PES 100 

 Fall 2000

Physics in Everyday Life

Lecture Notes

Thermodynamics: Heat Pumps and Heat Engines

Read Sections 7.1 and 7.2

ideal gas law material: p. 131-132

skip pages 310-314

Heat Pumps:

Use work to force heat to flow from cold to hot

Examples: Air conditioner, refrigerator


heat pump showing heat flowing from cold to hot
This appears to violate the 2nd Law !!


heat pump with electrical energy in
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 = house

HOT region = outside

HEAT PUMP = evaporator, compressor, condenser and liquid

air conditioner schematic

Heat Engines

 converts some thermal energy into work

example: automobile engine


heat engine schematic
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 fuel

COLD = 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 gasses

pressure 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 engine


Four steps in the cycle of an automobile engine cylinder:

  • Induction - put fuel in
  • Compression - raise pressure
  • Power - ignite fuel
  • Exhaust - get rid of waste products

    Repeat cycle

    cycle of pistons in engine


    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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