## CBSE Previous Year Solved  Papers  Class 12 Physics Outside Delhi 2015

Time allowed : 3 hours                                                                                           Maximum Marks: 70

General Instructions:

1.  All questions are compulsory. There are 26
questions in all.
2.  This question paper has five sections : Section A, Section B, Section C, Section D and Section E.
3. 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.
4.  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 weight age. You have to attempt only one of the choices in such questions.
5. You may use the following values of physical constants wherever necessary:

### SET I

SECTION-A

Question.1. Define the term self-inductance of a coil. Write its S.I. unit.
Answer : Self inductance of a coil is numerically equal to the amount of magnetic flux linked with the coil when unit current flows through the coil.
The S.I. unit of self inductance is henry (H) or weber per ampere.
1H = 1 Wb/A

Question.2. Why does bluish colour predominate in a clear sky ?
Answer: While light from the sun reaches the atmosphere that is comprised of the tiny particles of the atmosphere. These act as a prism and cause the different components to scatter. As blue light travels in shorter and smaller waves in comparison to the other colours of the spectrum, it is scattered the most, causing the sky to appear bluish.

Question.3. I-V graph for a metallic wire at two different temperatures, T1 and T2 is as shown in the figure. Which of the two temperature is lower and why ?

Answer : The slope of a V-I graph is given by the formula I/V=I/R. Thus, the smaller the slope larger is the resistance. As the resistance of a metal increases with the increase in temperature, so resistance at T2 is higher and T1 is lower.

Question.4. Which basic mode of communication is used for telephonic communication?
Answer: The basic mode of communication used in telephony is the point-to-point communications mode, which takes place through a link between a single transmitter and receiver.

Question.5. Why do the electrostatic field lines not form closed loops ?
Answer : Electrostatic field lines never form loops because they do not converge at only one point as in the case with magnetic field lines. They depend on +ve and -ve charges that can extend infinity in any particular direction.

SECTION-B

Question.6. When an electron in hydrogen atom jumps from the excited state to the ground state, how would the de-Broglie wavelength associated with the electron change ? Justify your answer.
Answer : In an hydrogen atom, the negatively charged electron revolves in a circular orbit around the heavy positively charged nuclear. The centripetal force required by the electron is produced by the attractive force exerted by the nucleus on it. The energy is radiated, when an electron jumps from higher to lower energy orbit and the energy is absorbed, when jumps from lower to higher energy orbit.
hv = Ef– Ei[ Ef> Ei] It is called Bohr’s frequency condition.

Question.7. Write two factors which justify the need of modulating a low frequency signal into high frequencies before transmission.
Answer : Two factors that justify the need for modulating low frequency signals to high frequency signals are
(a) increase in range of communication base band signals suffer from attenuation and so cannot be transmitted over long distances, and
(b) the antenna height and aperture is inversely proportional to the radiated signal frequency, implying that higher frequencies result in smaller antennas.

Question. 8. Use Kirchhoff’s rules to determine the potential difference between the points A and D when no current flows in the arm BE of the electric network shown in the figure.

Question. 9. You are given two converging lenses of focal lengths 1.25 cm and 5 cm to design a compound microscope. If it is desired to have a magnification of 30, find out the separation between the objective and the eyepiece.
OR
A small telescope has an objective lens of focal length 150 cm and eyepiece of focal length 5 cm. What is the magnifying power of the telescope for viewing distant objects in normal adjustment ?
If this telescope is used to view a 100 m tall tower 3 km away, what is the height of the image of the tower formed by the objective lens ?

Question.10. Calculate the shortest wavelength in the Balmer series of hydrogen atom. In which region (infra-red, visible, ultraviolet) of hydrogen spectrum does this wavelength lie ?

SECTION – C

Question.11. Calculate the potential difference and the energy stored in “* the capacitor C2 in the circuit shown in the figure. Given potential at A is 90 V, C1 = 20 μF, C2= 30 μF and C3 = 15 μF.

