2014 Chapter 24 - Thermodynamics
24.1 Absolute
Zero
The Greek roots of the word thermodynamics mean movement
of heat. The science of thermodynamics is based on the
conservation of energy and the fact that heat flows from
warmer areas to colder areas.
While there is no upper limit to how high temperature can go,
there is a lower limit. ”As the thermal motion of atoms in a
substance approaches zero, the kinetic energy of the atoms
approaches zero, and the temperature of the substance
approaches a lower limit.”
Absolute zero is the
lowest temperature a
substance can reach
and no more energy can
be taken from that
substance. The
discovery of absolute
zero is related to the
gas laws and the
contraction of all
gases. Absolute zero is
0°K and corresponds to
-273 °C.
24.2 First Law of James Joule showed that the
Thermodynamics flow of heat is the flow of
energy. He showed how work
could change the temperature of
water. We call the law of
conservation of energy applied to
thermal systems, the first law of
thermodynamics.
”The first law of
thermodynamics states, that
whenever heat is added to a
system, it transforms to an equal
amount of some other form of energy.”
Heat added to a system increases the internal energy of the
system and allows the system to do work. Heating an enclosed
system only increases the temperature. Heating a can with a
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24.3 Adiabatic
Processes
balloon attached would expand and do work but the
temperature of the system would be lower. (also gas laws:
pressure, temperature and volume)
Doing work on a system – such as compressing a gas, can
increase heat. Using a bicycle pump causes an increase in heat
on the pump. Releasing the air in a tire cools the gas.
When the volume of a gas is changed by compression or
expansion, without adding or losing heat externally, we say this
is an adiabatic process. The high compression ratios of diesel
engines ignite the fuel without any spark plugs.
”When work is done on a gas by adiabatically compressing it,
the gas gains internal energy and becomes warmer.”
When a gas expands – like in a cylinder – work is done giving up
energy and cooling the system. When we blow on food, with
pursed lips, air expands and cools off our food.
Changes in air pressure ~ pressure changes
We can see these
affects living on a
mountain in Los Alamos.
As air rises, there is less
pressure at the higher
altitudes. With less
pressure, the air
expands, and as it
expands, the air loses
energy and cools. Rising
air cools 10°C for every
1,000 m altitude. So if
go from 20°C in town to
the top of Pajarito
Mountain, we could
expect the temperature to be about 10°C there.
24.4 Second and When two objects are places near one another in thermal
Third Laws of
contact, heat will flow from the warmer object to the cooler
Thermodynamics one until they reach thermal equilibrium.
The second law of thermodynamics says that the natural flow
of heat of from hot to cold.
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24.5 Heat
Engines and the
Second Law
”The second law of thermodynamics states that heat will
never of itself flow from a cold object to a hot object.” For
heat to move from colder areas to warmer areas requires a
heat pump.
The third law of thermodynamics says that no system can
reach absolute zero. Scientist have approached by never
reached absolute zero.
Remember from chapter 9 that energy is the ability to do work.
When a high temperature substance is transformed to a lower
temperature, it loses energy. That lost energy has been
transformed to work. It is possible to turn mechanical energy
100% into heat, but heat
cannot be transformed
completely into mechanical
energy. A device that
changes internal (thermal)
energy into mechanical work
is a heat engine. As heat
flows from high
temperatures to low
temperatures, some of the
thermal energy is
transformed into
mechanical energy. The
remaining heat is expelled or
transformed to a low
temperature reservoir –
think exhaust.
”According to the second law of thermodynamics, no heat
engine can convert all heat input to mechanical energy
output.”
There will always be some heat that is not converted to
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mechanical energy. Carnot efficiency says that the ideal
efficiency of a heat engine is a fraction with the difference of
the hot and cold over the hot. Steam turbines are
superheated so that the temperature difference is greater and
the Carnot efficiency is increased. For turbine blades to turn,
there must be a pressure difference between the front of the
blades and the back. To have less pressure behind the blades,
the steam that passes through is condensed on the backside.
Heat engines are more efficient at higher temperatures, but
they can never be 100% efficient in producing mechanical
energy because of the 2nd Law of Thermodynamics.
24.6 Order
Tends to
Disorder
big dogs don’t
live as long nor
do larger people
24.7 Entropy
Lovington, NM has five new natural gas engines turning
electrical generators. These state of the art heat engines can
turn 42% of the potential chemical energy in natural gas to
electricity!
Chemical energy in gasoline is organized and usable. An engine
transforms some of this to mechanical energy (about 30-35%
for automobiles), some heats the engine and some is lost
through the exhaust system. All of the electrical energy from
lighting buildings is eventually degraded into heat energy that
is no longer usable.
”Natural systems tend to proceed toward a state of greater
disorder.”
Disordered energy can be reordered into usable energy only be
work output. Plants are able to take energy from sunlight and
order atoms into sugar molecules with potential energy.
Entropy is how
much chaos or
disorder is in a
system. Ordered
energy deteriorates
to disordered
energy. Ordered energy tends to be degraded.
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”According to the second law of thermodynamics, in the
long run, the entropy of a system always increases for natural
processes.” Living things extract energy from their
surroundings to decrease entropy. This increases entropy
elsewhere because
organisms are
extracting energy.
The second law of
thermodynamics is a
probability statement.
There can be
exceptions but they
are quite unlikely. It
is possible for a
system to become
more ordered but this
is so improbable that
is never seen.
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