Q1 of 805 Page 1

In the given figure, charge + Q is placed at the centre of a dotted circle. Work done in taking another charge + q from A to B is W1 and from B to C is W2. Which one of the following is correct: W1 > W2, W1 = W2 and W1 < W2?

In the figure, the dotted circle is an equipotential surface due to the inner charge + Q.


The voltage at all points of an equipotential surface is the same (i.e. VA = VB = VC)


We know that work done by a point charge in moving from one point to another W = q × ΔV


Work done by moving a charge from A to B is W1 = q × (VA-VB) = 0


Work done by moving a charge from B to C is W2 = q × (VB-VC) = 0


Total work is done in moving a charge from A to B and then B to C is W1 + W2 = 0.


Thus we can conclude that work done by moving a charge from one point to another on an equipotential surface is always zero.


More from this chapter

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26

(a) There are two sets of apparatus of Young’s double slit experiment. In set A, the phase difference between the two waves emanating from the slits does not change with time, whereas in set B, the phase difference between the two waves from the slits changes rapidly with time. What difference will be observed in the pattern obtained on the screen in the two set ups?

(b) Deduce the expression for the resultant intensity in both the above-mentioned set ups (A and B), assuming that the waves emanating from the two slits have the same amplitude A and same wavelength λ.


OR


(a)The two polaroids, in a given set up, are kept ‘crossed’ with respect to each other. A third polaroid, now put in between these two polaroids, can be rotated. Find an expression for the dependence of the intensity of light I, transmitted by the system, on the angle between the pass axis of first and the third polaroid. Draw a graph showing the dependence of I on ϴ.


(b)When an unpolarized light is incident on a plane glass surface, find the expression for the angle of incidence so that the reflected and refracted light rays are perpendicular to each other. What is the state of polarization, of reflected and refracted light, under this condition?

 27

a) Draw the circuit diagram to determine the characteristics of a p- n-p transistor in common emitter configuration.

b) Explain, using I-V characteristics, how the collector current changes with the base current. How can (i) output resistance and (ii) current amplification factor be determined from the I-V characteristics?


OR


a) Why are photodiodes preferably operated under reverse bias when the current in the forward bias is known to be more than that in reverse bias?


b) The two optoelectronic devices: - Photodiode and solar cell, have the same working principle but differ in terms of their process of operation. Explain the difference between the two devices in terms of (i) biasing, (ii) junction area and (iii) I-V characteristics.

 2

Plot a graph showing the variation of current ‘I’ versus resistance ‘R’, connected to a cell of emf E and internal resistance ‘r’.

 3

State the factors on which the refractive index of a material medium for a given wavelength depends.