文档介绍：Heat Engines, Heat Pumps, and Refrigerators
Getting something useful from heat
Spring 2007
1
Heat can be useful
Normally heat is the end-product of the flow/transformation of energy
remember examples from lecture #4 (coffee mug, automobile, bouncing ball)
heat regarded as waste: as useless end result
Sometimes heat is what we want, though
hot water, cooking, space heating
Heat can also be coerced into performing “useful”(., mechanical) work
this is called a “heat engine”
Spring 2007
2
Heat Engine Concept
Any time a temperature difference exists between two bodies, there is a potential for heat flow
Examples:
heat flows out of a hot pot of soup
heat flows into a cold drink
heat flows from the hot sand into your feet
Rate of heat flow depends on nature of contact and thermal conductivity of materials
If we’re clever, we can channel some of this flow of energy into mechanical work
Spring 2007
3
Heat Work
We can see examples of heat energy producing other types of energy
Air over a hot car roof is lofted, gaining ic energy
That same air also gains gravitational potential energy
All of our wind is driven by temperature differences
We already know about radiative heat energy transfer
Our electricity generation thrives on temperature differences: no steam would circulate if everything was at the same temperature
Spring 2007
4
Power Plant Arrangement
Heat flows from Th to Tc, turning turbine along the way
Spring 2007
5
Heat Engine Nomenclature
The symbols we use to describe the heat engine are:
Th is the temperature of the hot object
Tc is the temperature of the cold object
T = Th–Tc is the temperature difference
Qh is the amount of heat that flows out of the hot body
Qc is the amount of heat flowing into the cold body
W is the amount of “useful” mechanical work
Sh is the change in entropy of the hot body
Sc is the change in entropy of the cold body
Stot is the total change in entropy (entire system)
E is the entire amount of energy involve