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# CBSE Physics Past year paper - 2015, Class 12 Class 12 Notes | EduRev

## Class 12 : CBSE Physics Past year paper - 2015, Class 12 Class 12 Notes | EduRev

``` Page 1

CBSE XII  | PHYSICS
Board Paper – 2015

CBSE
Class XII Physics
Board Paper – 2015
Time: 3 hours                                                                                            Maximum Marks: 70

General Instructions:
(i) All questions are compulsory. There are 26 questions in all.
(ii) This question paper has five sections: Section A, Section B, Section, Section D and
Section E.
(iii) Section A contains five questions of one mark each, section B contains five questions
of two marks each, section C contains twelve questions of three marks each, section
D contains one value based question of four marks and section E contains three
questions of five marks each.
(iv) There is no overall choice. However, an internal choice has been provided in one
question of two marks, one question of three marks and all the three questions of five
marks weightage. You have to attempt only one of the choices in such questions.
(v) You may use the following values of physical constants wherever necessary:
c = 3 ? 10
8
m/s
h = 6.63 ? 10
-34
Js
e = 1.6 ? 10
-19
C
µ 0 = 4 p ? 10
-7
T mA
-l

? 0 = 8.854 ? 10
-12
C
2
N
-1
m
-2

0
1
4 ??
= 9 ? 10
9
N m
2
C
-2

m e = 9.1 ? 10
-31
kg
Mass of neutron = 1.675 ? 10
-27
kg
Mass of proton = 1.673 ? 10
-27
kg
Avogadro’s number = 6.023 ? 10
23
per gram mole
Boltzmann constant = 1.38 ? 10
-23
J K
-1

SECTION – A

Q. 1 Define the term ‘mobility’ of charge carriers. Write its S.I. unit  

Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  

Q. 3 The focal length of an equiconvex lens is equal to the radius of curvature of either face.
What is the refractive index of the material of the lens?  

Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an
external electric field E . 
Page 2

CBSE XII  | PHYSICS
Board Paper – 2015

CBSE
Class XII Physics
Board Paper – 2015
Time: 3 hours                                                                                            Maximum Marks: 70

General Instructions:
(i) All questions are compulsory. There are 26 questions in all.
(ii) This question paper has five sections: Section A, Section B, Section, Section D and
Section E.
(iii) Section A contains five questions of one mark each, section B contains five questions
of two marks each, section C contains twelve questions of three marks each, section
D contains one value based question of four marks and section E contains three
questions of five marks each.
(iv) There is no overall choice. However, an internal choice has been provided in one
question of two marks, one question of three marks and all the three questions of five
marks weightage. You have to attempt only one of the choices in such questions.
(v) You may use the following values of physical constants wherever necessary:
c = 3 ? 10
8
m/s
h = 6.63 ? 10
-34
Js
e = 1.6 ? 10
-19
C
µ 0 = 4 p ? 10
-7
T mA
-l

? 0 = 8.854 ? 10
-12
C
2
N
-1
m
-2

0
1
4 ??
= 9 ? 10
9
N m
2
C
-2

m e = 9.1 ? 10
-31
kg
Mass of neutron = 1.675 ? 10
-27
kg
Mass of proton = 1.673 ? 10
-27
kg
Avogadro’s number = 6.023 ? 10
23
per gram mole
Boltzmann constant = 1.38 ? 10
-23
J K
-1

SECTION – A

Q. 1 Define the term ‘mobility’ of charge carriers. Write its S.I. unit  

Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  

Q. 3 The focal length of an equiconvex lens is equal to the radius of curvature of either face.
What is the refractive index of the material of the lens?  

Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an
external electric field E . 

CBSE XII  | PHYSICS
Board Paper – 2015

Q. 5 What happens when a forward bias is applied to a p-n junction? 

SECTION – B
Q. 6                   
(a) Distinguish between ‘Analog’ and ‘Digital’ forms of communication.
