CBSE Physics Past year paper with solution - 2018, Class 12 Class 12 Notes | EduRev

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Class 12 : CBSE Physics Past year paper with solution - 2018, Class 12 Class 12 Notes | EduRev

 Page 1


XII CBSE - BOARD - MARCH - 2018
Date: 07.03.2018
CODE ( 55/1 ) SET -1
Physics - Solutions
SECTION - A
1. A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting
perpendicular to their paths. Which of them will move in a circular path with higher frequency ?
Ans. Electron as it has low mass.
2. Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.
Ans. (a) 
U V ? ?
(b) laser
3. Draw graphs showing variation of photoelectric current with applied voltage for two incident radiati ons of
equal frequency and different intensities. Mark the graph for the radiation of higher intensity.
Ans. Graph
I
P
I
1
I
2
I > 
1 
I
2
V
4. Four nuclei of an element undergo fusion to form a heavier nucleus, with release of energy. Which of the two
— the parent or the daughter nucleus — would have higher binding energy per nucleon?
Ans. Daughter nuclei B.E. > Parent nuclei B.E.
      ( ?stable)                  (unstable)
5. Which mode of propagation is used by short wave broadcast services ?
Ans. Sky waves
1
Page 2


XII CBSE - BOARD - MARCH - 2018
Date: 07.03.2018
CODE ( 55/1 ) SET -1
Physics - Solutions
SECTION - A
1. A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting
perpendicular to their paths. Which of them will move in a circular path with higher frequency ?
Ans. Electron as it has low mass.
2. Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.
Ans. (a) 
U V ? ?
(b) laser
3. Draw graphs showing variation of photoelectric current with applied voltage for two incident radiati ons of
equal frequency and different intensities. Mark the graph for the radiation of higher intensity.
Ans. Graph
I
P
I
1
I
2
I > 
1 
I
2
V
4. Four nuclei of an element undergo fusion to form a heavier nucleus, with release of energy. Which of the two
— the parent or the daughter nucleus — would have higher binding energy per nucleon?
Ans. Daughter nuclei B.E. > Parent nuclei B.E.
      ( ?stable)                  (unstable)
5. Which mode of propagation is used by short wave broadcast services ?
Ans. Sky waves
1
SECTION - B
6. Two electric bulbs P and Q have their resistances in the ratio of 1 : 2. They are connected in series across a
battery. Find the ratio of the power dissipation in these bulbs.
Ans.
1
2
P
Q
R
R
?
in series
P Q
I I ? ? ?
2
2
1
2
P P P
Q Q Q
P I R
P I R
? ?
7. A 10 V cell of negligible internal resistance is connected in parallel across a battery of emf 200 V and internal
resistance 38 ? as shown in the figure. Find the value of current in the circuit.
10V
200V
38 ?
Ans. 200 10 E ? ?
190 ?
190
38 ?
190
20
30
V
I A
R
? ? ?
(OR)
In a potentiometer arrangement for determining the emf of a cell, the balance point of the cell in open circuit
is 350 cm. When a resistance of 9 ? is used in the external circuit of the cell, the balance point shifts to 300
cm. Determine the internal resistance of the cell.
Ans. Potentiometer at open circuit 
1
350 ? ?
  9 R ?
 
2
300 ? ?
350
9 1
300
r
? ?
? ?
? ?
? ?
=1.5 ?
8. (a) Why are infra-red waves often called heat waves ? Explain.
Ans. Infrared produce heat or emitted from hot bodies.
2
Page 3


