The Heat Capacity of an object , C, is defined by, the heat needed to raise an object by one degree. The study of heat gained and lost in this manner is often called Calorimetry. Change of Phase and Latent Heat. These phase changes occur at definite temperatures, e. Note that for a system in thermal equilibrium if ice and water are present the temperature must be 0 0 C.
If heat is added, ALL the ice must melt before the temperature will rise above 0 0 C. Two objects are in thermal equilibrium if they are in close contact that allows either to gain energy from the other, but nevertheless, no net energy is transferred between them.
Even when not in contact, they are in thermal equilibrium if, when they are placed in contact, no net energy is transferred between them. If two objects remain in contact for a long time, they typically come to equilibrium. In other words, two objects in thermal equilibrium do not exchange energy.
Experimentally, if object A is in equilibrium with object B , and object B is in equilibrium with object C , then as you may have already guessed object A is in equilibrium with object C.
That statement of transitivity is called the zeroth law of thermodynamics. The first, second, and third laws of thermodynamics were already named and numbered then. The zeroth law had seldom been stated, but it needs to be discussed before the others, so Fowler gave it a smaller number. Consider the case where A is a thermometer.
The zeroth law tells us that if A reads a certain temperature when in equilibrium with B , and it is then placed in contact with C , it will not exchange energy with C ; therefore, its temperature reading will remain the same Figure. In other words, if two objects are in thermal equilibrium, they have the same temperature. A thermometer measures its own temperature. It is through the concepts of thermal equilibrium and the zeroth law of thermodynamics that we can say that a thermometer measures the temperature of something else, and to make sense of the statement that two objects are at the same temperature.
Then we can say that a system is in thermal equilibrium with itself if all parts of it are at the same temperature. We will return to the definition of a thermodynamic system in the chapter on the first law of thermodynamics. They are at the same temperature, and if they are placed in contact, no net heat flows between them. Combine the two to get a differential equation That being said, even with transient heat conduction problems, it helps to have some experience to get a quick answer without having to resort to numerical methods or complicated analytical approaches.
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