Thermal energy of a substance is the sum of the potential energy and kinetic energy of the particles that make it up. Let's say you had a hot cup of water and a cold cup of water, and then mixed them. Illustrate the behavior of the particles before and after thermal equilibrium is reached.

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Explanation:

When the two samples of water (hot water and cold water) are put in contact, thermal energy is transferred from the hot water to the cold water. This occurs because heat always flows from a hotter object to a colder object. The heat is transferred by collision between the molecules: the molecules of the hot water have on average more kinetic energy than the molecules of cold water, so when they collide to each other, the molecules of hot water transfer energy to the molecules of cold water. As a result, the kinetic energy of the molecules of cold water increases, and therefore the temperature of the cold water increases, while the temperature of the hot water decreases. This process lasts until the molecules of the two samples have same average kinetic energy: when this occurs, the two samples have.