Question.12. Find the relation between drift velocity and relaxation time of charge carriers in a conductor. A conductor of length L is connected to a d.c. source of emf ‘E’ . If the length of the conductor is tripled by stretching it, keeping ‘E’ constant, explain how its drift velocity would be affected.
Answer : The drift velocity of electrons can be formulated as :

where
a – average acceleration of the carrier
q = charge of the carrier
m* = effective mass of charge carrier
x = carrier lifetime = relaxation time
E = electric field strength
As the drift velocity is directly proportional to the current flowing through the conductor and as the current flowing through the conductor is inversely proportional to its length, so the drift velocity would be reduced by one third if the length of the conductor is tripled.

Question.13. State clearly how an unpolarised light gets linearly polarized when passed through a Polaroid.
(i) Unpolarised light of intensity I0 is incident on a Polaroid P1 which is kept near another Polaroid P2 whose pass axis is parallel to that of Pi. How will the intensities of fight, Ii and I2, transmitted by the Polaroids P1 and P2 respectively, change on rotating P1 without disturbing P2 ?
(ii) Write the relation between the intensities I1 and I2.
Answer : Polaroid filters are made of a special material that is capable of blocking of the two planes of vibration of an electromagnetic wave. In this sense, a Polaroid serves as a device that filter out one-half of the vibrations upon transmission of the light through the filter. When unpolarised light is transmitted through a Polaroid filter, it emerge with one-half the intensity and with vibrations in a single plane; it emerges as polarized light.

Question.14. Define modulation index. Why is its value kept, in practice, less than one ? A carrier wave of frequency 1.5 MHz and amplitude 50 V is modulated by a sinusoidal wave of frequency 10 kHz producing 50% amplitude modulation. Calculate the amplitude of the AM wave and frequencies of the side bands produced.
Answer : The modulation index mf, can be defined as the ratio between the amplitude of the modulating signal and the amplitude of the carrier signal.
A modulation index above 1 causes over-modulation, causing the carrier to experience 180° phase reversals. These reversals give rise to additional side and audio signal will be distorted during reception.

Question.15. A uniform magnetic field B is set up along the positive x-axis. A particle of charge ‘q’ and mass ‘m’ moving with a velocity v enters the field at the origin in X-Y plane such that it has velocity components both along and perpendicular to the magnetic field B. Trace, giving reason, the trajectory followed 0 by the particle. Find out the expression for the distance moved by the particle along the magnetic field in one rotation.

Question.16. (a) Determine the value of phase difference between the current and the voltage in the given series LCR circuit.

(b) Calculate the value of the additional capacitor which may be joined suitably to the capacitor C that would make the power factor of the circuit unity.

Question.17. Write the expression for the generalized form of Amperes Circuital law. Discuss its significance and describe briefly how the concept of displacement current is explained through charging/discharging of a capacitor in an electric circuit.
Answer : Generalized form of Ampere Circuital law:

It signifies that the source of magnetic field is not just due to the conduction electric currently due to flow of charge but also due to the time rate of change of electric field called displacement current.
Displacement Current : During charging and discharging of a capacitor the electric field between the plates will change so there will be a change of electric flux (displacement current) between the plates.

Question.18. Use Huygens principle to show how a plane wavelength propagates from a denser to rarer medium. Hence verify Snell’s law of refraction.

Question.19. Identify the gates P and Q shown in the figure. Write the truth table for the combination of the gates shown.

Name the equivalent gate representing this circuit and write its logic symbol.

Question.20. Draw a circuit diagram of a C.E. transistor amplifier. Briefly explain its working and write the expression for (i) current gain, (ii) voltage gain of the amplifier.

Question.21. (a) Write three characteristic properties of nuclear force.
(b) Draw a plot of potential energy of a pair of nucleons as a function of their separation. Write two important conclusions that can be drawn from the graph.
Answer : (a) (i) Nuclear forces are short range forces ;
(ii) Nuclear forces are primarily attractive and extremely strong;
(iii) Nuclear forces are charge independent.

