## Saturday, 26 July 2014

### CLASS 12 PHYSICS TEST PRACTICE 2

Practice Test Paper 2

Physics (Theory) – XII

Time allowed: 3 hours                                                                                                 Maximum marks: 70  Q. 1. An electron beam projected along +X-axis, experiences a force due to a magnetic field along the +Y-axis. What is the direction of the magnetic field? 1 Q. 2. Name an appropriate communication channel needed to send a signal of band-width 100 kHz over a distance of 8 km. 1 Q. 3. Ultraviolet light is incident on two photosensitive materials having work functions W1 and W2 (W1>W2). In which case will the kinetic energy of the emitted electrons be greater? Why? 1 Q. 4. Name the device which can represent digital data by analog signals and vice-versa. 1 Q. 5. Why is the conductivity of n-type semiconductor greater than that of the p-type semiconductor even when both of these have same level of doping? 1 Q. 6. Define the terms ‘Magnetic Dip’ and ‘Magnetic Declination’ with the help of relevant diagrams. 2 Q. 7. A 4mF capacitor is charged by a 200 V supply. The supply is then disconnected and the charged capacitor is connected to another uncharged 2 mF capacitor. How much electrostatic energy of the first capacitor is lost in the process of attaining the steady situation? 2 Q. 8. State the underlying principle of an a.c. generator. Write the relationship between the peak value and r.m.s. value of alternating voltage. 2 Q. 9. A voltage of 30 V is applied across a carbon resistor with first, second and third rings of blue, black and yellow colours respectively. Find the value of current through the resistor. 2 Q. 10. Define the term threshold frequency and work function in relation to photoelectric effect. 2 Q. 11. Explain, with the help of a circuit diagram, the use of n-p-n transistor as an amplifier in common emitter configuration. 2 Q. 12. Write the expression for frequency of an ideal LC circuit. In an actual circuit, why do the oscillations ultimately die away? 2 Q. 13. State Gauss’s theorem. Apply this theorem to obtain the expression for the electric field intensity at a point due to an infinitely long, thin, uniformly charged straight wire. 3 Q. 14. State the principle of potentiometer. Draw a circuit diagram used to compare the e.m.f. of two primary cells. Write the formula used. How can the sensitivity of a potentiometer be increased? 3 Q. 15. Define self-inductance and give its S.I. unit. Derive an expression for self-inductance of a long, air-cored solenoid of length l, radius r, and having N number of turns. 3 Q. 16. State Biot-Savart law. Apply it to calculate the magnetic field at a point near a current carrying long straight wire. 3 Q. 17. Sate the principle of potentiometer. With the help of circuit diagram, describe a method to find the internal resistance of a primary cell. 3 Q. 18. Explain the following terms: (a)    Ground waves (b)    Space waves (c)     Sky waves 3 Q. 19. Calculate the binding energy per nucleon (in MeV) of the nucleus . Given mass of = 1.00783 u, mass of = 1.00867 u, mass of = 55.934939 u, 1u = 931 MeV/c2. 3 Q. 20. What do the acronyms ‘LASER’ and ‘LED’ stand for? Name the factor which determines:   (a) Frequency, and   (b) Intensity of light emitted by LED. 3 Q. 21. What are coherent sources? How does the width of interference fringes in Young’s double slit experiment change when (a)    The distance between the slits and screen is decreased? (b)   The frequency of the source is increased? Justify your answer in each case. 3 Q. 22. Distinguish between analog and digital communication. Write any two modulation techniques employed for the digital data. Describe briefly any one of the techniques used. 3 Q. 23. On the basis of energy band diagrams distinguish between metals, insulators and semiconductors 3 Q. 24. (a)    Show that the decay rate ‘R’ of a sample of a radionuclide is related to the number of radioactive nuclei ‘N’ at the same instant by the expression R = lN. (b)   The half-life of against a-decay is 1.5x1017s. What is the activity of a sample of having 25x1020 atoms? 3 Q. 25. (a)    With the help of a schematic sketch of a cyclotron explain its working principle. Mention its two applications. What is the important limitation encountered in accelerating a light elementary particle such as electron to high energies? (b)   A particle of mass m and charge q moves at right angles to a uniform magnetic field. Plot a graph showing the variation of the radius of the circular path described by it with the increase in its (i) charge, (ii) kinetic energy, where, in each case other factors remain constant. Justify your answer.                                                                                                Or (a)    Using Biot-Savart law derive an expression of the magnetic field due to a current carrying loop at a point along the axis of the loop. (b)   A long straight conductor carries a steady current ‘I’. The current is uniformly distributed across its cross-section of radius ‘a’. Plot a graph showing the variation of the magnetic field ‘B’ produced by the conductor with distance ‘r’ from the axis of the conductor in the region (i) ra. 5 Q. 26. Show by a diagram the image formation of a point object by a thin double convex lens having radii of curvature R1 and R2. Hence derive the formula where f is the focal length and n is refractive index of material of the lens.                           Or State Huygen’s principle. Use Huygen’s construction to explain refraction of a plane wavefront at a plane surface. Draw diagrams to show the behaviour of a (a) convex lens, (b) concave mirror when a plane wavefront falls on it. 5 Q. 27. With the help of a neat and labelled diagram, explain the principle, construction and working of an a.c. generator.                                                                                         Or Explain, with the help of a neat and labelled diagram, the principle, construction and working of a transformer. 5