Chapter 6 Thermodynamics: The First Law Worksheets Answers
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12Chapter 6 Thermodynamics:
The First Law
Systems, States, and Energy (Sections 6.1–6.8)
Key Concepts
thermodynamics, statistical thermodynamics, system, surroundings, open system,
closed system, isolated system, work, internal energy, translational kinetic energy,
rotational kinetic energy, vibrational kinetic energy, mode of motion (degree of
freedom), equipartition theorem, internal energy of an ideal gas, heat, calorie, adiabatic
wall, diathermic (nonadiabatic) wall, first law of thermodynamics, state function,
expansion work, free expansion, nonexpansion work, reversible isothermal expansion,
reversible process, irreversible process, exothermic process, endothermic process,
calorimetry, calorimeter, heat capacity, specific heat capacity, molar heat capacity
6.1 Systems
Example 6.1a
Suggest a system, boundary, and surroundings for 10 moles of propane gas in a rigid metal
cylinder.
Answer
One choice is that the 10 moles of propane gas constitute the system, the metal cylinder and
everything outside of it the surroundings, and the inside walls of the cylinder a real boundary.
This is an appropriate way to study the properties of the gas alone.
Example 6.1b
Suggest a system, boundary, and surroundings for 500 mL of water in an open beaker.
Answer
To study the properties of water alone, it should constitute the system. The walls of the beaker
constitute a real, physical boundary and the phase boundary between water and air constitutes
an imaginary boundary. The surroundings consist of everything outside the boundaries
including the beaker, the platform on which it rests, and the atmosphere.
6.2 Work and Energy
Example 6.2a
Describe three commonplace examples of how work is done on or by a system.
Answer
Compression of a spring: If the spring is the system, work is done on the system by the
surroundings to compress the spring.
Compression of a gas mixture as in an automobile engine: If the gas mixture is the system,
work is done on the system by the surroundings (piston) to compress the gases.
Muscle contraction: As the muscle contracts, work is done on the surroundings.
Both mechanical and electrical work are involved as well as chemical processes.
Example 6.2b
A plumber of mass 65 kg carries a toolbox of mass 15 kg to a fifth floor walk-up apartment
15 m above ground level. Calculate the work required for this process. Recall that
(difference in potential energy) = mg∆h from Table 6.1 in the text.
Answer
The difference in potential energy between the fifth and ground floors is equal to the work
done by the plumber. The force is mg and the distance is ∆h. Both the plumber’s mass and
that of the toolbox must be elevated.
m = (65 + 15) kg = 80 kg
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