Goal of Problem Set #9:
This assignment is meant to help you understand:
- the relationships between ordered
and disordered energies,
- the statistical and probabilistic
nature of thermodynamics,
- the role of entropy in determining
what is likely to happen,
- the interplay of work, heat, and
entropy.
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| Your art gallery in New York's Soho district
has only been opened for two months, yet it is already attracting a steady
clientele of the city's moneyed elite. You are exhibiting works by up
and coming artists as well as old masters and having a wonderful time
of it.
Unfortunately, with all the changes in weather and
humidity outside, you are finding it hard to keep the artwork in tiptop
condition. To remedy the situation, you design a climate control system
that keeps the gallery at a fixed temperature and humidity that's optimal
for preservation. Whether the outside air is hot or cold, dry or moist,
your new system will fix it.
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1. When the outside air is hot
and moist, your system's first goal is to extract the excess moisture.
It does this by passing the air over a cold surface so that some of
the moisture condenses and runs down a drain as liquid water. Why
is this process inevitably going to (A) require electricity
(or an equivalent source of ordered energy) and (B) going to
require something to become hotter than the original outside air?
[Note: ignore the possibility of tapping into the neighbor's air conditioning
system or any other trick solution to your predicament. It's just
your gallery and the outside air.]
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2. Having chilled the air until
it contains only the perfect amount of moisture, you want to reheat
it to the ideal gallery temperature. You could use an electric heater
or a woodstove, but you have another source of heat: the same device
that was chilling the air in the first place. Instead of sending its
heat outdoors, you can use some of that heat to warm the chilled air
back to gallery temperature. But you have to be careful: if you use
all of the heat that's released by the chiller, the gallery air will
become hotter than the outdoor air! Why?
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3. You arrange your climate control
system so that as it pumps heat out of the new gallery air to chill
it and dry it, part of that heat goes to the outdoor air and part
to the indoor air. It turns out that sending some of this heat to
the cooler, indoor air (rather than the hotter, outdoor air) makes
the system more energy efficient--it uses less electricity! Why?
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4. On one particular day, the outside
air is exactly at gallery-air temperature. However, the outside air
is too moist to use directly. So your climate control system chills
it to extract some moisture and then reheats it back up to gallery
temperature. When all this is done, you have drier air in the gallery
and water running down the drain. Which system has more entropy: (1)
a gallery full of outside air (moist but at the right temperature)
or (2) a gallery full of dried air (drier and at the right temperature)
plus the extracted water in the drain?
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5. When the weather gets colder,
the climate control system must begin warming the gallery air. Amazingly
enough, it can do this relatively efficiently by transferring heat
from the outside air into the gallery. However, your system must use
electricity to carry out this heat transfer. Why won't the heat transfer
occur naturally?
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6. Instead of pumping heat into
the gallery air from the outside air, you could simply put an electric
space heater inside the gallery. A space heater turns electric energy
directly into thermal energy. Why would operating a space heater consume
more electric power than operating a heat pump?
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7. When the air outside is very
cold and dry, your climate control system must humidify the gallery
air. It lets pure water evaporate into the air and raises the moisture
content of that air. As this evaporation occurs, the temperature of
the air drops somewhat because thermal energy is used to turn the
liquid water into water vapor. But lowering the temperature of something
usually means lowering its entropy. Why doesn't this process of mixing
water and air to create slightly colder, moister air violate the second
law of thermodynamics (the one requiring that overall entropy never
decrease)?
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8. Keeping the gallery door closed
is important, so you install a sophisticated closing system that keeps
the air exchange between inside and outside to a minimum. Why will
such air exchanges always make your gallery consume extra electricity
(and money)?
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