time-dependent source term in Diffusion Equation

[R. Edwin Garcia]

Hi Dan,
I am trying to program a time-dependent diffusion-like equation that  
looks something like this:

\frac{\partial \phi}{\partial t} = \nabla \cdot M \nabla \phi + A*  
\sin (\omega t)

where $\phi$ is the spatially distributed variable, $M$ is the  
mobility, $A$ and $\omega$ are constants, and t is time.

I looked around, but I couldn't find an example: Would you have a  
suggestion on how to add a source term that has an explicit  
(sinusoidal) time-dependence?

Thanks,
Edwin