31. At a place on earth, horizontal component of earth’s magnetic field is $${B_1}$$ and vertical component of earth’s magnetic field is $${B_2}.$$ If a magnetic needle is kept vertical, in a plane making angle $$\alpha $$ with the horizontal component of magnetic field, then square of time period of oscillation of needle when slightly distributed is proportional to
A
$$\frac{1}{{\sqrt {{B_1}\cos \alpha } }}$$
B
$$\frac{1}{{\sqrt {{B_2}} }}$$
C
$$\frac{1}{{\sqrt {{{\left( {{B_1}\cos \alpha } \right)}^2} + B_2^2} }}$$
D
infinite
Answer :
$$\frac{1}{{\sqrt {{{\left( {{B_1}\cos \alpha } \right)}^2} + B_2^2} }}$$
32. At a temperature of $${30^ \circ }C,$$ the susceptibility of a ferromagnetic material is found to be $$\chi .$$ Its susceptibility at $${333^ \circ }C$$ is
A
$$\chi $$
B
$$0.5\,\chi $$
C
$$2\,\chi $$
D
$$11.1\,\chi $$
Answer :
$$0.5\,\chi $$
33. A diamagnetic substance is brought near a strong magnet, then it is
A
attracted by a magnet
B
repelled by a magnet
C
repelled by North pole and attracted by South pole
D
attracted by North pole and repelled by South pole
Answer :
repelled by a magnet
34. A short bar magnet of magnetic moment $$0.4\,J{T^{ - 1}}$$ is placed in a uniform magnetic field of $$0.16\,T.$$ The magnet is in stable equilibrium when the potential energy is
A
$$ - 0.64\,J$$
B
zero
C
$$ - 0.082\,J$$
D
$$ - 0.064\,J$$
Answer :
$$ - 0.064\,J$$
35.
A dip needle lies initially in the magnetic meridian when it shows an angle of dip $$\theta $$ at a place. The dip circle is rotated through an angle $$x$$ in the horizontal plane and then it shows an angle of dip $$\theta '.$$
Then $$\frac{{\tan \theta '}}{{\tan \theta }}$$ is
A
$$\frac{1}{{\cos x}}$$
B
$$\frac{1}{{\sin x}}$$
C
$$\frac{1}{{\tan x}}$$
D
$${\cos x}$$
Answer :
$$\frac{1}{{\cos x}}$$
36. A Rowland ring of mean radius $$15\,cm$$ has $$3500$$ turns of wire wound on a ferromagnetic core of relative permeability $$800.$$ What is the magnetic field $$B$$ in the core for a magnetising current of $$1.2\,amp$$ ?
A
$$4.48\,T$$
B
$$2.48\,T$$
C
$$5.48\,T$$
D
$$3.12\,T$$
Answer :
$$4.48\,T$$
37. A magnetising field of $$2 \times {10^3}\,amp/m$$ produces a magnetic flux density of $$8\pi \,Tesla$$ in an iron rod. The relative permeability of the rod will be -
A
$${10^2}$$
B
$${10^0}$$
C
$${10^4}$$
D
$${10^1}$$
Answer :
$${10^4}$$
38. The true value of angle of dip at a place is $${60^ \circ },$$ the apparent dip in a plane inclined at an angle of $${30^ \circ }$$ with magnetic meridian is
A
$${\tan ^{ - 1}}\frac{1}{2}$$
B
$${\tan ^{ - 1}}\left( 2 \right)$$
C
$${\tan ^{ - 1}}\left( {\frac{2}{3}} \right)$$
D
$${\tan ^{ - 1}}\left( {\frac{{\sqrt 3 }}{4}} \right)$$
Answer :
$${\tan ^{ - 1}}\left( 2 \right)$$
39. The magnetic field of earth at the equator is approximately $$4 \times {10^{ - 5}}T.$$ The radius of earth is $$6.4 \times {10^6}m.$$ Then the dipole moment of the earth will be nearly of the order of :
A
$${10^{23}}A{m^2}$$
B
$${10^{20}}A{m^2}$$
C
$${10^{16}}A{m^2}$$
D
$${10^{10}}A{m^2}$$
Answer :
$${10^{23}}A{m^2}$$
40. A $$25\,cm$$ long solenoid has radius $$2\,cm$$ and 500 total number of turns. It carries a current of $$15\,A.$$ If it is equivalent to a magnet of the same size and magnetization $$\vec M\left( {\frac{{{\text{magnetic moment}}}}{{{\text{volume}}}}} \right),$$ then $$\left| {\vec M} \right|$$ is:
A
$$30000\pi \,A{m^{ - 1}}$$
B
$$3\pi \,A{m^{ - 1}}$$
C
$$30000\,A{m^{ - 1}}$$
D
$$300\,A{m^{ - 1}}$$
Answer :
$$30000\,A{m^{ - 1}}$$