General Physics 1 Hongqun Zhang The Department of Physics, Beijing Normal University June 2005

The Second Law of Thermodynamics  Heat can travel from higher temperature area to lower temperature area spontaneously. But the reverse process must occur conditionally.

How Refrigerators Work? 1.Parts of A Refrigerator 2.The Refrigeration Cycle 3. A Relevant Question

Background Question:  Why does it feel cold when rubbing alcohol evaporates from your skin? evaporates from your skin?

Parts of a Refrigerator  Refrigerant (Freon)  Compressor  Heat-exchanging pipes (Condenser)  Expansion valve  Heat-exchanging pipes (Evaporator) Evaporator

The Refrigeration Cycle Higher temperature and higher pressure gas(p 2 =9.2bar, t 2 =46 o c) Lower temperature and higherpressure liquid (p 3 =9bar, t 3 =37.4 o c) Lower temperature and lower pressure liquid (p 4 =1.5bar, t 4 =-20 o c) Lower temperature andlower pressuregas(p 1 =1.5bar, t 1 =-10 o c)

Conclusion  In refrigeration cycle, Freon absorb heat from freezer compartment and release the heat from condenser to outside, with input of electrical work

Question:  Why does Freon liquid pass through expansion valve with the temperature decreasing ?