CHAPTER 3  >  OVERVIEW  >  CONCEPTS
CONCEPTS OF THERMAL PROCESSES (1)

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Thermal phenomena have to do with the flow and the storage of heat (technical term: entropy; entropy is the thermal fluid-like quantity) and with temperature (i.e., how warm bodies are)…
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Thermal phenomena are caused by entropy. Entropy plays a role analogous to fluids in fluid systems or charge in electrical systems. (There are differences: Charge creates electric fields, and charge can take positive and negative values.)
There is an important difference between entropy and charge or fluids: entropy can be produced (but not destroyed). We say that entropy is produced in irreversible processes (rubbing, fire, conduction of electricity, absorption and emission of radiation, conduction of entropy…).
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In communicating thermal elements (bodies in thermal contact), temperatures equilibrate. Temperature differences are driving forces for thermal processes. If disequilibration is to be created, entropy must be forced (pumped with the help of a heat pump).
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Entropy is stored in bodies, it can flow, and it can be created. The amount of entropy in a storage element can be changed by inflows and outflows and by production. This means that we can formulate a law of balance of entropy.
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Entropy exists at certain temperatures. In general, the temperature changes from point to point. If we move around a closed thermal path, the temperature goes up and down. Finally, we're back at the same temperature (thermal potential). Temperature differences along a closed path add up to zero.

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