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Read Sections 7.1 and 7.2ideal gas law material: p. 131-132
skip pages 310-314
Use work to force heat to flow from cold to hotExamples: Air conditioner, refrigerator
This appears to violate the 2nd Law !!
Second law is followed if energy in is not perfectly efficient. (Some energy converted to heat).The bigger the temperature difference between cold and hot, the more electrical energy is needed. (Like air conditioners on a hot day !!)
Example: air conditioner
Air conditioners make us of special liquids which absorb heat as they change from a liquid to a gas and release the heat again as they change from a gas to a liquid.
COLD region = houseHOT region = outside
HEAT PUMP = evaporator, compressor, condenser and liquid
converts some thermal energy into workexample: automobile engine
Entropy must NOT go down overall.=> amount of work heat engine can do is limited by amount of entropy increase of Cold region.
Bigger temperature difference between COLD and HOT allows more Work
Example: Automobile engine
HOT = burning fuelCOLD = outside air
WORK = pistons provide work to turn wheels
Ideal gas law:To understand how the automobile engine works, we need to understand how pressure and temperature effect gassespressure of a gas
- increases with increasing number of particles
- increases with decreaseing volume
- increases with temperature
in other words:
- push more gas into a can and the pressure increases
- squeeze a can and the pressure increases
- heat gas in a sealed can (pressure cooker) and the pressure increases
[demo: pressure vessel and hot air gun]
Examine on cylinder of an automobile engineFour steps in the cycle of an automobile engine cylinder:
Repeat cycle
Use the moving piston to turn wheels
One piston only does work during part of the cycle
Use four (or more) cylinders to keep work being done over more time
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