Page 1
IIT-JAM 2020
SECTION – A
MULTIPLE CHOICE QUESTIONS (MCQ)
Q1 – Q10 carry one mark each.
Q1. Which one of the following functions has a discontinuity in the second derivative at 0 x ? ,
where x is a real variable?
(a) ??
3
fxx ? (b) ?? fxxx ? (c) ? ? ? ?
cos fxx ? (d) ??
2
fxx ?
Ans. : (b)
Solution:
??
2
2
2 dx
xx
dx x
?
Assuming a function to be real.
Plots:
Q2. A collimated beam of laser light of wavelength 514nm is normally incident on a smooth
glass slab placed in air. Given the refractive indices of glass and air are 1.47 and 1.0 ,
respectively, the percentage of light intensity reflected back is
(a) 0 (b) 4.0 (c) 3.6 (d) 4.2
Ans. : (c)
Solution:
2
12
12
11.47
0.036 0.036 100 3.6%
11.47
nn
R
nn
?? ?? ??
?? ? ? ??
????
??
??
??
Q3. Two stationary point particles with equal and opposite charges are at some fixed distance
from each other. The points having zero electric potential lie on:
(a) A sphere (b) A plane (c) A cylinder (d) Two parallel planes
Ans. : (b)
Solution: All points on the plane are at equal distance from charges, so potential will be zero.
y
2
1
1 ? 2 ? 1 2
x
1 ?
2 ?
? ? from 2 to 2 x ?
Page 2
IIT-JAM 2020
SECTION – A
MULTIPLE CHOICE QUESTIONS (MCQ)
Q1 – Q10 carry one mark each.
Q1. Which one of the following functions has a discontinuity in the second derivative at 0 x ? ,
where x is a real variable?
(a) ??
3
fxx ? (b) ?? fxxx ? (c) ? ? ? ?
cos fxx ? (d) ??
2
fxx ?
Ans. : (b)
Solution:
??
2
2
2 dx
xx
dx x
?
Assuming a function to be real.
Plots:
Q2. A collimated beam of laser light of wavelength 514nm is normally incident on a smooth
glass slab placed in air. Given the refractive indices of glass and air are 1.47 and 1.0 ,
respectively, the percentage of light intensity reflected back is
(a) 0 (b) 4.0 (c) 3.6 (d) 4.2
Ans. : (c)
Solution:
2
12
12
11.47
0.036 0.036 100 3.6%
11.47
nn
R
nn
?? ?? ??
?? ? ? ??
????
??
??
??
Q3. Two stationary point particles with equal and opposite charges are at some fixed distance
from each other. The points having zero electric potential lie on:
(a) A sphere (b) A plane (c) A cylinder (d) Two parallel planes
Ans. : (b)
Solution: All points on the plane are at equal distance from charges, so potential will be zero.
y
2
1
1 ? 2 ? 1 2
x
1 ?
2 ?
? ? from 2 to 2 x ?
Q4. For a system undergoing a first order phase transition at a temperature
c
T , which one of the
following graphs best describes the variation of entropy (S ) as a function of temperature
(T )?
(a) (b) (c) (d)
Ans. : (d)
Solution:
P
g
S
T
? ??
??
??
?
??
specific entropy. In first order Phase transition first order derivative of
specific Gibbs free energy is discontinuous that is specific entropy.
Q5. In a photoelectric effect experiment, a monochromatic light source emitting photon with
energy greater than the work function of the metal under test is used. If the power of the
light source is doubled, which one of the following statements is correct?
(a) The number of emitted photoelectrons remains the same
(b) The stopping potential remains the same
(c) The number of emitted photoelectrons decreases
(d) The stopping potential doubles
Ans.: (b)
Solution: Stopping potential is not defined at power of light source
Q6. The figure below shows a cubic unit cell with lattice constant a . The shaded
crystallographic plane intersects the x -axis at 0.5a . The Miller indices of the shaded plane
are
(a) ?? 210
(b)
??
