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7A-CD-QuizzesSolns

# 7A-CD-QuizzesSolns - 7A-C/D Win 09 Quiz 1 DL Sec Last 6...

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7A -C/D Win 09 Quiz 1 DL Sec Last 6 digits ID # Name (Last, First) Grade 1. A 100 gram block of ice is placed on a 5 kg block of lead (Pb). The ice is initially at -10° C, while the Pb block is initially at +80° C. The two blocks are contained within a well-insulated box. Given that all of the ice will melt , construct an energy system diagram (including carefully stated initial and final conditions), write an appropriate energy conservation equation, and solve for the final equilibrium temperature. You must show your work clearly to receive credit! 2. The diagram to the right shows temperature vs. energy added for a 1-kg sample of an unknown pure substance. Give as much information as you can about the physical properties of the substance. Your answer should at least include a comparison of specific heats for different phases, description of interesting temperatures, and something about heats of melting and vaporization .  Please explain clearly but concisely! ∆Ε th = mc ∆Τ H 2 O:   ∆Η melting = 334 kJ/kg        liquid: c = 4.18 kJ/(kg K),  solid:  c= 2.05 kJ/(kg K) ∆Ε bond   = ( m) ∆Η Pb:      ∆Η melting =   23 kJ/kg        solid Pb:  c = 0.13 kJ/(kg K)

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Phy 7A Winter 2009 Quiz 1 Solution Michael Woods January 25, 2009 1) Initial: 0.1 kg of water at -10 C and 5 kg of lead at 80 C put into thermal contact. So from the diagram we found that E th,ice + ∆ E bond,H 2 O + ∆ E th,water + ∆ E th,lead = 0 (1) m I C I T ice + m I H I + m I C w T water + m Pb C T = 0 (2) - m I C I T i,ice + m I H I + m I C w T f + m C ( T f - 80 C ) = 0 (3) T f = 1 m I C w + m C ( m C T i,Pb - m I H ) (4) = 15 . 49 C (5) = 288 . 5 K (6) 1
2) We can read straight from the chart that the freezing point is 40 C and the boiling point is 210 C. The slopes of the transition regions can tell us about the speciﬁc heats in each phase of this substance. The solid phase has a small slope so we can see that when we add a lot of energy the energy only increases slightly. Thus the speciﬁc heat for the solid state is large since to raise one degree celsius or one kelvin we need to add a of energy. The liquid phase is very steep and has a very small speciﬁc heat. The gas phase is between the liquid and solid, and its speciﬁc heat is between that of the liquid and solid phases. The ﬂat region at 40 C represents the heat of melting while the ﬂat region at 210 C represents the heat of vaporization. We quantify the two by noting that the heat of melting is much larger (spans a much larger energy range) than the heat of vaporization. Note that this is probably verbose; more words than you needed to use on the quiz. 2

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7A-C/D Win 09 Quiz 2 DL Sec Last 6 digits ID # Name (Last, First) Grade 1. A skier begins sliding down a hill from an initial stop at a height of 30 m.
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7A-CD-QuizzesSolns - 7A-C/D Win 09 Quiz 1 DL Sec Last 6...

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