PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 18 Chapter 12 The Laws of Thermodynamics 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 Work done on a gas Adding heat Q can Change temperature Change state of matter Can also change U by doing work on the gas r r W F x F y PA y W P V Work done on the gas Change of Internal Energy U First Law of Thermodynamics U Q W Conservation of Energy Can change internal energy U by Adding heat to gas Q Doing work on gas W P V Note Work done by the gas Work done on the gas W by the gas P V Q U W by the gas Add heat Increase Int Energy Gas does work 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 1 950x105 Pa b If the gas expands such that the cylinder rises 12 0 cm what work was done by the gas 183 8 J c What amount of the work went into changing the gravitational PE of the piston 88 3 J d Where did the rest of the work go 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 P Isobaric V P constant Isovolumetric V constant W 0 Isothermal T constant U 0 ideal gas P V P V Adiabatic Q 0 P V Example 12 3a Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston isothermally compressing the gas to half its original volume b Pb is Pa a Greater than b Less than c Equal to U Q P V W by the gas P V Example 12 3b Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston isothermally compressing the gas to half its original volume b Tb is Ta a Greater than b Less than c Equal to U Q P V W by the gas P V Example 12 3c Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston isothermally compressing the gas to half its original volume b Wab is 0 a Greater than b Less than c Equal to U Q P V W by the gas P V Vocabulary Wab is work done by gas between a and b Example 12 3d Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston isothermally compressing the gas to half its original volume b Ub is Ua a Greater than b Less than c Equal to U Q P V W by the gas P V Example 12 3e Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston isothermally compressing the gas to half its original volume b Qab is 0 a Greater than b Less than c Equal to U Q P V W by the gas P V Vocabulary Qab is heat added to gas between a and b Example 12 4a Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston adiabatically compressing the gas to half its original volume b Pb is Pa a Greater than b Less than c Equal to U Q P V W by the gas P V Example 12 4b Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston adiabatically compressing the gas to half its original volume b Wab is 0 a Greater than b Less than c Equal to U Q P V W by the gas P V Example 12 4c Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston adiabatically compressing the gas to half its original volume b Qab is 0 a Greater than b Less than c Equal to U Q P V W by the gas P V Example 12 4d Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston adiabatically compressing the gas to half its original volume b Ub is Ua a Greater than b Less than c Equal to U Q P V W by the gas P V Example 12 4e Outside Air Room T Atm P A massive piston traps an amount of Helium gas as shown The piston freely slides up and down The system initially equilibrates at room temperature a Weight is slowly added to the piston adiabatically compressing the gas to half its original volume b Tb is Ta a Greater than b Less than c Equal to U Q P V W by the gas P …
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