“Being ignorant
is not so much a shame as being unwilling to learn.”
– Benjamin
Franklin
During March and April I have been unexpectedly busy
and therefore could not post any thing here. Today I give you a few multiple choice practice
questions on atomic physics and quantum effects
along with their solution. This section will be interesting to most of you.
Questions (with solution) in this section were posted on many occasions on this
site. You may access them by clicking on the label ‘atomic physics and quantum effects’
below this post. Here are the additional questions:
(1) The energy of the electron in the hydrogen atom in the ground state
is E1. When the hydrogen
atom is in its first excited state, the energy of its electron is E2. Suppose we have a large
number of hydrogen atoms in the ground state and radiation of frequency (E2 – E1) is incident on these atoms. What happens?
(a) All atoms will jump to the first excited
state.
(b) All atoms will continue to remain in the
ground state.
(c) Some of the atoms will jump to the first
excited state.
(d) Most of the atoms will jump to the second
excited state.
(e) Most of the atoms will get ionised.
Since the energy of each photon in the incident radiation is equal to the
energy difference between the ground state and the first excited state of the hydrogen
atom, the probability of absorption of energy is high so that some of the hydrogen
atoms will jump to the first excited state. The correct option is (c).
(2) The simple Bohr model is applicable to which one of the following?
(a) Atoms with light nuclei
(b) Atoms with heavy nuclei
(c) Helium atoms
(d) Doubly ionised lithium
(e) Uranium atoms
Doubly ionized lithium has just one electron moving around the nucleus
and hence it is hydrogen-like. Therefore the simple Bohr model is applicable to
it [Option (d)].
(3) A proton is moving through a region of space
where a constant electric field acts along its direction of motion. The de
Broglie wave length of the proton
(a) decreases with time
(b) increases with time
(c) remains constant
(d) varies in a sinusoidal manner
(e) goes on increasing first and soon becomes
constant
The de Broglie wave length λ is given by
λ = h/p where h is Planck’s constant and and p is the momentum.
Since the proton is
accelerated by the electric field, its momentum goes on increasing with time.
Therefore, the de Broglie wave length λ
of the proton decreases with time [Option (a)].
(4) An electron is projected at an acute angle with respect to a uniform
magnetic field so that it travels along a helical path. The de Broglie wave
length of the electron
(a) varies periodically
(b) increases with time
(c) remains constant
(d) decreases with time
(e) goes on decreasing first and attains a
constant value
A static magnetic field cannot change the kinetic energy of a charged
particle. The momentum of the electron therefore remains unchanged. Therefore
the de Broglie wave length of the electron remains constant.
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