Sure-shot questions for SA-2 (Class XII) Physics 1. Two point charges q and -2q are kept d distance apart. Find the location of the point relative to charge q at which potential due to this system of charges in zero. 2 2. Write three basic properties of photons which are used to obtain. Use this equation to draw a plot of maximum kinetic energy of the electrons emitted versus the frequency of incident radiation. 2marks 3. a).how is toroid different from a solenoid? 3 b).use Ampere s circuital law to obtain the magnetic field inside a toroid. c).show that an idea toroid, the magnetic field (i) inside the toroid (ii) outside the toroid at any point in the open space is zero. 4. Derive an expression for the magnetic moment (µ) of an electron revolving around the nucleus in terms of its angular momentum (L). What is the direction of the magnetic moment of the electron with respect to its angular momentum? 3 5. a).define the term activity of a sample of radioactive nucleus. Write its S.I. unit. 3 b).the half life of 238 92 U. Given that 1 g of 238 92 U contains 25.3x 10 20 atoms. 6. A metallic rod of length l and resistance R is rotated with a frequency v, with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius l, about an axis passing through the centre and perpendicular to the plane of the ring. A constant and uniform magnetic field B parallel to the axis is present everywhere. 5 a). Derive the expression for the induced emf and the current in the rod
b). Due to the presence of the current in the rod and of the magnetic field, find the expression for the magnitude and direction of the force acting on this rod. c). Hence obtain the expression, for the power required to rotate the rod. Draw a block diagram of a transmitter in a communication system. 7. A point object is placed in front of a double convex lens(of refractive index n = n 2 / n 1 with respect to air) with its spherical faces of radii of curvature R 1 and R 2. Show the path of rays due to refraction at first and subsequently at the second surface to obtain the formation of the real image of the object. Hence obtain the lens-maker s formula for a thin lens. 5 8. a).draw a labelled ray diagram showing the image formation of a distant object by a refracting telescope. Deduce the expression for its magnifying power when the final image is formed at infinity. b). The sum of focal length of the two lenses of a refracting telescope is 105 cm. The focal length of one lens is 20 times that of the other. Determine the total magnification of the telescope when the final image is formed at infinity. 5 9. Draw a block diagram of a transmitter in a communication system. 1 10. a). Deduce an expression for the frequency of revolution of a charged particle in a magnetic field and show that it is independent of velocity or energy of the particle. b).draw a schematic sketch of a cyclotron. Explain, giving the essential details of its construction, how it is used to accelerate the charged particles. 5 11. a).draw a labelled diagram of a moving coil galvanometer. Describe briefly its principle and working. b).(i)why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer?
(ii)increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Explain, giving reason. 5 12. A student obtains resistances 3, 4, 12 and 16 ohm using only two metallic resistors either separately of joined together. What is the value of resistance of each of the resistors? 1 13. What is the effect of temperature on the relaxation time of electrons in a metal? 1 14. Two wires of equal length, one of copper and the other of manganin have the same resistance, which wire is thicker? 1 15. State Kirchhoff s laws for electrical circuits. Derive the balance condition for a Wheatstone bridge using these laws. 2 16. State the principle of the device used for comparing the emf of two cells. Draw the relevant circuit diagram and explain how the emf s of the two cells is compared. How can the sensitivity of such a device be increased? In what way is this method different from the one using voltmeter for comparing the emf s? 2 17. Are the paths of electron straight lines between successive collisions (with positive ions of the metals) in the (i) absence of electric field (ii) presence of electric field? Establish a relation between drift velocity v d of an electron in a conductor of cross section A carrying current I and concentration n of free electrons per unit volume of conductor. Hence, obtain the relation between current density and drift velocity. 3 18. Define the term temperature coefficient of resistivity. Show graphically the variation of resistivity with temperature for nichrome/copper. 19. A voltage of 30 V is applied across a carbon resistor with first, second and third rings of blue, black and yellow colours respectively. Calculate the value of current, in ma through the resistor. 5
20. A potential difference of 2 Volt is applied between the points A and B shown in the network drawn in the figure. Calculate (i) equivalent resistance of the network across the points A and B and (ii) the magnitude of currents in the arms AFCEB and AFDEB. 5 21. The figure shows the V-I graph for a parallel and series combination of two resistors A and B. Which line represents the parallel combination? 2 V A B O I 22. The following circuit shows the use of potentiometer to measure the internal resistance of a cell (i) when the key K is open, how does the balance point change, if the current from the driver cell decreases, (ii) when the key K is closed, how does the balance point change, if R is increased, keeping the current from the driver cell constant. 2
23. Steel is preferred for making permanent magnets whereas soft iron is preferred for making electromagnets. Give one reason. 1 24. Why do magnetic lines of force prefer to pass through ferromagnetic substances than through air? 1 25. A small magnetic needle pivoted at the centre is free to rotate in a magnetic meridian. At what place will the needle be vertical? 1 26. An electron is moving along the +ve X-axis in the presence of uniform magnetic field along the +ve Y-axis. What is the direction of force acting on it? 27. Show mathematically that the cyclotron frequency does not depend upon the speed of the particle. 3 28. State the principle of working of a cyclotron. Write two uses of this machine. 3 29. Draw a schematic sketch of a cyclotron. State its working principle. Describe briefly how it is used to accelerate charged particles. Show that the period of a revolution of an ion is independent of its speed or radius of the orbit. Write two important uses of a cyclotron. 5 30. Depict the behaviour of magnetic field lines when (i) diamagnetic and (ii) paramagnetic material is placed in an external magnetic field. Mention briefly the properties of these materials which explain this distinguishing behaviour. 5 31. A short bar magnet of magnetic moment M= 0.32 JT -1 is placed in a uniform external magnetic field of 0.15 T. if the bar is free to rotate in the plane field, which orientations would correspond to its (i) stable and (ii) unstable equilibrium? What is the potential energy of the magnet in each case? 5 32. a).what happens if a bar magnet is cut into two pieces; (i)transverse to its length (ii) along its length? b). A magnetic needle in a uniform magnetic field experiences a torque but no net force. An iron nail near a bar magnet, however, experiences a force of attraction in addition to a torque. Why?
