weak form of transient heat conduction in solids

Hello all,

I'm trying to come up with a transient weak formulation of heat conduction through solids. The equation of interest is:

rho.C.dT/dt + del(q)=Q

I have already successfully implemented the steady state part (subdomain) of the heat equation in Comsol's Physics Interface Builder as k.del(T).test(del(T)) which was verified by Comsol's "ht"-module. However, I have trouble in defining the weak equation for the transient part (rho.C.dT/dt).

rho.C.Tt gives me an error, saying Failed to evaluate variable Jacobian. - Variable: Tt
rho.C.d(T,t) does not give me any comparable solution to Comsol's "ht"-module

Does anyone know what the weak form of the above equation is?

Many thanks in advance