Question.22. (a) Describe briefly three experimentally observed features
in the phenomenon of photoelectric effect.
(b) Discuss briefly how wave theory of light cannot explain these features.
OR
(a) Write the important properties of photons which are used to establish Einstein’s photoelectric equation.
(b) Use this equation to explain the concept of (i) threshold frequency and (ii) stopping potential.
Answer : (a) (i) The photoelectric effect will not occur when the frequency of the incident light is less than the threshold frequency. Different materials have different threshold frequencies and most elements have threshold frequencies in the ultraviolet region of the electromagnetic spectrum.
(ii) The maximum KE of a stream of photo electrons increases linearly with the frequency of the incident light above the threshold frequency.
(iii) The rate at which photo electrons are emitted from a photosensitive surface is directly proportional to the incidents light when the frequency is constant.
(b) Classical wave theory cannot explain :
(i) The existence of threshold frequency because it predicts that electrons would absorb enough energy to escape and there would not be any threshold frequency;
(ii) The almost immediate emission of photo electrons as, according to this theory, electrons require a period of time before sufficient energy is absorbed by it to escape from the metal; however, such a thing does hot happen practically; and
(iii) The independence of KE of photo electrons on intensity and dependence on frequency because it cannot explain why maximum KE is dependent on the frequency and independent of intensity.
OR
(a) The important property of photons that is useful in establishing Einstein’s photoelectric equation is their ability to hold on to the electrons of an atom by their forces of attraction.
(b) Einstein’s photoelectric equation states that:

SECTION – D

Question.23. One morning an old man walked bare-foot to replace the fuse wire in kit kat fitted with power supply mains for his house. Suddenly he screamed and collapsed on the floor. His wife cried loudly for help. His neighbour’s son Anil heard the cries and rushed to the place with shoes on. He took a wooden baton and used it to switch off the main supply.
(i) What the voltage and frequency of mains supply in India ?
(ii) These days most of the electrical devices we use require a.c. voltage. Why ?
(iii) Can a transformer be used to step up d.c. voltage ?
(iv) Write two qualities displayed by Anil by his action.
Answer : (i) The voltage and frequency of mains supply in India are 240 V and frequency is 50 Hz.
(ii) Most electrical devices require a.c. voltage because a.c. is available by default through the mains supply and also because d.c., is actually a one-way current, is available only through batteries.
(iii) No, a transformer cannot be used to set-up d.c. voltage because a transformer works on the magnetic effect of current and d.c. voltage does not passess any magnetic field.
(iv) The two qualities displayed by Anil are : (i) preventing himself from being the earthing conductor by putting on his shoes and (ii) switching off the mains connection to prevent the old main from acting as an earthing conductor.

SECTION – E

Question.24. (a) Define electric flux. Write its S.I. unit.
“Gauss’s law in electrostatics is true for any closed surface, no matter what its shape or size is”. Justify this statement with the help of a suitable example.
(b) Use Gauss’s law to prove that electric field inside a uniformly charged spherical shell is zero.
OR
(a) Derive the expression for the energy stored in a parallel plate capacitor. Hence obtain the expression for the energy density of the electric field.
(b) A fully charged parallel plate capacitor is connected across an uncharged identical capacitor. Show that the energy stored in the combination is less than that stored initially in the single capacitor.
Answer : (a) The electric flux through an area is defined as the electric field multiplied by the area of the surface projected on a plane, perpendicular to the field. Its S.I. unit is voltmeters (Vm) or Newton metres square per coulomb (Nm2 C-1). The given statement is justified because while measuring the flux, the surface area is more important than its volume on its size.
(b) To prove that the electric field inside a uniformly charged spherical shell is zero, we place a single positive point charge ‘q’ at the centre of an imaginary spherical surface with radius R. The field lines of this point radiate outside equally in all directions. The magnitude E of the electric field at every

Question.25. Explain, using a labelled diagram, the principle and working of a moving coil galvanometer. What is the (unction of (i) uniform radial magnetic field, (ii) soft iron core ?Define the terms (i) current sensitivity and (ii) voltage sensitivity of a galvanometer. Why does increasing the current sensitivity not necessarily increase voltage sensitivity ?

Question.26.

(ii) The different colours of white light will produce different interference patterns but the central bright fringes due to all colours are at the same positions. Therefore, the central bright fringe is white in colour. Since the wavelength of the blue light is smallest, the fringe closed on the either side of the central white fringe is blue and farthest is red. Beyond a few fringes, no clear fringe pattern is visible.
OR

So angle of deviation produced by a prism depends upon the angle of incidence, refracting angle of prism, and the material of the prism.

(b) A ray of light incident on face AB will just suffer internal reflection at the other face AC, if it gets incident on the face AC at an angle equal to the critical angle for the material of the prism. If critical angle for the material of prism is C.