210
(c) ?? 110
(d) ?? 102
S
c
T
T
S
c
T
T
S
c
T
T
S
c
T
T
x
y
a
z
Page 3
IIT-JAM 2020
SECTION – A
MULTIPLE CHOICE QUESTIONS (MCQ)
Q1 – Q10 carry one mark each.
Q1. Which one of the following functions has a discontinuity in the second derivative at 0 x ? ,
where x is a real variable?
(a) ??
3
fxx ? (b) ?? fxxx ? (c) ? ? ? ?
cos fxx ? (d) ??
2
fxx ?
Ans. : (b)
Solution:
??
2
2
2 dx
xx
dx x
?
Assuming a function to be real.
Plots:
Q2. A collimated beam of laser light of wavelength 514nm is normally incident on a smooth
glass slab placed in air. Given the refractive indices of glass and air are 1.47 and 1.0 ,
respectively, the percentage of light intensity reflected back is
(a) 0 (b) 4.0 (c) 3.6 (d) 4.2
Ans. : (c)
Solution:
2
12
12
11.47
0.036 0.036 100 3.6%
11.47
nn
R
nn
?? ?? ??
?? ? ? ??
????
??
??
??
Q3. Two stationary point particles with equal and opposite charges are at some fixed distance
from each other. The points having zero electric potential lie on:
(a) A sphere (b) A plane (c) A cylinder (d) Two parallel planes
Ans. : (b)
Solution: All points on the plane are at equal distance from charges, so potential will be zero.
y
2
1
1 ? 2 ? 1 2
x
1 ?
2 ?
? ? from 2 to 2 x ?
Q4. For a system undergoing a first order phase transition at a temperature
c
T , which one of the
following graphs best describes the variation of entropy (S ) as a function of temperature
(T )?
(a) (b) (c) (d)
Ans. : (d)
Solution:
P
g
S
T
? ??
??
??
?
??
specific entropy. In first order Phase transition first order derivative of
specific Gibbs free energy is discontinuous that is specific entropy.
Q5. In a photoelectric effect experiment, a monochromatic light source emitting photon with
energy greater than the work function of the metal under test is used. If the power of the
light source is doubled, which one of the following statements is correct?
(a) The number of emitted photoelectrons remains the same
(b) The stopping potential remains the same
(c) The number of emitted photoelectrons decreases
(d) The stopping potential doubles
Ans.: (b)
Solution: Stopping potential is not defined at power of light source
Q6. The figure below shows a cubic unit cell with lattice constant a . The shaded
crystallographic plane intersects the x -axis at 0.5a . The Miller indices of the shaded plane
are
(a) ?? 210
(b)
??
210
(c) ?? 110
(d) ?? 102
S
c
T
T
S
c
T
T
S
c
T
T
S
c
T
T
x
y
a
z
Ans. : (a)
Solution: (i) Intercepts along ,, xyz - axis
,,
2
a
xyaz ?? ??
(ii) Divide by lattice parameters
/2 1
,, ,1,
2
aa
aaa
?
??
(iii) Take reciprocal, 2,1,0
? The Miller indices is ?? 210
Q7. For a particle moving in a central potential, which one of the following statements is
correct?
(a) The motion is restricted to a plane due to the conservation of angular momentum
(b) The motion is restricted to a plane due to the conservation of energy only
(c) The motion is restricted to a plane due to the conservation of linear momentum
(d) The motion is not restricted to a plane
Ans. : (a)
Solution: For central force problem angular momentum J
?
is conserved and .0 rJ ?
?
?
which ensure
that motion of particle is confined in plane
Q8. Consider the motion of a quantum particle of mass m and energy E under the influence of
a step potential of height
0
V . If R denotes the reflection coefficient, which one of the
following statements is true?
(a) If
0
4
,1
3
EVR ?? (b)
0
4
,0
3
EVR ? ?
(c)
0
1
,1
2
EVR ?? (d)
0
1
,0.5
2
EVR ??
Ans. : (c)
Solution:
0
2
V
E ?
0
EV ?? , so 1 R ?
Therefore, option (c) is correct
x
0
V
Page 4
IIT-JAM 2020
SECTION – A
MULTIPLE CHOICE QUESTIONS (MCQ)
Q1 – Q10 carry one mark each.
