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Heat Pumps and Fridges
As explained in the discussion of the Carnot cycle, if a heat engine is operated in reverse, we have a device which has the effect of transferring internal energy from a body at a low temperature to one at a higher temperature.
Such a device is called a heat pump (or a refrigerator depending on what it is used for).  
  A heat pump or fridge can be represented by a similar diagram to the one used for the heat engine but with the arrows representing energy flows reversed.
 
An explanation of the operation of a real heat pump or fridge requires consideration of cooling caused by evaporation.  
 
The rate at which a liquid evaporates depends (amongst other things) on:  
1. the temperature of the liquid; increasing temperature increases the rate of evaporation
2. the pressure acting on its surface; decreasing pressure increases the rate of evaporation
 
   
The change of state, from liquid to vapour, requires energy, called latent heat*  
If the pressure acting on the surface of a liquid is decreased, the rate of evaporation will increase and the energy needed to produce this increased rate of change of state is taken from the surroundings.  
Thus, the increased rate of evaporation causes cooling.  
   
The diagram below shows the main parts of a refrigerator.  
   
   
   
   
  In the tubes around the freezer compartment, the pressure is decreased by the pump (there is a small section of the tube which is narrower than the rest).
  Rapid evaporation takes place here and latent heat of vaporization is taken in.
   
  In the tubes outside the refrigerator, the vapour is compressed so it condenses, giving out latent heat*.
   
   
   
   
   
   
 
A favourite question about refrigerators:  
A refrigerator is in a perfectly insulated room.  
Somebody leaves the door of the refrigerator open.  
Will the temperature of the room decrease, increase or stay the same?  
   
The phase change, liquid to vapour (inside the fridge), occurs at low temperature.  
During this phase change, latent heat of vaporization is taken in.  
   
When the vapour is compressed (in order to change it back into a liquid) there is an increase in temperature.  
In other words, the phase change, vapour to liquid (outside the fridge), occurs at high temperature.  
Latent heat of condensation is given out but also energy will be given out as the liquid cools down to the same temperature as the surroundings.  
Summarizing:  
Energy taken in during the vaporization is given by  
 
Energy given out during the condensation and cooling is given by  
 
where m is the mass of liquid/vapour, Lv is the specific latent heat of vaporization of the liquid, c is the specific heat capacity of the liquid and ΔT is the change in temperature of the liquid after condensation.  
   
Clearly, Q2 > Q1 so the answer to the question is...  
   
   
* I stated elsewhere that the word heat in physics is only used as a verb, to heat, heating... ok, this is "the exception that proves the rule"...  
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