Adding a dielectric to capacitors

[Physics197]

Homework Statement

Fig. 1 and 2 show a dielectric slab being inserted between the plates of one of two identical capacitors, capacitor 2. Select the correct answer to each of the statements below (enter I for `increases', D for `decreases', or S for `stays the same').

1. In Fig. 2, the potential difference between the plates of capacitor 1 _______ when the dielectric is inserted.
2. In Fig. 2, the charge on capacitor 2 _______ when the dielectric is inserted.
3. In Fig. 2, the capacitance of capacitor 1 _______ when the dielectric is inserted.
4. In Fig. 1, the potential energy stored in capacitor 2 _______ when the dielectric is inserted.

http://i.imagehost.org/view/0204/prob45

Homework Equations

The Attempt at a Solution

1. Stay the same - would it affect capacotor 1
2. stay the same, wouldn't the charge still be the sdame on the plates
3. stay the same - like 1
4. decrease - adding a dielectric decreases the potential, doesn't it

 

[Mindscrape]

I agree that the first capacitor shouldn't change. Rethink your answers to the other two.

 

[Physics197]

For 2, would the charge decrease because the electric field decreases (due to the dielectric).
and for 4,
would the potential increase, because the capitance would increase (C is proportional to K), and the energy is 0.5C(V)²

 

[Mindscrape]

So from circuits you know that all the elements in parallel will always have the same voltage. Given that the voltage of the capacitor with the dielectric remains the same, what must happen to the charge?