Q1. Which one of the following functions has a discontinuity in the second derivative at 0 x ? ,
where x is a real variable?
(a) ??
3
fxx ? (b) ?? fxxx ? (c) ? ? ? ?
cos fxx ? (d) ??
2
fxx ?
Ans. : (b)
Solution:
??
2
2
2 dx
xx
dx x
?
Assuming a function to be real.
Plots:
Q2. A collimated beam of laser light of wavelength 514nm is normally incident on a smooth
glass slab placed in air. Given the refractive indices of glass and air are 1.47 and 1.0 ,
respectively, the percentage of light intensity reflected back is
(a) 0 (b) 4.0 (c) 3.6 (d) 4.2
Ans. : (c)
Solution:
2
12
12
11.47
0.036 0.036 100 3.6%
11.47
nn
R
nn
?? ?? ??
?? ? ? ??
????
??
??
??
Q3. Two stationary point particles with equal and opposite charges are at some fixed distance
from each other. The points having zero electric potential lie on:
(a) A sphere (b) A plane (c) A cylinder (d) Two parallel planes
Ans. : (b)
Solution: All points on the plane are at equal distance from charges, so potential will be zero.
y
2
1
1 ? 2 ? 1 2
x
1 ?
2 ?
? ? from 2 to 2 x ?
Q4. For a system undergoing a first order phase transition at a temperature
c
T , which one of the
following graphs best describes the variation of entropy (S ) as a function of temperature
(T )?
(a) (b) (c) (d)
Ans. : (d)
Solution:
P
g
S
T
? ??
??
??
?
??
specific entropy. In first order Phase transition first order derivative of
specific Gibbs free energy is discontinuous that is specific entropy.
Q5. In a photoelectric effect experiment, a monochromatic light source emitting photon with
energy greater than the work function of the metal under test is used. If the power of the
light source is doubled, which one of the following statements is correct?
(a) The number of emitted photoelectrons remains the same
(b) The stopping potential remains the same
(c) The number of emitted photoelectrons decreases
(d) The stopping potential doubles
Ans.: (b)
Solution: Stopping potential is not defined at power of light source
Q6. The figure below shows a cubic unit cell with lattice constant a . The shaded
crystallographic plane intersects the x -axis at 0.5a . The Miller indices of the shaded plane
are
(a) ?? 210
(b)
??
210
(c) ?? 110
(d) ?? 102
S
c
T
T
S
c
T
T
S
c
T
T
S
c
T
T
x
y
a
z
Ans. : (a)
Solution: (i) Intercepts along ,, xyz - axis
,,
2
a
xyaz ?? ??
(ii) Divide by lattice parameters
/2 1
,, ,1,
2
aa
aaa
?
??
(iii) Take reciprocal, 2,1,0
? The Miller indices is ?? 210
Q7. For a particle moving in a central potential, which one of the following statements is
correct?
(a) The motion is restricted to a plane due to the conservation of angular momentum
(b) The motion is restricted to a plane due to the conservation of energy only
(c) The motion is restricted to a plane due to the conservation of linear momentum
(d) The motion is not restricted to a plane
Ans. : (a)
Solution: For central force problem angular momentum J
?
is conserved and .0 rJ ?
?
?
which ensure
that motion of particle is confined in plane
Q8. Consider the motion of a quantum particle of mass m and energy E under the influence of
a step potential of height
0
V . If R denotes the reflection coefficient, which one of the
following statements is true?
(a) If
0
4
,1
3
EVR ?? (b)
0
4
,0
3
EVR ? ?
(c)
0
1
,1
2
EVR ?? (d)
0
1
,0.5
2
EVR ??
Ans. : (c)
Solution:
0
2
V
E ?
0
EV ?? , so 1 R ?
Therefore, option (c) is correct
x
0
V
Q9. The Boolean function
?? ? ?
PQ P Q Q Q ?? is equivalent to:
(a) P (b) P (c) PQ (d) PQ
Ans. : (b)
Solution:
????
Y PQP QQQ ?? ?
? ? ? ?
.1 YPQPQ PPQPQ ?? ? ? ? ? ?
??
??
11 YP Q PQ P Q P ?? ? ? ? ? ?
Q10. Three point charges each carrying a charge q are placed on the vertices of an equilateral
triangle of side L . The electrostatic potential energy of the configuration is:
(a)
2
0
1
4
q
L ??
(b)
2
0
2
4
q
L ??
(c)
2
0
3
4
q
L ??
(d)
2
0
1 q
L ??
Ans. 10: (c)
Solution: Work done in placing charges
1
0 W ? ,
2
2
0
1
4
q
W
L ??
?
and
2
3
00
112
44
qq q
Wq
LL L ?? ??
??
???
??
??
Also Total work done ??
22
12 3
00
13
12
44
qq
WW W
LL ?? ??
? ??? ? ?
q
q q
?
? ?
L L
L
1
2 3
Page 5
IIT-JAM 2020
SECTION – A
MULTIPLE CHOICE QUESTIONS (MCQ)
Q1 – Q10 carry one mark each.
Q1. Which one of the following functions has a discontinuity in the second derivative at 0 x ? ,
where x is a real variable?
(a) ??
3
fxx ? (b) ?? fxxx ? (c) ? ? ? ?
cos fxx ? (d) ??
2
fxx ?
Ans. : (b)
Solution:
??
2
2
2 dx
xx
dx x
?
Assuming a function to be real.
Plots:
Q2. A collimated beam of laser light of wavelength 514nm is normally incident on a smooth
glass slab placed in air. Given the refractive indices of glass and air are 1.47 and 1.0 ,
respectively, the percentage of light intensity reflected back is
(a) 0 (b) 4.0 (c) 3.6 (d) 4.2
Ans. : (c)
Solution:
2
12
12
11.47
0.036 0.036 100 3.6%
11.47
nn
R
nn
?? ?? ??
?? ? ? ??
????
??
??
??
Q3. Two stationary point particles with equal and opposite charges are at some fixed distance
from each other. The points having zero electric potential lie on:
(a) A sphere (b) A plane (c) A cylinder (d) Two parallel planes
Ans. : (b)
Solution: All points on the plane are at equal distance from charges, so potential will be zero.
y
2
1
1 ? 2 ? 1 2
x
1 ?
2 ?
? ? from 2 to 2 x ?
Q4. For a system undergoing a first order phase transition at a temperature
c
T , which one of the
following graphs best describes the variation of entropy (S ) as a function of temperature
(T )?
(a) (b) (c) (d)
Ans. : (d)
Solution:
P
g
S
T
? ??
??
??
?
??
specific entropy. In first order Phase transition first order derivative of
specific Gibbs free energy is discontinuous that is specific entropy.
Q5. In a photoelectric effect experiment, a monochromatic light source emitting photon with
energy greater than the work function of the metal under test is used. If the power of the
light source is doubled, which one of the following statements is correct?
(a) The number of emitted photoelectrons remains the same
(b) The stopping potential remains the same
(c) The number of emitted photoelectrons decreases
(d) The stopping potential doubles
Ans.: (b)
Solution: Stopping potential is not defined at power of light source
Q6. The figure below shows a cubic unit cell with lattice constant a . The shaded
crystallographic plane intersects the x -axis at 0.5a . The Miller indices of the shaded plane
are
(a) ?? 210
(b)
??
210
(c) ?? 110
(d) ?? 102
S
c
T
T
S
c
T
T
S
c
T
T
S
c
T
T
x
y
a
z
Ans. : (a)
Solution: (i) Intercepts along ,, xyz - axis
,,
2
a
xyaz ?? ??
(ii) Divide by lattice parameters
/2 1
,, ,1,
2
aa
aaa
?
??
(iii) Take reciprocal, 2,1,0
? The Miller indices is ?? 210
Q7. For a particle moving in a central potential, which one of the following statements is
correct?
