Simple Nature - Light and Matter

Conservation of energy tells us ∆E = 0, soorm 1 c∆T 1 + m 2 c∆T 2 = 0∆T 1∆T 2= − m 2m 1= −4.0 .If T 1 has to change four times as much as T 2 , and the two finaltemperatures are equal, then the final temperature must be28 ◦ C.Note how only differences in temperature and energy appearedin the preceding example. In other words, we don’t have to makeany assumptions about whether there is a temperature at which allan object’s heat energy is removed. Historically, the energy andtemperature units were invented before it was shown that there issuch a temperature, called absolute zero. There is a scale of temperature,the Kelvin scale, in which the unit of temperature is thesame as the Celsius degree, but the zero point is defined as absolutezero. But as long as we only deal with temperature differences, itdoesn’t matter whether we use Kelvin or Celsius. Likewise, as longas we deal with differences in heat energy, we don’t normally haveto worry about the total amount of heat energy the object has. Instandard physics terminology, “heat” is used only to refer to differences,while the total amount is called the object’s “thermal energy.”This distinction is often ignored by scientists in casual speech, andin this book I’ll usually use “heat” for either quantity.We’re defining energy by adding up things from a list, which welengthen as needed: heat, light, motion, etc. One objection to thisapproach is aesthetic: physicists tend to regard complication as asynonym for ugliness. If we have to keep on adding more and moreforms of energy to our laundry list, then it’s starting to sound likeenergy is distressingly complicated. Luckily it turns out that energyis simpler than it seems. Many forms of energy that are apparentlyunrelated turn out to be manifestations of a small number of formsat the atomic level, and this is the topic of section 2.4.Discussion QuestionsA The ancient Greek philosopher Aristotle said that objects “naturally”tended to slow down, unless there was something pushing on them tokeep them moving. What important insight was he missing?2.1.2 Logical issuesAnother possible objection is that the open-ended approach todefining energy might seem like a kind of cheat, since we keep oninventing new forms whenever we need them. If a certain experimentseems to violate conservation of energy, can’t we just inventSection 2.1 Energy 75