Capacitors charge and voltage relationship

[physgrl]

Homework Statement

When the voltage across the two plates of a capacitor is increased then:

*a. the charge will increase

b. the charge will decrease

c. the capacitance will increase

d. the capacitance will decrease

Homework Equations

CV=Q

The Attempt at a Solution

I answered that the capacitance will decrease instead of the charge increasing because if the voltage is not constant ie not connected to a voltage source, the charge doesn't change but the capacitance does.

 

[BruceW]

what other rules do you know about capacitance? For example, do you know the equation for the capacitance between two parallel plates?

 

[technician]

The capacitance is determined by the area of the plates, their separation and the material between the plates.
The capacitance is not determined by the voltage or the chatge.
So what 2 quantities do depend on each other ?

 

[physgrl]

but can't the capacitance change with changes in charge and voltage? because I was told that when the voltage is constant charge and capacitance change and otherwise the voltage and capacitance change

 

[physgrl]

or is it like if capacitance changes then the other one changes, but they can both change without capacitance having to change

 

[BruceW]

As technician said, the capacitance of the capacitor is purely geometric, i.e. how it was made. So if you used a different capacitor, then the capacitance would change.

 

[physgrl]

ok, so changes in like distance of plates area etc. will change capacitance and X other characteristic but never the other way around

 

[BruceW]

What do you mean by 'X other characteristics'? But I think the answer is yes. We usually assume that the distance of plates and area etc of the capacitor are not changed.

 

[physgrl]

X as in charge or voltage

 

[BruceW]

Yes, that's right.

 

[BruceW]

If you look on wikipedia, there are more complicated capacitors whose capacitance is voltage-dependent or frequency-dependent. But I think it is usually assumed that the capacitance doesn't change when these things change.