Convection is the process in which heat is carried by the bulk movement of a fluid. During natural convection, the warmer, less dense part of a fluid is pushed upward by the buoyant force provided by the surrounding cooler and denser part. Forced convection occurs when an external device, such as a fan or a pump, causes the fluid to move. Conduction is the process whereby heat is transferred directly through a material, any bulk motion of the material playing no role in the transfer. The heat Q conducted during a time t through a bar of length L and cross-sectional area A is expressed as Q = ( kA ∆ T ) t / L , where ∆ T is the difference in temperature between the ends of the bar and k is the thermal conductivity of the material. Materials that have large values of k , such as most metals, are known as thermal conductors. Materials with small values of k , such as Styrofoam, are referred to as thermal insulators. Radiation
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