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Unformatted text preview: 1359 Chapter 18 Heat and the First Law of Thermodynamics Conceptual Problems 1 • Picture the Problem We can use the relationship T mc Q ∆ = to relate the temperature changes of bodies A and B to their masses, specific heats, and the amount of heat supplied to each. Express the change in temperature of body A in terms of its mass, specific heat, and the amount of heat supplied to it: A A A c m Q T = ∆ Express the change in temperature of body B in terms of its mass, specific heat, and the amount of heat supplied to it: B B B c m Q T = ∆ Divide the second of these equations by the first to obtain: B B A A A B c m c m T T = ∆ ∆ Substitute and simplify to obtain: ( )( ) 4 2 2 B B B B A B = = ∆ ∆ c m c m T T or A B 4 T T ∆ = ∆ *2 • Picture the Problem We can use the relationship T mc Q ∆ = to relate the temperature changes of bodies A and B to their masses, specific heats, and the amount of heat supplied to each. Relate the temperature change of block A to its specific heat and mass: A A A c M Q T = ∆ Relate the temperature change of block B to its specific heat and mass: B B B c M Q T = ∆ Chapter 18 1360 Equate the temperature changes to obtain: A A B B 1 1 c M c M = Solve for c A : B A B A c M M c = and correct. is ) ( b 3 • Picture the Problem We can use the relationship T mc Q ∆ = to relate the amount of energy absorbed by the aluminum and copper bodies to their masses, specific heats, and temperature changes. Express the energy absorbed by the aluminum object: T c m Q ∆ = Al Al Al Express the energy absorbed by the copper object: T c m Q ∆ = Cu Cu Cu Divide the second of these equations by the first to obtain: T c m T c m Q Q ∆ ∆ = Al Al Cu Cu Al Cu Because the object’s masses are the same and they experience the same change in temperature: 1 Al Cu Al Cu < = c c Q Q or Al Cu Q Q < and correct. is ) ( c 4 • Determine the Concept Some examples of systems in which internal energy is converted into mechanical energy are: a steam turbine, an internal combustion engine, and a person performing mechanical work, e.g., climbing a hill. *5 • Determine the Concept Yes, if the heat absorbed by the system is equal to the work done by the system. 6 • Determine the Concept According to the first law of thermodynamics, the change in the internal energy of the system is equal to the heat that enters the system plus the work done on the system. correct. is ) ( b Heat and the First Law of Thermodynamics 1361 7 • Determine the Concept . on in int W Q E + = ∆ For an ideal gas, ∆ E int is a function of T only. Because W on = 0 and Q in = 0 in a free expansion, ∆ E int = 0 and T is constant. For a real gas, E int depends on the density of the gas because the molecules exert weak attractive forces on each other. In a free expansion, these forces reduce the average kinetic energy of the molecules and, consequently, the temperature....
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This note was uploaded on 08/18/2010 for the course IMT 12253 taught by Professor Mikael during the Spring '10 term at École Normale Supérieure.
 Spring '10
 Mikael

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