XII CBSE - BOARD - MARCH - 2018
Date: 07.03.2018
CODE ( 55/1 ) SET -1
Physics - Solutions
SECTION - A
1. A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting
perpendicular to their paths. Which of them will move in a circular path with higher frequency ?
Ans. Electron as it has low mass.
2. Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.
Ans. (a) 
U V ? ?
(b) laser
3. Draw graphs showing variation of photoelectric current with applied voltage for two incident radiati ons of
equal frequency and different intensities. Mark the graph for the radiation of higher intensity.
Ans. Graph
I
P
I
1
I
2
I > 
1 
I
2
V
4. Four nuclei of an element undergo fusion to form a heavier nucleus, with release of energy. Which of the two
— the parent or the daughter nucleus — would have higher binding energy per nucleon?
Ans. Daughter nuclei B.E. > Parent nuclei B.E.
      ( ?stable)                  (unstable)
5. Which mode of propagation is used by short wave broadcast services ?
Ans. Sky waves
1
SECTION - B
6. Two electric bulbs P and Q have their resistances in the ratio of 1 : 2. They are connected in series across a
battery. Find the ratio of the power dissipation in these bulbs.
Ans.
1
2
P
Q
R
R
?
in series
P Q
I I ? ? ?
2
2
1
2
P P P
Q Q Q
P I R
P I R
? ?
7. A 10 V cell of negligible internal resistance is connected in parallel across a battery of emf 200 V and internal
resistance 38 ? as shown in the figure. Find the value of current in the circuit.
10V
200V
38 ?
Ans. 200 10 E ? ?
190 ?
190
38 ?
190
20
30
V
I A
R
? ? ?
(OR)
In a potentiometer arrangement for determining the emf of a cell, the balance point of the cell in open circuit
is 350 cm. When a resistance of 9 ? is used in the external circuit of the cell, the balance point shifts to 300
cm. Determine the internal resistance of the cell.
Ans. Potentiometer at open circuit 
1
350 ? ?
  9 R ?
 
2
300 ? ?
350
9 1
300
r
? ?
? ?
? ?
? ?
=1.5 ?
8. (a) Why are infra-red waves often called heat waves ? Explain.
Ans. Infrared produce heat or emitted from hot bodies.
2
(b) What do you understand by the statement, "Electromagnetic waves transport momentum"?
Ans. When electromagnetic waves hit body the mass is lost by the momentum is conserved. i.e., transfered
from EMW to body.
?
 EMW transports momentum.
9. If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric
emission and why ?
Metal Work function (eV)
1.92
2.15
3.20
4.17
Na
K
Ca
Mo
Ans.
( )
hc
E hv J
?
? ?
( )
hc
eV
e ?
?
 
34 8
9 19
6.6 10 3 10
412.5 10 1.6 10
?
? ?
? ? ?
?
? ? ?
      3 E eV ?
0
emission Na E ? ? ?
0
emission K E ? ? ?
0
Noemission Ka E ? ? ?
0
No emission Mo E ? ? ?
10. A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the
modulating signal in order to have a modulation index of 60%.
Ans.
m
c
A
A
? ?
15 15
60% , 100 25
60
C
C
V
A V
A
? ? ? ?
3
Page 4


XII CBSE - BOARD - MARCH - 2018
Date: 07.03.2018
CODE ( 55/1 ) SET -1
Physics - Solutions
SECTION - A
1. A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting
perpendicular to their paths. Which of them will move in a circular path with higher frequency ?
Ans. Electron as it has low mass.
2. Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.
Ans. (a) 
U V ? ?
(b) laser
3. Draw graphs showing variation of photoelectric current with applied voltage for two incident radiati ons of
equal frequency and different intensities. Mark the graph for the radiation of higher intensity.
Ans. Graph
I
P
I
1
I
2
I > 
1 
I
2
V
4. Four nuclei of an element undergo fusion to form a heavier nucleus, with release of energy. Which of the two
— the parent or the daughter nucleus — would have higher binding energy per nucleon?
Ans. Daughter nuclei B.E. > Parent nuclei B.E.
      ( ?stable)                  (unstable)
5. Which mode of propagation is used by short wave broadcast services ?
Ans. Sky waves
1
SECTION - B
6. Two electric bulbs P and Q have their resistances in the ratio of 1 : 2. They are connected in series across a
battery. Find the ratio of the power dissipation in these bulbs.
Ans.
1
2
P
Q
R
R
?
in series
P Q
I I ? ? ?
2
2
1
2
P P P
Q Q Q
P I R
P I R
? ?
7. A 10 V cell of negligible internal resistance is connected in parallel across a battery of emf 200 V and internal
resistance 38 ? as shown in the figure. Find the value of current in the circuit.
10V
200V
38 ?
Ans. 200 10 E ? ?
190 ?
190
38 ?
190
20
30
V
I A
R
? ? ?
(OR)
In a potentiometer arrangement for determining the emf of a cell, the balance point of the cell in open circuit
is 350 cm. When a resistance of 9 ? is used in the external circuit of the cell, the balance point shifts to 300
cm. Determine the internal resistance of the cell.
Ans. Potentiometer at open circuit 
1
350 ? ?
  9 R ?
 
