## CBSE Previous Year Solved  Papers  Class 12 Physics Delhi 2011

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 weightage. 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

Note : Except for the following questions, all the remaining question have been asked in Previous Set.
Question.1.A point charge is placed at point O as shown in the figure. Is the potential difference VA-VB positive, negative or zero, if Qis (i) positive (ii) negative

Answer : If Q is positively charged, VA-VB = positive If Q is negatively charged, VA-VB = negative.

Question.2.A plane electromagnetic wave travels in vacuum along Z-direction. What can you say about the direction of electric and magnetic field vector?
Answer : In electromagnetic wave, the electric field vector E and magnetic field vector g show their variations perpendicular to’ the direction of propagation of wave as well as perpendicular to each other.. As the electromagnetic wave is travelling along z-direction, hence E and B show their variation in xy-plane..

Question.3.A resistance R is connected across a cell of emf e and internal resistance r. A potentiometer now measures the potential
difference between the terminals of the cell as V. Write the expression for ‘r’ in terms of ε, V and R.

Question.4.The permeability of magnetic material is 0.9983. Name the type of magnetic materials it represents.

Question.5.Show graphically, the variation of the de-Broglie wavelength (λ) with the potential (V) through which an electron is accelerated from rest.

Question.6. In a transistor, dopping level .in base is increased slightly. How will it affect

1.  collector current and
2. base current?

1. Collector current decreases slightly
2.  Base current increases slightly

Question.7. Define the term ‘wattless current’.
Answer : Current flowing in a circuit without any net dissipation of power is called wattless current.

Question.8. When monochromatic light travels from one medium to another its wavelength changes but frequency remains the same, Explain.
Answer : Atoms (of the second medium) oscillate with the same (incident light) frequency and in turn, emit light of the same frequency.

Question.9. Two uniformly large parallel thin plates having charge densities +σ and – σ are kept in the X-Z plane at a distance ‘d apart. Sketch an equipotential surface due to electric field between the plates. If a particle of mass m and charge ‘-q’ remains stationary between the plates, what is the magnitude and direction of this field?
OR
Two small identical electrical dipoles AB and CD, each of dipole moment ‘p’ are kept at an angle of 120° as shown in the figure. What is the resultant dipole moment of this combination? If this system is subjected to electric field () directed along +X direction, what will be the magnitude and direction of the torque acting on this?

Question.10. A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip down at 60° with the horizontal. The horizontal component of the earth’s magnetic field at the place is known to be 0.4 G. Determine the magnitude of the earth’s magnetic field at the place.

Question.11. Figure shows two identical capacitors, C1 and C2, each of lpF capacitance connected to a battery of 6 V. Initially switch ‘S’ is closed. After sometime ‘s’ is left open and dielectric slabs of dielectric constant K = 3 are inserted to fill completely the space between the plates of the two capacitors. How will the (i) charge and (ii) potential difference between the plates of the capacitors be affected after the slabs are inserted?

Question.12. Two convex lenses of same focal length but of aperture A1 and A2 (A2 < A1), are used as the objective lenses in two astronomical telescopes having identical eyepieces. What is the ratio of their resolving power? Which telescope will you prefer and why? Give reason.

Question.13. Draw the output waveform at X, using the given inputs A and B for the logic circuit shown below. Also, identify the logic operation performed by this circuit.

Question.14. Name the semiconductor device that can be used to regulate an unregulated dc power supply. With the help of I-V characteristics of this device, explain its working principle.
Answer : A Zener diode is a specially designed diode which is operated in reverse breakdown region continuously with any damage. When Zener diode is operated in the reverse break down region, the voltage across it remains practically constant (Vz) for a large change in reverse current. Therefore, for any increase/decrease of the input voltage there is a increase/deer ease of the voltage drop across series resistance (Rs) without any change in the voltage across Zener diode,

Question.15. How are infrared waves produced? Why are these referred to as ‘heat wave’? Write their one important use.
Answer: Infrared rays are produced by hot bodies and molecules. Infrared waves are Called heat waves as they cause heating effect. Infrared waves are used to maintain earths warmth, in physical therapy, remote switches etc.

Question.16. Draw the transfer characteristic curve of a base biased transistor in CE configuration. Explain clearly how the active region of the V0 versus Vi– curve in a transistor is used as an amplifier.

Question.17.

