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Heat and Temperature

Resources: Denny Air and Water ch 8

Heat vs temperature

     Molecules in a gas, a liquid or a solid are always in motion.  Temperature is a measure of the average kinetic energy of those molecules.  There is a temperature, absolute 0 on the Kelvin scale, at which molecular motion stops.*  However, above this temperature, molecules are always moving.   The hotter an object is, the greater the molecular motion.  

     There are many ways to measure temperature.  The most common means convert a temperature response, measured by a thermometer, into a linear scale to characterize how hot or cold an object is.  Examples of some temperature responses include expansion of a liquid (either mercury or alcohol), the voltage across a thermocouple made by joining two dissimilar metals (in a digital thermometer) or the amount of infrared light given off (in optical thermometers).

     Heat is the total amount of thermal energy in an object.  Although it is related to temperature, it is not the same thing.  Heat depends on how big the object is and the more mass an object has, the more heat it can contain.  However, temperature is essentially independent of mass or size.  To put more energy into an object, it can be placed in the sun or over a hot flame.  The added heat increases the motion of the molecules and increases the object’s temperature.  Similarly, energy can be removed by placing the object in a snow bank or in a cold mountain stream.  This would slow molecular motion and decrease the object’s temperature.

     Animals often care what their temperature is.  Some animals adjust their temperature by absorbing heat from their environment.  These ectotherms control temperature by moving to where they can absorb more heat, such as into the sun, or where they can absorb less heat, such as into the shade.  As a result, their temperature fluctuates considerably through the day and night.  Other animals generate heat internally.  These endotherms burn fat and sugar to convert them to heat and so stabilize their body temperatures.  It is possible to be an ectotherm and find an environment with a relatively stable temperature to maintain a constant body temperature.  So while an ectotherm, such as a snake or turtle, can have a wildly varying body temperature as it moves in and out of the sun (making it a poikilotherm, poikilo- means varied or irregular), other ectotherms such as marine fish stay in water of quite stable temperature to keep their temperature nearly constant.  This makes some fish homeotherms (homeo- means same or similar).  All endotherms will be homeotherms as the point of generating internal energy is to maintain a constant temperature.

* This statement is a bit sloppy and is based on a purely classical description. As molecules slow down to where they are almost stopped, their quantum wave nature comes into play. The lowest possible energy for a molecule is not with everything stopped but more like the lowest normal mode of a standing wave. Even at T = 0K molecules have a small kinetic energy that cannot be removed.

Follow ons:

• Heat capacity
• Heat transfer
• Biology of heat transfer

J. Gouvea 8/21/13