# Mumbai University Previous year question papers

## I Year Examination Dec 2012

## Applied Physics

N.D. : (l) Question No.1 is compulsory.

(2) Attempt any three questions from cemaining questions No.2 to 6.

(3) Assume suitable data wherever required.

(4) Figures to the right indicate marks.

1.(a) Explain the tenn lattice parameters of Cubic crystal.

(b) WlJ.atis the probability of an electron being thermally excited to conduction band in silicon at 20°C. The band gap energy is 1·12e V; Boltzmann constant is 1·38 x 10-23 *JIk.*

*(c) *Mobility of holes is 0·Q25 m2N-sec~ What would be the re~istivity of P-type silico~. if th.e flaIl coefficient of the sample is 2·25 x I~-s m3/C ?

(d) Defin~ dielectrics, electric d~pole and polarizability.

(e) Dif(erentiate between soft and.hard magnetic materials.

(f) Define ‘Reverberation time’. WrIte sabine’s formula and explain the te~s in it.

(g)State the terms : magnetostriction effect; piezo-electric effec.t.

2(a) Explai_n. the_f_ormation-of ~ergy band~in-solids; With’neat-energyband diagraInS”exptairr extrinsic semiconductors.

(b) Draw the unit cell of HCP. What is its co-ordination number, atomic radius, and • effective number of atoms per unit cell. Also calculate its packing factor .

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 transfonner which kind of material will you prefer-_the one with small hysteresis area or the big one?

(b) Derive Bragg’s law. Calculate the glancing angle on the plane (100) for a crystal of rock salt (a= 2’125 AfT).Consider the case of 2nd order maximum and A = 0·S92Ao.

4. (a) Calculate the ntimber of atoms per unit cell ofa metal ~ving lattiCe parameter 2’9Ao and density 7·87 gm/cm3• Atomic weight of metal· is * S~’85, *Avagadro number is 6’02:3 x 1023/gm-mole.

(b) Prove that the Fermi level lies exactly at the centre of the forbidden energy gap in case of an intrinsic semiconductor.

(c) Explain ionic polarization and obtain polarizability (~).

5. (a) With neat diagram of a unit celJ, explain the structure of BaTi03·

(b) What is Hall etlect ? Derive expression for Hall voltage. 5

(c) Explain the absorption coefJicient of a hall. Calculate the change in intensity level WhC~l :- the intensity of sound increases 1000 times its original intensity.

6. (a) In what sense real crystals differ from ideal crystals? Explain the point defects in crystals.

(b) Explain construction and working of a solar cell. 5

(c) Find the natural frequency of vibration of quartz plate of thickness 2mm. Given Young’s modulus of quartz Y= 8 X 1010 N/m2, density of quartz is 2650 kg/m3• Caculate the change in thickness required if the same plate is used to produce ultrasonic waves of frequency 3MHz.