How to use user defined functions as physics's input? Thanks!

Hello, every one!
I am learnging to use COMSOL4.2 and want to add AC signal as my device's input.

Just as Eric told me to write an equation as the input.

(www.comsol.com/community/forums/general/thread/34060/#p94106)

I want to try another way.

Because there is a way to define a function( Model bulider--> Global Definition-->Function),

I can define a funciton (name wv1) as wv1= 5 *sin( 50*pi* t ).


My question is, how can I use this function as an AC input to a capacitor's input?

Thanks!