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Entropy and Second Law of Thermodynamics

• One-way processes: Some processes will happen only in a certain way: heat flows from hot to cold, not the other way around, and air spreads throughout a room, but never get concentrated in only the left half of the room. Energy conservation is satisfied even if a process goes the wrong way; the fact that it does not mean that there must be another property of the system that defines whether a process occurs. This is entropy.
• For an irreversible process in a closed system, entropy always increases; in a way, entropy change defines the arrow of time.
• If an amount of heat Q is added to a system at constant temperature T, the entropy changes by Q/T.
• The entropy statement of the second law of thermodynamics: The entropy of a closed system never decreases. For an irreversible process the change of entropy is positive; for a reversible process it is zero.