Heat Transfer, Conduction And Convection

The transfer of heat is normally from a high temperature object to a lower temperature object. Heat transfer changes the internal energy of both systems involved according to the First Law of Thermodynamics. Although internal energy will not spontaneously flow from a cold region to a hot region, it can be forced to do so by doing work on the system. Refrigerators and heat pumps are examples of heat engines which cause energy to be transferred from a cold area to a hot area

Conduction is heat transfer through solids and liquids by means of molecular agitation ( vibration and collision of molecules and free electrons ) within a material without any motion of the material as a whole. If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones. For heat transfer between two plane surfaces, such as heat loss through the wall of a house, the rate of conduction heat transfer is:

• Q = heat transferred in time =
• K = thermal conductivity of the barrier W/m.K.
• A = Area
• T = Temperatur
• d = thickness of barrier
  

Convection is heat transfer by mass motion of a fluid (a liquid or a gas) when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (see Ideal Gas Law). Hot water is likewise less dense than cold water and rises, causing convection currents which transport energy.
Convection can also lead to circulation in a liquid, as in the heating of a pot of water over a flame. Heated water expands and becomes more buoyant. Cooler, more dense water near the surface descends and patterns of circulation can be formed, though they will not be as regular as suggested in the drawing. when we heat water on a stove, the volume of water at the bottom of the pot will be warmed up by conduction from the metal of the pot and it will become less dense. Then, because it is less dense, it will shift upward to the surface of the volume of water and will displace to the upper -less hot and denser- mass of water

Formula of Convection:

q = hA (Ts - T ∞)

Where h is the constant for the convective heat transfer coefficient, A is the area implied, and Ts - T ∞ is the difference between the final temperature and the initial temperature.

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