31.

Taking the earth to be a spherical conductor of diameter 12.8×10³ km. Its capacity will be

• 711 μF

• 611 μF

• 811 μF

• 511 μF

In junction diode, the holes are because of

• Missing electrons

• extra electrons

• protons

• neutrons

The half-life of a radioactive substance is 3.6 days. How much of 20 mg of this radioactive substance will remain after 36 days?

• 0.0019 mg

• 1.019 mg

• 1.109 mg

• 0.019 mg

The kinetic energy of an electron is 5eV. Calculate the de-Broglie wavelength associated with it (h=6.6×10^-34 Js, m_e =9.1×10^-31 kg)

• 5.47A^o

• 10.9A^o

• 2.7A^o

• None of these

The energy in MeV is released due to transformation of 1kg mass completely into energy is (c=3×10⁸ m/s)

• 7.625×10MeV

• 10.5×10²⁹MeV

• 2.8×10^-28MeV

• 5.625×10²⁹MeV

20 kV potential is applied across X-ray tube, the minimum wavelength of X-ray emitted will be

• 0.62 A^o

• 0.37 A^o

• 1.62 A^o

• 1.31 A^o

In the CB mode of a transistor, when the collector voltage is changed by 0.5 volt, the collector current changes by 0.05 mA. The output resistance will be

• 10 kΩ

• 20 kΩ

• 5 kΩ

• 2.5 kΩ

A student can distinctly see the object upto a distance 15 cm. He wants to see the blackboard at a distance of 3m. Focal length and power of lens used respectively will be

• -4.8cm, -3.3D

• -5.8cm, -4.3D

• -7.5cm, -6.3D

• -15.8cm, -6.33D

A thin glass prism (μ =15) in the position of minimum deviation deviates the monochromatic light ray by 10^o, the refracting angle of prism is

• 20^o

• 10^o

• 30^o

• 45^o

Two thin lenses of powers 12 D and-2D respectively are placed in contact, the power, focal length and nature
respectively will be

• 8D, 0.8m, convex

• 14D, 0.5m, convex

• 5D, 0.2m, convex

• 10D, 0.1m, convex