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Unformatted text preview: ENGR 145: Chemistry of Materials SI Practice Problems (Exam IV) 1. You drill a 20 mm diameter hole into a piece of 1020 steel ( α = 11.7x10-6 °C-1 ). You desire to place a 19.95 mm aluminum rod ( α = 23.6x10-6 °C-1 ) into the steel. a. To what temperature must you heat the system to ensure a tight fit? 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? Assume the rod has its initial diameter. c. What is the electrical conductivity of the aluminum when it is first placed into the temperature chamber? 2. The following concept-based questions relate to electrical conductivity. a. Six extrinsic semiconductors chemistries are listed below. Circle all of the following that can be classified as n-type semiconductors. i. Al doped Ge iv. As doped Si ii. P doped InSb v. In doped GaAs iii. Si doped Ge vi. N doped Si b. 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 _____ Aluminum Oxide _____ 18 wt% Cr - 8 wt% Ni stainless steel _____ Intrinsic Si at 273K _____ Copper wire formed by deformation _____ In doped Si _____ Elemental copper _____ Low carbon steel c. 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. Justin μ Josh μ d. Two equal length cylindrical metallic samples are placed in identical circuits. Both samples are 42.0 cm long. Sample A has a radius of 5.0 cm and Sample B has a radius of 2.5 cm. When a 12 V voltage is applied, Sample A has a current of 1.333 A passing through it and Sample B has a current of 3.5 A passing through it. State the relationship (“<”,”>”, or “=”) between the resistivities of the samples....
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This document was uploaded on 04/28/2010.
- Spring '09