)^2 \over 12 } $ $ \left ( \therefore I = { 1 \over 12} ML^2 \right) $ $ = { 1 \over n} \times \left( { 1 \over n} \right) ^2 $ $ = { 1 \over n^3 } time $ $ Magnetic moment M = pole strength \times length$ $ = { 1 \over n} time $ $ T = \sqrt [ 2 \pi ] { I \over MH } $ $ = \sqrt { {1 \over n^3 } \over {1 \over n} } time$ $ = {1 \over n } time $ $ \therefore T^1 = { T \over n} sec $

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The plane of a dip circle is set in the geographic meridian and the apparent dip is $ \delta _1 $ . It is then set in a vertical plane per perpendicular to the geographic meridian. The appartent dip angle is $ \delta _2 $ . The declination at the place is

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Explanation

$ tan \delta_1 = { V \over Hcos \theta } $ $ tan \delta_2 = { V \over Hcos (90 ^\circ - \theta ) } = { V \over Hsin \theta } $ $ { tan \delta_1 \over tan \delta_2 } = { sin \theta \over cos \theta } $ ${ tan \delta_1 \over tan \delta_2 } = tan \theta $ $ \theta = tan^{-1} \left( tan \delta_1 \over tan \delta_2 \right) $

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The coercivity of a bar magnet is 100 A/m. It is to be demagnetised byplacing it inside a solenoid of length 100 cm and number of turms 50. The current flowing through the solenoid will be

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Explanation

Coercivity H = 100 A/m ; l = 100 cm = 1m ; n =50 $ As H = nI \Rightarrow I = {H \over n} ={ 100 \over 50} = 2 Amp $

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The angles of dip at two places are $30^\circ $ and $ 45 ^\circ $ . The ratio of horizontal components of earth's magnetic field at the two places will be

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Explanation

$ H_1 = B cos \delta_1 ; H_2 = B cos \delta_2 $ $ \therefore { H_1 \over H_2 } = { cos 30^ \circ \over cos 45^ \circ } $ $ = { \sqrt 3 \over \sqrt 2 } $

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Read the assertion and reason carefully to mark the correct option out of the options given below. Assertion : We cannot think of magnetic field configuration with three poles. Reason : A bar magnet does exert a torque on itself due to its own field.

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Read the assertion and reason carefully to mark the correct option out of the options given below. Assertion : If a compass needle be kept at magnetic north pole of the earth, the compass needle may stay in any direction. Reason : Dip needle will stay vertical at the north pole.

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