31.

The work done in increasing the size of a soap film from 10 cm x 6 cm to 10 cm x 11 cm is 3 x 10^-4 J. The surface tension of the film is

• 1.5 × 10^-2 N/m

• 3 × 10^-2 N/m

• 6.0 × 10^-2 N/m

• 11 × 10^-2 N/m

The outer sphere of a spherical air capacitor is earthed. For increasing its capacitance

• vacuum is created between two spheres

• dielectric material is filled between the two
  spheres

• the space between two spheres is increased

• the earthing of the outer sphere is removed

When a galvanometer is shunted by resistance S, its current capacity increases n times. If the same galvanometer is shunted by another resistance S', its current capacity will increase by n' is given by

• $\frac{(\mathrm{n}+1)\mathrm{S}}{\mathrm{S}\text{'}}$

• $\frac{\mathrm{S} (\mathrm{n}-1)+ \mathrm{S}\text{'}}{\mathrm{S}\text{'}}$

• $\frac{\mathrm{n}+\mathrm{S}}{\mathrm{S}\text{'}}$

• $\frac{\mathrm{S}(\mathrm{n}-1) + \mathrm{S}\text{'}}{\mathrm{S}\text{'}}$

A 100 mH coil carries a current of 1 A. The energy stored in the magnetic field is

• 0.1 J

• 0.05 J

• 0.5 J

• 1 J

A copper rod is suspended in a non-homogeneous magnetic field region. The rod when in equilibrium will align itself

• in the region where magnetic field is strongest

• in the region where magnetic field is weakest and parallel to direction of magnetic field there

• in the direction in which it was originally suspended

• in the region where magnetic field is weakest and perpendicular to the direction of magnetic field there

What is the area of the plates of a 3F parallel plate capacitor, if the separation between the plates is 5 mm?

• 1.694 × 10⁹ m²

• 4.529 × 10⁹ m²

• 9.281 × 10⁹ m²

• 12.981 × 10⁹ m²

A potentiometer has a uniform potential gradient across it. Two cells connected in series (i).to support each other and (ii).to oppose each other, are balanced over 6m and 2m, respectively on the potentiometer wire. The emf's of the cells are in the ratio of

• 1 : 2

• 1 : 1

• 3 : 1

• 2 : 1

A condenser of capacitance 6 µF was originally charged to 10 V. Now potential difference is made 20 V. The increase in potential energy is

• 3×10^-4 J

• 6×10^-4 J

• 9×10^-4 J

• 12×10^-4 J

If there is no deflection in the galvanometer connected in a circuit shown in the figure, then the ratio of lengths AC/CB is

• 4:1

• 1:4

• 1:1

• 2:1

In moving coil galvanometer, the magnetic field used is

• non-uniform

• radial

• uniform

• None of these