21.

Minimum and maximum values of Poisson's ratio for a metal lies between:

• - ∞ to + ∞ 

• 0 to 1

• - ∞ to 1

• 0 to 0.5

A wire of diameter 1 mm breaks under a tension of 1000 N. Another wire, of same material as that of the first one, but of diameter 2 mm, breaks under a tension of :

• 500 N

• 100 N

• 1000 N

• 4000 N

In a capillary tube experiment, a vertical, 30 cm long capillary tube is dipped in water. The water rises upto a height of 10 cm due to capillary action. If this experiment is conducted in a freely falling elevator, the length of the water column becomes :

• 10 cm

• 20 cm

• 30 cm

• zero

An aeroplane gets its upward lift due to a phenomenon described by the :

• Archimedes principle

• Bernoulli's principle

• Buoyancy principle

• Pascal law

A wooden block is taken to the bottom of a deep, calm lake of water and then released. It rises up with a :

• constant acceleration

• decreasing aceleration

• constant velocity

• decreasing velocity

A hollow conducting sphere is placed in an electric field produced by a point charge placed at P as shown in the figure. Let V_A, V_B, V_C be the potentials at points A, B and C respectively. Then :

• V_C > V_B

• V_B > V_c

• V_A > V_B

• V_A = V_C

The difference between soft and hard X-rays is of :

• velocity

• intensity

• frequency

• polarization

The capacity of an isolated conducting sphere of radius R is proportional to :

• R²

• 1/R²

• 1/R

• R

Two spheres of radii R₁ and R₂ respectively are charged and joined by a wire. The ratio of the electric field of spheres is :

• $\frac{{\mathrm{R}}_2^2}{{\mathrm{R}}_1^2}$

• $\frac{{\mathrm{R}}_1^2}{{\mathrm{R}}_2^2}$

• $\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}$

• $\frac{{\mathrm{R}}_1}{{\mathrm{R}}_2}$

If a positive charge is shifted from a low potential region to a high potential region, the electric potential energy :

• decreases

• increases

• remains the same

• may increase or decrease