How to use parameters between different physics?

Hello, my name is Wang Jiujiang from University of Macau.
I am using COMSOL to simulate a capacitor, after I simulated
using DC voltage, I want to add AC voltage.

After I add a physics--circuit and then 'voltage source', I cann't
add the source to the driving voltage of capacitor.

Can anyone give me some hints for this problem?

Thanks !

Wang Jiujiang