Law of Conservation of Energy Definition

Energy Is Neither Created Nor Destroyed

Energy can be neither created nor destroyed.
Energy can be neither created nor destroyed. Mmdi, Getty Images

The law of conservation of energy is a physical law which states energy cannot be created or destroyed, but may be changed from one form to another. Another way of stating the law is to say the total energy of an isolated system remains constant or is conserved within a given frame of reference.

In classical mechanics, conservation of mass and conversation of energy are considered to be two separate laws. However, in special relativity, matter may be converted into energy and vice versa, according to the famous equation E = mc2. Thus, it's more appropriate to say mass-energy is conserved.

Example of Conservation of Energy

For example, if a stick of dynamite explodes, the chemical energy contained within the dynamite changes into kinetic energy, heat, and light. If all of this energy is added together, it will equal the starting chemical energy value.

Consequence of Conservation of Energy

One interesting consequence of the law of conservation of energy is that it means perpetual motion machines of the first kind are not possible. In other words, a system must have an external power supply in order to continuously deliver unlimited energy to its surroundings.

It's also worth noting, it's not always possible to define conservation of energy because not all systems have time translation symmetry. For example, conservation of energy may not be defined for time crystals or for curved spacetimes.