Code: D-04                                                                    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. Mica is a

(A)  Dielectric material.                        (B)  Insulating material.

(C)   Magnetic material.                       (D)  Both insulating and dielectric

                                                                  material.

b. The conductivity of copper is less than that of silver by

(A) 5 – 10 %.                                     (B)  50 – 60 %.

(C) 80 – 90 %.                                    (D)  20 – 30 %.

             c.   A ferrite core has less eddy current loss than an iron core because ferrites have

(A) High resistance.                            (B)  Low resistance.

(C)  Low permeability.                         (D)  High hysteresis.

             d.   Transformer cores are laminated with

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

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

             e.   For silicon the forbidden gap is

(A) 1.1ev.                                          (B)  0.25ev.

(C)  0.5ev.                                          (D)  0.7ev.

             f.    Plastics are

(A) Good conductors of heat.             (B)  Good conductors of electricity.

(C)  Bad conductors of electricity.        (D)  High density.

             g.   In order to obtain p-type germanium it should be doped with a

(A) Trivalent impurity.                         (B)  Tetravalent impurity.

(C)  Pentavalent impurity.                     (D)  Any of the above will do.       

             h.   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.

             i.    The heating elements of electric irons are made of

(A) Copper.                                       (B) Nichrome.

(C) Constantan.                                   (D) Aluminium.

             j.    The most malleable, ductile low resistivity material is 

(A)  Copper.                                       (B)  Aluminium.

(C)  Silver.                                          (D) Iron.

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   Explain the effect of a dielectric on the behaviour of a capacitor.                          (8)

             b.   Give the properties and applications of porcelain and cotton.                               (8)

Q.3     a.   Explain the electron gas model of a metal.                                                           (8)

             b.   Explain the suitability of copper and aluminium as electrical conducting materials.                 (8)

Q.4     a.   Explain the factors affecting hysteresis loss and permeability.                           (5+5)

b.   Give the properties and applications of permanent magnet materials.                    (6)

Q.5     a.   Explain how materials can be classified with reference to different band structures.              (8)

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

  Q.6     a. Explain the structure of germanium and silicon.                                                    (8)

             b.   Explain how the magnetic field affects superconductors.  Also give three applications of superconductors.                                                              (8)

Q.7     a.   Explain the magnetisation-curves of magnetic materials.                                       (8)

b.   Explain the significance of the loss tangent of a dielectric material.                        (8)

Q.8     a.   Give the properties and uses of Ferro electric materials.                                      (6)

             b.   Give the properties and applications of  asbestors.                                              (4)

c.   The resistivity of pure copper is 1.56 micro-ohm –cm.  An alloy of copper contains 1 atomic percent nickel has a resistivity of 2.81 micro-ohm –cm.  An alloy of copper containing 3-atomic percent silver has a resistivity of 1.98 micro-ohm –cm.  What is the resistivity of an alloy of copper containing 2 atomic percent nickel and 2 atomic percent silver?       (6)

Q.9     a.   Explain the effect of temperature on the conductivity of a semiconductor.             (8)

b.   Explain the terms

(i) Mobility.

(ii) Doping.

(iii) Diffusion.

(iv) Piezoelectricity.                                                                          (8)