Heat is a form of energy that is transferred between two objects or systems that are at different temperatures. The transfer of heat can be accomplished through a variety of mechanisms, including conduction, convection, and radiation. Heat transfer is a very important process in many scientific and engineering disciplines, such as thermodynamics, HVAC (heating, ventilation, and air conditioning), and mechanical engineering.
The study of heat transfer is divided into three main categories: conduction, convection, and radiation. Each mode of heat transfer involves the transfer of energy from one place to another by either the movement of particles or the emission of electromagnetic waves.
Conduction is the transfer of heat by direct contact between molecules. In solids, this occurs via atomic collisions. In fluids (liquids and gases), it occurs via molecular collisions. Convection is the transfer of heat by the circulation or movement of fluids (liquid or gas). Radiation is the transfer of heat by electromagnetic waves; this includes infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. All three modes can occur simultaneously in various combinations depending on the situation.
Heat always flows from a hotter object to a cooler object until both objects reach thermal equilibrium—the same temperature—at which point no further heat flow takes place. The rate at which heat flow occurs depends on several factors:
* The difference in temperature between the two objects (ΔT): The larger the ΔT ,the greater the rateofheat flow Rate ∝ ΔT
* The typeof material(s) involved: Materials with good thermal conductivity (e.g., metals) conduct heat better than those with poor thermal conductivity (e.g., plastics). Rate ∝ Conductivity
* The cross-sectional area over which heating takes place: A wider cross section conducts more heat than a narrower one for any given material because there are more opportunities for molecules to collide across any given distance .A = Cross – Sectional Area Rate ∝ 1/A