1.  Define modulation index.
2. Why is the amplitude of modulating signal kept less than the amplitude of carrier wave?

1.  Modulation index is defined as the ratio of amplitude of modulating signal to amplitude of carrier wave. It is given by
2.  The amplitude of modulating signal is kept less than the amplitude of carrier wave to avoid noise.

Question.18. A current is induced in coil C1due to the motion of current carrying coil C2. (a) Write any two ways by which a large deflection can be obtained in the galvanometer G. (b) Suggest an alternative device to demonstrate the induced current in place of a galvanometer.

Answer : (a) Any two ways to obtain large deflection in G :
(i) Moving C2 faster towards C1
(ii)Insertion of soft iron core in C1
(b) Alternative device that can be used in place of galvanometer is LED.

Question.19. Define the terms (i) drift velocity, (ii) relaxation time. A conductor of length L is connected to a dc”source of emf 8. If this conductor is replaced by another conductor of same material and same area of cross-section but of length 3L, how will the drift velocity change?
Answer : (i) Drift velocity : The average velocity with which the free electrons drift towards positive terminal under the influence of an external field is called drift velocity.
(ii) Relaxation time : Average time interval between two successive collisions of an electron with the ions / atoms of the conductor.

Question.20.Using Gauss’s law obtain the expression for the electric field due to a uniformly charged thin spherical shell of radius R at a point outside the shell. Draw a graph showing the variation of electric field with r, for r > R and r < R.
Answer : Electric field at point P at a distance V’outside the spherical shell.

Question.21.An electron and a photon each have a wavelength 1.00 nm. Find
(a) their momenta,
(b) the energy of the photon and
(c) the kinetic energy

Question.22. Draw a schematic diagram showing the (i) ground wave (ii) sky wave and (iii) space wave propagation modes for em waves. ,
Write the frequency range for each of the following :
(ii) Television
(iii) Satellite communication

Question.23. Describe Youngs double slit experiment to produce interference pattern due to a monochromatic source of light. Deduce the expression for the fringe width.
OR
Use Huygens principle to verify the laws of refraction.
Answer : S is a narrow slit (of width about 1 mm) illuminated by a monochromatic source of light. At a suitable distance (= 10 mm) from S, two slits S1 and S2 are placed parallel to S. When a screen is placed at a large distance (about 2m) from the slit S1 and S2, alternate dark and bright bands appear on the screen. These are the interference bands or fringes. The band disappear when either slit is covered.

Explanation : According to Huygens principle, the monochromatic source of light illuminating the slit S tends out spherical wavefront.
The two waves of same amplitude and same frequency super impose on each other, Dark fringes appear on the screen when the crest of one wall falls on the trough of other and they neutralize the effect of each other. Bright fringes appear on the screen when the crest of one wave coincides with the crest of other and they reinforce each other.
Expression for the fringe width :
Let d = distance between slits S1 and S2
D = distance of screen from two slits and
x = distance between the central maxima
‘O’ and observation point P.

Now the intensity at point P is maximum :or minimum according as the path difference is an integral multiple of wavelength or an odd integral multiple of half wavelength.
OR
Wavefront : The continuous locus of all the particles of a medium, which are vibrating in the same phase is called wavefront.
Laws of refraction : Let PP’ represent the surface separating medium 1 and medium 2 as shown in fig.

Which is Snell’s Law of refraction of light (first law)
Second Law : Incident wavefront, refracted wave front, normals all lie in the same plane.

Question.24. (a) Describe briefly, with the help of suitable diagram, how the transverse nature of light can be demonstrated by the phenomenon of polarization.
(b) When unpojarized light passes from air to a transparent medium, under what condition does the reflected light get polarized?
(a) When a polaroid P1 is rotated in the path of an unpolarised light, there is no change in transmitted intensity. v The light transmitted through polaroid P1 is made to pass through polaroid P2. On rotating polaroid P2, in path of light transmitted from P1 we notice a change in intensity of transmitted light. This shows the light transmitted from P1 is polarized. Since light can be polarized, it has transverse
nature.

(b) Whenever unpolarised light is incident from air to a transparent medium at an angle of incidence equal to polarizing angle, the reflected light gets fully polarized.

Question.25. The energy levels of a hypothetical atom are shown below. Which of the shown transitions will result in the emission of a photon of wavelength 275 nm?
Which of these transitions correspond to emission of radiation of (i) maximum and (ii) minimum wavelength?

