First Law of Thermodynamics

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Frst Law of Thermodynamcs Readng: Chapter 18, Sectons 18-7 to 18-11 Heat and Work When the

1 Frst Law of Thermodynamcs Readng: Chapter 18, Sectons 18-7 to Heat and Work When the pston s dsplaced by ds, force exerted by the gas = F = pa, work done by the gas: dw Fds ( pa)( ds) p( Ads) p d. When the volume of the gas changes from to f, total work done: W dw f p d. Ths work done s represented by the area under the p- curve n the p- dagram between ponts and f. 1

2 When the work s done n a thermodynamc cycle, the net work done by the system s equal to the area of the cycle enclosed by the cycle. (a) The work W done by a system as t goes from an ntal state to a fnal state f. Work W s postve when the system s volume ncreases. (b) W s stll postve, but now greater. (c) W s stll postve, but now smaller. (d) W can be even smaller (path cdf) or larger (path ghf). (e) The system goes from state f to state as the gas s compressed to less volume by an external force. (f) The net work W net done by the system durng a complete cycle s represented by the enclosed area. 2

The Frst Law of Thermodynamcs E nt Q W, where E nt = change n nternal energy of the system, Q = heat absorbed by the system, W = work done by the system. For a dfferental change, de nt dq dw.

3 The Frst Law of Thermodynamcs E nt Q W, where E nt = change n nternal energy of the system, Q = heat absorbed by the system, W = work done by the system. For a dfferental change, de nt dq dw. The frst law of thermodynamcs s an extenson of the conservaton of energy. See Youtube Rubber Band Heat Engne Specal Cases Adabatc processes: No transfer of heat e.g. the process occurs too rapdly, the system s thermally nsulated. Then Q = 0, leadng to E nt W. 3

If the gas s allowed to expand, ts nternal energy decreases. If the gas s compressed, ts nternal energy ncreases. Constant-volume processes: No work s done. Then W = 0, leadng to E nt Q.

4 If the gas s allowed to expand, ts nternal energy decreases. If the gas s compressed, ts nternal energy ncreases. Constant-volume processes: No work s done. Then W = 0, leadng to E nt Q. If heat s added to the system, ts nternal energy ncreases. If heat s removed, ts nternal energy decreases. Cyclcal Processes: When a cycle s complete, t forms a closed loop n the p- dagram. Snce the nternal energy s an ntrnsc property of the system, t returns to the ntal state. Then E nt = 0, leadng to Q W. Free Expanson: Snce the system s nsulated, Q = 0. Snce the gas expands freely nto vacuum, no work s done, W = 0. Ths leads to E nt 0. 4

Examples 18-5 Let 1.00 kg of lqud water at 100 o C be converted to steam at 100 o C by bolng at standard atmospherc pressure (1.00 atm or 1.01 10 5 Pa). The volume changes from an ntal value of 1.

5 Examples 18-5 Let 1.00 kg of lqud water at 100 o C be converted to steam at 100 o C by bolng at standard atmospherc pressure (1.00 atm or Pa). The volume changes from an ntal value of m 3 as a lqud to m 3 as steam. (a) How much work s done by the system durng ths process? (b) How much heat must be added to the system durng the process? (c) What s the change n the nternal energy of the system durng the bolng process? (The latent heat of vaporzaton of water s 2256 kjkg 1.) f (a) W p d p f ) ( p f d 5 3 ( )( ) J 169 kj (ans) (b) Q L m 1 (2256kJkg )(1kg) 2256 kj 2260 kj (ans) (c) Usng the frst law of thermodynamcs, E nt Q W kj (ans) 5

6 Problem 82. A cylnder contans gas and s closed by a movable pston. The cylnder s kept submerged n an ce-water mxture. The pston s quckly pushed down from poston 1 to poston 2 and then held at poston 2 untl the gas s agan at the temperature of the ce-water mxture; t then s slowly rased back to poston 1. If 100 g of ce s melted durng the cycle, how much work has been done on the gas? (L F = 333 kj kg -1 ) 6

Thermodynamics Second Law Entropy

Thermodynamics Second Law Entropy Thermodynamcs Second Law Entropy Lana Sherdan De Anza College May 8, 2018 Last tme the Boltzmann dstrbuton (dstrbuton of energes) the Maxwell-Boltzmann dstrbuton (dstrbuton of speeds) the Second Law of