(b) Explain briefly two commonly used applications of the ‘Internet’.
Q. 7 Given the ground state energy E 0 = - 13.6 eV and Bohr radius a 0 = 0.53
o
A . Find out how
the de Broglie wavelength associated with the electron orbiting in the ground state
would change when it jumps into the first excited state. 

Q. 8 State Bohr’s postulate of hydrogen atom which successfully explains the emission lines
in the spectrum of hydrogen atom.
Use Rydberg formula to determine the wavelength of H
? line.
[Given: Rydberg constant R = 1.03 × 10
7
m
-1
] 

Q. 9 State the two Kirchhoff’s rules used in electric networks. How are there rules justified?


Q. 10 Write the important characteristic features by which the interference can be
distinguished from the observed diffraction pattern. 
OR
Explain the basic differences between the construction and working of a telescope
and a microscope.  

SECTION – C

Q. 11 Light of intensity ‘I’ and frequency ‘ ?’ is incident on a photosensitive surface and
causes photoelectric emission. What will be the effect on anode current when (i) the
intensity of light is gradually increased, (ii) the frequency of incident radiation is
increased, and (iii) the anode potential is increased? In each case, all other factors
remain the same.
Explain, giving justification in each case.  

Q. 12 When is a transistor said to be in active state? Draw a circuit diagram of a p-n-p
transistor and explain how it works as a transistor amplifier. Write clearly, why in
the case of a transistor (i) the base is thin and lightly doped and (ii) the emitter is
heavily doped.  

Page 3

CBSE XII  | PHYSICS
Board Paper – 2015

CBSE
Class XII Physics
Board Paper – 2015
Time: 3 hours                                                                                            Maximum Marks: 70

General Instructions:
(i) All questions are compulsory. There are 26 questions in all.
(ii) This question paper has five sections: Section A, Section B, Section, Section D and
Section E.
(iii) Section A contains five questions of one mark each, section B contains five questions
of two marks each, section C contains twelve questions of three marks each, section
D contains one value based question of four marks and section E contains three
questions of five marks each.
(iv) There is no overall choice. However, an internal choice has been provided in one
question of two marks, one question of three marks and all the three questions of five
marks weightage. You have to attempt only one of the choices in such questions.
(v) You may use the following values of physical constants wherever necessary:
c = 3 ? 10
8
m/s
h = 6.63 ? 10
-34
Js
e = 1.6 ? 10
-19
C
µ 0 = 4 p ? 10
-7
T mA
-l

? 0 = 8.854 ? 10
-12
C
2
N
-1
m
-2

0
1
4 ??
= 9 ? 10
9
N m
2
C
-2

m e = 9.1 ? 10
-31
kg
Mass of neutron = 1.675 ? 10
-27
kg
Mass of proton = 1.673 ? 10
-27
kg
Avogadro’s number = 6.023 ? 10
23
per gram mole
Boltzmann constant = 1.38 ? 10
-23
J K
-1

SECTION – A

Q. 1 Define the term ‘mobility’ of charge carriers. Write its S.I. unit  

Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  

Q. 3 The focal length of an equiconvex lens is equal to the radius of curvature of either face.
What is the refractive index of the material of the lens?  

Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an
external electric field E . 

CBSE XII  | PHYSICS
Board Paper – 2015

Q. 5 What happens when a forward bias is applied to a p-n junction? 

SECTION – B
Q. 6                   
(a) Distinguish between ‘Analog’ and ‘Digital’ forms of communication.
(b) Explain briefly two commonly used applications of the ‘Internet’.
Q. 7 Given the ground state energy E 0 = - 13.6 eV and Bohr radius a 0 = 0.53
o
A . Find out how
the de Broglie wavelength associated with the electron orbiting in the ground state
would change when it jumps into the first excited state. 

Q. 8 State Bohr’s postulate of hydrogen atom which successfully explains the emission lines
in the spectrum of hydrogen atom.
