Q12 of 26 Page 1

(a) A parallel plate capacitor (C1) having charge Q is connected, to an identical uncharged capacitor C2 in series. What would be the charge accumulated on the capacitor C2?
(b) Three identical capacitors each of capacitance 3mF are connected, in turn, in series and in parallel combination to the common source of V volt. Find out the ratio of the energies stored in two configurations.

(a) Here, capacitor C1 is charged having charge Q. When this is connected to an uncharged capacitor, C2, the charge of C1 transfers to C2 till equilibrium is reached. This means till they have equal charges. Hence, the charge on C2 after connection is Q/2.

(b) Given, the capacitance of each capacitor, C=3

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10

In the ground state of hydrogen atom, its Bohr radius is given as 5.33x10-11 m. The atom is excited such that the radius becomes 21.23x10-11 m. Find (i) the value of the principal quantum number and (ii) the total energy of the atom in this excited state.

OR


The energy levels of a hypothetical atom are shown below. Which of the shown transitions will result in the emission of photon of wavelength 275 nm?


 11

Derive an expression for the electric field intensity at a point on the equatorial line of an electric dipole of dipole moment p and length 2a. What is the direction of this field?

 13

(a) Define the term intensity of radiation in photon picture.
(b) Plot a graph showing the variation of photo current vs collector potential for three different intensities I1>I2>I3, two of which (I1 and I2) have the same frequency n and the third has frequency n1>n.
(c) Explain the nature of the curves on the basis of Einsteins equation.

 14

(i) Explain briefly the process of emission of light by a Light Emitting Diode (LED).
(ii) Which semiconductors are preferred to make LEDs and why?
(iii) Give two advantages of using LEDs over conventional incandescent lamps.