Chapter 1 Conservation of Mass - Light and Matter

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Heat energy can be converted to light energy. Very hot objects glow visibly, and even objects that aren’t so hot give off infrared light, a color of light that lies beyond the red end of the visible rainbow. This photo was made with a special camera that records infrared light. The man’s warm skin emits quite a bit of infrared light energy, while his hair, at a lower temperature, emits less. 74 Chapter 2 Conservation of Energy that can do the opposite, turning heat into nothing. You might think that a refrigerator was such a device, but actually your refrigerator doesn’t destroy the heat in the food. What it really does is to extract some of the heat and bring it out into the room. That’s why it has big radiator coils on the back, which get hot when it’s in operation. If it’s not possible to destroy or create heat outright, then you might start to suspect that heat was a conserved quantity. This would be a successful rule for explaining certain processes, such as the transfer of heat between a cold Martini and a room-temperature olive: if the olive loses a little heat, then the drink must gain the same amount. It would fail in general, however. Sunlight can heat your skin, for example, and a hot lightbulb filament can cool off by emitting light. Based on these observations, we could revise our proposed conservation law, and say that there is something called heatpluslight, which is conserved. Even this, however, needs to be generalized in order to explain why you can get a painful burn playing baseball when you slide into a base. Now we could call it heatpluslightplusmotion. The word is getting pretty long, and we haven’t even finished the list. Rather than making the word longer and longer, physicists have hijacked the word “energy” from ordinary usage, and give it a new, specific technical meaning. Just as the Parisian platinum-iridium kilogram defines a specific unit of mass, we need to pick something that defines a definite unit of energy. The metric unit of energy is the joule (J), and we’ll define it as the amount of energy required to heat 0.24 grams of water from 20 to 21 degrees Celsius. (Don’t memorize the numbers.) 2 Temperature of a mixture example 1 ⊲ If 1.0 kg of water at 20 ◦ C is mixed with 4.0 kg of water at 30 ◦ C, what is the temperature of the mixture? ⊲ Let’s assume as an approximation that each degree of temperature change corresponds to the same amount of energy. In other words, we assume ∆E=mc∆T , regardless of whether, as in the definition of the joule, we have ∆T = 21 ◦ C-20 ◦ C or, as in the present example, some other combination of initial and final temperatures. To be consistent with the definition of the joule, we must have c = (1 J)/(0.24 g)/(1 ◦ C) = 4.2×10 3 J/kg· ◦ C, which is referred to as the specific heat of water. 2 Although the definition refers to the Celsius scale of temperature, it’s not necessary to give an operational definition of the temperature concept in general (which turns out to be quite a tricky thing to do completely rigorously); we only need to establish two specific temperatures that can be reproduced on thermometers that have been calibrated in a standard way. Heat and temperature are discussed in more detail in section 2.4, and in chapter 5. Conceptually, heat is a measure of energy, whereas temperature relates to how concentrated that energy is.
Conservation of energy tells us ∆E = 0, so or m1c∆T1 + m2c∆T2 = 0 ∆T1 ∆T2 = − m2 m1 = −4.0 . If T1 has to change four times as much as T2, and the two final temperatures are equal, then the final temperature must be 28 ◦ C. Note how only differences in temperature and energy appeared in the preceding example. In other words, we don’t have to make any assumptions about whether there is a temperature at which all an object’s heat energy is removed. Historically, the energy and temperature units were invented before it was shown that there is such a temperature, called absolute zero. There is a scale of temperature, the Kelvin scale, in which the unit of temperature is the same as the Celsius degree, but the zero point is defined as absolute zero. But as long as we only deal with temperature differences, it doesn’t matter whether we use Kelvin or Celsius. Likewise, as long as we deal with differences in heat energy, we don’t normally have to worry about the total amount of heat energy the object has. In standard 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, and in this book I’ll usually use “heat” for either quantity. We’re defining energy by adding up things from a list, which we lengthen as needed: heat, light, motion, etc. One objection to this approach is aesthetic: physicists tend to regard complication as a synonym for ugliness. If we have to keep on adding more and more forms of energy to our laundry list, then it’s starting to sound like energy is distressingly complicated. Luckily it turns out that energy is simpler than it seems. Many forms of energy that are apparently unrelated turn out to be manifestations of a small number of forms at the atomic level, and this is the topic of section 2.4. Discussion Questions A The ancient Greek philosopher Aristotle said that objects “naturally” tended to slow down, unless there was something pushing on them to keep them moving. What important insight was he missing? 2.1.2 Logical issues Another possible objection is that the open-ended approach to defining energy might seem like a kind of cheat, since we keep on inventing new forms whenever we need them. If a certain experiment seems to violate conservation of energy, can’t we just invent Section 2.1 Energy 75
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Chapter 1 Conservation of Mass - Light and Matter

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