Conservation Laws

Energy is a state function that can be described in many forms. The most basic form of energy is kinetic energy, which is the energy of motion. A moving object has energy solely due to the fact that it is moving. However, many non-moving objects contain energy in the form of potential or stored energy. A boulder on the top of a cliff has potential energy. This implies that the boulder could convert this potential energy into kinetic energy if it were to fall off the cliff. A stretched bow and arrow have potential energy also. This implies that the stored energy in the bow could be converted into the kinetic energy of the arrow, after it is released. Stored energy may be more Figure 3. Illustration by Hans & Cassidy. Courtesy of Gale Group. complicated than these mechanical examples, however, as in the stored electrical energy in a car battery. We know that the battery has stored energy because this energy can be converted into the kinetic energy of a cranking engine. There is stored chemical energy in many substances, for example gasoline. Again we know this because the energy of the gasoline can be converted into the kinetic energy of a car moving down the road. This stored chemical energy could alternately be converted into thermal energy by burning the gasoline and using the heat to increase the temperature of a bath of water. In all these instances, energy can be converted from one form to another, but it is always found that the total energy remains constant.

In certain instances, even mass can be converted into energy. For example, in a nuclear reactor the nucleus of the uranium atom is split into fragments. The sum of the masses of the fragments is always less than the original uranium nucleus. What happened to this original mass? This mass has been converted into thermal energy which heats the water to drive steam turbines which ultimately produces electrical energy. As first discovered by Albert Einstein (1879-1955), there is a precise relationship defining the amount of energy that is equivalent to a certain amount of mass. In instances where mass is converted into energy, or visa versa, this relationship must be taken into account.

In general, therefore, there is a universal law of conservation of energy and mass that applies to all of nature. The sum of all the forms of energy and mass in the universe is a certain amount which remains constant. As is the case for angular momentum, the energies of the electrons that surround the nucleus of the atom can possess only certain discrete values. And again, even with these constraints, the conservation of energy and mass is always obeyed.