A magnetic field of 2x10⁻² T acts at right angles to a coil of area 100 cm² with 5O turns. The average emf induced in the coil is 0.1 V. When it is removed from the field in time t,  the value of t is

• 0.1 s

• 0.01 s

• 1 s

• 10 s

A pan containing a layer of uniform thickness of ice is placed on a circular turntable with its centre coinciding with the centre of the turntable. The turntable is now rotated at a constant angular velocity about a vertical axis passing through its centre and the driving torque is withdrawn. There is no friction between the table and the pivot. The pan rotates with the table. As the ice melts

• the angular velocity of the system decreases

• the angular velocity of the system increases

• the angular velocity of the system remains unchanged

• the moment of inertia of the system decreases

The thermo-emf of a themocouple is 25µV /°C at room temperature. A galvanometer of 40Ω resistance, capable of detecting current as low as 10^-5 A, is connected with the thermocouple. The smallest temperature difference that can be detected by this system is

• 16°C

• 12°C

• 8°C

• 20°C

304.

A particle of mass M and charge Q moving with velocity $\overrightarrow{\mathrm{v}}$ describes a circular path of radius R when subjected to a uniform transverse magnetic field of induction B. The work done by the field when the particle completes one full circle is.

• $\left(\frac{{\mathrm{Mv}}^2}{\mathrm{R}}\right)2\mathrm{\pi R}$

• zero

• BQ 2$\mathrm{\pi }$R

• BQv 2$\mathrm{\pi }$R

The sensitiveness of a moving coil galvanometer can be increased by decreasing the

• number of turns in the coils

• area of the coil

• magnetic field

• couple per unit twist of the suspension

The earth's magnetic induction at a certain point is 7 x 10^-5 Wb/m². This is to be annuled by the magnetic induction at the centre of a circular conducting loop of radius 5 cm. The required current in the loop is

• 0.56 A

• 5.6 A

• 0.28 A

• 2.8 A

Susceptibility of a magnetic field is

• $\mathrm{{\rm X}} = \frac{\mathrm{l}}{\mathrm{H}}$

• $\mathrm{{\rm X}} = \frac{\mathrm{B}}{\mathrm{H}}$

• $\mathrm{{\rm X}} = \frac{\mathrm{M}}{\mathrm{V}}$

• $\mathrm{{\rm X}} = \frac{\mathrm{M}}{\mathrm{H}}$

An electron is moving with velocity v in the direction of magnetic field B, then force acting on electron is

• zero

• e (v x B)

• e (B x v)

• 200 J

A wire carrying current i and other carrying is in the same direction produce a magnetic  field B at the midpoint. What will be the field when 2i current is switched off ?

• B/2

• 2B

• B

• 4B

Two particles A and B of masses m and 2m have charges q and 2q respectively. Both  particles moving with velocities v₁ and v₂ respectively in the same direction and enter the same magnetic field B acting normally to their direction of motion. If the two forces F_A and F_B acting on them are in the ratio of 1 : 2, the ratio of their velocities is

• 2 : 1

• 3 : 2

• 2 : 3

• 1 : 1