Thanks for your effort.

What  i want is to solve an unsteady state ode (du/dt=c, say c is a dependent variable in the subdomain next to the boundary) at the boundary of a geometry.

I have written a n weak form in the boundary setting using boundary weak form.

But it is giving an error failed to evaluate variable Jacobian.

Please read this and comment on this.

thanks

Vivek

If I'm getting it right... I think you should do a component coupling operator over that boundary an evaluate the desired variable, then activate the advance physics option and add an ODE, there you can add a name of the variable (u) and define the ODE as ut-c=0

Thanks all for your kind support.

Do you have any idea about  how to write species balance equation in weak form in comsol? with dirichlet boundary condition and indulation boundary condition.

Thanks

Vivek

I am solving Drift diffusion using PDE in comsol but getting negative value of co concentration?

Thanks

Hi Avinash Gupta , if this is occurring only at the beginning then use segregated approach and limit the minimum concentration to 0 (using segregated approach limit node). Make sure other things are correctly defined beforehand.

Thanks, Noman, I am trying what you recommended