Heat, Motion, Forces, and Energy

Temperature, Thermal Energy, and Heat Transfer

To understand thermal physics, it is essential to distinguish between temperature and thermal energy. Temperature is a measure of the average kinetic energy of the particles within a substance. In contrast, thermal energy is associated with the random motion and interactions of a substance's particles; it depends on the amount of substance, its temperature, composition, and phase. Because of this, an object with a larger mass can hold more thermal energy than a smaller object, even when both substances share the exact same temperature. When substances absorb heat, thermal expansion causes most materials to increase in volume as their particles move more energetically.

Heat naturally flows spontaneously from a warmer object to a cooler object. This transfer occurs through three distinct mechanisms: conduction, convection, and radiation. Conduction is the transfer of thermal energy through direct contact, driven by interactions between neighboring particles and, in metals, by the flow of mobile electrons. Because of these mobile electrons, metals are generally good conductors of thermal energy. Many nonmetal solids are thermal insulators, but some, such as diamond, conduct heat well. Thermal insulators, such as air, foam, and wood, slow the rate of heat transfer, keeping hot items warm or cool items cold.

Convection is the transfer of heat by the bulk movement of a fluid, which can be driven either by an external force or by density differences. When heated, most fluids expand and become less dense. This decrease in density causes the warmer fluid to rise, while cooler, denser fluid sinks under gravity, establishing a continuous circulating current. Radiation transfers thermal energy across empty space by electromagnetic waves without requiring any physical matter. The way an object interacts with radiant energy depends on its surface properties: dark and matte surfaces absorb radiant heat much more effectively than light and reflective surfaces.

When thermal energy enters or leaves a substance, it does not always cause a change in temperature. During a phase change in a pure substance at constant pressure, heat is absorbed or released while the substance's temperature remains constant. The thermal energy transferred during this transition goes entirely into changing the physical state of the matter rather than altering the average kinetic energy of its particles.