Definition · Plain-language
Law of conservation of energy
The law of conservation of energy states that energy can neither be created nor destroyed; it can only be transformed from one form into another or transferred between objects.
The step most authors miss
Doing CRediT right? Don’t stop at the statement.
A CRediT statement credits you inside one paper. The recognition CRediT was built for happens when those roles are tied to you, persistently. Sign in with your ORCID — free — and claim your CRediT contributions on casrai.org, the home of the standard. They become a verified, portable part of your identity, not a line that disappears into one PDF.
Free: claim your contributions, then export a journal-ready CRediT statement, schema.org structured data, JATS XML, CSV or BibTeX — and preview your public profile. A membership publishes that profile publicly and verifies the journals you serve.
Energy is transformed, never lost
The law of conservation of energy is one of the most fundamental principles in all of physics. It states that within an isolated system — one that exchanges nothing with its surroundings — the total quantity of energy never changes. Energy constantly changes form: chemical energy in fuel becomes heat and motion in an engine, electrical energy becomes light and heat in a bulb, gravitational potential energy becomes kinetic energy as a ball falls. In every case the energy is converted or moved, but the grand total is conserved. Nothing in nature has ever been observed to create or destroy energy.
The first law of thermodynamics
Applied to heat and work, conservation of energy becomes the first law of thermodynamics: the change in a system’s internal energy equals the heat added to it minus the work it does. This is why a perpetual motion machine of the “first kind” — one that produces energy from nothing — is impossible, and why every engine is limited by the energy you put in. Einstein’s famous E = mc² extends the law, showing that mass is itself a form of energy; the conserved quantity is really mass-energy together, which is what is converted in nuclear reactions and in the Sun.
Why energy still “runs out”
If energy is always conserved, why do we worry about wasting it or running out of it? The answer is that energy can be conserved in total yet degraded in usefulness. Every real process converts some energy into low-grade heat that disperses into the surroundings and can no longer do useful work — a tendency described by the second law of thermodynamics and the concept of entropy. So “using energy” really means converting concentrated, useful energy into diffuse, useless heat. The energy is not destroyed, but its quality falls, which is why efficiency and energy conservation in the everyday sense still matter.
Key facts
At a glance
- Definition: the total energy of an isolated system stays constant
- Core idea: energy is transformed or transferred, never created or destroyed
- Also known as: the first law of thermodynamics
- Consequence: perpetual motion machines (of the first kind) are impossible
- Extension: E = mc² shows mass is a form of energy (mass-energy is conserved)
- Caveat: energy is conserved but can degrade into less useful heat (entropy)
Common misconceptions
What people often get wrong
Often heard: Using energy destroys it, which is why we run out of it.
Actually: Energy is never destroyed. “Using” it converts concentrated, useful energy into dispersed low-grade heat that can no longer do useful work — its quality falls, but the total is conserved.
Often heard: A clever enough machine could create energy from nothing.
Actually: Conservation of energy forbids it. A perpetual motion machine that generates net energy is impossible; every machine is limited by the energy supplied to it.
Often heard: Conservation of energy and conservation of mass are completely separate laws.
Actually: Einstein showed mass and energy are interchangeable through E = mc². The truly conserved quantity is mass-energy, which is converted in nuclear reactions.
Going deeper