Use Rydberg formula to determine the wavelength of H
? line.
[Given: Rydberg constant R = 1.03 × 10
7
m
-1
] 

Q. 9 State the two Kirchhoff’s rules used in electric networks. How are there rules justified?


Q. 10 Write the important characteristic features by which the interference can be
distinguished from the observed diffraction pattern. 
OR
Explain the basic differences between the construction and working of a telescope
and a microscope.  

SECTION – C

Q. 11 Light of intensity ‘I’ and frequency ‘ ?’ is incident on a photosensitive surface and
causes photoelectric emission. What will be the effect on anode current when (i) the
intensity of light is gradually increased, (ii) the frequency of incident radiation is
increased, and (iii) the anode potential is increased? In each case, all other factors
remain the same.
Explain, giving justification in each case.  

Q. 12 When is a transistor said to be in active state? Draw a circuit diagram of a p-n-p
transistor and explain how it works as a transistor amplifier. Write clearly, why in
the case of a transistor (i) the base is thin and lightly doped and (ii) the emitter is
heavily doped.  

CBSE XII  | PHYSICS
Board Paper – 2015

Q. 13
(a) State three important factors showing the need for translating a low frequency
signal into a high frequency wave before transmission.
(b) Draw a sketch of a sinusoidal carrier wave along with a modulating signal and show
how these are superimposed to obtain the resultant amplitude modulated wave. 

Q. 14 You are given three circuit elements X, Y and Z. When the element X is connected
across an a.c. source of a given voltage, the current and the voltage are in the same
phase. When the element Y is connected in series with X across the source, voltage is
ahead of the current in phase by /4 ? . But the current is ahead of the voltage in
phase by /4 ? when Z is connected in series with X across the source. Identify the
circuit elements X, Y and Z.
When all the three elements are connected in series across the same source,
determine the impedance of the circuit.
Draw a plot of the current versus the frequency of applied source and mention the
significance of this plot.  

Q. 15 plot a graph showing the variation of current density (j) versus the electric field (E)
for two conductors of different materials. What information from this plot regarding
the properties of the conducting material, can be obtained which can be used to
select suitable materials for use in making (i) standard resistance and (ii)
connecting wires in electric circuits?
Electron drift speed is estimated to be of the order of mm s
-1
. Yet large current of
the order of few amperes can be set up in the wire. Explain briefly. 

Q. 16 State Biot – Savart law. Deduce the expression for the magnetic field at a point on the
axis of a current carrying circular loop of radius ‘R’ distant ‘x’ from the centre. Hence,
write the magnetic field at the centre of a loop. 

Q. 17 What dose a polaroid consist of? Show, using a simple polaroid, that light waves are
transverse in nature. Intensity of light coming out of a polaroid does not change
irrespective of the orientation of the pass axis of the polaroid. Explain why. 

Q. 18 How is a Zener diode fabricated? What causes the setting up of high electric field
even for small reverse bias voltage across the diode?
Describe, with the help of a circuit diagram, the working of Zener diode as a voltage
regulator. 
OR
(a) Explain with the help of a diagram, how depletion region and potential barrier
are formed in a junction diode.
(b) If a small voltage is applied to a p-n junction diode, how will the barrier potential
be affected when it is (i) forward biased, and (ii) reveres biased?  
Page 4

CBSE XII  | PHYSICS
Board Paper – 2015

CBSE
Class XII Physics
Board Paper – 2015
Time: 3 hours                                                                                            Maximum Marks: 70

General Instructions:
(i) All questions are compulsory. There are 26 questions in all.
(ii) This question paper has five sections: Section A, Section B, Section, Section D and
Section E.
(iii) Section A contains five questions of one mark each, section B contains five questions
of two marks each, section C contains twelve questions of three marks each, section
D contains one value based question of four marks and section E contains three
questions of five marks each.
(iv) There is no overall choice. However, an internal choice has been provided in one
question of two marks, one question of three marks and all the three questions of five
marks weightage. You have to attempt only one of the choices in such questions.
(v) You may use the following values of physical constants wherever necessary:
c = 3 ? 10
8
m/s
h = 6.63 ? 10
-34
Js
e = 1.6 ? 10
-19
C
µ 0 = 4 p ? 10
-7
T mA
-l

? 0 = 8.854 ? 10
-12
C
2
N
-1
m
-2

0
1
4 ??
= 9 ? 10
9
N m
2
C
-2

m e = 9.1 ? 10
-31
kg
Mass of neutron = 1.675 ? 10
-27
kg
Mass of proton = 1.673 ? 10
-27
kg
Avogadro’s number = 6.023 ? 10
23
per gram mole
Boltzmann constant = 1.38 ? 10
-23
J K
-1

SECTION – A

Q. 1 Define the term ‘mobility’ of charge carriers. Write its S.I. unit  

Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  

Q. 3 The focal length of an equiconvex lens is equal to the radius of curvature of either face.
What is the refractive index of the material of the lens?  

Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an
external electric field E . 

CBSE XII  | PHYSICS
Board Paper – 2015

Q. 5 What happens when a forward bias is applied to a p-n junction? 

SECTION – B
Q. 6                   
(a) Distinguish between ‘Analog’ and ‘Digital’ forms of communication.
(b) Explain briefly two commonly used applications of the ‘Internet’.
Q. 7 Given the ground state energy E 0 = - 13.6 eV and Bohr radius a 0 = 0.53
o
A . Find out how
the de Broglie wavelength associated with the electron orbiting in the ground state
would change when it jumps into the first excited state. 

Q. 8 State Bohr’s postulate of hydrogen atom which successfully explains the emission lines
in the spectrum of hydrogen atom.
Use Rydberg formula to determine the wavelength of H
? line.
[Given: Rydberg constant R = 1.03 × 10
7
m
-1
] 

Q. 9 State the two Kirchhoff’s rules used in electric networks. How are there rules justified?


Q. 10 Write the important characteristic features by which the interference can be
distinguished from the observed diffraction pattern. 
OR
Explain the basic differences between the construction and working of a telescope
and a microscope.  

SECTION – C

Q. 11 Light of intensity ‘I’ and frequency ‘ ?’ is incident on a photosensitive surface and
causes photoelectric emission. What will be the effect on anode current when (i) the
intensity of light is gradually increased, (ii) the frequency of incident radiation is
increased, and (iii) the anode potential is increased? In each case, all other factors
remain the same.
Explain, giving justification in each case.  

Q. 12 When is a transistor said to be in active state? Draw a circuit diagram of a p-n-p
transistor and explain how it works as a transistor amplifier. Write clearly, why in
the case of a transistor (i) the base is thin and lightly doped and (ii) the emitter is
heavily doped.  

CBSE XII  | PHYSICS
Board Paper – 2015

Q. 13
(a) State three important factors showing the need for translating a low frequency
signal into a high frequency wave before transmission.
(b) Draw a sketch of a sinusoidal carrier wave along with a modulating signal and show
how these are superimposed to obtain the resultant amplitude modulated wave. 

Q. 14 You are given three circuit elements X, Y and Z. When the element X is connected
across an a.c. source of a given voltage, the current and the voltage are in the same
phase. When the element Y is connected in series with X across the source, voltage is
ahead of the current in phase by /4 ? . But the current is ahead of the voltage in
phase by /4 ? when Z is connected in series with X across the source. Identify the
circuit elements X, Y and Z.
When all the three elements are connected in series across the same source,
determine the impedance of the circuit.
Draw a plot of the current versus the frequency of applied source and mention the
significance of this plot.  

