SET-3

EXAM DATE: 05/03/2019

Code No. 55/1/3

Class XII Physics (Theory) (CBSE 2019)

Time: 3 Hrs.

Max. Marks: 70

GENERAL INSTRUCTIONS:

1. All questions are compulsory. There are 27 questions in all.
2. This question paper has four sections: Section A, Section B, Section C and Section D.
3. Section A contains five questions of one mark each, Section B contains seven questions of two marks each, Section C contains twelve questions of three marks each, and Section D contains three questions of five marks each.

1. There is no overall choice. However, internal choices have been provided in two questions of one mark, two questions of two marks, four questions of three marks and three questions of five marks weightage. You have to attempt only one of the choices in such questions.

1. You may use the following values of physical constants wherever necessary.

<math>c = 3 \times 10^8 \text{ m/s}</math>

<math>h = 6.63 \times 10^{-34} Js</math>

<math>e = 1.6 \times 10^{-19} C</math>

<math>\mu_0 = 4\pi \times 10^{-7} \text{ T m A}^{-1}</math>

<math>\varepsilon_0 = 8.854 \times 10^{-12} \,\mathrm{C}^2 \mathrm{N}^{-1} \mathrm{m}^{-2}</math>

<math>\frac{1}{4\pi\epsilon_0} = 9 \times 10^9 \text{ N m}^2\text{C}^{-2}</math>

Mass of electron (<math>m_e</math>) = 9.1 × 10^-31 kg

Mass of neutron = <math>1.675 \times 10^{-27}</math> kg

Mass of proton = <math>1.673 \times 10^{-27}</math> kg

Avogadro's number = <math>6.023 \times 10^{23}</math> per gram mole

Boltzmann constant = <math>1.38 \times 10^{-28} \text{ JK}^{-1}</math>

Section-A

1. Distinguish between unpolarized and linearly polarized light. 1
2. Define the term "threshold frequency", in the context of photoelectric emission. 1

OR

Define the term "Intensity" in photon picture of electromagnetic radiation.

1. How is the drift velocity in a conductor affected with the rise in temperature? 1
2. In sky wave mode of propagation, why is the frequency range of transmitting signals restricted to less than 30 MHz? 1

OR On what factors does the range of coverage in ground wave propagation depend?

1. Draw the pattern of electric field lines when a point charge +q is kept near an uncharged conducting

plate. 1

Section-B

1. (a) Define the terms, (i) threshold frequency and (ii) stopping potential in photoelectric effect. 2
2. Plot a graph of photocurrent versus anode potential for a radiation of frequency v and intensities <math>I_1</math> and <math>I_2(I_1 < I_2)</math>
3. Why a signal transmitted from a TV tower cannot be received beyond a certain distance? Write the expression for the optimum separation between the receiving and the transmitting antenna. 2
4. State Bohr's quantization condition of angular momentum. Calculate the shortest wavelength of the Bracket series and state to which part of the electromagnetic spectrum does it belong. 2

OR Calculate the orbital period of the electron in the first excited state of hydrogen atom.

1. Why is wave theory of electromagnetic radiation not able to explain photo electric effect? How does photon picture resolve this problem? 2
2. Two bulbs are rated (P₁, V) and (P₂, V). If they are connected (i) in series and (ii) in parallel across a supply V, find the power dissipated in the two combinations in terms of <math>P_1</math> and <math>P_2</math>. 2
3. Obtain the expression for the ratio of the de-Broglie wavelengths associated with the electron orbiting in the second and third excited states of hydrogen atom. 2

1. A charged particle q is moving in the presence of a magnetic field B which is inclined to an angle 30° with the direction of the motion of the particle. Draw the trajectory followed by the particle in the

presence of the field and explain how the particle describes this path. 2