CBSE Class 12 Physics Paper 2018 (page 6)

CBSE Class 12 Physics Paper 2018

Page 6

Section E

Question 24:
(a) Define electric flux. Is it a scalar or a vector quantity?

A point charges q is at a distance of d/2 directly above the centre of a square of side d, as shown in the figure. Use Gauss' law to obtain the expression for the electric flux through the square.

[figure to be inserted]

(b) 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.


Question 24:
(a) Use Gauss' law to derive the expression for the electric field (E) due to a straight uniformly charged infinite line of charge density λ C/m.
(b) Draw a graph to show the variation of E with perpendicular distance 'r' from the line of charge.
(c) Find the work done in bringing a charge q from perpendicular distance r1 to r2 (r2 > r1).

Question 25:
(a) 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 'ω' in a magnetic field B, directed perpendicular to the axis of rotation.
(b) An aeroplane is flying horizontally form 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 × 10(to the power –4) T and the angle of dip is 30º.


Question 25:
A device X is connected across an ac source of voltage V = V0 sin ωt. The current through X is given as I = Io sin (ωt + π/2)
(a) Identify the device X and write the expression for its reactance.
(b) Draw graphs showing variation of voltage and current with time over one cycle of ac, for X.
(c) How does the reactance of the device X vary with frequency of the ac? Show this variation graphically.
(d) Draw the phasor diagram for the device X.

Question 26:
(a) Draw a ray diagram to show image formation when the concave mirror produces a real, inverted and magnified image of the object.
(b) Obtain the mirror formula and write the expression for the linear magnification.
(c) Explain two advantages of a reflecting telescope over a refracting telescope.


Question 26:
(a) Define a wavefront. Using Huygens' principle, verify the laws of reflection at a plane surface.
(b) 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.
(c) When a tiny circular obstalce 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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