Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.
The four fundamental modes of heat transfer illustrated with a campfire
Red-hot iron object, transferring heat to the surrounding environment through thermal radiation
Lightning is a highly visible form of energy transfer and is an example of plasma present at Earth's surface. Typically, lightning discharges 30,000 amperes at up to 100 million volts, and emits light, radio waves, X-rays and even gamma rays. Plasma temperatures in lightning can approach 28,000 kelvins (27,726.85 °C) (49,940.33 °F) and electron densities may exceed 1024 m−3.
Nucleate boiling of water.
The term "thermal energy" is used loosely in various contexts in physics and engineering, generally related to the kinetic energy of vibrating and colliding atoms in a substance. It can refer to several different physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, defined as a type of energy transfer ; and the characteristic energy of a degree of freedom, , in a system that is described in terms of its microscopic particulate constituents.
Thermal radiation in visible light can be seen on this hot metalwork, due to blackbody radiation.
