Simple Nature - Light and Matter

it becomes possible to answer at one stroke the questions posedat the beginning of the section. Yes, it is at least theoreticallypossible to remove all the heat from an object. The coldest possibletemperature, known as absolute zero, is that at which all the atomshave zero velocity, so that their kinetic energies, K = (1/2)mv 2 , areall zero. No, there is no maximum amount of heat that a certainquantity of matter can have, and no maximum to the temperaturescale, since arbitrarily large values of v can create arbitrarily largeamounts of kinetic energy per atom.The kinetic theory of heat also provides a simple explanation ofthe true nature of temperature. Temperature is a measure of theamount of energy per molecule, whereas heat is the total amount ofenergy possessed by all the molecules in an object.There is an entire branch of physics, called thermodynamics,that deals with heat and temperature and forms the basis for technologiessuch as refrigeration. Thermodynamics is discussed in moredetail in chapter 5, and I’ve provided here only a brief overview ofthe thermodynamic concepts that relate directly to energy.2.4.2 All energy comes from particles moving or interacting.If I stretch the spring in figure c and then release it, it snaps tautagain. The creation of some kinetic energy shows that there musthave been some other form of energy that was destroyed. What wasit?We could just invent a new type of energy called “spring energy,”study its behavior, and call it quits, but that would be ugly. Arewe going to have to invent a new forms of energy like this, overand over? No: the title of this book doesn’t lie, and physics reallyis fundamentally simple. As shown in figure d, when we bend orstretch an object, we’re really changing the distances between theatoms, resulting in a change in electrical energy. Electrical energyisn’t really our topic right now — that’s what most of the secondhalf of this book is about — but conceptually it’s very similar togravitational energy. Like gravitational energy, it depends on 1/r,although there are some interesting new phenomena, such as theexistence of both attraction and repulsion, which doesn’t occur withgravity because gravitational mass can’t be negative. The real pointis that all the apparently dissimilar forms of energy in figure d turnout to be due to electrical interactions among atoms. Even if wewish to include nuclear reactions (figure e) in the picture, there stillturn out to be only four fundamental types of energy:kinetic energy (including heat)gravitational energyelectrical and magnetic energynuclear energyAstute students have often asked me how light fits into this pic-c / The spring’s energy is reallydue to electrical interactionsamong atoms.d / All these energy transformationsturn out at the atomiclevel to be due to changes in thedistances between atoms thatinteract electrically.Section 2.4 Atomic Phenomena 111