{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Solutions 4-23-08 (practice exam 4)

Solutions 4-23-08 (practice exam 4) - ENGR 145 Chemistry of...

Info icon This preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 2
Image of page 3

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 4
Image of page 5

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 6
Image of page 7

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 8
Image of page 9
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: ENGR 145: Chemistry of Materials SI Practice Problems (Exam IV) 1. You drill a 20 mm diame er hole into a piece of 1020 steel ((1 = 11.7x10-6 °C-1). You desire to place a. ' mm aluminum rod ((1 = 23.6);10'6 °C'1) into the steel. a. To What temperature must you beat the system to ensure a tight fit? cementum .04'0-0 1. d\‘(‘1£_'["3 =fi k = 9-9 A<IE~TL> l— in lcbn'. at :- ‘LO (“4 $104“) (1;- lfi + 20 W55 (13.1,xlo‘L3 (t- ZSB + M5 2.4 :un + Sout. 2% °C b. Assume the aluminum rod (k = 237 W / m K) is suddenly removed and placed inside a temperature chamber at 25°C. If the rod is one meter long, what is the total heat loss from the rod? MSW"- coA has dts mild Am“. = - Al J-L M q \L éx ‘~ ‘Blmv «mash-A A—l'. it " b (/0 “l: «on to 37; mm “wow W has “Mal w1s 6 ~ 5.0m up" 3:} *(ZT‘ “"3353 C03 2 SWOOO w W: c. What is the electrical conductivity of the aluminum when it is first placed into the temperature chamber? -AL -7 _ J; L'c’l “" 6‘ LT Luu x up" 1%”; (w. +1433 2. The following concept-based questions relate to electrical conductivity. b. C. [a (“in d. c = ”gm 5 \q)’ a. Six extrinsic semiconductors chemistries are listed below. Circle all of the following that can be classified as n-type semiconductors. i. A1 doped Ge @ As doped s1 @ P doped InSb v. In doped GaAs iii. Si doped Ge @ N doped s1 No other material property has as much variability across material categories as does electrical conductivity. Based on your knowledge of the mechanisms for causing and limiting conductivity, rank the following eight materials from highest conductivity to lowest conductivity. _?_ Intrinsic Si at 300K L Copper wire formed by 3 Aluminum Oxide deformation . 9 18 wt% Cr - 8 wt% Ni 5 In doped Si stainless steel I Elemental copper 6 Intrinsic Si at 273K 5 Low carbon steel Josh and Justin both leave Wade at exactly 6:42 PM for the exam review in Schmitt. Josh arrives in Schmitt at 6:58 PM. Justin, however, slips on a number of low-friction surfaces en-route and arrives at 7:02 PM. State the relationship (“<”,”>”, or “=”) between Josh’s and Justin’s mobilities. 1” Justin < ”Josh ' p L Two equal length cylindrical metallic samples are placed in identical AA z’fi (‘33 circuits. Both samples are 42.0 cm long. Sample A has a radius of 5.0 cm A A , .0074 ,,."‘ and Sample B has a radius of 2.5 cm. When a 12 V voltage is applied, A P ( 1 Sample A has a current of 1.333 A passing through it and Sample B has a B a ' ’ r3) current of 3.5 A passing through it. State the relationship “<”,”>”, or “=”) A 9 ’ ' 00 20:” between the resistivities of the samples. RA; "’/ 21.: 'z" ; 7 J1 pA > ’03 A : $132334» c. 0014) . qz : 6"“ 2 .V/ ———’“’ ~ '“"‘ . . ‘3 '. ’9’ 1'” ‘ g"Vi-“LIJ’IMM In the year 2000, Alan Heeger, Alan MacDiarmid, and H1dek1 Shlrakawa J1 .h shared the Nobel Prize in Chemistry for their 1976 research on conductive organic polymers. Circle the polymer structure below that is most likely to result in conductive properties. \, H | /o\ I -—C' C H III [CRE— I l H H R CWJuCiTM dl mots have delocalneol bowl on Clea?» 3. A metal box is fitted with a hinged bottom which is closed Via a magnetic catch (made from iron). At room temperature, the magnetic force of attraction is equal to the force of gravity on the bottom. a. Suppose the box is placed inside a temperature chamber. If the temperature is decreased by 200°C, What do you expect to happen? What if the temperature is increased instead? (Ignore temperature effects on other material properties). flew-o} LVN-“*3 ro-obe‘ZJS mbfiwt?