Capacitors charge and voltage relationship

1. Jan 31, 2012

physgrl

1. The problem statement, all variables and given/known data

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

2. Relevant equations

CV=Q

3. 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.

2. Jan 31, 2012

BruceW

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

3. Jan 31, 2012

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 ?

4. Jan 31, 2012

physgrl

but cant 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

5. Jan 31, 2012

physgrl

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

6. Jan 31, 2012

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.

7. Jan 31, 2012

physgrl

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

8. Jan 31, 2012

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.

9. Jan 31, 2012

physgrl

X as in charge or voltage

10. Jan 31, 2012

BruceW

Yes, that's right.

11. Jan 31, 2012

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.