Science Matters by Robert M. Hazen Read Free Book Online Page B

your hand out toward a fire and you feel warmth, even though neither convection nor conduction is operating. Heat reaches you by
radiation
. In this case, infrared radiation (an invisible cousin of ordinary light) travels from the fire to your hand, carrying energy in the process. Every object in the universe gives off heat by radiation. Indeed, for something like a satellite or a star in the vacuum of space, radiation is the
only
way that heat can be given off.
Heat, Temperature, and Absolute Zero
    The words “temperature” and “heat” are often used interchangeably, but scientists think of the two terms in quite distinct ways. Heat refers to the total amount of atomic kinetic and potential energy in a material. Two gallons of ice water hold twice as much heat energy as one gallon. Temperature, on the other hand, is a relative term. Two objects are at the same temperature if no heat flows between them. Put a pound of metal into a gallon of ice water and they will soon be at the same temperature—32° Fahrenheit. But the metal and ice water do not hold the same amount of heat energy, because it takes more energy to make the atoms of water vibrate.
    The time and temperature display at your local bank probably gives temperatures in degrees Fahrenheit and degrees Celsius, two scales in common use. The choice of which scale to use is arbitrary—there is no “correct” way to report temperature. All you have to do is pick two easily reproducible temperature reference points, assign a number to each, and then split the gap between the numbers into convenient intervals that you call “degrees.” The Celsius scale, for example, uses the freezing and boiling points of water as its two reference points, calling the former zero, the latter 100, and defining a degree Celsius as a hundredth of the interval between the two. The Fahrenheit scale works the same way, with zero signifying the coldest temperature Daniel Fahrenheit could produce in his laboratory back in 1717 and 100 being his best determination of human body temperature. Today, the Fahrenheit scale is defined in terms of the freezing and boiling points of water (32° and 212° respectively), and human body temperature is the more familiar 98.6°F.
    Insofar as there is a “scientific” scale, however, it is what scientistscall the Kelvin scale, named after William Thomson, Lord Kelvin (1824–1907), one of the founders of thermodynamics. The degree in the Kelvin scale is the same as a degree Celsius, but, unlike other scales, it is anchored by the only temperature that refers to a fundamental physical process.
    The zero in the Kelvin scale is taken to be absolute zero—the lowest temperature attainable by any natural system. In the nineteenth century, absolute zero was pictured as the temperature at which all atomic motion stopped—at which everything just froze. Today, the laws of quantum mechanics have changed this picture slightly, and we define absolute zero as the temperature at which no more heat can be extracted from a system. Either way, absolute zero is cold. It measures-273.16°C (or-453°F). On the Kelvin scale, water freezes at 273.16 K, room temperature is about 300 K, and a wood fire ignites at about 650 K.
BAD NEWS—THE SECOND LAW
    The first law tells us that energy can be converted from one form to another and that the total amount of energy in a closed system is fixed, but it says nothing about whether a particular store of energy can be used to do anything useful. There is, for example, a great deal of energy stored in the vibrational energy of water in the ocean. According to the first law, that energy could, in principle, be used to power a ship. The fact that no one has devised a ship to tap this reservoir of energy is a consequence of the second law of thermodynamics. This law places limitations on the ways that heat can be converted to useful work, and, in the process, produces a gloomy picture of the evolution of the universe.
    Like other