**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.

**OR**

**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.

**OR**

**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.

**OR**

**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.

***** The End *****