Misleading Poor Posting Boned - Fundamentals of Materials | ENGR 220, Exams of Materials science

Download Misleading Poor Posting Boned - Fundamentals of Materials | ENGR 220 and more Exams Materials science in PDF only on Docsity! ENGR 220: Final Exam Name: Section_______ Spring 2009 R. Knight (Print) 2 hours. Closed-book, closed-notes exam. Useful formulae, constants and a periodic table are attached (at the end). You may also use a calculator and ruler. Please show all working for full credit. 1. For the following questions choose the correct answer and/or fill in the blanks, as necessary. i) 2 pts. The ability of crystalline materials, such as iron, to exist in different forms at different temperatures is known as . ii) 3 pts. Ionic bonding is an example of primary / secondary (circle one) bonding, and usually occurs between atoms with similar / large differences in (circle one) electronegativity. iii) 4 pts. In the cesium chloride (CsCl) unit cell there are cations and anions, because (show calculations etc.) . iv) 3 pts. For the HCP crystal structure there are atoms per unit cell. This number was determined as follows (show calculations). v) 2 pts. Dislocations in metals are an example of a point / linear / surface (circle one) defect. vi) 4 pts. The coordination number (CN) for octahedral interstitial sites is higher / lower (circle one) than that for the tetrahedral sites. There are octahedral sites per FCC unit cell. That number was obtained as follows: (show calculation). vii) 3 pts. Interstitial diffusion generally proceeds faster / slower (circle one) than vacancy diffusion because . viii) 3 pts. The equation for Fick’s first law of diffusion is and in essence relates to _______________________________________. ix) 4 pts. Graphite is often used as a solid lubricant because . x) 3 pts. Most materials interact to some degree with their environment. In the case of metals this is known as whereas in the case of polymers it is known as . xi) 2 pts. A syndiotactic polymer has all of its side groups located on the same side / on alternate sides / randomly (circle one). xii) 3 pts. Tg, the glass transition temperature, is the temperature below which polymers become rubbery / glassy (i.e. brittle) (circle one). A polymer used for an ice cube tray should be above its Tg in use because . xiii) 4 pts. Galvanic corrosion involves a reaction at the and a reaction at the . xiv) 4 pts. The electrical resistance of a wire of length L will be the same / higher / lower (circle one) as that of a wire 2L in length. The same can / cannot (circle one) be said of the resistivity because . xv) 3 pts. Doping of Si with B produces an intrinsic/extrinsic/superconducting (circle one) -type semiconductor. The majority carriers in this type of semiconductor are . xvi) 3 pts. Mankind produces <30% / >80% / >95% (circle one) of its energy from fossil fuels. xvii) 3 pts. There are 0.35 / 3.5 / 35 / 350 (circle one) tons of gold in a cubic mile of ocean. /53 ENGR 220: Final Exam Name: Section_______ Spring 2009 R. Knight (Print) 2 hours. 2. (a) 5 pts. Compute the magnetic flux density in a solenoid 200 mm long, having 500 turns, carrying a current of 300 A, and located in a vacuum. (b) 15 pts. A junkyard wishes to use a coil of the dimensions given above to pick up steel scrap, but needs a magnetic field of 1.6 Tesla to do the job. As the designer, you have an iron-silicon alloy available which has the B-H characteristics shown in the Figure below. Compute the current required to generate the required magnetic field if a core of this material is placed within the coil. (c) 10 pts. On the axes shown below, draw the B-H plot (make sure to label the axes) you would expect to obtain for a ferromagnetic material at room temperature. Label the key points and explain your sketch, i.e. explain why you drew it the way you did, and explain what is happening at the magnetic dipole scale. /30 ENGR 220: Final Exam Name: Section_______ Spring 2009 R. Knight (Print) 2 hours. 5. (a) 5 pts. In the context of electrical conduction in solids, briefly explain the meaning of (i) electron drift velocity, and (ii) electron mobility. (b) 10 pts. An n-type extrinsic semiconductor has an electron concentration of 5 x 10 17 m -3 . If the electron drift velocity is 350 m/s under an applied electric field of 1000 V/m, calculate the electrical conductivity of the material. • Bonus: 15 pts. (9%) /15 (30) ENGR 220: Final Exam Name: Section_______ Spring 2009 R. Knight (Print) 2 hours. 6. (a) 5 pts. Compare and contrast thermoplastic and thermosetting polymers on the basis of (i) how their mechanical properties change on heating, and (ii) their typical structures. (b) 15 pts. Is it possible to have a poly(vinyl chloride) (PVC) homopolymer with the molecular weight data given below, and a degree of polymerization of 1120? Explain why or why not. Show all working for full credit. Molecular Weight Range (g/mol) wi xi 8,000-20,000 0.02 0.05 20,000-32,000 0.08 0.15 32,000-44,000 0.17 0.21 44,000-56,000 0.29 0.28 56,000-68,000 0.23 0.18 68,000-80,000 0.16 0.10 80,000-92,000 0.05 0.03 /20