What is entropy in simple terms?

Entropy measures disorder or randomness. High entropy means energy is spread out evenly (like room-temperature coffee). Low entropy means energy is concentrated (like hot coffee). Nature tends toward higher entropy—heat flows from hot to cold, not reverse, without work input.

Why can't entropy decrease?

The Second Law of Thermodynamics states entropy of an isolated system never decreases. Locally, entropy can decrease (like refrigeration), but this requires work that increases entropy elsewhere more. The universe's total entropy always increases.

How is entropy related to heat engine efficiency?

Maximum efficiency = 1 - T_cold/T_hot (Carnot efficiency). Entropy explains why: some energy must be rejected as waste heat to increase entropy. No real engine reaches Carnot efficiency because irreversibilities create additional entropy.

What's the difference between entropy and enthalpy?

Entropy (S) measures disorder (J/K). Enthalpy (H) measures total heat content (J). They're related through Gibbs free energy: G = H - TS. A reaction is spontaneous when ΔG < 0, which can happen with positive entropy change (increased disorder) even if endothermic.

エントロピー Converter

About Entropy Conversion

Entropy measures the degree of disorder or randomness in a thermodynamic system—a fundamental concept that determines which processes can occur spontaneously and which cannot. It quantifies how much energy in a system is unavailable to do useful work, increasing as energy spreads and disperses. The inexorable increase of entropy in isolated systems (the Second Law of Thermodynamics) defines the arrow of time and explains why heat flows from hot to cold, not reverse.

The SI単位 is joules per kelvin (J/K). Entropy governs the maximum efficiency of heat engines (Carnot efficiency), determines chemical reaction spontaneity through Gibbs free energy, and appears in statistical mechanics through Boltzmann's formula S = k ln(W). 理解する entropy is essential for thermodynamic cycle analysis, chemical equilibrium calculations, heat engine and refrigeration design, and information theory.

Our converter handles all standard entropy units used in physics, chemistry, and engineering thermodynamics.

Common Entropy Conversions

変換元	変換先	乗数
J/K	kJ/K	0.001
kJ/K	J/K	1,000
J/K	cal/K	0.2388
cal/K	J/K	4.184
J/K	BTU/°R	0.000527
BTU/°R	J/K	1,899
kJ/K	kcal/K	0.2388
J/K	erg/K	10⁷

Entropy 単位リファレンス

Joule per kelvin (J/K) – The SI単位 for entropy, representing energy dispersal per unit temperature. This is an extensive property—doubling the amount of material doubles the entropy. Used throughout thermodynamics, physical chemistry, and statistical mechanics. The Boltzmann constant k = 1.38 × 10⁻²³ J/K sets the scale between microscopic and macroscopic entropy.

Kilojoule per kelvin (kJ/K) – Convenient for larger systems and engineering calculations where entropy values in J/K would be unwieldy large numbers. Common in process thermodynamics, power plant analysis, and industrial heat transfer calculations. Steam tables often use kJ units for practical convenience.

Calorie per kelvin (cal/K) – Older unit still encountered in chemistry literature and some thermodynamic tables. 1 cal/K = 4.184 J/K (exactly, by definition of the thermochemical calorie). Historical chemistry data and some food science applications use calorie-based entropy.

BTU per degree Rankine (BTU/°R) – US engineering unit for entropy, used in HVAC design, power plant engineering, and American steam tables. 1 BTU/°R ≈ 1899 J/K. Rankine is the absolute temperature scale with 華氏-sized degrees, maintaining unit consistency in US thermodynamic calculations.

Entropy unit (e.u.) – Traditional chemistry unit equal to 1 cal/(mol·K) or approximately 4.184 J/(mol·K). Found in older thermodynamic tables and physical chemistry literature. Largely superseded by SI単位s in modern practice.