Point Charge

Hello all

I want to define point charges (lets say some circles as positive Or negative charges) in Comsol. Can anybody give me some idea how i can do this? what I have in my mind is:

define a constant as Q and equal that to a number (like 1) and then place Q or -Q as the value in space charge density when defining material properties.

However I face in to some problems:

1- I cam only find surface charge density (and not space) in 3d.
2- when i choose Q as surface charge density it seems it doesn't work

anybody have any idea?

Bahar