Diffusion is the process of transfer of molecules either due to concentration difference or bulk motion. Diffusion is of two types: one is molecular diffusion and another is convective
diffusion. The molecular diffusion happens due to concentration difference
while convective diffusion occurs due to bulk motion of particles and medium.
For example (assume unidirectional diffusion) if a
person has put perfume on his body, when he and observer both are either
stationary or moving with same velocity, then the flux of perfume molecules (the no. of particles passing through unit area) with reference to observer
is only due to concentration difference i.e. molecular diffusion flux (JA) and if there is relative motion between person and observer, then diffusion of molecules have both
the effects: molecular as well as convective (due to relative motion) i.e. total flux (NA).
Total Flux =
molecular flux + convective flux
NA =
JA + cA.v
cA.vA
= cA(vA
– v) + cA.v
Where vA
is the avg. velocity of all A like species and v is the avg. velocity of
the carrier relative to observer means average of carrier velocity and
velocity of A molecules. In the above example, vA is the velocity of all perfume
particles and v is the avg. velocity of the person and perfume particles. So, the
difference cA(vA – v) corresponds to molecular diffusion and cA.v
corresponds to convective
diffusion. If a person is moving
with perfume put on body and other person is stationary with perfume put on his body, then the difference
between the two fluxes corresponds to convective diffusion. The other example
can be taken of the fragrance particles of air freshener, the particles are diffusing through molecular diffusion and if
air blows, then there will be combined effect of molecular diffusion and convective diffusion.
In distillation column, the flow of liquid and vapor streams leads to molecular as well as convective diffusion.
super cool content....doubts clarified!
ReplyDeleteVery very nice explanation,Sunil sir...thank u
ReplyDeleteGreat simplified text ,to understand
ReplyDelete