Lecture18 - PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 18...

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Unformatted text preview: PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 18 The Laws of Thermodynamics The Laws of Thermodynamics Chapter 12 Principles of Thermodynamics Energy is conserved FIRST LAW OF THERMODYNAMICS Examples: Engines (Heat -> Mechanical Energy) Friction (Mechanical Energy -> Heat) All processes must increase entropy SECOND LAW OF THERMODYNAMICS Entropy is measure of disorder 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 Internal Energy ! U Work done on the gas W = r F " # r x = F ( $# y ) = ( PA )( $# y ) W = " P # V First Law of Thermodynamics " U = Q + W Note: (Work done by the gas ) = - (Work done on the gas ) Conservation of Energy Can change internal energy ! U by Adding heat to gas: Q Doing work on gas: W = " P # V Q = " U + W by the gas Add heat => Increase Int. Energy & Gas does work W by the gas = + P " V Example 12.1 A cylinder of radius 5 cm is kept at pressure with a piston of mass 75 kg. a) What is the pressure inside the cylinder? b) If the gas expands such that the cylinder rises 12.0 cm, what work was done by the gas ? c) What amount of the work went into changing the gravitational PE of the piston? d) Where did the rest of the work go? 1.950x10 5 Pa 183.8 J 88.3 J Compressing the outside air Example 12.2a A massive copper piston traps an ideal gas as shown to the right. The piston is allowed to freely slide up and down and equilibrate with the outside air. The pressure inside the cylinder is _________ the pressure outside. a) Greater than b) Less than c) Equal to Example 12.2b A massive copper piston traps an ideal gas as shown to the right. The piston is allowed to freely slide up and down and equilibrate with the outside air. The temperature inside the cylinder is __________ the temperature outside. a) Greater than b) Less than c) Equal to Example 12.2c A massive copper piston traps an ideal gas as shown to the right. The piston is allowed to freely slide up and down and equilibrate with the outside air. If the gas is heated by a steady flame, and the piston rises to a new equilibrium position, the new pressure will be _________ than the previous pressure. a) Greater than b) Less than c) Equal to Some Vocabulary Isobaric P = constant Isovolumetric V = constant W = 0 Isothermal T = constant ! U = 0 (ideal gas) Adiabatic Q = 0 V V V V P P P P A massive piston traps an amount of Helium gas as shown. The piston freely slides up and down.slides up and down....
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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 - PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 18...

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