{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Quiz7_Solution

# Quiz7_Solution - Quiz 7 Monday March 24‘“ 2008 PHY 252...

This preview shows pages 1–2. Sign up to view the full content.

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

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Quiz 7. Monday, March 24‘“, 2008 PHY 252 Name. SOLu—nozu 1. Two blocks of copper, each exactly of 1 kg mass, are placed in a well- insulated container so that no heat can flow in or out of the system (:12. Bl°°k l BIOCI‘ 2 it is a closed system). Block 1 is initially at temperature Tl =60.0 °C, block 2 is initially at temperature T2 = 20.0 °C. Cu Cu 1.0 kg 1.0 kg (a) Calculate the equilibrium temperature Tf. (1 point) 600 DC 200 0C From lsi: law Qt°t=o = Q‘+Qz QI+ at. = MIC-i (T; -T;) + MIC1CTg-T;) ‘-‘-‘ O :2) T3, = “‘31; IMICITI -.-_-_ éCTl-iT-g) = 41(60‘c.+2.o°c) 2' 40°C — 'A‘ (b) Calculate the change in e‘ii‘ti’cfﬁy of the system, ASJr once equilibrium is reached. (CCll =390J/kgoK) (1 point) Téd Tédﬂ T 1" ASed» ='- A5. + A5,. = J .3: + 3 __.=== mgj‘il: +M1ctj‘ﬂ’ "r. T 1.‘ T T 'T‘ T T ‘ I. Astot = nigh-1%- +“:Ca1H-Ii 7:. “cauLAI—E ‘ Ta, TIT}, 1. as“ = (1:3)(aaomna.n)u[_&_m_ -.-. l-é'J'IK *- (z‘i‘SHCSBI) (c) Now consider the system before equilibrium is reached. After some time a certain amount of heat Q is transferred from block 1 to block 2. The temperature of block 1 has fallen by an amount AT , and the temperature of block 2 has risen by an equal amount AT (since they have the same mass and specific heat, and heat is conserved by the 1st law), but neither has yet reached Tf. Derive down an expression for the entropy change of the system AS in terms of AT , m, CONT“ and T2. (1 point) Frau lsi law Ql = - at ‘= -MCAT 11-51“ “Ru-r AS ::.- AS.+ASL == ‘73-! + 9L9.- :_ WICcu +1“ EAT) T‘ T T T T. 1-I I. ((1) Now solve for the value of AT that maximizes AS . (Hint, solve d(AT) = 0 ), and comment brieﬂy on the physical insight revealed by your result. (2 points) Differentiate AS: dcas) :: urn—GEE. + lid—‘1‘. = a forum: orm‘u‘ cl CAT) Fr! "AT Tad-HAT ' % -TI+AT +T1+AT =0 ==> AT = 1"“ : 50°C“2°°‘ -..-. 206° '2- 2 “There is atumiuﬂ Poiulr for AS when col-tickle T5 . To elect-J: ‘15- N.” is a. Maxi-mum or CLICAS) .__ ._ WIng “(6“ h.- atcm‘ ("n—ATP (mm: Eiuilibr‘fum CorrespauaLs to 'the. maximum eu‘Lropd change that? is canalsten‘t wit-L.__'l:ke "its!- low . =‘T.-AT = 60-10 = 40°C Mimmum 6! :Hereu'tliat'e ad q in < O alwaﬂL .‘. 4-5 .15 mxﬁﬂﬂhd‘ﬁ Quiz 7. Monday, March 24‘“, 2008 PHY 252 2. An ideal heat engine has a theoretical efﬁciency of 25.0 % when the high temperature reservoir is at TH = 127 °C . (a) What is the temperature of the low temperature reservoir, TL ? (1 point) e , .__= l __ “Tc. Ideal. 3F: 0.25. ___ l.._ -r¢_ a 1.. :5 @119: trooth—oar) 2?3N2?-K ‘HDOK Tc = 300 k .....__.. + In one cycle, a real engine extracts 250. J of heat from the hot reservoir while doing 40. J of work. (b) What is the actual efﬁciency of the engine? (1 point) ._. _. T ._.. ecu-114.4 ‘ 19/— —- “'0' = 0.14 :2 16% -— =14 Qu 2 s 0.3 (c) How much heat per cycle is exhausted to the cold reservoir? (1 point) [Q4 : [QH[_[w] = 230.2:— A—oq‘ = 210:: i4 3. 1.00 g of ice at 0.0 °C melts in a large lake of water whose temperature is very slightly above 0.0 °C. Estimate the entropy change of the ice? (LF = 333 J/ g) (2 points) AS"; '2 7-7 5"— ~= ..M_L_§ 56-:- a.“ pretend e T T .1. Ast'ce = C "005) “_(_33__3 We) :. 1.22 T/K 9(- 2? 3 K What is the entropy change of the ice—lake system? (1 bonus point) 'qu. tux—.th macaw from rum at 0°C, .‘. Aside :: -— :LF 2:. -- 1.7.2. T/K an Aswan = 45,-“ +AsMe := L22 -r.22 = OU/k* 1e. JUL» imam; Pro-mass L4 rem—AM. ...
View Full Document

{[ snackBarMessage ]}

### Page1 / 2

Quiz7_Solution - Quiz 7 Monday March 24‘“ 2008 PHY 252...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online