Chapter 4: Matter and Energy in the Universe

Portrait of Benjamin Thompson, Count Rumford.

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.

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.

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.

William Thomson, 1st baron Kelvin.

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!