2
300 ? ?
350
9 1
300
r
? ?
? ?
? ?
? ?
=1.5 ?
8. (a) Why are infra-red waves often called heat waves ? Explain.
Ans. Infrared produce heat or emitted from hot bodies.
2
(b) What do you understand by the statement, "Electromagnetic waves transport momentum"?
Ans. When electromagnetic waves hit body the mass is lost by the momentum is conserved. i.e., transfered
from EMW to body.
?
 EMW transports momentum.
9. If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric
emission and why ?
Metal Work function (eV)
1.92
2.15
3.20
4.17
Na
K
Ca
Mo
Ans.
( )
hc
E hv J
?
? ?
( )
hc
eV
e ?
?
 
34 8
9 19
6.6 10 3 10
412.5 10 1.6 10
?
? ?
? ? ?
?
? ? ?
      3 E eV ?
0
emission Na E ? ? ?
0
emission K E ? ? ?
0
Noemission Ka E ? ? ?
0
No emission Mo E ? ? ?
10. A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the
modulating signal in order to have a modulation index of 60%.
Ans.
m
c
A
A
? ?
15 15
60% , 100 25
60
C
C
V
A V
A
? ? ? ?
3
SECTION - C
11. Four point charges Q, q, Q and q are placed at the corners of a square of side ‘a’ as shown in the figure.
Q
q
q Q
a
Find the
(a) resultant electric force on a charge Q, and
Ans. There will be three forces on charge Q
1 2
0
1
4
qQ
F
a ? ?
?
Q
q
q Q
a
a
a
a
2 a
2 2
0
1
4
qQ
F
a ? ?
?
2
3 2 2
0 0
1 1
4 4 2 ( 2 )
QQ Q
F
a a ? ? ? ?
? ?
F
3
Q
F
1
F
2
1
F and 
2
F are perpendicular to each other so their resultant will be
2 2
1 2 1 2
2 cos90 F F F F F ? ? ? ? ?
1 2
F F ?
2
0
1
2
4
qQ
F
a ? ?
? ?
3
F and resultant of 
1
F and 
2
F will be in same direction
Net force
3
F F F ? ? ?
2 2
0
2
4 2
Q q Q
F
a a ? ?
? ?
? ?
? ?
? ?
2
0
2 2
4 2
Q q Q
a ? ?
? ?
?
?
? ?
? ?
2
0
2 2
4 2
Q q Q
F
a ? ?
? ?
?
?
? ?
? ?
4
Page 5


XII CBSE - BOARD - MARCH - 2018
Date: 07.03.2018
CODE ( 55/1 ) SET -1
Physics - Solutions
SECTION - A
1. A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting
perpendicular to their paths. Which of them will move in a circular path with higher frequency ?
Ans. Electron as it has low mass.
2. Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.
Ans. (a) 
U V ? ?
(b) laser
3. Draw graphs showing variation of photoelectric current with applied voltage for two incident radiati ons of
equal frequency and different intensities. Mark the graph for the radiation of higher intensity.
Ans. Graph
I
P
I
1
I
2
I > 
1 
I
2
V
4. Four nuclei of an element undergo fusion to form a heavier nucleus, with release of energy. Which of the two
— the parent or the daughter nucleus — would have higher binding energy per nucleon?
Ans. Daughter nuclei B.E. > Parent nuclei B.E.
      ( ?stable)                  (unstable)
5. Which mode of propagation is used by short wave broadcast services ?
Ans. Sky waves
1
SECTION - B
6. Two electric bulbs P and Q have their resistances in the ratio of 1 : 2. They are connected in series across a
battery. Find the ratio of the power dissipation in these bulbs.
Ans.
1
2
P
Q
R
R
?
in series
P Q
I I ? ? ?
2
2
1
2
P P P
Q Q Q
P I R
P I R
? ?
7. A 10 V cell of negligible internal resistance is connected in parallel across a battery of emf 200 V and internal
resistance 38 ? as shown in the figure. Find the value of current in the circuit.
10V
200V
38 ?
Ans. 200 10 E ? ?
190 ?
190
38 ?
190
20
30
V
I A
R
? ? ?
(OR)
In a potentiometer arrangement for determining the emf of a cell, the balance point of the cell in open circuit
is 350 cm. When a resistance of 9 ? is used in the external circuit of the cell, the balance point shifts to 300
cm. Determine the internal resistance of the cell.
Ans. Potentiometer at open circuit 
1
350 ? ?
  9 R ?
 
