Quick conceptual question about disconnecting battery

[xchococatx]

If there is a parallel plate capacitor, connected to a battery, which is then disconnected, there is NO voltage now, but "Q" (charge) on the capacitor remains the same. YES OR NO? I think yes.

 

[gneill]

If there is a parallel plate capacitor, connected to a battery, which is then disconnected, there is NO voltage now, but "Q" (charge) on the capacitor remains the same. YES OR NO? I think yes.

What is your reasoning?

 

[xchococatx]

Well basically I'm thinking that a capacitor stores charges (or energy) so, onces its all charged up by a battery, I don't see how it could lose those charges...even when the battery is disconnected. (immediately anyways, it wouldn't be all gone I don't think) Why I was thinking about this was because if the battery is now disconnected, and you wanted to calculate the work needed to insert a dielectric, fitting perfectly between the 2 plates, since there is no voltage, I don't see how you could calculate the work unless you used the original charge of the capacitor (without dielectric). (By means of E=Q^2/2C) So in order to use the original charge of the capacitor, I figured charge has to stay in the capacitor even if the battery is disconnected.

 

[gneill]

Well basically I'm thinking that a capacitor stores charges (or energy) so, onces its all charged up by a battery, I don't see how it could lose those charges...even when the battery is disconnected. (immediately anyways, it wouldn't be all gone I don't think) Why I was thinking about this was because if the battery is now disconnected, and you wanted to calculate the work needed to insert a dielectric, fitting perfectly between the 2 plates, since there is no voltage, I don't see how you could calculate the work unless you used the original charge of the capacitor (without dielectric). (By means of E=Q^2/2C) So in order to use the original charge of the capacitor, I figured charge has to stay in the capacitor even if the battery is disconnected.

Okay, your reasoning is pretty good. Note that the charged capacitor will maintain a voltage (potential difference) according to Q = C*V, even without an external voltage source applied. Of course, if the capacitance changes (due, say, to the insertion of a dielectric) the voltage will change accordingly if the charges have nowhere to go.

 

[asteriatic]

Yes, your understanding is correct. When a battery is disconnected from a parallel plate capacitor, the voltage across the capacitor drops to zero, but the charge on the capacitor remains the same. This is because the charge on a capacitor is stored on the plates and is not affected by the presence or absence of a voltage source. The charge will only change if there is a change in the capacitance or if the capacitor is discharged.