## Applied Physics 1 - Dec 2012

### First Year Engineering (Semester 1)

TOTAL MARKS: 60

TOTAL TIME: 2 HOURS
(1) Question 1 is compulsory.

(2) Attempt any **three** from the remaining questions.

(3) Use suitable data wherever required.

(4) Figures to the right indicate full marks.

### Attempt any FIVE.

**1(a)** Explain term lattice parameters of cubic crystal.(3 marks)
**1(b)** What is the probability of an electron being thermally excited to conduction band in silicon at 20^{o}C. The band gap energy is 1.12eV; Boltzmann constant is 1.38 x 10^{-23} JIk.(3 marks)
**1(c)** Mobility of holes is 0.025 m^{2} /V-sec What would be the resistivity of P-type silicon . if the Hall coefficient of the sample is 2.25 x 10^{-5} m^{3} /C ?(3 marks)
**1(d)** Define dielectrics, electric dipole, polarizability.(3 marks)
**1(e)** Difference between soft and hard magnetic materials(3 marks)
**1(f)** Define Reverberation time. Write Sabine's formula and explain terms in it.(3 marks)
**1(g)** State the terms : magnetostriction effect; piezoelectric effect(3 marks)
**2(a)** Explain the formation of energy band in solids. With neat energy band diagram explain extrinsic semiconductors.(8 marks)
**2(b)** Draw the unit cell of HCP. What is its co-ordination number, atomic radius, effective number of atoms per unit cell. Also calculate its packing factor .(7 marks)
**3(a)** What is hysteresis? Draw a hysteris loop for ferromagnetic material and explain the various important points on it. What is the technical significance of the area enclosed under it. For a transformer which kind of material will you prefer-the one with small hysteresis area or the big one ?(8 marks)
**3(b)** Derive Bragg's law. Calculate the glancing angle on the plane (100) for a crystal of rock salt (a = 2.125 A^{0} .Consider the case of 2nd order maximum and A = 0.592 A^{0}.(7 marks)
**4(a)** Calculate the number of atoms per unit cell of a metal having lattice parameter 2.9A^{0} and density 7.87 gm/cm3. Atomic weight of metal is 55.85, Avagadro number is 6.023 x 10^{23} /gm-mole.(5 marks)
**4(b)** Prove that the Fermi level lies exactly at the centre of the forbidden energy gap in case of an intrinsic semiconductor.(5 marks)
**4(c)** Explain ionic polarization and obtain polarizability.(5 marks)
**5(a)** With neat diagram of a unit cell, explain the structure of BaTi0_{3}.(5 marks)
**5(b)** What is Hall effect ? Derive expression for Hall voltage.(5 marks)
**5(c)** Explain the absorption coeficient of a hall. Calculate the change in intensity level if the intensity of sound increases 1000 times its original intensity.(5 marks)
**6(a)** In what sense real crystals differ from ideal crystals? Explain the point defects in crystals.(5 marks)
**6(b)** Explain construction and working of a solar cell.(5 marks)
**6(c)** Find the natural frequency of vibration of quartz plate of thickness 2mm. Given Young's 5 modulus of quartz Y = 8 x 10^{10} N/m^{2}, density of quartz is 2650 kg/m^{3}. Calculate the change in thickness required if the same plate is used to produce ultrasonic waves of frequency 3MHz.(5 marks)