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

Physics Class 12

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  [1] 
 
Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  [1] 
 
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?  [1] 
 
Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an 
external electric field E . [1] 
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  [1] 
 
Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  [1] 
 
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?  [1] 
 
Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an 
external electric field E . [1] 
  
 
CBSE XII  | PHYSICS 
Board Paper – 2015 
 
     
Q. 5 What happens when a forward bias is applied to a p-n junction? [1] 
 
SECTION – B 
Q. 6                   [2] 
(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. [2] 
 
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
] [2] 
 
Q. 9 State the two Kirchhoff’s rules used in electric networks. How are there rules justified? 
 [2] 
 
Q. 10 Write the important characteristic features by which the interference can be 
distinguished from the observed diffraction pattern. [2] 
OR 
Explain the basic differences between the construction and working of a telescope 
and a microscope.  [2] 
  
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.  [3] 
 
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.  [3] 
 
 
 
 
 
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  [1] 
 
Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  [1] 
 
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?  [1] 
 
Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an 
external electric field E . [1] 
  
 
CBSE XII  | PHYSICS 
Board Paper – 2015 
 
     
Q. 5 What happens when a forward bias is applied to a p-n junction? [1] 
 
SECTION – B 
Q. 6                   [2] 
(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. [2] 
 
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
] [2] 
 
Q. 9 State the two Kirchhoff’s rules used in electric networks. How are there rules justified? 
 [2] 
 
Q. 10 Write the important characteristic features by which the interference can be 
distinguished from the observed diffraction pattern. [2] 
OR 
Explain the basic differences between the construction and working of a telescope 
and a microscope.  [2] 
  
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.  [3] 
 
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.  [3] 
 
 
 
 
 
  
 
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. [3] 
 
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.  [3] 
 
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. [3] 
 
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. [3] 
 
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. [3] 
 
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. [3] 
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?  [3] 
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  [1] 
 
Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  [1] 
 
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?  [1] 
 
Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an 
external electric field E . [1] 
  
 
CBSE XII  | PHYSICS 
Board Paper – 2015 
 
     
Q. 5 What happens when a forward bias is applied to a p-n junction? [1] 
 
SECTION – B 
Q. 6                   [2] 
(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. [2] 
 
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
] [2] 
 
Q. 9 State the two Kirchhoff’s rules used in electric networks. How are there rules justified? 
 [2] 
 
Q. 10 Write the important characteristic features by which the interference can be 
distinguished from the observed diffraction pattern. [2] 
OR 
Explain the basic differences between the construction and working of a telescope 
and a microscope.  [2] 
  
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.  [3] 
 
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.  [3] 
 
 
 
 
 
  
 
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. [3] 
 
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.  [3] 
 
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. [3] 
 
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. [3] 
 
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. [3] 
 
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. [3] 
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?  [3] 
  
 
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 
 (d) Radio waves 
 How are infra-red waves produced? What role dose infra-red radiation play in (i) 
maintain the Earth’s warmth and (ii) physical therapy?  [3] 
 
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 
 Justify your answer by writing the necessary expressions.  [3] 
 
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.  [3] 
 
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?  [3] 
 
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. 
 Answer the following questions:  [4] 
 (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  [1] 
 
Q. 2 In a series LCR circuit, V L = V C ? V R. What is the value of power factor?  [1] 
 
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?  [1] 
 
Q. 4 Write a relation for polarisation P of a dielectric material in the presence of an 
external electric field E . [1] 
  
 
CBSE XII  | PHYSICS 
Board Paper – 2015 
 
     
Q. 5 What happens when a forward bias is applied to a p-n junction? [1] 
 
SECTION – B 
Q. 6                   [2] 
(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. [2] 
 
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
] [2] 
 
Q. 9 State the two Kirchhoff’s rules used in electric networks. How are there rules justified? 
 [2] 
 
Q. 10 Write the important characteristic features by which the interference can be 
distinguished from the observed diffraction pattern. [2] 
OR 
Explain the basic differences between the construction and working of a telescope 
and a microscope.  [2] 
  
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.  [3] 
 
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.  [3] 
 
 
 
 
 
  
 
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. [3] 
 
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.  [3] 
 
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. [3] 
 
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. [3] 
 
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. [3] 
 
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. [3] 
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?  [3] 
  
 
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 
 (d) Radio waves 
 How are infra-red waves produced? What role dose infra-red radiation play in (i) 
maintain the Earth’s warmth and (ii) physical therapy?  [3] 
 
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 
 Justify your answer by writing the necessary expressions.  [3] 
 
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.  [3] 
 
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?  [3] 
 
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. 
 Answer the following questions:  [4] 
 (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.  [5] 
 
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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