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Eti 2106: Material Science Question Paper
Eti 2106: Material Science
Course:Bachelor Of Science In Mechatronics
Institution: Dedan Kimathi University Of Technology question papers
Exam Year:2010
KIMATHI UNIVERSITY COLLEGE OF TECHNOLOGY
DATE: February 2012 TIME: 2 HOURS
ETI 2106: MATERIAL SCIENCE
INSTRUCTIONS
1. This paper consists of five questions.
2. Answer Question One and any other two questions.
3. Unless otherwise specified; assume free space conditions and all symbols have their usual
meaning.
12
0 e 8.854 10 F/m ? ? ?
7
0 µ 4p 10 H/m ? ? ?
QUESTION ONE (30 MARKS)-COMPULSORY
(a) Find the conductivity of intrinsic silicon at 300K. Given that; (6 marks)
(i) ni at 300K is1.5?1010 / cm3 ,
(ii) the mobility of electrons and holes in silicon are 1300 2 cm /V ? s and 500
2 cm /V ? s respectively and
(iii) the number of Si atoms per cubic cm equals 22 5?10 .
(iv). If donor type impurity is added to the extent of one atom per 8 10 silicon atoms,
find the conductivity.
(v). If acceptor type impurity is added to the extent of one impurity atom per 8 10
silicon atoms, find the conductivity.
b) (i). Find the resistivity of intrinsic silicon at 300 degree K. Given that;
? i n at 300 degrees K is 10 3 1.5?10 / cm .
? the mobility of electrons and holes in silicon are 3800 2 cm /V ? s and 1800 2 cm /V ? s
respectively and
? the number of Si atoms per cubic cm equals 21 5?10 . [3 marks]
(ii). If donor impurity is added to the extent of one impurity atom per 8 10 silicon atoms, find
the resistivity. [3 marks]
(iii). If acceptor impurity is added to the extent of one impurity atom per 8 5?10 silicon
atoms, find the conductivity [3 marks]
c) Define the following terms (2 marks each)
i. Fillers
ii. Fibers;
iii. Plasticizers;
iv. Dyes and pigments
v. Thermoplastics
d) With the aid of well labeled diagrams, explain how temperature affects the conductivity of
intrinsic and extrinsic semiconductors. [5 marks]
QUESTION TWO.
(i).A uniform silver wire has a resistivity of 16 1.4 10 cm ? ? ? ? at room temperature. An electric
field of 50V/m exists along the length of the wire. If the number of free electrons is
5.8?1028 /m3 . Calculate the drift velocity of electrons, their mobility and relaxation time. [4
marks]
(ii).State three factors which affect the magnitude of the coercive force and hence the remanence
flux density in a ferrite material. Therefore discuss how a high coercity is achieved in permanent
magnets. [5 marks]
b) Describe clearly what electrical conductivity is, state the two factors the electrical conductivity
depends on, and give its formula and units (6 marks)
c) Define the following terms as used in material science. [5 marks]
(i). Remanence magnetization
(ii). Coercive force
(iii). Magnetic anisotropy
(iv). Electrostriction
(v) . Dielectric
QUESTION THREE
(a). Explain why power transformer cores are made from cold rolled grain oriented (CRGO) silicon
steel alloy. [7 marks]
(b). A 2µF capacitor is connected across a 500Volts d.c supply. If it contains mica as the dielectric
material having relative dielectric constant 5 r ? ? . find;
(i). The energy stored in the capacitor. [3 marks]
c). (b) Discuss four important requirements of a good insulating material. (4 marks)
(c) Given the electrical resistivity of a material is
3 3.0?10 ??m at room temperature
0 27 C .
The conductivity is
1 1 0 2.67 m at 250 C ? ? ? . Evaluate the size of the energy gap. (6 marks)
QUESTION FOUR
(a) List the Classification of magnetic materials (5 marks)
(b) Define amorphous Polymers (2 marks)
(c) Describe the preparation of amorphous polymer (2 marks)
(d) mention one application of super conductor (1 mark
An intrinsic semiconductor has a conductivity of 1 1 390 m ? ? ? at 0 5 C and 1 1 1010 m ? ? ? at 0 25 C .
(i). What is the size of the energy gap in eV? (3 marks)
(ii). What is the conductivity at 0 15 C ? (4 marks)
(d) There are many causes of resistivity in metals, mention at least three of them (3 marks)
QUESTION FIVE
(a) List six properties of glass (12 marks)
(b) State two reasons why silver is the best metallic conductor of electricity. Hence list four causes of
resistivity in metals. (6 marks)
(c) What are the applications of Superconductors (3 mark)
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