# Board Paper of Class 12 2020 Physics - Solutions

*All questions are compulsory.***:**

*This question paper is divided into 4 Sections, A, B, C and D as follows*

**Section A***Question number 1 is of twelve marks. All parts of this question are compulsory.*

**Section B***Question numbers 2 to 12 carry 2 marks each with two questions having internal choice.*

**Section C***Question numbers 13 to 19 carry 3 marks each with two questions having internal choice.*

**Section D***Question numbers 20 to 22 are long-answer type questions and carry 5 marks each. Each question has an internal choice.*

*The intended marks for questions are given in brackets [ ].*

*All working, including rough work, should be done on the same sheet as and adjacent to the rest of the answer.*

*Answers to sub parts of the same question must be given in one place only. A list of useful physical constants is given at the end of this paper.*

*A simple scientific calculator without a programmable memory may be used for calculations.*

Useful Constant and Relation:

1u | = | 931MeV |

- Question 1
**(a)****Choose the correct alternative (a), (b), (c) or (d) for each of the questions given below:**

(i) A point charge ‘*q*’ is kept at each of the vertices of an equilateral triangle having each side ‘*a*’. Total electrostatic potential energy of the system is:(a) $\left(\frac{1}{4\pi {\epsilon}_{0}}\right)\frac{3{q}^{2}}{{a}^{2}}$

(b) $\left(\frac{1}{4\pi {\epsilon}_{0}}\right)\frac{3q}{a}$

(c) $\left(\frac{1}{4\pi {\epsilon}_{0}}\right)\frac{3{q}^{2}}{{a}^{}}$

(d) $\left(\frac{1}{4\pi {\epsilon}_{0}}\right)\frac{3q}{{a}^{2}}$

(ii)**Curie**temperature is the temperature above which:(a) a ferromagnetic substance behaves like a paramagnetic substance.

(b) a paramagnetic substance behaves like a diamagnetic substance.

(c) a ferromagnetic substance behaves like a diamagnetic substance.

(d) a paramagnetic substance behaves like a ferromagnetic substance.

(iii) In an**astronomical telescope**of**refracting**type:(a) Objective should have small focal length.

(b) Objective should have large focal length.

(c) Eyepiece should have large focal length.

(d) Both objective and eyepiece should have large focal length.

(iv) In**photoelectric effect**experiment, the slope of the graph of the**stopping potential**versus**frequency**gives the value of:(a) $\frac{h}{e}$

(b)*h*

(c)$\frac{e}{h}$

(d) $\frac{hc}{e}$

(v) In a nuclear reactor,**cadmium**rods are used as:(a) Control rods

(b) Fuel rods

(c) Coolant

(d) Moderator**(b)**Answer the following questions**briefly**and to the point:

(i) State**Gauss**’ theorem.

(ii) A metallic wire having a resistance of 20Ω is bent in order to form a complete circle. Calculate the resistance between**any two**diametrically opposite points on the circle.

(iii) How can a moving coil galvanometer be converted into a**voltmeter**?

(iv) Write**Biot-Savart’s law**in vector form.

(v) What is the**phase difference**between any two points lying on the**same**wavefront?

(vi) Name the physical**principle**on the basis of which**optical fibres**work.

(vii) What is**Pair production**