Q. 15 plot a graph showing the variation of current density (j) versus the electric field (E)
for two conductors of different materials. What information from this plot regarding
the properties of the conducting material, can be obtained which can be used to
select suitable materials for use in making (i) standard resistance and (ii)
connecting wires in electric circuits?
Electron drift speed is estimated to be of the order of mm s
-1
. Yet large current of
the order of few amperes can be set up in the wire. Explain briefly. 

Q. 16 State Biot – Savart law. Deduce the expression for the magnetic field at a point on the
axis of a current carrying circular loop of radius ‘R’ distant ‘x’ from the centre. Hence,
write the magnetic field at the centre of a loop. 

Q. 17 What dose a polaroid consist of? Show, using a simple polaroid, that light waves are
transverse in nature. Intensity of light coming out of a polaroid does not change
irrespective of the orientation of the pass axis of the polaroid. Explain why. 

Q. 18 How is a Zener diode fabricated? What causes the setting up of high electric field
even for small reverse bias voltage across the diode?
Describe, with the help of a circuit diagram, the working of Zener diode as a voltage
regulator. 
OR
(a) Explain with the help of a diagram, how depletion region and potential barrier
are formed in a junction diode.
(b) If a small voltage is applied to a p-n junction diode, how will the barrier potential
be affected when it is (i) forward biased, and (ii) reveres biased?  

CBSE XII  | PHYSICS
Board Paper – 2015

Q. 19 Arrange the following electromagnetic wave in the order of their increasing
wavelength:
(a) ?- rays
(b) Microwaves
(c) X-rays
How are infra-red waves produced? What role dose infra-red radiation play in (i)
maintain the Earth’s warmth and (ii) physical therapy?  

Q. 20 Explain briefly the process of charging a parallel plate capacitor when it is connected
across a d.c. battery.
A capacitor of capacitance ‘C’ is charged to ‘V’ volts by a battery. After some time the
battery is disconnected and the distance between the plates is doubled. Now a slab of
dielectric constant, 1 < k < 2, is introduced to fill the space between the plates. How
will the following be affected?
(a) The electric field between the plates of the capacitor
(b) The energy stored in the capacitor

Q. 21 Write symbolically the nuclear ß
+
decay process of
11
6
C . Is the decayed product X an
isotope or isobar of (
11
6
C )? Given the mass values m (
11
6
C ) = 11.011434 u and
m (X) = 11.009305 u. Estimate the Q-value in this process.  

Q. 22 An object is placed 15 cm in front of a convex lens of focal length 10 cm. Find the
nature and position of the image formed. Where should a concave mirror of radius
of curvature 20 cm be placed so that the final image is formed at the position of the
object itself?  

SECTION – D

Q. 23 Ajit had a high tension tower erected on his farm land. He kept complaining to the
authorities to remove it as it was occupying a large portion of his land. His uncle,
who was a teacher, explained to him the need for erecting these towers for efficient
transmission of power. As Ajit realised its significance, he stopped complaining.
(a) Why is it necessary to transport power at high voltage?
(b) A low power factor implies large power loss. Explain.
(c) Write two values each displayed by Ajit and his uncle.

Page 5

CBSE XII  | PHYSICS
Board Paper – 2015

CBSE
Class XII Physics
Board Paper – 2015
Time: 3 hours                                                                                            Maximum Marks: 70

General Instructions:
(i) All questions are compulsory. There are 26 questions in all.
(ii) This question paper has five sections: Section A, Section B, Section, Section D and
Section E.
(iii) Section A contains five questions of one mark each, section B contains five questions
of two marks each, section C contains twelve questions of three marks each, section
D contains one value based question of four marks and section E contains three
questions of five marks each.
(iv) There is no overall choice. However, an internal choice has been provided in one
question of two marks, one question of three marks and all the three questions of five
marks weightage. You have to attempt only one of the choices in such questions.
