Heat and other forms of energy can be converted from one form to another. Heat for industrial processes, for human comfort, and for many other purposes is obtained through the conversion of chemical, electric, and nuclear energy into heat energy. Steam engines, gasoline engines, and certain other devices make it possible to convert heat energy into mechanical energy to accomplish work.

In the conversion of energy from one form to another or in the use of energy to produce work, some energy is always lost as heat. An electric motor, for example, loses some of its energy through friction in its bearings and electrical resistance in its wires. The energy is lost as heat that “leaks” into the surrounding air and cannot be used to produce work. Scientists express the amount of heat available to do work and the amount of heat that is lost in energy conversions in terms of a quantity called entropy.

Heat is changed into mechanical energy by the following thermodynamic process: (1) Heat causes a substance to expand. For example, water is heated to form steam. (2) The expanding substance exerts pressure on some object, such as a piston or a wheel. (3) The object moves. Heat has accomplished work by overcoming the resistance of the object, and the object (piston or wheel) has acquired mechanical energy—the type of energy associated with the motion of bodies. The amount of mechanical work (measured in such units as foot-pounds) that can be performed by a given amount of heat (measured in such units as calories or Btu's) is called the mechanical equivalent of heat.

A piston or wheel, once set in motion by steam pressure, may then accomplish work of its own, by imparting motion to another body. A heat engine is a device for converting heat into mechanical energy for the performance of work. Examples of heat engines are the steam turbines used to generate electricity, the internal combustion engine of an automobile, and the jet engines of aircraft.

Mechanical energy can be used to remove heat energy from a substance so that it becomes cooler than its surroundings. Mechanical energy is used in this way by air conditioners and mechanical refrigerators. A heat pump is a mechanical device that can be used either to add or remove heat energy.

Chemical energy, the type of energy found in food and fuels, is changed into heat energy by combustion (burning). A flame and the heat of the human body are examples of useful heat obtained by chemical energy conversion.

When an electric current is opposed by a resisting substance, some of the electricity is converted into heat. This kind of energy conversion takes place in an arc-welding device and in the coils of an electric heater or hot plate.

Under certain conditions heat may be converted into electricity, as in the thermocouple, a device for measuring temperature.

Nuclear energy is converted into heat and other forms of energy during fission and fusion, two types of nuclear reactions. In a power reactor, the conversion of nuclear energy into heat is carefully controlled and is used to produce steam for generating electricity. When a nuclear weapon is detonated, a large amount of nuclear energy is converted into heat very rapidly, causing a massive explosion.