Differentiation

Hi all,

Is it possible to differentiane an expression in variable field itself.

I have defined a variable: es.modE = (es.Ex)^2+(es.Ex)^2+(es.Ex)^2 (where es.Ex is x component of electric field etc..)

now, I need to define another variable, lets say : f_DEP = partial derivative of (es.modE) wrt x.

how can i express f_DEP in the variable field. I am using COMSOL4

plz reply asap