Why does charge stay constant when a dielectric is inserted in a capacitor?

[maiad]

Homework Statement

When a dielectric is inserted in a capacitor, why does the charge remain unchanged while the potential decreases and capacitance increase?

Homework Equations

The Attempt at a Solution

i'm not really about this one, but is it because the charges accumulate on the surface area and since surface area doesn't change after an insertion of a dielectric, the charges won't change since area didn't change? But then again... Capacitance is a capacitors ability to hold a charge in a electric field and it increase so you can see my dilemma lol.

 

[maiad]

O never mind there was no power source ;P but on that note, let's say if there was a power source, how would it change the answer?