Hence transition B would result in the emission of a photon of wavelength 275 nm.
(i) Transition A corresponds to maximum wavelength .
(ii)Transition D corresponds to minimum wavelength

Question.26. State the law of radioactive decay. Plot a graph showing the number (N) of undecayed nuclei as a function of time (t) for a given radioactive sample having half lifeT1/2.
Depict in the plot the number of undecayed nuclei at (i) t = 3 T1/2. and (ii) t = 5 T1/2..
Answer : The number of nuclei undergoing decay per unit time, at any instant, is proportional to the total number of nuclei in the sample at that instant.

Question.27. In the circuit shown, R1 = 4 Ω, R2 = R3 = 15 Ω, R4 = 30Ω and E = 10 V. Calculate the equivalent resistance of the circuit and the current in each resistor.

Question.28.State Biot-Savart law, giving the mathematical expression for it. Use this law to derive the expression for the magnetic field due to a circular coil carrying current at a point along its
How does a circular loop carrying current behave as a magnet?
OR
With the help of a labelled diagram, state the underlying principle of a cyclotron. Example clearly how it works to accelerate the charged particles.
Show that cyclotron frequency is independent of energy of the particle. Is there an upper limit on the energy acquired by the particle? Give reason.

Working : High frequency oscillator maintains moderate alternating potential difference between the dees. This potential difference establishes an electric field that reverse its direction periodically. Suppose a positive ion of moderate mass produced at the centre of the dees, finds D2 at negative potential. It gets accelerated towards it. A uniform magnetic field, normal to the plane of the dees, makes it move in a circular track. Particle t races a semicircular track and returns back to the region between the dees. The moment it arrives in the region electric field reverses its direction and accelerate the charge towards. D1. This way charge keeps on getting accelerated until it is removed out of the dees.
Centripetal force, neede d by the charged particle to move in circular track, is provided by the magnetic field.

Thus, frequency of revolut/ion is independent of the energy of the particle. Yes, there is an upper limit on the energy acquired by the charged particle. The charged part icle gains maximum speed when it moves in a path of radius equal to the radius of the dees.

Question.29. (a) Draw a ray diagram to show refraction of a ray of monochromatic light passing through a glass prism.
Deduce the expression for the refractive index of glass in terms of angle of prism and angle of minimum deviation.
(b) Explain briefly how the phenomenon of total internal reflection is used in fibre: optics.

Here the ray of light propagation from a rarer medium of refractive index ( n1) to a denser medium of refractive index ( n2) is incident on the convex side of spherical refracting surface of radius of curvature R.
(b) Draw a ray diagram to show the image formation by a concave mirror when the object is kept between its focus and the pole. Using this diagram, derive the magnification formula for the image found.

Each optical fibre consists of a core and cladding, refractive index of the material of the core is higher than that of cladding. When a signal, in the form of light, is directed into the optical fibre, at an angle greater than the critical angle, it undergoes repeated total internal reflections along the length of the fibre and comes out of it at the other end with almost negligible loss of intensity.

Question.30.(i) With the help of a labelled diagram, describe briefly the underlying principle and working of a step up transformer.
(ii) Write any two sources of energy loss in a transformer.
(iii) A step up transformer converts a low input voltage into a
high output voltage. Does it violate law of conservation of energy? Explain.
OR
Derive an expression for the impedance of a series LCR circuit connected to an AC supply of variable frequency. Plot a graph showing variation of current with the frequency of the applied voltage.
Explain briefly how the phenomenon of resonance in the circuit can be used in the tuning mechanism of a radio or a TV set.

Principle : It works on the principle of mutual induction. When alternating current is passed through a coil an induced emf is set up in the neighbouring coil.
Working : When an alternating voltage is applied to the primary, the resulting current produces an alternating magnetic flux which links the secondary and induces an emf in it. We consider an ideal transformer in which the primary has negligible resistance and all the flux in the core links with both primary and secondary windings. Let (ø) be the flux in each turn in the core at any time t due to current in rlie primary when a voltage Vp is applied to it.
Then the induced emf or voltage Es, in the secondary with

(ii) Sources of energy loss in transformer are : Joule’s loss in the resistan ce of windings, loss due to eddy currents.
(iii) No. A step up transformer steps up the voltage while it steps down the current. So the input and output power remain srime (provided there is no loss). Hence there is no violation of the principle of energy conservation.
OR
impedance (Z) in LCR circuit. The effective resistance offered by a series LCR circuit to the flow of current is called its impedance. It is denoted by Z.

