Physics | CBSE Board | Grade 12 | 2012
Q. Draw a labelled ray diagram of a reflecting telescope. Mention its two advantages over the refracting telescope.
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Q. You are given three lenses L1, L2 and L3 each of focal length 20cm. An object is kept at 40cm in front of L1, as shown. The final real image is formed at the focus I of L3. Find the separations between L1, L2 and L3.

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Q. Describe briefly with the help of a circuit diagram, how the flow of current carriers in a pnp transistor is regulated with emitter-base junction forward biased and base-collector junction reverse biased.
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Q. A proton and an electron have same kinetic energy. Which one has greater de-Broglie wavelength and why?
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Q. (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.

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Q. For the same value of angle of incidence, the angles of refraction in three media A, B and C are 15o, 25o and 35o respectively. In which medium would the velocity of light be minimum?
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Q. 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?
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Q. (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 bream of light consisting of two wavelengths, 800nm and 600nm is used to obtain the interference fringes in a Young's double slit experiment on a screen placed 1.4m away. If the two slits are separated by 0.28nm, calculate the least distance from the central bright maximum where the bright fringes of the two wavelengths coincide.
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Q. Draw a plot potential energy of a pair of nucleons as a functions of their separations. Mark the regions where the nuclear force is (i) attractive and (ii) repulsive. Write any two characteristic features of nuclear forces.
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Q. Define the terms (i) 'cut-off voltage' and (ii) 'threshold frequency' in relation to the phenomenon of photoelectric effect.
Using Einstein's photoelectric equations show how the cut-off voltage and threshold frequency for a given photosensitive material can be determined with the help of a suitable plot/graph.
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Q. 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.
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Q. Two wires of equal length, one of copper and the other of manganin have the same resistance. Which wire is thicker?
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Q. Mention the two characteristic properties of the material suitable for making core of a transformer.
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Q. 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.
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Q. 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? Explain ?
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Q. In the given block diagram of a receiver, identify the boxes labelled as X and Y and write their functions.

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Q. 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.
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Q. What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves?
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Q. 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.
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Q. A light bulb is rated 100W for 220V ac supply of 50Hz. Calculate
(i) The resistance of the bulb;
(ii) The rms current through the bulb.
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Q. How does the angular separation between fringes in single slit diffraction experiment change when the distance of separation between the slit and screen is doubled?
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Q. A charge q is placed at the centre of a cube of side l. What is the electric flux passing through each face to the cube?
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Q. Calculate the value of the resistance R in the circuit shown in the figure so that the current in the circuit is 0.2A. What would be the potential difference between points B and E?

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Q. 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?
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Q. 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?

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Q. (a) Write the expression for the force F, acting on a charged particle of charge q, moving with a velocity v in the presence of both electric field E and magnetic field B. Obtain the condition under which the particle moves undeflected through the fields.
(b) A rectangular loop of size l×b carrying a steady current I is placed in a uniform magnetic field B. Prove that the torque τ acting on the loop is given by τ=m×B where m is the magnetic moment of the loop.
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Q. What will be the directions of induced current in each coil when A bar magnet is moved in the direction indicated by the arrow between two coils PQ and CD.

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Q. 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?

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Q. 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.
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Q. 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 in rotating it through the angle of 180.
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