Physics – Important Question Bank for CBSE Class XII (HSC) Board Exam 2019

PHYSICS – IMPORTANT QUESTION BANK

1. Find the work done in bringing a charge q from perpendicular distance r1 to r2 (r2 > r1).
2. Draw a graph to show the variation of E with perpendicular distance r from the line of charge.
3. Use Gauss’ law to derive the expression for the electric field.
4. If the point charge is now moved to a distance‘d’ from the centre of the square and the side of the square is doubled, explain how the electric flux will be affected.
5. Define electric flux. Is it a scalar or a vector quantity?
6. A point charge q is at a distance of d/2 directly above the centre of a square of side d. Use Gauss’ law to obtain the expression for the electric flux through the square.
7. State the principle of an ac generator and explain its working with the help of a labelled diagram. Obtain the expression for the emf induced in a coil having N turns each of cross-sectional area A, rotating with a constant angular speed ‘w’ in a magnetic field B, directed perpendicular to the axis of rotation.
8. An aeroplane is flying horizontally from west to east with a velocity of 900 km/hour. Calculate the potential difference developed between the ends of its wings having a span of 20 m. The horizontal component of the Earth’s magnetic field is 5 x 10^(–4) T and the angle of dip is 30.
9. Explain with an example, how power loss is reduced if the energy is transmitted over long distances as an alternating current rather than a direct current.
10. An iron ring of relative permeability r has windings of insulated copper wire of n turns per metre. When the current in the windings is I, find the expression for the magnetic field in the ring.
11. The susceptibility of a magnetic material is 0·9853. Identify the type of magnetic material. Draw the modification of the field pattern on keeping a piece of this material in a uniform magnetic field.
12. Using the concept of free electrons in a conductor, derive the expression for the conductivity of a wire in terms of number density and relaxation time. Hence obtain the relation between current density and the applied electric field E.
13. A symmetric biconvex lens of radius of curvature R and made of glass of refractive index 1·5, is placed on a layer of liquid placed on top of a plane mirror as shown in the figure. An optical needle with its tip on the principal axis of the lens is moved along the axis until its real, inverted image coincides with the needle itself. The distance of the needle from the lens is measured to be x. On removing the liquid layer and repeating the experiment, the distance is found to be y. Obtain the expression for the refractive index of the liquid in terms of x and y.
14. State Bohr’s postulate to define stable orbits in hydrogen atom. How does de Broglie’s hypothesis explain the stability of these orbits?
15. A hydrogen atom initially in the ground state absorbs a photon which excites it to the n = 4 level. Estimate the frequency of the photon.
16. What kind of fringes do you expect to observe if white light is used instead of monochromatic light?
17. If one of two identical slits producing interference in Young’s experiment is covered with glass, so that the light intensity passing through it is reduced to 50%, find the ratio of the maximum and minimum intensity of the fringe in the interference pattern.
18. A radioactive isotope has a half-life of 10 years. How long will it take for the activity to reduce to 3·125% ?
19. In a potentiometer arrangement for determining the emf of a cell, the balance point of the cell in open circuit is 350 cm. When a resistance of 9  is used in the external circuit of the cell, the balance point shifts to 300 cm. Determine the internal resistance of the cell.
20. What do you understand by the statement, ‘‘Electromagnetic waves transport momentum’’?
21. A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the modulating signal in order to have a modulation index of 60%.
22. Define a wavefront. Using Huygens’ principle, verify the laws of reflection at a plane surface.
23. 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
24. A hydrogen atom initially in the ground state absorbs a photon which excites it to the n = 4 level. Estimate the frequency of the photon.
25. The figure shows a ray of light falling normally on the face AB of an equilateral glass prism having refractive index 3/2, placed in water of refractive index 4/3. Will this ray suffer total internal reflection on striking the face AC? Justify your answer.
26. Define the term ‘conductivity’ of a metallic wire. Write its SI unit.
27. Why are infra-red waves often called heat waves? Explain.
28. A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting perpendicular to their paths. Which of them will move in a circular path with higher frequency ?
29. When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the obstacle. Explain why.

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