“ otétr "M L “A" sin“ H‘ é QM g“ “Amen. 3- ’l 1 W‘“ nga. *k‘ ‘03 ‘LD oft“ GS X'Qrtl c§ gist m\-we iwo; El ‘WQ. Ww‘gkofil‘a gawk“! OVL‘CDM‘K "EM: b. Suppose you placed the box in a magnetic field and slowly increased the field strength until magnetic saturation. You then remove the magnetic field and repeat the experiment in part (a). What might you observe? 1k“, fhwmn \s ‘Cmowa 04. Mx-iums, O.V\¢ he». .Lhe, disreoi gC skrunsimumb JcN— mguwiw mmcu‘i 6R Ox c. Could you perform the experiment in (a) if the catch were made out of titanium (a paramagnetic material)? If not, Why not? N‘Mbuwi‘s M m Qawmbfl “tit ww ltu \o.\$ go." be. MAD C QC! \w) “(91% “:3 ova \\ gm. \A. $0 'l'ke 2X emu“... i3 (equ\f2. O *‘t‘A \* O '89} -\(o b?— eat-Yorvnfid. wov ‘A 2. Indium (III) Antimonide (InSb) is a semiconductor with a very small band gap of 0.17 eV. As a result, it is commonly used in infrared detectors due to the fact that only light waves with wavelength greater than that of the visible spectrum (such as infrared light) can promote electrons from the valence band to the conduction band. The mobility of holes in InSb is 7.7 m2/V-s and the mobility of holes is 0.07 mZ/V-s. a. Calculate the conductivity of intrinsic InSb if the hole concentration at room temperature is 2.0 x 1016 cm'3. (HINT: Watch the units!!) M: 2.0xlOlQ/Cm5 O": nlel(fle+flk) n : 2.0;:10 11/.“3 02 szlo‘zwt‘Xh bx/o""c)(7.> 4 .0? m"/u.s) 0’12ng” LR“)— Calculate the conductivity of extrinsic InSb if it is n-doped with a 2.7 x 1022 In-3 concentration of arsenic donors. At this dopant concentration, the conductivity effect from intrinsic holes can be assumed to be neglibible. Nd: 2.7x101zm'3 0': and: + p614». 9‘6 ”Wife 0"- NJ (He 0‘: (1.7ylouh")(I.bx/0“Qc)(7,7.“1/V‘s) ! -l I 0' =— ’5 3 2. (1” ( p ’w) In the InSb were instead p-dope to t e same concentration as part b with the same assumptions, would the conductivity calculated be less than or greater than that of part b? Without any calculation, how do you know? 1L1 Canlucimly ‘Ffom p'acap'us would (at less 1316:4442 flu<€fle We have seen in class that above a certain temperature, even in an extrinsic semiconductor, the concentration of electrons behaves as if the material were intrinsic. This occurs when the intrinsic concentration of M holes as predicted by the Boltzmann relationship exceeds the dopant concentratlon. If the dens1ty of states in the conductlon band is 4.2 x 1022 m‘3 and the density of states in the valence band is 7.3 X 1024 m'3, and the semiconductor is n-doped to the concentration in part b, what calculate the temperature where the concentration of electrons begins to exhibit intrinsic behavior. h -— :"rNol 5. Use the stress strain curve below to answer the following questions. Note that the dashed curve labeled “Actual Stress” is not used. -- Mm! sires: -.. E 'L. stress 5' [tips per sq in] wrimm‘e 5 a. What is the tensile strength of soft steel? (94 V99: b. What is the yield strength of hard steel? 3‘? \Lm c. If a 10 m bar of hard steel were stretched to breaking, What would its length be at break? g: .04? AL: ,oH‘+ (t0«\ = Aim .9“- :- (0.47 M d. Identify the elastic region for hard steel (on the diagram above) and calculate the elastic and shear moduli, assuming a Poisson ratio of 0.30. W 0= [1 1!? bqslx E (.006) (two E: (Lew ‘4?“ Ga 44"“) k?“ 25-. 24cm) e. Draw typical stress-strain curves for 1) a metal, 2) a ceramic, 3) a plastic polymer, and 4) an elastic polymer. Highlight and explain the important differences. 