# Lect06 - Le cture6 Exam s and Proble s ple m Heat capacity...

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Lecture 6, p. 1 Lecture 6 Examples and Problems Thermal diffusion Thermal conductivity Irreversibility Random Walk and Particle Diffusion Counting and Probability Microstates and Macrostates The meaning of equilibrium Hot Cold 9 spins -9 -7 -5 -3 -1 1 3 5 7 9 (m) m

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Lecture 6, p. 2 Exercise: Spacecraft Heat Shields This false-color view of Titan (moon of Saturn) is a composite of images captured by Cassini's infrared camera, which can penetrate some of Titan's clouds. Light and dark regions in the upper left quadrant are unknown types of terrain on Titan's surface. The Huygens spacecraft entered the atmosphere on Jan. 14, 2005, initially traveling at ~ 6 km/s . After decelerating from friction, the heat shield was jettisoned, and three parachutes were deployed to allow a soft landing. What is the temperature rise on entry , assuming that half of the thermal energy goes into the ship (and half to the atmosphere)? Assume c steel = 500 J/kg-K .
Lecture 6, p. 3 v = 6 km/s c = 500 J/kg-K Use conservation of energy (1 st law of thermodynamics). Half of the initial kinetic energy becomes internal thermal energy. The problem is that steel melts at ~1700 K! For this reason, the heat shield is not made of steel, but rather a ceramic that burns off (“ablates”). Also, the ceramic has a very low thermal conductivity! 2 2 3 2 1 1 2 2 (6 10 m/s) 18,000 K! 4 4 500 J/kg-K = = × = × = = mv C T cm T v T c Solution Note that m cancels.

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Lecture 6, p. 4 Exercise: Heat Capacity Two blocks of the same material are put in contact. Block 1 has m 1 = 1 kg , and its initial temperature is T 1 = 75° C . Block 2 has m 2 = 2 kg , and T 2 = 25° C . What is the temperature after the blocks reach thermal equilibrium? 1 2
Lecture 6, p. 5 Solution Two blocks of the same material are put in contact. Block 1 has m 1 = 1 kg , and its initial temperature is T 1 = 75° C . Block 2 has m 2 = 2 kg , and T 2 = 25° C . What is the temperature after the blocks reach thermal equilibrium? The two blocks have the same (unknown) specific heat.

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## This note was uploaded on 02/21/2011 for the course PHYS 213 taught by Professor Staff during the Spring '08 term at University of Illinois, Urbana Champaign.

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Lect06 - Le cture6 Exam s and Proble s ple m Heat capacity...

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