This preview shows page 1. Sign up to view the full content.
Unformatted text preview: um possible efficiency is the efficiency of the Carnot cycle: e = 1 (TL /TH) = 1 [(215 K)/(425 K)] = 0.494 = 49.4%. Yes, there is something fishy, because his claimed efficiency is not possible. 20.62
(a) The efficiency of the Carnot cycle is eCarnot = 1 (TL/TH) = 1 [(358 K)/(773 K)] = 0.537 = 53.7%. Thus we have e/eCarnot = 15%/53.7% = 0.28, so e = (28%)eCarnot . (b) The rate at which heat flows into the engine is QH/t = (100 hp)(746 W/hp) = 7.46 104 W. The useful power, which moves the car, is W/t = eQH/t = (0.15)(7.46 104 W) = 1.12 104 W. The rate at which heat is exhausted is QL/t = QH/t W/t = [7.46 104 W 1.12 104 W](3600 s/h) = 2.28 108 J/h = 5.45 104 kcal/h. UNIVERSITY OF CALIFORNIA, BERKELEY Physics Department 7b, Liphardt Exam #1 Name __________________________ Disc. Section ____________________ SID ____________________________ #0 (20)______ #1 (25)______ #2 (25)______ #3 (25)______ #4 (25)______ Total _______ of 120 The relative weight of each problem is indicated next to the problem number. Most credit will be given for algebraic work. Do not insert numerical values until you have a final algebraic answer inside a box. If you don't know a particular constant, use a symbol to get partial credit. It is almost impossible to award partial credit if you insert numbers too early. If you get stuck on one problem, go on to the next and come back to the difficulties later in the exam period. Do not leave early until you have completed everything. Do not quit! Never, never, never quit!! As usual, before you start to manipulate equations and numbers you should think for a moment and make sure that you have a general sense of the important features of a problem. Finally, please do not violate the first and second laws! kB = 1.38 x 10-23 J/K NA = 6 x 1023 Spring Term 2006 There is no need to compute precise numerical results ball-park values are ok. Points will be deducted for answers with insane units (e.g. the final temperature is 19 kg*m). Stuck? Approximate, approximate, a...
View Full Document
- Fall '08