2
300 ? ?
350
9 1
300
r
? ?
? ?
? ?
? ?
=1.5 ?
8. (a) Why are infra-red waves often called heat waves ? Explain.
Ans. Infrared produce heat or emitted from hot bodies.
2
(b) What do you understand by the statement, "Electromagnetic waves transport momentum"?
Ans. When electromagnetic waves hit body the mass is lost by the momentum is conserved. i.e., transfered
from EMW to body.
?
 EMW transports momentum.
9. If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric
emission and why ?
Metal Work function (eV)
1.92
2.15
3.20
4.17
Na
K
Ca
Mo
Ans.
( )
hc
E hv J
?
? ?
( )
hc
eV
e ?
?
 
34 8
9 19
6.6 10 3 10
412.5 10 1.6 10
?
? ?
? ? ?
?
? ? ?
      3 E eV ?
0
emission Na E ? ? ?
0
emission K E ? ? ?
0
Noemission Ka E ? ? ?
0
No emission Mo E ? ? ?
10. A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the
modulating signal in order to have a modulation index of 60%.
Ans.
m
c
A
A
? ?
15 15
60% , 100 25
60
C
C
V
A V
A
? ? ? ?
3
SECTION - C
11. Four point charges Q, q, Q and q are placed at the corners of a square of side ‘a’ as shown in the figure.
Q
q
q Q
a
Find the
(a) resultant electric force on a charge Q, and
Ans. There will be three forces on charge Q
1 2
0
1
4
qQ
F
a ? ?
?
Q
q
q Q
a
a
a
a
2 a
2 2
0
1
4
qQ
F
a ? ?
?
2
3 2 2
0 0
1 1
4 4 2 ( 2 )
QQ Q
F
a a ? ? ? ?
? ?
F
3
Q
F
1
F
2
1
F and 
2
F are perpendicular to each other so their resultant will be
2 2
1 2 1 2
2 cos90 F F F F F ? ? ? ? ?
1 2
F F ?
2
0
1
2
4
qQ
F
a ? ?
? ?
3
F and resultant of 
1
F and 
2
F will be in same direction
Net force
3
F F F ? ? ?
2 2
0
2
4 2
Q q Q
F
a a ? ?
? ?
? ?
? ?
? ?
2
0
2 2
4 2
Q q Q
a ? ?
? ?
?
?
? ?
? ?
2
0
2 2
4 2
Q q Q
F
a ? ?
? ?
?
?
? ?
? ?
4
(b) potential energy of this system.
Ans.
Q
q
q Q
2 2
4
2 2
KQq Kq KQ
W
a a a
? ? ?
0
1
4
K
? ?
?
OR
(a) Three point charges q, – 4q and 2q are placed at the vertices of an equilateral triangle ABC of side ‘l’
as shown in the figure. Obtain the expression for  the magnitude of the resultant electric force acting on
the charge q.
A
B C
q
–4q
2q
l
Ans. 1 2
0
1 (4 )( )
4
q q
F
? ?
?
? ? ?
?
2
1 2
0
1 4
4
q
F
? ?
?
?
A
B C
q
–4q
2q
120°
2
F
1
F
60°
2
1 2
0
1 q
F
? ?
?
?
2 2
0
1 ( )(2 )
4
q q
F
? ?
?
? ? ?
?
2
2 2
0
1
2
q
F
? ?
?
?
angle between 
1
F
? ? ?
 and 
2
F
? ? ?
 is 120°
1 2
1 2 1 2
2 cos120 F F F F F ? ? ? ?
? ?
5
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