Physics Delhi (Set 2)

General Instructions:

Read the following instructions very carefully and strictly follow them:

1. This question paper comprises four sections – A, B, C and D.
2. There are 37 questions in the question paper. All questions are compulsory.
3. Section A: Q. no. 1 to 20 are very short-answer type questions carrying 1 mark each.
4. Section B : Q. no. 21 to 27 are short-answer type questions carrying 2 marks each.
5. Section C: Q. no. 28 to 34 are long-answer type questions carrying 3 marks each.
6. Section D : Q. no. 35 to 37 are also long answer type questions carrying 5 marks each.

1. There is no overall choice in the question paper. However, an internal choice has

been provided in two questions of one mark, two questions of two marks, one question of three marks and all the three questions of five marks. You have to attempt only one of the choices in such questions.

1. However, separate instructions are given with each section and question, wherever necessary.
2. Use of calculators and log tables is not permitted.
3. 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>\epsilon_0 = 8.854 \times 10^{-12} \,\mathrm{C}^2 \,\mathrm{N}^{-1} \,\mathrm{m}^{-2}</math>

<math>4\pi\varepsilon_0 = 9 \times 10^9 \,\mathrm{N} \,\mathrm{m}^2 \,\mathrm{C}^{-2}</math>

Mass of electron (m_e) = <math>9.1 \times 10^{-31}</math> 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^{-23} \text{ JK}^{-1}</math>

Question 1

The wavelength and intensity of light emitted by a LED depend upon

1. forward bias and energy gap of the semiconductor
2. energy gap of the semiconductor and reverse bias
3. energy gap only
4. forward bias only

Solution:

The wavelength and intensity of light emitted by an LED depends on both energy gap and bias of the diode. Only when the diode is forward biased, it emits photons. The wavelength of the emitted light depends on the energy gap of the semiconductor.

Hence, the correct answer is option (a).

Question 2

The graph showing the correct variation of linear momentum (p) of a charge particle with its de-Broglie wavelength (<math>\lambda</math>) is - P P λ λ λ λ (b) (a) (d) (c)

Solution:

The relation between de-Broglie wavelength and the momentum is:

<math display="block">\lambda = \frac{h}{p}</math><math display="block">\Rightarrow p = \frac{h}{\lambda}</math>

By the above relation, we can conclude that the graph between the momentum and the de-Broglie wavelength is a rectangular hyperbola.

Hence, the correct answer is option (b).

Question 3

The selectivity of a series LCR a.c. circuit is large, when

1. L is large and R is large
2. L is small and R is small
3. L is large and R is small
4. <math>L = R</math>

Solution: Selectivity of a circuit depends on the quality of resonance. The quality factor is given by: