**ANNA UNIVERSITY, CHENNAI – 600 025**

## MODEL QUESTION PAPER

**B.E./B.TECH DEGREE EXAMNATIONS**

**First Semester**

**PH131 PHYSICS I**

Time: 3 hours Maximum Marks:100

Answer All Question

PART – A (10 x 2 = 20 Marks)

- How do temperature and impurities affect the elasticity of a material?
- A wire of length 1 m and radius 1 mm is rigidly clamped at one end. Calculate the couple required to twist the free end by 45°. The rigidity modulus of the material of the wire is 200 Gpa.
- Define sound intensity level.
- The volume of a hall is 1000m
^{3. }The area of the walls, floor and ceiling are 400m^{2}, 100m^{2}, and 100m^{2 }respectively. The sound absorption coefficients of the wall, floor and ceiling are 0.025, 0.05, 0.10 respectively. Calculate the reverberation time of the hall. - What are reversible processes?
- What is a cyclic process? What is the change in internal energy of a system during a cyclic process?
- Why is antireflection coating given on to a material.
- A sextant is used to measure the height of a tower. The angle subtended by the tower is 56°. The sextant is moved 10m towards the tower from is initial position and now the angle subtended by the tower is 59°. Calculate the height of the tower.
- A quartz crystal of thickness of 0.001 meter is vibrating at resonance. Calculate the fundamental frequency given that E = 7.9 x 10
^{10}N/m^{2 }and p = 2650 kg/m^{3 }for quartz. - What is the difference between step index and graded index fiber.

**PART – B **(5×16 = 80 Marks)

11. i) What are the characteristics of laser light? (2 Marks)

ii) Describe the working of He-Ne laser. (8 Marks)

iii) Derive an expression for the numerical aperture of an optical fiber. (6 Marks)

12.a)i) Derive an expression for the couple per unit twist at the free end of a cylinder

whose other end is rigidly fixed. (10 Marks)

ii) Discuss with a neat circuit diagram the working of a Pirani gauge. (6 Marks)

(OR)

b)i) Derive an expression for the period of oscillation of a torsion pendulum. (8 Marks)

ii) Using the expression for the period of oscillation of a torsion pendulum explain how

you would find the moment of inertia of the disc of the pendulum. (8 Marks)

13.a)i) Derive Sabine’s formula for the reverberation time of a hall. (10 Marks)

ii) Briefly outline how the reverberation time of a hall is experimentally determined. (4

Marks)

iii) What is the percentage change in intensity represented by one decibel? (2 Marks)

(OR)

b)i) Discuss with a neat circuit diagram how ultrasonic waves are generated by the piezo

electric method. (10 Marks)

ii) What are the harmful effects caused by noise on human beings. (4 Marks)

iii) If the intensity of sound is increased by a factor of 30, by how many decibel is the

sound level increased. (2 Marks)

14.a)i) Give a detailed account of the Forbe’s method of determining the thermal conductivity of a

good conductor. (10 Marks)

ii) Show that the area of the T-S diagram of a carnot engine represents the useful work done

by the engine in one cycle. (4 Marks)

iii) Using the I law of thermodynamics show that adiabatic expansion causes cooling, while

adiabatic compression causes heating. (2 Marks)

(OR)

b)i) Discuss with necessary theory how the thermal conductivity of glass is determined

experimentally. (10 Marks)

ii) Derive an expression for the efficiency of an Otto cycle. (6 Marks)

15.a)i) Arrive at the stress-optic law involving absolute changes in indices of refraction and

relative stress-optic coefficients for plane stress condition. (4 Marks)

ii) Explain with necessary theory how the thickness of a thin mica sheet is determined using

the Michelson’s interferometer. (6 Marks)

iii) Describe the working of a scanning electron microscope. (6 Marks)

(OR)

b)i) How is the sample prepared for analysing the microstructure of a sample using the

metallurgical microscope. (4 Marks)

ii) How is the height of a building determined using a sextant. Arrive at the formula used.

(6 Marks)

iii) Discuss the working of a Lummer-Brodhum Photometer. (6 Marks)

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