Code: DE04                                                                   Subject: ENGINEERING MATERIALS

Time: 3 Hours                                                                                                     Max. Marks: 100

NOTE: There are 9 Questions in all.

· Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

· Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

· Any required data not explicitly given, may be suitably assumed and stated.

Q.1       Choose the correct or best alternative in the following:                                         (2x10)

a. Ferro electric materials are characterised by

(A)  Very high degree of polarisation.  

                   (B)  A sharp dependence of polarisation on temperature.

(C)   Non-linear dependence of the charge Q on the applied voltage.                                

(D) All the above.

b. In a ferromagnetic material the state of flux density is as follows when external magnetic field is applied to it.

(A) Increased                                     (B)  Decreased

(C)  Remains unchanged                      (D)  Becomes zero

             c.   Heating elements of irons are made of

(A) Copper.                                       (B)  Nichrome.

(C)  Constantan.                                  (D)  Aluminium.

             d.   Materials which can store electrical energy are called

(A)  Magnetic materials.                     (B) Dielectric materials.

(C)  Semi conductors.                         (D) Super conductors.       

             e.   Transformer cores are laminated with

(A) Low carbon steel.                        (B)  Silicon sheet steel.

(C)  Nickel alloy steel.                         (D)  Chromium sheet steel.

             f.    Barrier potential in a P-N junction is caused by

(A) Thermally generated electrons and holes.       

(B) Diffusion of majority carriers across the junction.

(C)  Migration of minority carriers across the junction.       

(D)  Flow of drift current.

             g.   Holes are the majority carriers in    

(A) P-type semiconductor.                 (B)  N-type semiconductor.

(C)  Superconductors.                         (D)  Insulators.      

             h.   Dielectric materials are

(A) Insulating materials.                       (B) Semiconducting materials.

                   (C) Magnetic materials.                        (D) Ferroelectric materials.

             i.    The property due to which the resistance of some metal or compound vanishes under certain conditions is

(A)  Semiconductivity.                         (B)  Superconductivity.

(C)  Curie point.                                  (D)  Magnetostriction.       

             j.    A pure semiconductor under ordinary conditions, behaves like

(A)  a conductor.                                 (B) an insulator.

(C)  a magnetic material.                      (D) a ferroelectric material.

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   Explain the suitability of copper and aluminium that is used as electrical conducting materials.                                                                      (8)

     b.   Calculate the diameter of copper wire of length 100 metres used as winding material in a transformer such that the resistance of the whole winding is 2 ohms.  Calculate the diameter of wire if aluminium is to be used for the above winding, resistance remaining the same.  Given the resistivity of Cu is  ohm-metre and of AI is  ohm-metre.                                                                   (8)

Q.3     a.   What is superconductivity? Explain the effect of magnetic field on superconductors. Also give three applications of superconductors.          (8)

b.   Explain Ionic and Oriental polarization. (8)                                                                        

Q.4     a.   Give the properties and applications of porcelain and transformer oil.                  (8)

b.   Explain the electron gas model of a metal.                                                           (8)

Q.5     a.   What are ferrites?  Why is it used in high frequency devices?                               (6)

     b.   Explain various magnetic materials with example.                                                (6)

c.   Give the properties and application of permanent magnetic materials.                   (4)

  Q.6     a. Draw the symbols of MOSFET.  Explain the construction of MOSFET with a suitable diagram.                                                                 (8)

             b.   Explain junction transistors (npn and pnp)                                                           (8)

Q.7     a.   What is a dielectric material?  Explain electric break down.                                 (8)

             b.   Explain dielectric constant and dielectric losses.                                                  (8)

Q.8     a.    Explain the terms:  mobility, doping, diffusion, ferroelectricity.                              (8)

             b.   Draw the hysteresis loop for a soft magnetic material and compare it with the hysteresis loop of hard magnetic material. Also give example of each.                                                     (8)

Q.9           Write a note on:

(i)   Classification of materials on the basis of energy band.                                     

(ii)  Germanium and Silicon.                                                                              (16)