Physics
Physics
Problem
Consider the following student statements about what occurs when a hot metal rod is placed into cool water and the system eventually reaches thermal equilibrium. I. At equilibrium, the average kinetic energies of the particles in the metal and the water are equal. II. No collisions occur between particles of the two substances once equilibrium is reached. III. The net transfer of energy as heat is zero at equilibrium, even though individual collisions still occur. Which of the following is correct? a) II and III only b) I only c) I and III only d) I and II onlyIntuitive explanation
Thermal equilibrium is about matching temperatures, not about particles stopping their motion or collisions ceasing—those continue forever. The key insight is distinguishing between net heat flow (which stops at equilibrium) and individual molecular collisions (which never stop). Statement I confuses equal temperature with equal kinetic energy per particle; Statement II ignores that molecular motion is constant; Statement III correctly captures that equilibrium means zero net energy transfer while microscopic activity persists.
Intuitive explanation
Thermal equilibrium is about matching temperatures, not about particles stopping their motion or collisions ceasing—those continue forever. The key insight is distinguishing between net heat flow (which stops at equilibrium) and individual molecular collisions (which never stop). Statement I confuses equal temperature with equal kinetic energy per particle; Statement II ignores that molecular motion is constant; Statement III correctly captures that equilibrium means zero net energy transfer while microscopic activity persists.