Science (English Medium) Class 12 - CBSE Important Questions for Physics

Using Bohr’s postulates for hydrogen atom, show that the total energy (E) of the electron in the stationary states tan be expressed as the sum of kinetic energy (K) and potential energy (U), where K = −2U. Hence deduce the expression for the total energy in the n^th energy level of hydrogen atom.

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. 

Answer the following question.
State Bohr's quantization condition of angular momentum. Calculate the shortest wavelength of the Bracket series and state to which part of the electromagnetic spectrum it belongs. 

Answer the following question.
Calculate the orbital period of the electron in the first excited state of the hydrogen atom.

Use Bohr’s model of hydrogen atom to obtain the relationship between the angular momentum and the magnetic moment of the revolving electron.

Calculate the de-Broglie wavelength associated with the electron revolving in the first excited state of the hydrogen atom. The ground state energy of the hydrogen atom is −13.6 eV.

Draw a graph showing the variation of the number of particles scattered (N) with the scattering angle θ in the Geiger-Marsden experiment. Why only a small fraction of the particles are scattered at θ > 90°?

State Bohr's postulate to explain stable orbits in a hydrogen atom. Prove that the speed with which the electron revolves in n^th orbit is proportional to `(1/"n")`.

A narrow beam of protons, each having 4.1 MeV energy is approaching a sheet of lead (Z = 82). Calculate:

1. the speed of a proton in the beam, and
2. the distance of its closest approach

A hydrogen atom makes a transition from n = 5 to n = 1 orbit. The wavelength of photon emitted is λ. The wavelength of photon emitted when it makes a transition from n = 5 to n = 2 orbit is ______.

Write the ionisation energy value for the hydrogen atom.

How is the size of a nucleus found experimentally? Write the relation between the radius and mass number of a nucleus.

A nucleus with mass number A = 240 and BE/A = 7.6 MeV breaks into two fragments, each of A = 120 with BE/A = 8.5 MeV. Calculate the released energy.

Calculate the energy in fusion reaction:

`""_1^2H+_1^2H->_2^3He+n`, where BE of `""_1^2H`23He=7.73MeV" data-mce-style="position: relative;">=2.2323He=7.73MeV MeV and of `""_2^3He=7.73 MeV`

Asha's mother read an article in the newspaper about a disaster that took place at Chernobyl. She could not understand much from the articles and asked a few questions from Asha regarding the article. Asha tried to answer her mother's questions based on what she learnt in Class XII Physics.

(a) What was the installation at Chernobyl where the disaster took place? What according to you, was the cause of this disaster?

(b) Explain the process of release of energy in the installation at Chernobyl.

(c) What according to you, were the values displayed by Asha and her mother?

 In a typical nuclear reaction, e.g.

`"_1^2H+"_1^2H ->"_2^3He + n + 3.27 \text { MeV },`

although number of nucleons is conserved, yet energy is released. How? Explain.

Write the relationship between the size of a nucleus and its mass number (A)?

Distinguish between isotopes and isobars.

Two nuclei have different mass numbers A₁ and A₂. Are these nuclei necessarily the isotopes of the same element? Explain.

A heavy nucleus P of mass number 240 and binding energy of 7.6 MeV per nucleon splits into two nuclei Q and R of mass number 110 and 130 and binding energy per nucleon of 8.5 MeV and 8.4 MeV respectively. Calculate the energy released in fission.