Heat transfer, the movement of thermal energy from one place to another, can occur through three main mechanisms: conduction, convection, and radiation.
Conduction is the transfer of heat through a solid material by the collision of atoms and molecules. The heat energy is transferred from atom to atom through vibrations and collisions, without any bulk motion of the material. The efficiency of heat transfer by conduction is affected by the thermal conductivity of the material, which is a measure of how well a material conducts heat. Metals generally have high thermal conductivity, while insulators like wood and plastic have low thermal conductivity.
Convection is the transfer of heat by the movement of a fluid or gas. It occurs when a fluid is heated, causing it to expand and become less dense. The less dense fluid then rises, while cooler, denser fluid sinks. This creates a circulating pattern, where heat is transferred from one part of the fluid to another by the motion of the fluid. The efficiency of heat transfer by convection is affected by the thermal conductivity and viscosity of the fluid. Fluids with high thermal conductivity and low viscosity, such as water, are better heat transfer agents than fluids with low thermal conductivity and high viscosity, such as oil.
Radiation is the transfer of heat through electromagnetic waves, such as infrared radiation. This type of heat transfer does not require a medium to travel through, and can occur in a vacuum. Radiation is the mechanism by which heat is transferred between objects that are not in direct contact with each other. The efficiency of heat transfer by radiation is affected by the emissivity of the surfaces. Emissivity is a measure of how well a surface emits thermal radiation. A surface with high emissivity will emit more thermal radiation than a surface with low emissivity.
All three types of heat transfer can occur simultaneously in different ways and combinations, in different situations and environments.