What happens to heat flow when thermal equilibrium is reached?

• A. Heat flow increases over time.
• B. Heat flow ceases and does not resume.
• C. Heat flow can continue intermittently in cycles.
• D. Heat flow stops once equilibrium is reached.

Answer: (D)

Heat flow is driven by a temperature difference. When two objects reach thermal equilibrium, their temperatures are equal, so there is no driving gradient left. Without a gradient, the net rate of heat transfer becomes zero, and heat flow stops. There may still be tiny microscopic energy exchanges, but they cancel out and there is no overall heat transfer. If external conditions later create a new temperature difference, heat flow can resume, but in unchanged equilibrium it is zero. The other ideas imply ongoing, cyclic, or increasing transfer, which doesn't occur once the system is at equilibrium.