Capacitor Energy and Charge Relationship

[victorializ]

Homework Statement

If you increase the charge on a parallel plate capacitor from 3μC to 9μC and increase the plate separation from 1mm to 3mm, the energy stored in the capacitor changes by a factor of...

Homework Equations

U = 1/2QV
or 1/2CV^2

The Attempt at a Solution

I'm not sure which equation to even use, I'm not to great when it comes to differentiating between electric field and energy of the capacitor.

I think I should probably use 1/2QV but i don't have a voltage to work with so I'm confused.

 

[dauto]

Use the relation between charge Q, Voltage V, and Capacitance C to eliminate the voltage from the expression for the energy.

 

[victorializ]

Use the relation between charge Q, Voltage V, and Capacitance C to eliminate the voltage from the expression for the energy.

so U =1/2 Q^2/C

i'm still not sure how i would find capacitance without using the C=Q/V and not having a voltage?
also does the distance between the plates changing not matter?

 

[mfb]

The capacitance changes if you change the distance, and you should know a formula that connects the two.

 

[victorializ]

The capacitance changes if you change the distance, and you should know a formula that connects the two.

oh alright i thought that the permitivity of free space had to be used when a dielectric was present only but that works thanks!