6. An engineering application where optical properties are of significant importance is in grocery checkout scanner covers. These covers must be optically transparent and transmit a large amount of the laser intensity from the scanner itself in order to effectively “read” the grocery bar code. a. In order to allow for proper bar code scanning, the transparent covering must not absorb light from a red laser with a wavelength of 700 nm. What is the minimum band gap energy that the material can have to avoid absorption by the promotion of electrons from the valence band to the conduction band? //\_ 700 :w 'an g z (6 “2219,10 3 ”J'leix/D M’s) E‘): i" ‘U‘ —_ 700 up”? an E6 -. Linda“ 3: [177(V b. Another important property of the bar code scanner cover is that a large amount of the intensity of the laser must pass through the scanner cover. ”5 What is the maximum absorption coefficient that a material can have ’ ‘ c..." ‘0 to allow 95% of the light intensity to pass through a 1.0 cm plate of the 7““ material? I A (a ‘K’ ) '57,: ,‘lg 7/" .01.. 17:10 eFP(-flK) [/61 S513 m"; 7. The following concept-based questions relate to mechanical properties. a. Answer whether the following statements are TRUE or FALSE. 0 The relationship E = 2G(1 + v) is true for all materials. TRUE 0 Elastic strain is an example of microscopic chemistry giving rise to macroscopic behavior. FALSE b. Circle all of the following that are true of plastic deformation? @ Plastic deformation involves the breaking and reforming of bonds ii. The transition from elastic to plastic deformation occurs , suddenly Upon release of a load, some fraction of deformation is recovered 1v. The onset of plastic deformation can be identified on a stress- strain curve by looking for curvature 0. Circle all of the following that are true of the mechanical behavior of polymers? i. The stress-strain curve for polymers is relatively independent of temperature ® Unlike in metals, the tensile strength of polymer may be less than the yield strength iii. Necking is more likely to occur in highly crystalline polymers than in semicrystalline or amorphous ones iv. The stress-strain curve for polymers is relatively independent of the rate of deformation 8. A number of graphs that we have seen in-lecture are presented below. Beside each graph are three theoretical explanations that may or may not describe the underlying reason for the behavior observed in the graphs. Circle all of the explanations that are true. a. Electron & hole mobility in extrinsic silicon as a function of extrinsic impurity concentration I. Hole mobility decreases as extrinsic concentration increases 1k as a result of the Si being p-type. b" i l ! @D Electron mobility is higher than hole mobility because holes are almost always scattered more than electrons. Mobility (mEJ‘V s‘, dimes“ m2; a An increase in impurities : ways increases the scattering of electrons and holes leading to the decrease in mobility at high concentrations. b. Saturation magnetization of pure iron and magnetite plotted as a function of material temperature 0 aonmperali‘o:(fl 1200 1600 25,000 I. Pure iron has a larger magnetization for all temperatures because it is diamagnetic. !" o a 8 ._. in N o ‘0 8 15.000 II. Magnetite (Fe304) has a lower magnetization for all temperatures because it is anitferromagnetic. 0.5 5.000 L——> Fen-l Majnt “C II. Both materials have >200 o zoo we soo soo woo magnetizations that decrease Temperature ( ‘0) 10.000 Salutation magnetization. quo" Aim) 0 Saturation flux dens-1y. Bx (gauss) rapidly as they approach their Curie temperatures because the materials begin to exhibit paramagnetic behavior. L——~> 14‘s Farrow-952M: c. Electrical resistivity of a metal versus temperature Na: VH‘F?‘ velocrly is Terseratu': ( F2 (1m 3130 .905) we 0 in? I. Resistivity increases Wlth ' increasing temperature because A 4 electrons move slower between (1’0" “9‘ ""’ 5114 332 am. My scattering events at high ‘Tle uelacfly A: fi‘ 4 . aim... N. , temperatures than low it .p e {col/u, temperatures. .454! F islocations caused by eformation scatter electrons and thus lead to an increase in resistivity with deformation. Emma: lumen-Ma ‘1!) V Farm? :nppm V 705; Mm 1m may {7’ 3,0 Tsmucraturni @Pure copper 1s the least sistive because additions of nickel create additional electron scattering locations. ...
View Full Document

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern