A Physics Digest—Part 3: Heat

All matter we’re familiar with possesses some amount of heat, which means its atoms and molecules constantly jiggle about. The hotter something is, the greater the jiggle. Atoms in a solid can jiggle only so much, before they break loose and form a liquid. Heat it some more and atoms jiggle even more and spread out into a gas.

Temperature: it’s a measure of the thermal energy of something—it’s a way to quantify of the amount of kinetic (jiggling) energy of the atoms and molecules. The calorie: it’s a measure of the amount of heat energy it takes to raise the temperature of a body. We convert food calories (heat energy) into the energy of walking or doing the work of lifting things (like our bodies off the couch).

Heat always moves from hotter to colder bodies (cold doesn’t move); and does so in three ways. (1) Conduction—warmer jiggling atoms in a solid pass their jiggle on down the line. Good conductors (metals) pass it on faster than insulators. (2) Convection—hotter molecules and atoms in a liquid or gas move to cooler places and warm them. (3) Radiation—heat energy gets transferred by electromagnetic waves (we’ll look at them later).

Now comes the hard part: thermodynamics. No undergraduate class gave me more grief than thermo. Thermodynamics is the study of heat and how it’s transformed into mechanical energy. It’s a macroscopic discipline—not caring about the jiggling of individual atoms, just the net impact of what they do in concert. Thermodynamics is the basic study of how engines transform heat into useful energy: your car, a refrigerator, a nuclear power plant.

One of the basic concepts in thermodynamics is the temperature of absolute zero (there is no maximum temperature). Energy can always be extracted from any warm body; but when we get that body down to absolute zero, there is no energy at all (no more jiggle). Thus it’s the absolute ground floor for all heat calculations.

Similar to Newton’s discoveries for forces, thermodynamicists have discovered two basic laws. The First Law of Thermodynamics describes the conservation of energy—that it can neither be created nor destroyed, just transformed from one type into another. Thus when we add heat to a system we can then transform it into various forms of energy, knowing we can account for every portion without something mysteriously disappearing.

The Second Law of Thermodynamics tells us that heat always flows from hot to cold locations—always downhill. Thus energy always dissipates, is always deteriorating into less useful forms; eventually into waste heat. Entropy is a measure of this disorder.