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4.8 Heat and Temperature

Two hundred years ago scientists were puzzled by another type of energy: heat. In many ways heat seems like a fluid. It flows from one place to another; some objects absorb and give off a lot of heat and others absorb or give off very little. But if heat is a fluid, then every object must contain a fixed amount. An American cannon maker called Count Rumford showed that this was not true. He observed that a lot of heat was generated when boring the barrel of a cannon. But blunt tools generated more heat than sharp tools. So heat is just the result of friction: the energy of motion when two surfaces rub against each other. For a quick example, rub your hands together briskly. Even two pieces of ice make heat when they are rubbed together.

Picture of James Prescott Joule. Click here for original source URL.

Can heat or thermal energy be quantified? It certainly can. In 1840, James Prescott Joule did a famous experiment. The British physicist arranged a weight so that it turned paddles in water as it dropped. He suspected that the spinning paddles would heat the water and he was right. Joule measured a rise in the temperature of the water each time he dropped the weight. "The work done by the weight of one pound dropping through 772 feet... will, if spent producing heat by friction in the water, raise the temperature of one pound of water by one degree Fahrenheit," he concluded. Joule's simple experiment showed that kinetic energy, or energy of motion, can be converted directly into heat.

Joule's Heat Apparatus used for measuring heat, 1845. Click here for original source URL.

What are the implications of Joule's famous experiment? Moving particles have kinetic energy, even on the scale of molecules. Think of the molecules and atoms as tiny particles, darting around and hitting other particles. Each one has a mass and a velocity, hence a kinetic energy. The motion of the paddles in Joule's experiment caused all the water molecules to move faster. Heat is just a microscopic form of kinetic energy.

The temperature of an ideal?monatomic?gas?is related to the average?kinetic energy?of its?atoms. In this animation, the?size?of?helium?atoms relative to their spacing is shown to scale under 1950?atmospheres?of pressure. These atoms have a certain, average speed (slowed down here two?trillion?fold from room temperature). Click here for original source URL.

English botanist Robert Brown. Click here for original source URL.

Where does the idea of temperature come from? We all have an idea of "hot" and "cold" but what aspect of matter does temperature actually measure? The answer surprises many people when they first learn it. Temperature is merely a measurement of the motion of molecules and atoms. (For convenience, let's refer only to molecules, since an atom is a special class of molecule with only one atom instead of several. When we say "molecule" in the following discussion, you can think of either molecules or atoms.) The higher the temperature, the faster the molecules are moving. The colder the temperature, the slower the molecules are moving. The English botanist Robert Brown made this visible in 1827 when he looked at pollen suspended in water through a microscope. The pollen particles were in constant random motion. They were kept in motion by the constant buffeting of the water molecules all around them.

Notice that thermal energy and temperature are not the same thing. A drop of boiling water and a cup of boiling water have the same temperature, but one clearly has more heat energy, as you would notice by the change each caused on your skin. Temperature does not measure the total amount of heat in an object. It is a measure of the microscopic motions of the molecules or atoms.

William Thomson, 1st baron Kelvin. Click here for original source URL.

Scientists use a different temperature scale than those you are used to. In the United States most people still use the archaic Fahrenheit temperature scale, introduced almost 300 years ago. Most of the world uses the Celsius (or centigrade) scale, where 0° C is the freezing point of water and 100° C is the boiling point of water at sea level. But matter can clearly be colder than the freezing point of water, so there must still be some heat below 0° C. Scientists use a physical scale of temperature, where zero corresponds to no motion of atoms or molecules whatsoever. This is the Kelvin scale, named after Lord Kelvin (William Thomson), a famous English physicist who made many made many contributions to our understanding of heat. You can see from the many examples in physics — force in Newtons, power in Watts, temperature in Kelvins, energy in Joules — that one way we commemorate scientists is by naming a unit of measurement after them. Unfortunately for modern physicists, the most important units have already been named!

A Kelvin is the same size as a degree Celsius, but the zero point of the scale is far lower. On the Kelvin temperature scale, water boils at 373 K and freezes at 273 K and room temperature is at about 295 K. To go from a temperature in Kelvins to a temperature in degrees Celsius, add 273. When temperatures are very high, say in the millions of degrees, the difference between the Kelvin and Celsius scales is negligible. Absolute zero is 0 K or -273° C. Experimenters in the laboratory have chilled materials to within a millionth of a degree of absolute zero.

To sum up, all atoms and molecules are in constant motion. This is true of any form of matter, whether a solid, a liquid, or a gas. We call the description of materials in terms of microscopic motions the kinetic theory of matter. Heat is a measure of the amount of thermal energy contained in these microscopic motions. Temperature is a measure of the kinetic energy of the individual atoms or molecules. Scientists often call this description the kinetic theory of matter. The theory tells you how to calculate the speeds and energies of molecules corresponding to different temperatures. Such equations in turn are useful in predicting what kinds of phenomena will happen in planets, stars, and other materials at different temperatures.