8.1 Air Conditioners
8.1 Air Conditioners
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Air Conditioners - How Things Work | PHYS 140, Study notes of Classical Physics
Material Type: Notes; Class: How Things Work; Subject: Physics; University: University of Illinois - Urbana-Champaign; Term: Spring 2009;
Typology: Study notes
Pre 2010
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8.1 Air Conditioners
8.1 Air Conditioners
Big question:
Big question:
Why is partof an airconditioneroutside?
Heat Machines
Heat Machines Air conditioners (this lecture)
- use work
to
transfer heat from
cold
to
hot
- “heat pumps”
Automobiles (next lecture)
- use flow of heat from
hot
to
cold
to
do work
- “heat engines”
Thermodynamics
Thermodynamics • Rules governing movement of thermal
energy
- Relationships between
- thermal energy and mechanical work– disordered energy and ordered energy
- Codified in the laws of thermodynamics
- thermal energy and mechanical work– disordered energy and ordered energy
st st
Law
Law
Law about Conservation of Energy Law about Conservation of Energy
Internal energy Internal energy
thermal + stored energies
Heat in Heat in
heat transferred into the object
Work out Work out
outside work done by the object
“Change in
internal energy
equals
heat
in minus
work out
You can add energy
You can add energy
into the air in a piston
into the air in a piston
By compressing it
Or by heating it directly
Order versus Disorder
Order versus Disorder
- It is easy to convert
ordered energy intothermal (disordered)energy
- It is hard to convert
thermal energy intoordered energy
- Statistically, order
→
disorder is one-way
Entropy
Entropy
- Entropy is a measure of an object’s disorder
- Includes thermal disorder and structural
disorder (think solid vs. liquid)
- Disorder of isolated system never decreases• Entropy can move or be transferred
Stephen Hawking is a rapper! Stephen Hawking is a rapper!
Did you know Did you know
“Entropy” from Fear of a Black Hole
also available on hisgreatest hits album, A Brief History of Rhyme
Mixmaster DeMarco’s non-offensive version
The Heat Pump
The Heat Pump
Unnatural heat flow Unnatural heat flow
Natural heat flow
Natural heat flow Heat naturally flows from hot to
cold
- Remove heat from hot object:
entropy decreases
- Add heat to cold object:
entropy increases
Entropy of combined system
increases
A joule of thermal energy ismore disordering to a coldobject than to a hot object
heat
Unnatural heat flow
Unnatural heat flow •
Heat does NOT naturally flowcold to hot
- Removing heat from cold object
decreases entropy
- Adding heat to hot object
increases entropy
More entropy is removed thanadded
- The same amount of heat causes a
greater change on cooler object’sentropy.
Therefore, some ordered energymust become disordered
Air conditioner
Air conditioner
Evaporator:
located in room air
- transfers heat from room air to
fluid
Condenser:
located in outside air
- transfers heat from fluid to
outside air
Compressor:
located in outside air
- does work on fluid and creates
entropy
Uses a working fluid Uses a working fluid