(a) The motion is restricted to a plane due to the conservation of angular momentum
(b) The motion is restricted to a plane due to the conservation of energy only
(c) The motion is restricted to a plane due to the conservation of linear momentum
(d) The motion is not restricted to a plane
Ans. : (a)
Solution: For central force problem angular momentum J
?
is conserved and .0 rJ ?
?
?
which ensure
that motion of particle is confined in plane
Q8. Consider the motion of a quantum particle of mass m and energy E under the influence of
a step potential of height
0
V . If R denotes the reflection coefficient, which one of the
following statements is true?
(a) If
0
4
,1
3
EVR ?? (b)
0
4
,0
3
EVR ? ?
(c)
0
1
,1
2
EVR ?? (d)
0
1
,0.5
2
EVR ??
Ans. : (c)
Solution:
0
2
V
E ?
0
EV ?? , so 1 R ?
Therefore, option (c) is correct
x
0
V
Q9. The Boolean function
?? ? ?
PQ P Q Q Q ?? is equivalent to:
(a) P (b) P (c) PQ (d) PQ
Ans. : (b)
Solution:
????
Y PQP QQQ ?? ?
? ? ? ?
.1 YPQPQ PPQPQ ?? ? ? ? ? ?
??
??
11 YP Q PQ P Q P ?? ? ? ? ? ?
Q10. Three point charges each carrying a charge q are placed on the vertices of an equilateral
triangle of side L . The electrostatic potential energy of the configuration is:
(a)
2
0
1
4
q
L ??
(b)
2
0
2
4
q
L ??
(c)
2
0
3
4
q
L ??
(d)
2
0
1 q
L ??
Ans. 10: (c)
Solution: Work done in placing charges
1
0 W ? ,
2
2
0
1
4
q
W
L ??
?
and
2
3
00
112
44
qq q
Wq
LL L ?? ??
??
???
??
??
Also Total work done ??
22
12 3
00
13
12
44
qq
WW W
LL ?? ??
? ??? ? ?
q
q q
?
? ?
L L
L
1
2 3
Q11 - Q30 carry two marks each.
Q11. Which of the following statements is correct?
Given
??
!
!!
n
n
m mn m
??
?
??
?
??
is the binomial coefficient
(a)
22 4 4
cos cos cos sin cos sin ...
24
nn n
nn
n?? ? ? ? ?
??
?? ??
?? ? ?
?? ??
?? ??
(b)
133
sin cos sin cos sin ...
13
nn
nn
n??? ??
??
?? ??
?? ?
?? ??
?? ??
(c)
22 44
cos cos cos sin cos sin ....
24
nn n
nn
n? ? ?? ??
??
?? ??
?? ? ?
?? ??
?? ??
(d)
22 44
sin cos cos sin cos sin ....
24
nn n
nn
n?? ? ? ? ?
??
?? ??
?? ? ?
?? ??
?? ??
Ans. : (a)
Solution: Let’s invoke De Moivre’s theorem here. We have
?? ?? ? ? ? ? ? ?
cos sin cos sin
n
ni n i ? ?? ? ?? ?
Since n is a positive integer, the binomial theorem holds,
?? ?? ??
cos sin
n
i ? ? ?
Hence, by expanding, we have
?? ?? ?? ?? ? ? ??
? ?
?? ? ?
1222
1
cos sin cos cos sin cos sin ....
12
n
nn n
nn
ini i ?? ? ? ? ? ?
??
?
?? ? ? ? ? ?
?
Since
23 4 5
1, , 1, ,.... ii ii ii ?? ?? ? ? , we have
?? ?? ?? ??
? ?
?? ??
22
1
cos sin cos cos sin
12
n
nn
nn
i ? ?? ? ?
?
? ?
?? ? ?
?
?
?
??? ?? ?
?? ??
44
12 3
cos sin ....
12 3 4
n
nn n n
??
?
? ?? ?
???
?
?? ?
?
?? ??
? ? ? ? ? ?
?? ? ?
133
12 3
cos sin cos sin ....
12 3
nn
nn n n
in ?? ? ?
??
?? ?? ?
?? ? ? ?
??
??
??
By equating the real and imaginary parts, we obtain
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