What is the second law of thermodynamics?

• A. Heat cannot flow from hot to cold.
• B. Heat energy can be created and destroyed.
• C. Heat energy can only flow from a body having a high temperature to a body having a lower temperature.
• D. All processes are reversible.

Answer: (C)

The second law describes the direction of spontaneous energy dispersion and the growth of entropy in systems. In everyday terms, heat tends to move from hotter objects to cooler ones, and it won’t flow the other way on its own without some external influence. That makes the statement about heat energy flowing from a higher-temperature body to a lower-temperature body the best description of this law, because it captures the natural direction of heat transfer and the idea that energy spreads until temperatures equalize.

Think of it this way: you can’t rely on heat to spontaneously rise from a cold object to a hot one, and processes that do try to push energy opposite this direction require work or additional forces. Also, the second law is consistent with entropy increasing in isolated systems, which explains why perfect, all-heat-to-work conversion isn’t achievable and why real processes are often irreversible.