I am not sure how to program per time step, because I think in general COMSOL chooses it's own time steps. What is your variable T? I can immagine that A(T) would have to depend on the size of your time step, so is T the size of your time step? I would linearize A(T), so it would be approximately equal to A(0)+T*(dA/dT)_0, which always holds if your time step is sufficiently small ((dA/dT)_0 is dA/dT calculated at T=0). Then you would have
(F(t+T)-F(t))=A(T)=A(0)+T*(dA/dT)_0
dividing both sides by T you get
(F(t+T)-F(t))/T=A(0)/T+(dA/dT)_0
The left hand side is the definition of the differential, so you would get
dF/dt=A(0)/T+(dA/dT)_0
Which shows that A should approach 0 when your time step approaches 0, otherwise A(0)/T will be infinite. So A(0)=0 so
dF/dt=(dA/dT)_0
I would say that such a differential equation can be implemented somewhere in your model.
(F(t+T)-F(t))=A(T)=A(0)+T*(dA/dT)_0
dividing both sides by T you get
(F(t+T)-F(t))/T=A(0)/T+(dA/dT)_0
The left hand side is the definition of the differential, so you would get
dF/dt=A(0)/T+(dA/dT)_0
Which shows that A should approach 0 when your time step approaches 0, otherwise A(0)/T will be infinite. So A(0)=0 so
dF/dt=(dA/dT)_0
I would say that such a differential equation can be implemented somewhere in your model.