Final answer:
When thermal energy is applied to a substance, its atoms or molecules gain kinetic energy, increasing their movement and causing the substance's temperature to rise. This is because temperature is a measure of the average kinetic energy of the particles. Conversely, a loss of thermal energy will result in a decrease in temperature.
Step-by-step explanation:
When thermal energy is applied to a substance, the kinetic energy of its atoms or molecules increases. This increase in kinetic energy translates into various forms of motion—such as translations (straight-line motions), vibrations, or rotations—depending on the state of the substance. As a result, the temperature of the substance rises, because temperature is a measure of the average kinetic energy of the particles.
For example, when heating water in a pot, the thermal energy from the stove increases the water molecules' motion, leading to an increase in the temperature of the water. Provided that there is no phase change or chemical reaction taking place, this trend of kinetic energy increasing with thermal energy input holds true for all substances.
The concept of internal energy (U or E) is central here, as it represents the total of all forms of energy within a substance, including both kinetic and potential energy of its particles. When thermal energy is lost from a substance, the converse happens—the kinetic energy of its particles decreases, and its temperature drops.