# 12th Physics Paper Solutions Set 2 : CBSE All India Previous Year 2012

General Instructions:
(i) All questions are compulsory.
(ii) There are 30 questions in total. Question Nos. 1 to 8 are very short answer type questions and carry one mark each.
(iii) Question Nos. 9 to 18 carry two marks each, question 19 to 27 carry three marks each and question 28 to 30 carry five marks each.
(iii) 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 three questions of five marks each. You have to attempt only one of the choice in such questions.
(iv) Use of calculators is not permitted.
Q1 :

For the same value of angle of incidence, the angles of refraction in three media A, B and C are 15Â°, 25Â° and 35Â° respectively. In which medium would the velocity of light be minimum?

As light travels from a rarer to denser medium it bends towards the normal as its speed decreases. So, if the bending is more, the speed of the light would be less in that medium, compared to other media. As the angle of refraction is measured with respect to the normal, the ray making the least angle of refraction would bend more and the speed of light would be minimum in that case. So, the correct option is medium A where refracting angle is 15Â°.

Q2 :

A charge “q”™ is placed at the centre of a cube of side l. What is the electric flux passing through two opposite faces of the cube?

We can calculate the flux passing through the opposite faces by using the Gauss’s Law. It is given as Now, the flux passing through all the six surfaces would be And the flux passing through each surface would be And finally, the flux passing through opposite two faces would be Q3 :

A bar magnet is moved in the direction indicated by the arrow between two coils PQ and CD. Predict the directions of induced current in each coil. According to Lenz”™s law the polarity of the induced emf is such that it opposes the change in magnetic flux responsible for its production.

Since North Pole of bar magnet is receding away from the coil so the right end of the coil will develop South Pole i.e. induced current as seen from the left end will be anticlockwise. Again, since South Pole is pushing towards the second coil so the left end of the coil will develop South Pole in order to repel it and decrease the flux i.e. induced current as seen from the left end will be clockwise. Q4 :

Two wires of equal length, one of copper and the other of manganin have the same resistance. Which wire is thicker?

Q5 :

A proton and an electron have same velocity. Which one has greater de-Broglie wavelength and why?

Q6 :

Mention the two characteristic properties of the material suitable for making core of a transformer.

Q7 :

What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves?

Q8 :

In a single-slit diffraction experiment, the width of the slit is made double the original width. How does this affect the size and intensity of the central diffraction band?

Q9 :

A test charge “q”™ is moved without acceleration from A to C along the path from A to B and then from B to C in electric field E as shown in the figure. (i) Calculate the potential difference between A and C. (ii) At which point (of the two) is the electric potential more and why? Q10 :

An electric dipole is held in a uniform electric field.

(i) Show that the net force acting on it is zero.

(ii) The dipole is aligned parallel to the field. Find the work done in rotating it through the angle of 180Â°.

Q11 :

State the underlying principle of a transformer. How is the large scale transmission of electric energy over long distances done with the use of transformers?

Q12 :

A capacitor of capacitance “C”™ is being charged by connecting it across a dc source along with an ammeter. Will the ammeter show a momentary deflection during the process of charging? If so, how would you explain this momentary deflection and the resulting continuity of current in the circuit? Write the expression for the current inside the capacitor.

Q13 :

An object AB is kept in front of a concave mirror as shown in the figure. (i) Complete the ray diagram showing the image formation of the object.

(ii) How will the position and intensity of the image be affected if the lower half of the mirror”™s reflecting surface is painted black?

Q14 :

Draw a labeled ray diagram of a reflecting telescope. Mention its two advantages over the refracting telescope.

Q15 :

Describe briefly with the help of a circuit diagram, how the flow of current carriers in a p-n-p transistor is regulated with emitter-base junction forward biased and base-collector junction reverse biased.

Q16 :

In the given block diagram of a receiver, identify the boxes labeled as X and Y and write their functions. Q17 :

A light bulb is rated 100 W for 220 V ac supply of 50 Hz. Calculate

(i) The resistance of the bulb;

(ii) The rms current through the bulb.

OR

An alternative voltage given by V = 140 sin 314t is connected across a pure resistor of 50 Î©. Find

(i) The frequency of the source.

(ii) The rms current through the resistor.

Q18 :

A circular coil of N turns and radius R carries a current I. It is unwound and rewound to make another coil of radius R/2, current I remaining the same. Calculate the ratio of the magnetic moments of the new coil and original coil.

