Describe Fick's first law of diffusion. A plate of iron is exposed to a carburizing atmosphere on one side and a decarburizing atmosphere on the other side at 700 degree C. Under steady state condition calculate the diffusion flux of carbon through the plate, if the concentration of carbon at position of 5 mm and 10 mm beneath the carburizing surface are 1.2 kg/m3 and 0.8 kg/m3, respectively. Assume a diffusion coefficient of 3 * 1011 m2/s at this temperature.

Fick's First Law of Diffusion: The rate at which particles diffuse in steady state conditions i.e. when there are no changes in the system with time, the rate of diffusion is proportional to the concentration gradient dC/dx, this is known as Fick's First Law.

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From the Fick's first law,

J= -D dC/dx

is utilized to determine the diffusion flux.

Substitution of the value above into this expression yields.

J = -D CA-CB/xA-xB

where, D = 3 * 10-11 m2/s

CA = 1.2 kg/m3, CB = 0.8 kg/m3

xA = 5 * 10-3 m ; xB = 10-2 m

J = -D CA-CB/xA-xB = -(3 * 10-11) 1.2-0.8 kg/m3/(5 * 10-3-10-2) m

J = 2.4 * 10-9 kg/m2-s