Heat Energy I: Heat Transfer

Heat is defined in physics as the transfer of heat energy over well-defined boundaries around a thermodynamic system. Thermodynamic free energy is the amount of work that a thermodynamic system can perform.







Heat transfer, any or all of several phenomena, is considered a mechanism that transfers energy and entropy from one location to another. The specific mechanisms are often referred to as conduction, convection, radiation, and conduction.




i. Conduction

Conduction involves the transfer of energy and entropy between adjacent molecules, usually a slow process. Heat conduction occurs when hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring particles. In other words, when adjacent atoms vibrate with each other, or when electrons move from one atom to another, heat is transmitted through conduction.

Conduction is the most important means of transferring heat inside a solid or between solid objects in thermal contact. Fluids-especially gases-are less conductive. Thermal contact conductance is the study of the heat conduction between the solids in contact.

The process of transferring heat from one place to another without particle movement is called conduction, for example, when the hand is placed on a cold water glass-heat is transmitted from warm skin to cold glass, but if the steady-state conduction of the hand is an idealized conduction model, this happens when the temperature difference driving the conduction is constant, so after a period of time, the temperature spatial distribution in the conduction object will not change further.

ii. Convection

Convection involves the movement of a heated fluid (such as air) and is usually a fairly fast process. Convection heat transfer, or convection for short, is the transfer of heat from one place to another through the movement of the fluid.

This process is basically the transfer of heat through mass transfer. The bulk movement of the fluid enhances heat transfer in many physical situations, such as (for example) between the solid surface and the fluid. Convection is usually the main form of liquid and gas heat transfer.

Although sometimes discussed as the third method of heat transfer, convection is usually used to describe the combined effect of heat conduction (diffusion) within the fluid and heat transfer through the bulk fluid flow.


iii. Radiation


Radiation refers to the transmission of energy as electromagnetic radiation, the emission from the heating surface to the absorption of another surface, which is a process that does not require a medium to transmit energy. Radiation heat transfer is the transfer of energy through thermal radiation, that is, electromagnetic waves.

It occurs in a vacuum or any transparent medium(solid or fluid or gas). Due to the random movement of atoms and molecules in matter, all objects emit thermal radiation at temperatures above absolute zero. Since these atoms and molecules are composed of charged particles (protons and electrons), their movement causes electromagnetic radiation to be emitted, thus taking away energy.

Radiation is usually only important in engineering applications for very hot objects or objects with large temperature differences. When the size of the distance between the object and the separation of them is large and compared with the wavelength of thermal radiation, the transfer rate of radiant energy is best described by the Stefan-Boltzmann equation

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