What is thermal equilibrium?

• A. When heat flows from low to high temperature.
• B. When two substances reach the same temperature and heat flow stops.
• C. When both substances are heated equally.
• D. When heat flows from a single object to its surroundings.

Answer: (B)

Thermal equilibrium is reached when two substances in contact have the same temperature, so there is no net heat transfer between them. Heat always flows from hotter to cooler regions, and it continues until the temperature difference disappears. Once both substances share the same temperature, the driving force for heat flow vanishes, and the system is balanced. Heating both equally or continuously sending heat to the surroundings would not, by itself, establish this balance in the connected system; equilibrium specifically means equal temperatures with no further net heat exchange.