c).must every magnetic configuration have a north pole and a south pole? What about the field due to a toroid? d).two identical looking iron bars A and B are given, one of which is definitely known to be magnetized. (We do not know which one.) How would one ascertain whether or not both are magnetised? If only one is magnetized, how does one ascertain which one? 5 33. Define the term self inductance. Write its SI unit. Give two factors on which the self inductance of an air core coil depends. 2 34. State Lenz s law. Show that Lenz s law is in accordance with the law of conservation of energy. 2 35. How are eddy currents produced? Mention two applications of eddy currents. 2 36. Draw graphs showing the variation of photoelectric current with anode potential of a photocell for (i) the same frequency but different intensities I 1 > I 2 > I 3 of incident radiation (ii) the same intensity but different frequencies v 3 > v 2 > v 1 of incident radiation. Explain why the saturation current is independent of the anode potential. 3 37. Why are de-brogile waves associated with a moving football not visible? The wavelength of photon and the de-broglie wavelength of an electron have the same value. Show that energy of the photon is 2ƛmc/h times the kinetic energy of the electron, where m, c and h have their usual meanings. 3 38. Draw the energy band diagram of p-type and n-type semiconductors and hence write the differences between p and n type. 2 39. Draw a plot showing the variation of photoelectric current with collector plate potential for two different frequencies,v 1 > v 2, of incident radiation having the same intensity. In which case will the stopping potential be higher? Justify your answer. 3
40. Draw a graph showing the variation of potential energy of a pari of nucleons as a function of their separation. Indicate the regions in which nuclear force is (i) attractive, and (ii) repulsive 3 41. Draw a plot of binding energy per nucleon (BE/A) as a function of mass number A. write two important conclusions that can be drawn regarding the nature of nuclear force. 5 42. a). Briefly explain with the help of a circuit diagram how input and output characteristics of n-p-n transistor in CE configuration are obtained. b). Draw the transfer characteristics of a transistors in CE configuration. Explain how it is used in the working of the transistor as an amplifier and a switch. 5 43. a).define term depletion layer and barrier potential for a p-n junction. How does (i) an increase in the doping concentration and (ii) biasing across the junction, affect the width of the depletion layer? b). Draw the circuit diagram of a p-n diode used as a half- wave rectifier. Explain its working. 5 44. a).figure shows a long straight wire of a circular cross-section of radius a carrying steady current I. The current is uniformly distributed across this cross-section. Derive the expressions for the magnetic field in the region r < a and r > a.
b). Distinguish between diamagnetic and ferromagnetic materials in terms of (i) suspectibility and (ii) their behaviour in a nonuniform magnetic field 5 45. Write the truth table for the combination of the gates shown. Name the gates used. 2 46. What is space wave propagation? Which two communication methods make use of this mode of propogation? If the sum of the heights of transmitting and receiving antennae in line of sight of communication is fixed at h, show that the range is maximum when the two antennae have a height h/2 each. 3 47. Identify the logic gates marked P and Q in the given circuit. Write the truth table for the combination. 2 48. a).write Einstein s photoelectric equation. Mention the underlying properties of photons on the basis of which this equation is obtained. b).write two important observations of photoelectric effect which can be explained by Einstein s equation. 3
49. The figure given below shows the block diagram of a generalized communication system. Identify the element labelled X and write its function. 1 50. In the meter bridge experiment, balance point was observed at J with AJ = 1 I. The value of R and X are doubled and then interchanged. What would be the new position of balance point? II. If the galvanometer and battery are interchanged at the balance position, how will the balance point get affected? 3