Laws of Thermodynamics

Definition

The fundamental physical laws governing how heat, work, and energy change and flow in any system.

Key points

• The zeroth law states that if two bodies are each in thermal equilibrium with a third, they are in equilibrium with each other; this is the basis of temperature measurement.
• The first law is the conservation of energy: energy can be converted from one form to another but cannot be created or destroyed, so heat added to a system equals the increase in internal energy plus work done.
• The second law states that heat flows naturally from a hotter body to a colder one and never the reverse on its own; the disorder (entropy) of an isolated system always tends to increase.
• The third law states that as temperature approaches absolute zero (minus 273.15° Celsius, or 0 kelvin), the entropy of a perfect crystal approaches zero, and absolute zero can never be fully reached.
• These laws explain why no engine can be 100 percent efficient and why perpetual-motion machines are impossible.

Why it matters for CAPF

The first law as conservation of energy, the second law on the direction of heat flow and entropy, and absolute zero are standard physics facts, and the impossibility of perpetual motion is a frequent conceptual question.

Common confusion

The first law (energy is conserved) does not forbid perpetual-motion machines by itself; it is the second law (entropy increases, some energy is always lost) that makes them impossible. Heat flows from hot to cold by itself; moving it the other way (as in a refrigerator) needs external work.

Related concepts

concept heat transfer modes, concept simple machines, concept newtons laws

Parent note

physics everyday