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Chapter 24 Thermodynamics: 1st Law of Thermodynamics
What is
thermodynamics?
Thermodynamics is the study of heat and its transformations into
mechanical energy.
What is the focus of
Thermodynamics?
Focus is off the molecular level and on the macroscopic level
State the first law of
thermodynamics.
Energy is not created or destroyed
Also called the law of conservation of energy
What happens when
energy leaves a
system?
When energy leaves a system, external work is done
Work = F x d Internal energy decreases
How can the energy of
a system increase?
When external work is done on a system, internal energy increases.
What is an adiabatic
process?
Adiabatic Process: changing system energy without heat transfers.
Describe how the
energy changes during
this process.
Expansion: decreases temperature (energy) in a system
Contraction: increases Temperature (energy) in a system
What is the change in
temperature that occurs
as an air mass rises?
The temperature drop for a rising air mass is about 10ºC for every 1 km
increase in elevation.
How is a Chinook wind
created?
Chinook winds are created when descending dry, cold air warms.
If work is done adiabatically on a system, will the internal energy of the system increase or
decrease? If work is done by a system will the internal energy increase or decrease?
Summary
If work is done by a system will the internal energy decrease
Chapter 24 Thermodynamics: 2nd Law of Thermodynamics (parts I-III)
Part I
State the 2nd Law of
Thermodynamics.
Heat will never of itself flow from a colder object to a hot object.
How can you make heat
flow from cold to hot?
Heat can be made to flow in the reverse direction only with
External Effort
What is a heat engine?
Heat engine: any device that changes internal energy into mechanical
work
Work can create heat, but heat can never be completely
What does the 2nd Law of converted into work.
Part II
Thermodynamics say
about heat engines?
In all heat engines only some of the heat that flows will be converted to
mechanical energy. Some will be lost as heat to the surrounding area
What is thermal pollution?
Thermal Pollution: Unwanted waste heat expelled into the environment.
What is a heat reservoir?
A heat reservoir is an area where energy is stored in a heat engine
Heat Sink: Place where heat travels to.
Part II (continued)
What does the 2nd Law of
thermo say about
conversion of work to
mechanical energy?
How is Ideal Efficiency
of a heat engine
calculated?
When work is done by a heat engine only some of the input
heat can be converted into work, the rest is expelled as waste
heat.
What temperature scale is
used in this calculation?
Kelvin scale is used to calculate efficiency
How can efficiency of a
heat engine be increased?
Natural systems tend to proceed toward a state of greater disorder.
Entropy will increase. Work input decreases the entropy in a system
What is entropy?
Entropy: A measure of disorder in a system
Part III
What does the 2nd Law of
thermodynamics tell us
about entropy?
Natural systems tend to proceed toward a state of greater disorder
Entropy will increase. Work input decreases the entropy in a system
Distinguish between the first and second laws in terms of whether or not exceptions occur.
Summary
The first law has no exceptions. The second law, because it is dealing with probability,
may have exceptions. For example, it is possible, though not probable for entropy to
decrease through random chance. Also, energy can be made to flow from cold to hot if
external work is done on it.