Heat flows from hot to cold

The first statement of the 2nd law of thermodynamics - heat flows spontaneously from a hot to a cold body - tells us that an ice cube must melt on a hot day, rather than becoming colder.

An explanation for this form of the 2nd law can be obtained from Newton's laws and our microscopic description of the nature of temperature. We have already seen that the flow of heat through conduction occurs when fast (hot) atoms collide with slow (cool) atoms, transferring some of their kinetic energy in the process. One might wonder why the fast atoms don't collide with the cool ones and subsequently speed up, thereby gaining kinetic energy as the cool ones lose kinetic energy - this would involve the spontaneous transfer of heat from a cool object to a hot one, in violation of the 2nd law. The answer lies in energy and momentum conservation in a collision - one can show, using these two principles, that in a collision between two objects which conserves energy (called an elastic collision the faster object slows down and the slower object speeds up.

      It is important to emphasize that this statement of the 2nd law applies to the spontaneous flow of heat from hot to cold. It is possible, of course, to make a cool object in a warm place cooler - this is what a refrigerator does - but this involves the input of some external energy. As such, the flow of heat is not spontaneous in this case. The generic way that this works is pictured below.

 

Figure 8.6: A generic heat pump

A useful analogy in this regard is to think of heat flowing from hot to cold objects as running ``down hill'', which is what objects naturally do in Newtonian mechanics. It is possible to make objects go up hill, but only by doing external work on them. This movement of heat from a cool to a warm reservoir through some external work is the basis of the following three devices.

• In a refrigerator, the cool reservoir is the inside of the refrigerator, and the warm reservoir is the room itself. From this, one can see that leaving a refrigerator door open will not cool off the room that it is in.
• In an air conditioner, the cool reservoir is the inside of a house, and the warm reservoir is the outside. This is used to cool a house in the summer.
• In a heat pump, the cool reservoir is the outside of a house, and the warm reservoir is the inside. This can be used to warm a house in the winter. The heat pump is thus just the reverse of an air conditioner, and indeed some heat pumps have a switch which allows them to function as an air conditioner in the summer.

A practical refrigerator cycle uses a special liquid which, when the pressure is reduced, evaporates to become a gas. Such a cycle is illustrated below.

 

Figure 8.7: A refrigerator cycle

In this cycle, liquid enters the refrigerator in a region of low pressure, where it evaporates to become a gas, absorbing heat in the process. This gas then passes through a pump into a region of high pressure, where it condenses to become a liquid, thereby releasing heat. This cycle thus requires a special liquid which evaporates and condenses within the given operating pressure and temperature regions; these liquids, such as Freon, usually require special care in their handling and disposal.