Laws of Heat Transfer

Fourier’s law of conduction

Rate of heat transfer by conduction along a given direction is directly poportional to the area of the heat transfer lying perpendicular to the direction of heat transfer.
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In the above expression, -ve sign denotes that as x↑ T↓
“Heat always flows in direction of downhill of temperature.”

k is thermal conductivity. It is thermophysical property of material slab

Newton’s law of cooling

Rate of heat transfer by convection between a solid and surrounding fluid is directly proportional to the temperature difference between them and is also directly proportional t0 the area of contact or area of exposure between them.
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where, h is convective heat transfer coefficient
Unlike ‘k’, ‘h’ is not a property of the material but it depends upon some of the thermophysical properties of the fluid like density, μ, specific heat.  

Stefan-Boltzmann law of radiation

Radiation energy emitted from the surface of the black body per unit time and per unit area is directly proportional to the fourth power of the absolute temperature of the body.
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where, σ is Stefan-Boltzmann constant

Black Body 

Black body is the body which absorbs all the thermal radiation incident or falls upon it.
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A thermally black body absorbing all incident radiation may not appear black in color to human eye e.g. ice and snow
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A narrow cavity in a hollow spherical container can be treated as the black body since all the radiation entering the cavity is practically absorbed by it after multiple reflections.

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