First law of thermodynamics

The first law of thermodynamics is that energy can neither be created nor destroyed; it can change only from one form to another. The law forms the basis of the principle of conservation of energy. This means that anything that uses energy is changing the energy from one kind of energy to another. For example, exercising changes energy from food into kinetic (motion) energy. Another example: In the Sun (or any star), nuclear fusion changes mass into heat and light (electromagnetic radiation), which travels to Earth and is used by plants to create food (chemical energy) by photosynthesis, which can be eaten by animals allowing them to move (kinetic energy). Energy only ever changes its form; it is neither created nor destroyed. This is why perpetual motion machines do not exist and could never exist; it would break a fundamental law of physics.

People can use the changes to do work that is useful.^[1] Examples of forms of energy in classical mechanics include heat, light, kinetic (movement) or potential energy. However, in modern physics it is considered that there are only two types of energy - mass and kinetic energy, although this may not be helpful to those not familiar with more complex physics.

The law means that the total energy of the universe (or any Closed system) is a constant. However, energy can be transferred from one part of the universe to another.

The most common wording of the first law of thermodynamics used by scientists is:

1. ↑ 1st Law of Thermodynamics^{[permanent dead link]} Ohio State University. Accessed July 2011