החוק השני של התרמודינמיקה

Heat flows spontaneously from hot to cold objects because this process increases the entropy of the universe. When a hot object transfers heat to a cooler one, the entropy increase in the surroundings exceeds the entropy decrease in the system, making the overall change positive. Reverse heat flow would decrease universal entropy and never occurs spontaneously.

The second law states that entropy of the universe increases for all spontaneous processes. This means ΔS of the universe must be greater than zero. Entropy measures energy dispersal, so processes where energy becomes more dispersed in the final state than the initial state are spontaneous and thermodynamically favorable.

Spontaneity depends on whether the entropy of the universe increases. When ice melts, the system's entropy increases, but the surroundings must also contribute positively. At low temperatures, heat released during freezing causes a larger entropy increase in the surroundings than at high temperatures, making freezing spontaneous only below 0°C.

The entropy change of the surroundings is directly proportional to heat transferred and inversely proportional to temperature. At lower temperatures, the same amount of heat produces a greater entropy change in the surroundings. This relationship explains why processes like freezing become spontaneous only at sufficiently low temperatures.

The first law states energy is conserved during transfer between system and surroundings. However, energy conservation alone does not determine heat flow direction. The second law explains directionality: heat flows spontaneously only when universal entropy increases, providing the missing criterion for predicting spontaneous processes.

A system reaches equilibrium when the entropy change of the universe equals zero. This occurs when the system and surroundings are at essentially the same temperature, so entropy increases in one exactly balance entropy decreases in the other. At equilibrium, no net spontaneous change occurs in either direction.

A process can be spontaneous even when the system's entropy decreases if the surroundings' entropy increases sufficiently. For example, water freezing has negative ΔS for the system, but the heat released increases surroundings' entropy. If the total universal entropy change is positive, the process remains spontaneous.