Q19 :

You are given three lenses L1, L2 and L3 each of focal length 15 cm. A object is kept at 20 cm in front of L1, as shown. The final real image is formed at the focus “I”™ of L3. Find the separation between L1, L2 and L3. Q20 :

Define the terms (i) “cut-off voltage”™ and (ii) “threshold frequency”™ in relation to the phenomenon of photoelectric effect.

Using Einstein”™s photoelectric equation shows how the cut-off voltage and threshold frequency for a given photosensitive material can be determined with the help of a suitable plot/graph.

Q21 :

Mention three different modes of propagation used in communication system. Explain with the help of a diagram how long distance communication can be achieved by ionospheric reflection of radio waves.

Q22 :

Deduce the expression for the electrostatic energy stored in a capacitor of capacitance 'C' and having charge 'Q'.

How will the (i) energy stored and (ii) the electric field inside the capacitor be affected when it is completely filled with a dielectric material of dielectric constant K?

Q23 :

Calculate the value of the resistance R in the circuit shown in the figure so that the current in the circuit is 0.2 A. What would b the potential difference between points A and B? Q24 :

A series LCR circuit is connected to an ac source. Using the phasor diagram, derive the expression for the impedance of the circuit. Plot a graph to show the variation of current with frequency of the source, explaining the nature of its variation.

Q25 :

Define relaxation time of the free electrons drifting in a conductor. How is it related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.

Q26 :

Draw a plot of potential energy of a pair of nucleons as a function of their separations. Mark the regions where the nuclear force is (i) attractive and (ii) repulsive. Write any two characteristic features of nuclear forces.

Q27 :

In a Geiger-Marsden experiment, calculate the distance of closest approach to the nucleus of Z = 75, when a Î±-particle of 5 MeV energy impinges on it before it comes momentarily to rest and reverses its direction.

How will the distance of closest approach be affected when the kinetic energy of the Î±-particle is doubles?

OR

The ground state energy of hydrogen atom is −13.6 eV. If an electron make a transition from an energy level −0.85 eV to −1.51 eV, calculate the wavelength of the spectral line emitted. To which series of hydrogen spectrum does this wavelength belong?

Q28 :

(a) Describe briefly, with the help of a diagram, the role of the two important processes involved in the formation of a p-n junction.

(b) Name the device which is used as a voltage regulator. Draw the necessary circuit diagram and explain its working.

OR

(a) Explain briefly the principle on which a transistor-amplifier works as an oscillator. Draw the necessary circuit diagram and explain its working.

(b) Identify the equivalent gate for the following circuit and write its truth table. Q29 :

(a) Write the expression for the force, , acting on a charged particle of charge ‘q’, moving with a velocity in the presence of both electric field and magnetic field . Obtain the condition under which the particle moves undeflected through the fields.

(b) A rectangular loop of size l x b carrying a steady current I is placed in a uniform magnetic field . Prove that the torque acting on the loop is give by where is the magnetic moment of the loop.

OR

(a) Explain, giving reasons, the basic difference in converting a galvanometer into (i) a voltmeter and (ii) an ammeter.

(b) Two long straight parallel conductors carrying steady currents I1 and I2 are separated by a distance 'd'. Explain briefly, with the help of a suitable diagram, how the magnetic field due to one conductor acts on the other. Hence deduce the expression for the force acting between the two conductors. Mention the nature of this force.

Q30 :

(a) In Young's double slit experiment, derive the condition for

(i) constructive interference and

(ii) destructive interference at a point on the screen.

(b) A beam of light consisting of two wavelengths, 800 nm and 600 nm is used to obtain the interference fringes in a Young's double slit experiment on a screen placed 1 Â· 4 m away. If the two slits are separated by 0Â·28 mm, calculate the least distance from the central bright maximum where the bright fringes of the two wavelengths coincide.

OR

(a) How does an unpolarized light incident on a polaroid get polarized? Describe briefly, with the help of a necessary diagram, the polarization of light by reflection from a transparent medium.

(b) Two polaroids “A”™ and “B”™ are kept in crossed position. How should a third polaroid “C”™ be placed between them so that the intensity of polarized light transmitted by polaroid B reduces to 1/8th of the intensity of unpolarized light incident on A?