surface exchange b.c.
 

I have done a number of models with the built in physics modes of Comsol 3.3 Now I need a special bc that's not builtin and need some advice.

For a 1-d model, the bc (boundary condition) is for diffusion flux determined by a surface exchange coefficient:

j=k (c1-c2*B)

where j is the flux (=-Ddc/dx on left and right of the boundary) k is const c1 is concentration on left of boundary c2 is concentration on right of boundary B is const

I thought of exploiting the builtin boundary equations where one has

n(N1-N2)=N0+k(cb-c)

and setting cb appropriately, but this isnt right as my bc is still conservative (what flows out of one side flows into the next) so n(N1-N2)=0.

The boundary concentrations are apparently available thru the 'up' and 'down' functions to access the values of c just before and after a boundary , how do i define a bc to use these? Do I need to use 'weak form' if I only want to set the tangential derivative j=k (c1-c2*B)=-D dc/dx

Also is there a spherical symmetry model setup?