Bluffton University

NSC111: Physics/Earth/Space
Resource page: Thermodynamics

III. The First Law of Thermodynamics

In the 19th Century, several scientists noticed that heat could not be a material substance: unlimited amounts of heat could be generated by applying mechanical energy, as with a rotary drill.

It became apparent that energy was a quantifiable thing, and that it could be converted from one form to another. James Prescott Joule found the mechanical equivalent of heat, as you will do in the laboratory. Heat can also be converted into mechanical work, or work into electricity, and so forth.

The underlying assumption was and is that, just as mass remains constant unless more is added or removed, you can't get out more energy than you put in. This was eventually demonstrated to everyone's satisfaction, and came to be known as the

First Law of Thermodynamics

In an isolated system, the total amount of energy is conserved.

The First Law can be stated in other ways:

  • Energy cannot be created or destroyed, only changed from one form to another.
  • When heat flows to or from a system, the system gains or loses an amount of energy equal to the heat transferred.
  • You cannot get more energy out of a system than the energy you put in plus the energy of the system.
Energy can be obtained from
A primary source of energy
When the energy obtained from a source is used in its original form, the source is said to be primary. An example is using the heat from a stove to boil water.

A secondary source of energy
When energy obtained is converted into another form before use, the energy source is said to be secondary. An example is using the heat from boiling water to turn a turbine; the heat is converted into rotary motion (mechanical energy or work).
The distinction between "primary" and "secondary" sources is fluid in practice. For example, an internal combustion engine is usually treated as a primary source of rotary work: it is called a primary energy source when it is used to turn the wheels of a car -- using the mechanical energy directly -- but a secondary source when it is used to turn an electrical generator -- converting mechanical energy into electrical energy.

Nevertheless, an internal combustion engine is actually a heat engine, a secondary source of energy that converts the heat of burning fuel into mechanical motion.

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Copyright © 2001 by Daniel J. Berger. This work may be copied without limit if its use is to be for non-profit educational purposes. Such copies may be by any method, present or future. The author requests only that this statement accompany all such copies. All rights to publication for profit are retained by the author.