"Men often become what they believe
themselves to be. If I believe I cannot do something, it makes me incapable of
doing it. But when I believe I can, then I acquire the ability to do it even if
I didn't have it in the beginning

– Mahatma Gandhi

AP Physics 2012
exams are just a few days away. Your final preparations for the exam must be in
full swing and there is no time to waste. Today I give you a few multiple
choice practice questions on atomic physics and quantum effects. Questions in
this section posted earlier on this site (with solution) can be accessed by
clicking on the label ‘atomic physics and quantum effects’ given below this
post. Or, you may try a search for ‘atomic physics and quantum effects’ using
the search box provided on this page.

(1) The adjoining
figure shows the graphical relation between the frequency of incident radiation
and the magnitude of stopping potential in the case of metals A and B. Note
that the straight line graphs have the same slope. Which one among the
following statements is correct in this case?

(a) A and B have the same work function

(b) A and B have the same threshold wave length

(c) Maximum kinetic energy of photoelectron in
the case of metals A and B is directly proportional to the increment in
frequency of incident radiation over the threshold frequency.

(d) Metal B is a better photosensitive material
than metal A.

(e) For a given change in the frequency of
incident radiation, the changes in stopping potentials are different for metals
A and B

Since we have
straight line graphs for both metals, maximum
kinetic energy of photoelectron is directly proportional to the increment in
frequency of incident radiation over the threshold frequency. Therefore option
(c) is correct.

(2) The de Broglie
wave length of a particle can be reduced to half its initial value by changing
its kinetic energy to

(a) half the initial value

(b) twice the initial value

(c) three times the initial value

(d) four times the initial value

(e) a quarter of the initial value

Kinetic energy

*E*of a body of mass*m*is given by*E*=

*p*

^{2}/2

*m*where

*p*is the momentum

[This follows from

*E*= ½*mv*^{2}=*m*^{2}*v*^{2}/2*m*. Here*v*is the velocity and*mv = p*]
The above relation
shows that the kinetic energy becomes four times when the momentum is doubled.

The de Broglie wave
length

*λ*is given by*λ = h/p*where

*h*is Planck’s constant.

Therefore, the de Broglie wave length of a particle can be
reduced to half its initial value by changing its momentum to

*twice**the initial value. Evidently the kinetic energy of the particle the becomes four times the initial value [Option (d)].*
[Suppose the above question is modifie as
follows:

A particle has de Broglie wave length

*λ*when its kinetic energy is*E*.*What additional kinetic energy is to be aded to it in order to reduce the de Broglie wave length to**λ/*2?
(a)

*E*
(b) 2

*E*
(c) 3

*E*
(d) 4

*E*
(e)

*E/*4
The answer is 3

*E*since you are asked to find the*additional*kinetic energy].
(3) Uranium (atomic
number 92) has an isotope of mass number 235. It can undergo successive
disintegrations to get transformed into lead (

_{82}Pb^{207}). How many*α-*particles and*β-*particles are*emitted during this transformation?*
(a)

*α =*7,*β =*4
(b)

*α =*4,*β =*3
(c)

*α =*7,*β =*0
(d)

*α =*7,*β =*7
(e)

*α =*4,*β =*7
Beta particle
emission does not affect the mass number. In order to reduce the mass number by
28 (from 235 to 207), the number of

*α-*particles to be emitte must be 7. Since each*α-*particles carries two fundamental units of positive charge, the atomic number of the end product gets reduced by 14. But the final product (_{82}Pb^{207}) has its atomic number reduced by 10 only. The extra 4 units must be obtained by the emission of*four**β-*particles. The correct option therefore ia (a).
[Note that when a

*β-*particles (electron) is emitted from the nucleus, the nuclear charge increases by one unit. This happens as a result of the transformation of a neutron in the nucleus into a proton].
(4) Fundamental
forces in nature are gravitational force, electromagnetic force, nuclear force
and weak force. If these forces act over very short distances of the order of
nuclear dimensions, how do you arrange them in decreasing order (starting with
the strongest?

(a) Gravitational force, electromagnetic force,
nuclear force, weak force

(b) Electromagnetic force, gravitational force,
nuclear force, weak force

(c) Electromagnetic force, nuclear force, weak force,
gravitational force

(d) Gravitational force, nuclear force, electromagnetic force, weak
force

(e) Nuclear force, electromagnetic force, weak force, gravitational
force

The correct option
is (d).

[Don’t get carried
away by the term ‘weak force’. The weakest force is gravitational force where
as the strongest is nuclear force].

(5) Two protons are
separated by a distance of 50 Ǻ. If the electromagnetic force between them is F

_{1}and the nuclear force between them is F_{2}, which one among the following is the most reliable statement?
(a) F

_{1}>> F_{2}
(b) F

_{2}>> F_{1}
(c) F

_{1}> F_{2}
(d) F

_{2}> F_{1}
(e) F

_{2}= F_{1}
This question is
similar to question No. (4) in the sense that it is meant for checking your
knowledge of nuclear physics. The correct option is (a). Nuclear force is a very short range force. At a
separation of 50 Ǻ which is very large compared to the size of a nucleus,
nuclear force (strong interaction) between two protons is negligible compared
to the electrostatic force. Therefore the correct option is (a).

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