The second law of thermodynamics
The second law of thermodynamics can be stated in many ways:
- Heat will not spontaneously flow from a cold body to a warm one; i.e., putting a cube of ice in a cup of water won't make the water boil.
- The sum change in entropy in closed system, or in an open system and its surroundings, will be positive.
- Mathematically with the formula ΔSuniv = ΔSsys + ΔSsurr > 0, where ΔS is a change in entropy, "sys" is the system, "surr" its surroundings, and "univ" the combination of both of these, the universe.
When it comes to an open system and its surroundings, this law does not mandate a positive change in entropy in both the system and its surroundings. The change in either may be negative, but in the other the change must be such that the total change in entropy is positive. This allows ice to form, among other things; ice has a lower entropy than water. The formation of ice is compensated by an increase in entropy in its surroundings.