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The Laws of Thermodynamics Explained

Tap a law to see what it states.

Zeroth Law of Thermodynamics

States that if two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other β€” this is the basic principle that makes temperature measurement meaningful and consistent.

First Law of Thermodynamics

Energy cannot be created or destroyed, only converted from one form to another or transferred between systems β€” essentially a statement of the conservation of energy applied to thermal systems.

Second Law of Thermodynamics

The total entropy (disorder) of an isolated system will never decrease over time, meaning energy tends to spread out and become less useful for doing work, which is why heat naturally flows from hot to cold and not the reverse.

Third Law of Thermodynamics

As a system's temperature approaches absolute zero, its entropy approaches a minimum, fixed value β€” in practical terms, this means absolute zero can be approached but never actually fully reached.

Why entropy always increases

The second law of thermodynamics, often summarized as "entropy always increases" in an isolated system, is one of the most fundamental and far-reaching principles in physics β€” it explains everything from why a hot cup of coffee cools down (rather than a room spontaneously heating a cup further) to why perpetual motion machines are physically impossible.

Frequently Asked Questions

Does the second law of thermodynamics mean order can never increase anywhere?

No β€” order can increase locally, such as in a living organism or a refrigerator, but this always requires energy input from outside the system, and the total entropy of the system plus its surroundings still increases overall.

Why is it said that absolute zero can never actually be reached?

The third law implies that reaching absolute zero would require removing an infinite amount of energy from a system, which is physically impossible, so scientists can only get extremely close to it in laboratory conditions.