Heating Elements Electric
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Heat Transfer Basics
Heat transfer is the movement of energy due to a temperature difference. There are three physical mechanisms or modes of heat transfer; conduction, convection, radiation. Heat normally flows from an area of higher temperature to a region of lower temperature. Heat can be made to flow from a cooler area to a hotter area, but this is not a spontaneous phenomena, work must be done on the system. Commonly found examples of this are refrigerators and heat pumps.
The conduction mode of heat transfer occurs when there is a temperature difference in a stationary medium. On a molecular level, the high temperature area has a higher vibrational energy, and this energy is transferred molecule to molecule to the cooler region. There is no movement in the bulk media. An example of this is a coffee cup. When it is filled with hot coffee, the inside wall temperature is raised. The increased vibrational energy is diffused through the wall until the warmth of the coffee is felt on the outside surface.
Convection occurs when heat is transferred due to diffusion and bulk motion, most commonly between a fixed surface and a moving fluid, liquid or gas. Convection can be further subdivided into free convection and forced convection. For free convection, the flow of the fluid is induced by buoyancy forces, whereas in forced convection the fluid flow is due to some outside means such as a fan, blower, or pump. An example of free convection is the draft felt by a oven door. At the oven door surface, heat is diffused into the air. The increased temperature of the air causes it to expand. As it expands, it has a lower density than the cooler surrounding air causing it to rise. As the air moves up, heat is transported away from the oven door. An example of forced convection can be found under the hood with the car radiator. Air is forced by a fan over the fins of the radiator which has been heated by the engine coolant. Heat is diffused into the air as it comes into contact with the surface of the radiator, and is then transported away by the bulk motion of the air flow.
Thermal radiation occurs when thermal energy is exchanged via electromagnetic waves. A bulk media is not required in between the hot and cool regions for heat transfer to occur with radiation. For example, the earth receives thermal energy from the sun via radiation even though the void of space is in between. Radiation becomes the dominant mode of transfer at higher temperatures. For example, when standing in front of a camp fire you are warmed primarily by radiation. Heat is not diffused to you by conduction because the air surrounding you is still cool. Heat is transferred to the air by convection, but just in the draft above the fire which rises upwards. The warmth you feel when standing beside the fire is from infrared radiation intercepted by your clothing.
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