Monday, January 22, 2018

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Elementary Thermodynamics - Heat versus Temperature

Most people use the words heat and temperature interchangeably, as though they meant the exact same thing. This is unfortunate for every student of thermodynamics, who must first deconstruct this false conception and replace it with one more scientifically accurate before any progress may be made.

When people say something is “hot,” what they really mean is that the object has a high temperature. Temperature is a direct function of molecular motion within an object or a fluid sample. This is usually easier to visualize for a gas, where the individual molecules have great freedom of motion.

Heat, by contrast, is an expression of thermal energy transfer. By placing a pot of water over a fire, we are adding heat to that pot (transferring thermal energy to the water), the effect of which is to raise its temperature (make the water molecules’ motions more vigorous). If that same pot is taken away from the fire and allowed to cool, its loss of heat (transferring energy out of the water to the surrounding air) will result in its temperature lowering (the individual water molecules slow down).

Heat gain or loss often results in temperature change, but not always. In some cases heat may be gained or lost with negligible temperature change – here, the gain or loss of heat manifests as physical changes to the substance other than temperature. One example of this is the boiling of water at constant pressure: no matter how much heat is transferred to the water, its temperature will remain constant at the boiling point (100 degrees Celsius at sea level) until all the water has boiled to vapor. The addition of thermal energy to the boiling water does not raise its temperature, but rather goes into the work of breaking molecules apart from each other so that the liquid turns into vapor.

Heat transfer can only happen, though, where there is a difference of temperature between two objects. Thermal energy (heat) flows from the “hotter” (higher-temperature) substance to the “colder” (lower-temperature) substance. To use the boiling water example, the only way to get heat transfer into the water is to subject the water to a hotter substance (e.g., a flame, or a hot electric heating element).

Much more attention will be directed to the concepts of heat and temperature in the sub-articles of Elementary Thermodynamics.

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