31.

A coil of inductance 300 mH and resistance 2 0 is connected to a source of voltage 2 V. The current reaches half of its steady state value in

• 0.05 s

• 0.1 s

• 0.15 s

• 0.3 s

For a metallic wire, the ratio $\frac{\mathrm{V}}{\mathrm{i}}$ ( V = applied potential difference and i = current flowing) is

• independent of temperature

• increases as the temperature rises

• decreases as the temperature rises

• increases or decreases as temperature rises depending upon the metal 

In radioactive decay process, the negatively charged emitted -particles are 

• the electrons present inside the nucleus

• the electrons produced as a result of the decay of neutrons inside the nucleus

• the electrons produced as a result of collisions between atoms

• the electrons orbiting around the nucleus

The frequency of a light wave in a material is 2 x 10¹⁴ Hz and wavelength is 5000 A^o.  The refractive index of material will be

• 1.40

• 1.50 

• 3.00

• 1.33

Two radioactive substances A and B have decay constants 5 λ and λ respectively. At t = 0 they have the same number of nuclei. The ratio of number of nuclei of A to those of B will be ${\left(\frac{1}{\mathrm{e}}\right)}^2$ after a time interval

• $\frac{1}{4\mathrm{\lambda }}$

• 4λ

• 2λ

• $\frac{1}{2\mathrm{\lambda }}$

The refractive index of glass is 1.520 for red light and 1.525 for blue light. Let D₁ and D₂ be angles of minimum deviation for red and blue light respectively in a prism of this glass. then,

• D₁ < D₂

• D₁ = D₂

• D₁ can be less than or greater than D₂ depending upon the angle of prism

• D₁ > D₂

If M_o is the mass of an oxygen isotope ₈O¹⁷ , M_P and M_n are the masses of a proton and a neutron, respectively, the nuclear binding energy of the isotope is

• ( M_o - 8 M_P ) c² 

• ( M_o - 8 M_P - 9 M_c ) c²

• M_o C²

• (M_o - 17 M_n ) c²

A sound absorber attenuates the sound level by 20 dB. The intensity decreases by a factor of

• 1000

• 10000

• 10

• 100

In a laboratory four convex lenses L₁ ,L₂ , L₃ and L₄ of focal lengths 2, 4, 6 and 8 cm,respectively are available. Two of these lenses form a telescope of length 10 cm and magnifying power 4. The objective and eye lenses are respectively

• L₂ , L₃

• L₁, L₄

• L₁ , L₂

• L₄ , L₁

A symmetric double convex lens is cut in two equal parts by a plane perpendicular to the principal axis. If the power of the original lens is 4D, the power of a cut lens will be

• 2D

• 3D

• 4D

• 5D