Conservation of Energy

A coal plant burns fuel and produces electricity. A politician says 67% of the fuel's energy is "wasted." A physicist says 0% was wasted: every joule is accounted for, just not all in the form you wanted. The politician is speaking about useful output. The physicist is stating a law of nature.

The law of conservation of energy states that energy cannot be created or destroyed, only converted from one form to another. The total energy in a closed system is constant. When coal burns, chemical energy converts to thermal energy (heat), then to kinetic energy (spinning turbine), then to electrical energy (generator output). At each step, some energy converts to forms that are not useful (waste heat, friction, sound), but the total never changes.

This law is the foundation of all energy accounting. It means every energy flow diagram must balance: energy in equals energy out, always. When a system appears to lose energy, the "missing" energy has converted to a form you are not tracking (usually low-grade heat). When a system appears to gain energy, you have not accounted for an input.

The practical consequence: you cannot get more energy out of a system than you put in. Efficiency is always less than 100% because some conversion to heat is unavoidable (the Second Law of Thermodynamics, covered separately).

Worked Example

A gas turbine takes in natural gas with 300 MW of chemical energy. It produces 120 MW of electricity and exhausts 180 MW of heat.

• Check conservation. 120 + 180 = 300 MW. Energy in = energy out.
• Calculate efficiency. 120/300 = 40%.

A salesperson claims a new device produces 110% efficiency. What does conservation of energy tell you?

The claim is physically impossible. No device can output more energy than it receives. The salesperson is either measuring incorrectly, defining efficiency misleadingly, or making a false claim.

Conservation of energy is the single most powerful tool for catching errors in energy claims: if the numbers do not balance, something is wrong.

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