The given graph showing’variation of curr ent with the frequency of the applied voltage.
The radio and TV receiver sets are the practical applications of series resonant circuits. Signals of several different frequencies
are available in air. By turning the tuning knob of the radio set, we vary the frequency of the LC circuit till it mat ches the frequency of the desired signal.

A series resonant circuit allows maximum current through it. So, it is called acceptor circuit. A series resonance circuit is that circuit in which inductance L, capacitance C and resistance R are connected in series. The impedance of this circuit has a minimum value and the current through the circuit is maximum.

### SET II

Note : Except for the following questions, all the rem aining question have been asked in Previous Set.

Question.2. The susceptibility of magnetic material is 1.9 x 10 -5. Name the type of magnetic materials it represents.
Answer : Paramagnetic substance because susceptibility of magnetic material is positive.

Question.4. A plane electromagnetic wave travels in vacuum along x-direction. What can you say about the direction of elect tic and magnetic field vectors?
Answer: The electric and magnetic field vectors are in YZ-plane.

Question.10. A magnet is quickly moved in the direction indicated by am arrow between coils C1and C2 as shown in figure. What will be the direction of induced current in each coil as seen, from the magnet? Justify your answer.

Answer: For coil C1: As the N-pole of the magnet is moving away from the coil C1, the end of the coil will behave as S-pole so as to oppose the motion of the magnet. Therefore, looking from the end, the current in the coil C1 will be in clockwise direction.
For coil C2 : The end of the coil should behave as S-pole so as to repel the approaching magnet. Looking from the end, the direction of current in the coil of C2 will be clockwise direction.

Question.11. Figure shows two identical capacitors C1 and C2 each of 2μF capacitance, connected to a battery of 5V. Initially switch ‘S’
is dosed. After sometime ‘S’ is left open and dielectric slabs of dielectric constant k = 5 are inserted to fill completely the space between the plates of the two capacitors. How will the (i) charge and (ii) potential difference between the plates of the capacitors be affected after the slabs are inserted?

Question.16. How is forward biasing different from reverse biasing in a p-n junction diode?
Answer: Forward Biasing : The positive terminal of the external battery is connected to p-side and negative terminal of battery to n-side of p-n junction. The forward bias voltage oppose the potential barrier. Due to this, the potential barrier is reduced and hence the depletion becomes thin.
Reverse biasing : The negative terminal of the external battery is connected to p-side and positive terminal of battery to n-side of p-n junction. The reverse bias voltage supports the potential barrier. Due to this, the potential barrier is increased. The resistance of p-n junction becomes high.

Question.20. In the circuit shown, R1 = 4 Ω, R2 = R3= 5 Ω, R4 = 10 Ω, and E = 6V Work out the equivalent resistance of the circuit and the current in each resistor.

Question.23. An electron and a photon each have a wavelength of 1.50 nm. Find.
(i) their momenta
(ii) the energy of the photon
(iii) the kinetic energy of the electron

### SET III

Note : Except for the following questions, all the remaining question have been asked in Previous Set.

Question.2.A plane electromagnetic wave travels in vacuum along y-direction. What can you say about the direction of electric and, magnetic field vectors?
Answer : For an electromagnetic wave propagating along T-direction, the electric and magnetic field vectors vary sinusoidally along X-direction and Z-direction respectively.

Question.7.The susceptibility of a magnetic material is -4.2 x 10 -6 Name the type of magnetic materials it represents.

Question.9.Explain how a depletion region is formed in a junction diode.
Answer : Depletion region is formed in a junction diode ‘ because depletion region is created around the p-n junction w ho is devoid of free charge carriers and has immobile ions. It is created due to diffusion of majority carriers across the junction when p-n junction is formed.

Question.14. Figu re shows two identical capacitors C1and C2 each of 1.5 μF capacitance, connected to a battery of 2V. Initially switch
S’ is closed. After sometime ‘S’ is left open and dielectric slabs of dielectric constant K=2 are inserted to fill completely the space between the plates of the two capacitors. How will the (i) charge ahd (ii) potential difference between the plates of the capacitors be affected after the slabs are inserted?

Question.16. Write the truth table for the logic circuit shown below and identify the logic operation by this circuit.

Question.17. Predict the polarity of the capacitor when the two magnets are quickly moved in the directions marked by arrows.

Answer: In this situation shown, A will become positive with respect to B, as current induced is in clockwise direction.

Question.23. In the circuit shown, R1 = 2 Ω, R2 = R3 = 10 Ω, R4 = 20 Ω, and E = 6V. Work out the equivalent resistance of the circuit and the current in each resistor.