What really happens when charging a capacitor?

[iampaul]

What really happens when charging a capacitor??

Do the charges move from plate to plate or
do the charges in the wire only move towards the plates----> "During charging electrons flow from the negative terminal of the power supply to one plate of the capacitor and from the other plate to the positive terminal of the power supply."
http://www.schoolphysics.co.uk/age1...ext/Capacitor_charge_and_discharge/index.html

I have read in my textbook that the work done in charging a capacitor is based on charges moving against the field between the plates and U=Q²/2C. How can i compute for U using the 2nd ?

 

[sophiecentaur]

Charges don't move 'across the gap' or that would be conduction / a short circuit.

When a source of PD is connected to the capacitor, an excess of electrons will turn up on the negative plate and there will be fewer electrons on the positive plate (same numbers in each case because there is still the same 'zero' charge on the whole circuit). The amount of this charge imbalance (Q) will depend upon
1. The applied voltage (V)
2. The area of the plates
3. The spacing between them and the nature of the insulating material between them (dielectric)

2 and 3, together, go to make up the 'Capacity' (C) of the capacitor.

Charges will flow from the supply until the voltage across the capacitor is the same as that of the supply (equilibrium). Note: there will always be a finite resistance in the circuit so this flow of charge will take a finite amount of time to flow.

This is described by the very simple equation
Q=CV

 

[tiny-tim]

hi iampaul!

Do the charges move from plate to plate or
do the charges in the wire only move towards the plates----> "During charging electrons flow from the negative terminal of the power supply to one plate of the capacitor and from the other plate to the positive terminal of the power supply."

no charge moves across the gap (unless the breakdown voltage is reached, in which case it ceases to operateasa capacitor)

the charges move on or off the plates via the wire

… the work done in charging a capacitor is based on charges moving against the field between the plates and U=Q²/2C. How can i compute for U using the 2nd ?

you wouldn't, you wouldn't know the details of the field that way

however, you do know that a purely electric field is conservative, which means it has a scalar potential (a general electromagnetic field has a vector potential also), and the work done is independent of the path taken

so it doesn't matter that the electrons go the long way round …

the work done is the same as if they had jumped the gap! ​

(what i don't know the answer to: there's usually a magnetic field associated with the electric field … why doesn't that make it slightly non-conservative, and slightly spoil the calculation? … anyone? )