Science (English Medium) कक्षा १२ - CBSE Question Bank Solutions for Physics

A rod of rest length L moves at a relativistic speed. Let L' = L/γ. Its length

(a) must be equal to L'
(b) may be equal to L
(c) may be more than L' but less than L
(d) may be more than L

When a rod moves at a relativistic speed v, its mass ________________ .

A filter transmits only the radiation of wavelength greater than 440 nm. Radiation from a hydrogen-discharge tube goes through such a filter and is incident on a metal of work function 2.0 eV. Find the stopping potential which can stop the photoelectrons.

The earth revolves round the sun due to gravitational attraction. Suppose that the sun and the earth are point particles with their existing masses and that Bohr's quantization rule for angular momentum is valid in the case of gravitation. (a) Calculate the minimum radius the earth can have for its orbit. (b) What is the value of the principal quantum number n for the present radius? Mass of the earth = 6.0 × 10⁻²⁴ kg. Mass of the sun = 2.0 × 10³⁰ kg, earth-sun distance = 1.5 × 10¹¹ m.

Consider a neutron and an electron bound to each other due to gravitational force. Assuming Bohr's quantization rule for angular momentum to be valid in this case, derive an expression for the energy of the neutron-electron system.

Suppose in an imaginary world the angular momentum is quantized to be even integral multiples of h/2π. What is the longest possible wavelength emitted by hydrogen atoms in visible range in such a world according to Bohr's model?

By what fraction does the mass of a spring change when it is compressed by 1 cm? The mass of the spring is 200 g at its natural length and the spring constant is 500 N m⁻¹.

Find the increase in mass when 1 kg of water is heated from 0°C to 100°C. Specific heat capacity of water = 4200 J kg⁻¹ K⁻¹.

Find the loss in the mass of 1 mole of an ideal monatomic gas kept in a rigid container as it cools down by 100°C. The gas constant R = 8.3 J K⁻¹ mol⁻¹.

By what fraction does the mass of a boy increase when he starts running at a speed of 12 km h⁻¹?

A 100 W bulb together with its power supply is suspended from a sensitive balance. Find the change in the mass recorded after the bulb remains on for 1 year.

The energy from the sun reaches just outside the earth's atmosphere at a rate of 1400 W m⁻². The distance between the sun and the earth is 1.5 × 10¹¹ m.
(a) Calculate the rate  which the sun is losing its mass.
(b) How long will the sun last assuming a constant decay at this rate? The present mass of the sun is 2 × 10³⁰ kg.

An electron and a positron moving at small speeds collide and annihilate each other. Find the energy of the resulting gamma photon.

Find the mass, the kinetic energy and the momentum of an electron moving at 0.8c.

Through what potential difference should an electron be accelerated to give it a speed of (a) 0.6c, (b) 0.9c and (c) 0.99c?

Find the speed of an electron with kinetic energy (a) 1 eV, (b) 10 keV and (c) 10 MeV.

What is the kinetic energy of an electron in electron volts with mass equal to double its rest mass?

Find the speed at which the kinetic energy of a particle will differ by 1% from its nonrelativistic value \[\frac{1}{2} m_o v^2 .\]

Obtain Bohr’s quantisation condition for angular momentum of electron orbiting in nth orbit in hydrogen  atom on the basis of the wave picture of an electron using de Broglie hypothesis. 

In which of the following systems will the wavelength corresponding to n = 2 to n = 1 be minimum?