# Lecture18 - PH YSI CS 231 I N TRODUCTORY PH YSI CS I Lectur...

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Unformatted text preview: PH YSI CS 231 I N TRODUCTORY PH YSI CS I Lectur e 18 The Laws of Thermodynamics The Laws of Thermodynamics Chapter 12 Principles of Thermodynamics • Ener gy is conser ved • FI RST LAW OF TH ERMODYN AMI CS • Examples: • Engines (H eat -> Mechanical Ener gy) • Fr iction (Mechanical Ener gy -> H eat) • All pr ocesses must incr ease entr opy • SECON D LAW OF TH ERMODYN AMI CS • Entr opy is measur e of disor der • Engines can not be 100% efficient • Adding heat Q can: • Change temperature • Change state of matter • Can also change ∆ U by doing work on the gas Work done on a gas Change of I nter nal Ener gy ∆ U Wor k done on the gas W = r F D r x = F (- D y ) = ( PA )(- D y ) W = - P D V First Law of Thermodynamics ∆ U = Q + W N ote: (Wor k done by the gas ) = - (Wor k done on the gas ) • Conser vation of Ener gy • Can change inter nal ener gy ∆ U by • Adding heat to gas: Q • Doing wor k on gas: W = - P D V Q = D U + W by the gas Add heat => I ncr ease I nt. Ener gy & Gas does wor k W by the gas = + P D V Example 12.1 A cylinder of r adius 5 cm is kept at pr essur e with a piston of mass 75 kg. a) What is the pr essur e inside the cylinder ? b) I f the gas expands such that the cylinder r ises 12.0 cm, what wor k was done by the gas ? c) What amount of the wor k went into changing the gr avitational PE of the piston? d) Wher e did the r est of the wor k go? 1.950x10 5 Pa 183.8 J 88.3 J Compr essing the outside air Example 12.2a A massive copper piston tr aps an ideal gas as shown to the r ight. The piston is allowed to fr eely slide up and down and equilibr ate with the outside air . The pr essur e inside the cylinder is _________ the pr essur e outside. a) Gr eater than b) Less than c) Equal to Example 12.2b A massive copper piston tr aps an ideal gas as shown to the r ight. The piston is allowed to fr eely slide up and down and equilibr ate with the outside air . The temper atur e inside the cylinder is __________ the temper atur e outside. a) Gr eater than b) Less than c) Equal to Example 12.2c A massive copper piston tr aps an ideal gas as shown to the r ight. The piston is allowed to fr eely slide up and down and equilibr ate with the outside air . I f the gas is heated by a steady flame, and the piston r ises to a new equilibr ium position, the new pr essur e will be _________ than the pr evious pr essur e. a) Gr eater than b) Less than c) Equal to Some Vocabulary • I sobar ic • P = constant • I sovolumetr ic • V = constant • W = 0 • I sother mal • T = constant • ∆ U = 0 (ideal gas) • Adiabatic • Q = 0 V V V V P P P P A massive piston tr aps an amount of H elium gas as shown. The piston fr eely H elium gas as shown....
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## This note was uploaded on 07/25/2008 for the course PHY 231 taught by Professor Smith during the Spring '08 term at Michigan State University.

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Lecture18 - PH YSI CS 231 I N TRODUCTORY PH YSI CS I Lectur...

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