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THERMAL PHYSICS
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Temperature
If a hot* body is in thermal contact with a cold body, energy is transferred from the hot body to the cold body.
This is shown by the fact that the hot body "cools down" and the cold body "warms up".  
 
Hot body: the cup of coffee.  
Cold "body": the air in the room, the table etc.  
   
If there is no other source of energy, the energy transfer eventually stops.  
At this stage we say that the two bodies have reached a state of thermal equilibrium  
   
Note that energy only flows, unaided, from hot to cold.**  
   
Now consider a third body, which is first brought into contact with the cup of coffee (and allowed to come into thermal equilibrium with it) and is later allowed to reach a state of thermal equilibrium with the air in the room.  
If the state of the third body does not change when moved from the coffee to the air it is reasonable to predict that the coffee and the air are in thermal equilibrium with each other.  
   
This situation is considered in the zeroth§ law of thermodynamics, which is stated as follows.  
If body A is in thermal equilibrium with body C and body B is also in thermal equilibrium with body C then body A will be in thermal equilibrium with body B.
 
   
To simplify this statement a bit we introduce a "new" word... temperature.  
To save writing "is in thermal equilibrium with", we substitute "at the same temperature as" (not much of a saving, I agree but... sounds more "down to earth"...)  
We thus come close to a definition of the word temperature by saying that if energy flows unaided from body A to body B, then body A is at a higher temperature than body B and when the energy flow stops, they are at the same temperature.  
   
We can measure temperatures by using anything which experiences a measurable change in some physical property when energy flows into or out of it.  
Such a device is called a thermometer (and is the "third body" mentioned above).  
   
There are many different types of thermometer, each having its own temperature scale.  
The physical properties used to define temperature scales include:  
- the volume of a quantity of liquid (mercury or alcohol)  
- the electrical resistance of a piece of metal  
- the pressure of a gas in a constant volume container  
- the electrical conductivity of a semi-conductor device (for example a diode)...  
   
   
* For definitions of cold and hot:
i) hold a piece of ice in your hand...that’s cold
ii) put your hand into boiling water (only joking...don’t do it) that’s hot...
 
**This is an important point to remember; easily forgotten because... it's so obvious, from "everyday experience", though less obvious why it is the case.   
There is more thermal energy in the air in the room, the table etc than in the cup, so why doesn't the energy flow (unaided) the other way?  
For now, let's suggest that it flows from bodies where it is more concentrated to bodies where it is less concentrated.  
§ The name is a not a joke: the law was formally stated after the first law of thermodynamics but was considered to be more fundamental so it naturally became…the zeroth law.  
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