CBSE Physics sample paper class XII 2008

http://www.cbseguess.com/-------------------------------------------------------------------------------------------------------www.cbseguess.com Other Educational Portals www.icseguess.com | www.ignouguess.com | www.dulife.com | www.magicsense.com Sample Paper – 2008 Class – XII Subject – Physics Max Marks = 70 Min Marks = 27 Section-A 1. Two protons A and B are placed between two parallel plates having potential difference V as shown in the figure. Will these protons experience equal or unequal forces? 2. In an electric field an electron is kept freely. If the electron is replaced by a proton, what will be the relationship between the forces experienced by them? 3. Why is voltmeter is always connected in parallel with a circuit element across which voltage is to be measured? 4. You are given three bulbs of 25, 40 and 60 watt. Which of them has lowest resistance? 5. What characteristic of the particle has been established by the Davisson and Germer experiment? 1 x 5 = 5 Section-B 1. An electron and proton are moving in the same direction and possess same kinetic energy. Find the ratio of the de-Broglie wavelengths associated with these particles. 2. How do the photoelectric current and the kinetic energy of the photoelectrons emitted in a photocell vary if the intensity of the incident radiation is doubled? 3. A uniform wire of resistance is shaped into regular n sided polygon where n is even. Find the equivalent resistance between (1) opposite corners of the polygon (2) adjacent corners of the polygon. 4. A wire of resistance R is stretched, so that its radius decreases to, by a factor of n, calculate its new resistance. http://www.cbseguess.com/-------------------------------------------------------------------------------------------------------www.cbseguess.com Other Educational Portals www.icseguess.com | www.ignouguess.com | www.dulife.com | www.magicsense.com 5. Humid air breaks down (its molecules becomes ionized) in an electric field of 3.0 x 106 N/C. In that field, what is the magnitude of the electrostatic force on (a) an electron and (b) an ion with a single electron missing? 6. n small drops of same size are charged to v volt each. They coalesce to form a bigger drop. Calculate the capacity and potential of the bigger drop. 7. A sphere S1 of radius r1 encloses a total charge Q. If there is another concentric sphere S2 of radius r2 (>r1) and there be no additional charges S1 and S2, find the ratio of electric flux through S1 and S2. 2 x 7 = 14 Section-C 1. A spherical conducting shell of inner radius r1 and outer radius r2 has a charge Q. (a) Show that the field inside the cavity is zero. (b) A charge q is placed at the centre of the shell. What is the surface charge density on the inner and outer surfaces of the shell? (c) Is the electric field inside the cavity ( with no charge) zero, even if the shell is not spherical; but has any irregular shape? Explain 2. Can we create an electric field in which all the lines of force are parallel but their density increases continuously in a direction perpendicular to the lines of force? 3. Using gauss’s law derive the expression for the electric field intensity due to line charge distribution with linear charge density λ. 4. Derive an expression for the capacitance of the parallel plate capacitor having conducting slab inbetween the plates. 5. A wire of resistance 5Ω is drawn out so its length is increased by twice its original length. Calculate its new resistance. 6. http://www.cbseguess.com/-------------------------------------------------------------------------------------------------------www.cbseguess.com Other Educational Portals www.icseguess.com | www.ignouguess.com | www.dulife.com | www.magicsense.com Six equal resistances each of 4 ohm are connected to form a network as shown in Fig. What is the resistance between A and B. 7. Can you interchange the the positions of the battery in the auxiliary circuit and cell whose emf is to be determined in potentiometer circuit diagram. 8. A photon and electron have got the same de-Broglie wavelength which has greater kinetic energy? Explain. 9. What are the laws of photoelectric effect? 10. Why wave theory got failed in explaining the photoelectric effect? 11. A radio transmitter operates at a frequency of 880 khz and power of 10kW. Find the number of photons emitted per sec. 12. Define the terms (1) work function (2) threshold frequency and (3) stopping potential, with reference to photoelectric effect. 3 x 12 = 36 Section-D 1. Discuss briefly the principle, construction and working of the Van-de-Graff electrostatic generator. How is the leakage of charge minimized from the generator? 2. Ernest Rutherford said: In order to form some idea of the forces required to deflect an α particle through a large angle, consider an atom as containing a point charge Ze at its centre and surrounded by a distribution of negative electricity –Ze uniformly distributed within a sphere of radius R. The electric field E…. at a distance r from the centre for a point inside the atom is E = Ze∕4πε0(1/r2 – r/R3 ) OR An electron is constrained to the central axis of the ring of charge of radius R. Show that the electrostatic force of the electron can cause it to oscillate through the centre of the ring with an angular frequency ω = √eq/4πε0mR3 3. Derive an expression for the drift velocity of the electrons in a good conductor in terms of the relaxation time of electrons. A wire of resistance 4R is cut into four equal parts. Each part is stretched to twice its original length. If all the four wires after stretching are connected in parallel, Calculate resistance of combination. OR Write the principle of potentiometer. How will you compare e.m.f of two cells using potentiometer . Explain with a circuit diagram. 5 x 3 = 15