(v) You may use the following values of physical constants wherever necessary:
c = 3 ? 10
8
m/s
h = 6.63 ? 10
-34
Js
e = 1.6 ? 10
-19
C
µ 0 = 4 p ? 10
-7
T mA
-l

? 0 = 8.854 ? 10
-12
C
2
N
-1
m
-2

0
1
4 ??
= 9 ? 10
9
N m
2
C
-2

m e = 9.1 ? 10
-31
kg
Mass of neutron = 1.675 ? 10
-27
kg
Mass of proton = 1.673 ? 10
-27
kg
Avogadro’s number = 6.023 ? 10
23
per gram mole
Boltzmann constant = 1.38 ? 10
-23
J K
-1

SECTION – A

Q. 1 Define the term ‘mobility’ of charge carriers. Write its S.I. unit  

Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  

Q. 3 The focal length of an equiconvex lens is equal to the radius of curvature of either face.
What is the refractive index of the material of the lens?  

Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an
external electric field E . 

CBSE XII  | PHYSICS
Board Paper – 2015

Q. 5 What happens when a forward bias is applied to a p-n junction? 

SECTION – B
Q. 6                   
(a) Distinguish between ‘Analog’ and ‘Digital’ forms of communication.
(b) Explain briefly two commonly used applications of the ‘Internet’.
Q. 7 Given the ground state energy E 0 = - 13.6 eV and Bohr radius a 0 = 0.53
o
A . Find out how
the de Broglie wavelength associated with the electron orbiting in the ground state
would change when it jumps into the first excited state. 

Q. 8 State Bohr’s postulate of hydrogen atom which successfully explains the emission lines
in the spectrum of hydrogen atom.
Use Rydberg formula to determine the wavelength of H
? line.
[Given: Rydberg constant R = 1.03 × 10
7
m
-1
] 

Q. 9 State the two Kirchhoff’s rules used in electric networks. How are there rules justified?


Q. 10 Write the important characteristic features by which the interference can be
distinguished from the observed diffraction pattern. 
OR
Explain the basic differences between the construction and working of a telescope
and a microscope.  

SECTION – C

Q. 11 Light of intensity ‘I’ and frequency ‘ ?’ is incident on a photosensitive surface and
causes photoelectric emission. What will be the effect on anode current when (i) the
intensity of light is gradually increased, (ii) the frequency of incident radiation is
increased, and (iii) the anode potential is increased? In each case, all other factors
remain the same.
Explain, giving justification in each case.  

Q. 12 When is a transistor said to be in active state? Draw a circuit diagram of a p-n-p
transistor and explain how it works as a transistor amplifier. Write clearly, why in
the case of a transistor (i) the base is thin and lightly doped and (ii) the emitter is
heavily doped.  

CBSE XII  | PHYSICS
Board Paper – 2015

Q. 13
(a) State three important factors showing the need for translating a low frequency
signal into a high frequency wave before transmission.
(b) Draw a sketch of a sinusoidal carrier wave along with a modulating signal and show
how these are superimposed to obtain the resultant amplitude modulated wave. 

Q. 14 You are given three circuit elements X, Y and Z. When the element X is connected
across an a.c. source of a given voltage, the current and the voltage are in the same
phase. When the element Y is connected in series with X across the source, voltage is
ahead of the current in phase by /4 ? . But the current is ahead of the voltage in
phase by /4 ? when Z is connected in series with X across the source. Identify the
circuit elements X, Y and Z.
When all the three elements are connected in series across the same source,
determine the impedance of the circuit.
Draw a plot of the current versus the frequency of applied source and mention the
significance of this plot.  

Q. 15 plot a graph showing the variation of current density (j) versus the electric field (E)
for two conductors of different materials. What information from this plot regarding
the properties of the conducting material, can be obtained which can be used to
select suitable materials for use in making (i) standard resistance and (ii)
connecting wires in electric circuits?
Electron drift speed is estimated to be of the order of mm s
-1
. Yet large current of
the order of few amperes can be set up in the wire. Explain briefly. 

