# What really happens when charging a capacitor?

• iampaul
In summary, when charging a capacitor, 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. Charges do not move across the gap, but instead move on or off the plates through the wire. The work done in charging a capacitor is based on charges moving against the field between the plates, and can be calculated using the equation U=Q^2/2C. However, this calculation may not take into account the effects of the magnetic field associated with the electric field.
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 ?

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

hi iampaul!
iampaul said:
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? )

## 1. What is a capacitor?

A capacitor is an electronic component that stores and releases electrical energy. It is made up of two conducting plates separated by an insulating material, called a dielectric.

## 2. How does a capacitor store energy?

When a capacitor is connected to a power source, such as a battery, one plate becomes positively charged and the other becomes negatively charged. This creates an electric field between the plates, which stores the energy.

## 3. What happens when a capacitor is charged?

When a capacitor is charged, electrons move from one plate to the other, creating an electric potential difference between the plates. As more charge accumulates, the potential difference increases and the capacitor becomes fully charged.

## 4. Can a capacitor hold an unlimited amount of charge?

No, a capacitor has a maximum charge capacity, determined by its capacitance and the voltage applied to it. If the charge exceeds this limit, the capacitor can become damaged or even explode.

## 5. What happens when a capacitor is fully charged?

When a capacitor is fully charged, it has reached its maximum charge capacity and can no longer store any more energy. If a power source is still connected, the capacitor will maintain its charge until it is discharged through a circuit or some other means.

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