lecture18 - Physics 231 Lecture 18 Lecture 18 Main points...

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Physics 231 ecture 8 Lecture 18 Main points of today’s lecture: Heat and heat capacity: T cm Q Work in thermodynamic processes: First Law of Thermodynamics: rocesses V P W system V P Q U Processes – cyclic: isobaric: – isovolumetric: isothermal: adiabatic: 0 V 0 P 0 T 0 Q 0 T V P
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wo containers of the Checking Understanding Two containers of the same gas (ideal) have these masses and mperatures: temperatures: Which gas has atoms with the largest average thermal energy? Which container of gas has the largest thermal energy? A. P, Q B. P, P C. Q, P D. Q, Q Slide 11-21
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The ideal gas is an example of a thermal system. In thermal systems, temperature is an important state variable. Each thermal system there are a set of state variables that are sufficient to completely describe the macroscopic state of the system. –e x am p le: ideal gas (P,T,N), (V,T,N), (P,E th ,N) and (E th ,V,N) are examples of a set of state variables. U is the thermal energy. For ideal gass E th =3/2Nk B T is the internal energy. Zeroeth law of thermodynamics: Two macroscopic systems are in thermal equilibrium if and only if they are at the same temperature. In a thermal process, a macroscopic system changes its state variables in a smooth and controlled manner. Some common cases are: – Isobaric process: P=0; (The pressure remains the same.) – Isovolumetric process: V=0; (The volume remains the same.) – Adiabatic process: Q=0; (The system is thermally isolated.) – Isothermal process: T=0; (The temperature remains the same.)
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Heat and heat capacity We saw that for atoms or molecules in an ideal gas <KE>=3/2k B T. In general, the atoms or molecules in matter increase in energy if the object is heated. This is heat energy. t th th t fl h t bj t t ld bj t l l Heat Q is the energy that flows a hot object to a cold object solely because of the difference in T. When the heat flow is sufficient, the two objects reach the same temperature and we say they are in thermal equilibrium at the same T. For a mono-atomic ideal gas at constant volume 333 E E E Nk T Nk T Nk T T th th,f th,i b f b i b f i bm o l a r Q 222 33 QN k Tn R T n c T 22 c molar is the heat capacity per mole at constant volume For many other materials, the relationship between heat transferred and temperature change is given by: T cm Q object. the of capacity heat total the and mass the is mass unit per capacity heat specific the is cm m c
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Example Helium (He), a monoatomicgas, fills a 0.010 m 2 container. The pressure of the gas is 6.2x10 5 Pa. How long would a 0.25 hp engine have to run (1 hp=746 W) to produce an amount of energy equal to the ermal energy f this gas? thermal energy of this gas?
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This note was uploaded on 05/03/2011 for the course PHYS 231 taught by Professor Nagy during the Fall '11 term at Michigan State University.

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lecture18 - Physics 231 Lecture 18 Lecture 18 Main points...

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