Q. 16 State Biot – Savart law. Deduce the expression for the magnetic field at a point on the
axis of a current carrying circular loop of radius ‘R’ distant ‘x’ from the centre. Hence,
write the magnetic field at the centre of a loop. 

Q. 17 What dose a polaroid consist of? Show, using a simple polaroid, that light waves are
transverse in nature. Intensity of light coming out of a polaroid does not change
irrespective of the orientation of the pass axis of the polaroid. Explain why. 

Q. 18 How is a Zener diode fabricated? What causes the setting up of high electric field
even for small reverse bias voltage across the diode?
Describe, with the help of a circuit diagram, the working of Zener diode as a voltage
regulator. 
OR
(a) Explain with the help of a diagram, how depletion region and potential barrier
are formed in a junction diode.
(b) If a small voltage is applied to a p-n junction diode, how will the barrier potential
be affected when it is (i) forward biased, and (ii) reveres biased?  

CBSE XII  | PHYSICS
Board Paper – 2015

Q. 19 Arrange the following electromagnetic wave in the order of their increasing
wavelength:
(a) ?- rays
(b) Microwaves
(c) X-rays
How are infra-red waves produced? What role dose infra-red radiation play in (i)
maintain the Earth’s warmth and (ii) physical therapy?  

Q. 20 Explain briefly the process of charging a parallel plate capacitor when it is connected
across a d.c. battery.
A capacitor of capacitance ‘C’ is charged to ‘V’ volts by a battery. After some time the
battery is disconnected and the distance between the plates is doubled. Now a slab of
dielectric constant, 1 < k < 2, is introduced to fill the space between the plates. How
will the following be affected?
(a) The electric field between the plates of the capacitor
(b) The energy stored in the capacitor

Q. 21 Write symbolically the nuclear ß
+
decay process of
11
6
C . Is the decayed product X an
isotope or isobar of (
11
6
C )? Given the mass values m (
11
6
C ) = 11.011434 u and
m (X) = 11.009305 u. Estimate the Q-value in this process.  

Q. 22 An object is placed 15 cm in front of a convex lens of focal length 10 cm. Find the
nature and position of the image formed. Where should a concave mirror of radius
of curvature 20 cm be placed so that the final image is formed at the position of the
object itself?  

SECTION – D

Q. 23 Ajit had a high tension tower erected on his farm land. He kept complaining to the
authorities to remove it as it was occupying a large portion of his land. His uncle,
who was a teacher, explained to him the need for erecting these towers for efficient
transmission of power. As Ajit realised its significance, he stopped complaining.
(a) Why is it necessary to transport power at high voltage?
(b) A low power factor implies large power loss. Explain.
(c) Write two values each displayed by Ajit and his uncle.

CBSE XII  | PHYSICS
Board Paper – 2015

SECTION – E

Q. 24
(a) Deduce the expression for the potential energy of a system of two charges q 1 and q 2
located
1
r and
2
r , respectively, in an external electric field.
(b) Three point charges, + Q + 2Q and – 3Q are placed at the vertices of an equilateral
triangle ABC of side l. If these charges are displaced to the mid-point A 1, B 1 and C 1,
respectively, find the amount of the work done in shifting the charges to the new
locations.

OR
Define electric flux. Write its S.I unit.
State and explain Gauss’s law. Find out the outward flux to a point charge +q placed at
the centre of a cube of side ‘a’. Why is it found to be independent of the size and shape
of the surface enclosing it? Explain.  

Q. 25
(a) Define self-inductance of a coil. Obtain an expression for the energy stored in a
solenoid of self-inductance ‘L’ when the current though it grows from zero to ‘I’.
(b) A square loop MNOP of side 20 cm is placed horizontally in a uniform magnetic field
acting vertically downwards as shown in the figure. The loop is pulled with a
constant velocity of 20 cm s
-1
till it goes out of the field.

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## Physics Class 12

204 videos|288 docs|125 tests

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