# Require clarification on how charging a capacitor works

1. Aug 15, 2014

### needingtoknow

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

When it comes to charging a capacitor I'm under the impression that when a parallel plate capacitor is hooked up to a battery, that electrons simply flow from the negative terminal to the plate that is connected to the negative terminal, causing that plate to become the "negative plate" and electrons that are on the plate connected to the positive terminal will flow towards the positive terminal making that plate the "positive plate". This process occurs until the voltage of the capacitor matches the voltage of the battery. This is basically what the diagram that I attached is trying to explain.

I have two questions with respect to this explanation:

1. When I was reading up on charging a capacitor, I read that work is required to transfer charge from the positive plate to the negative plate and that transfer of electrons from one plate to another is what charges the battery, clearly not what I explained above in the first paragraph. I am obviously missing something here. I would appreciate your help in clarifying the process of charging a capacitor.

2. There is a space between the first plate and the second plate, which means the circuit is incomplete UNLESS what is happening is that electrons are being transferred from the "negative plate" to the "positive plate". Is that true?

Thank you very much!

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2. Aug 15, 2014

### needingtoknow

Actually I think when my book said that the electrons were being transferred from the positive plate to the negative plate they meant indirectly by travelling all the way around the voltage source rather than simply crossing the capacitor (b/c I now realize that that is unlikely due to the presence of the dielectric)

3. Aug 15, 2014

### phinds

Yes, if electrons flow across the dielectric, that is called breakdown and is a bad thing.

4. Aug 15, 2014

### BiGyElLoWhAt

Not necessarily, it depends what you're trying to do. I don't really have a specific example for when breakdown in a capacitor would be useful, but I personally have reverse biased many-a-diode, operated them at breakdown, and used them as photo detectors. Definitely not a bad thing there.

5. Aug 15, 2014

### needingtoknow

Dielectric capacitors are also better this way right because they prevent less electrons from passing across from one plate to the other, causing a greater build up of charge?

6. Aug 15, 2014

### phinds

I don't know what you mean. What is a "dielectric capacitor" as opposed to any other capacitor? ALL capacitors have a dielectric, whether it is air, wax, oil, vacuum, ceramic, or tiny little green men holding the plates apart. There is SOMETHING between the two plates and that is called the dielectric.

7. Aug 15, 2014

### phinds

I find it hard to believe that dielectric breakdown in a capacitor could ever be a good thing.

8. Aug 15, 2014

### AlephZero

That is also misleading, if it gives you the wrong idea that the electrons move at high speed all the way round the circuit. For typical components that you would find in battery powered electronic circuits, each individual electron will only move a fraction of a millimeter when you charge the capacitor. You are not moving electrons all the way from one plate to the other. You are just pushing the electrons on one plate slightly closer together, and pulling the electrons on the other plate slightly further apart, compared with the positive charges which can't move around because they are part of the nuclei of the atoms in the plates.

9. Aug 15, 2014

### needingtoknow

My mistake, you are right phinds I meant a dielectric insulator such as glass or rubber. That is actually pretty interesting AlephZero, so the electrons only move a fraction of a millimeter? I've always thought the opposite. I will definitely look into that now thank you!

10. Aug 15, 2014

### phinds

Yeah, lots of folks find that surprising. I forget the exact speed but it is something on the order of inches per hour. REALLY slow. It's called "drift velocity". CURRENT flows really fast so it's confusing.

11. Aug 15, 2014

### Staff: Mentor

I suppose one might argue that lightning arresters / overvoltage protectors make use of dielectric breakdown in the form of a capacitor of sorts. Also glow discharge lamps for another example. Of course we don't call these things capacitors because they're intended for another purpose, but they've still got a capacitor-like structure at their heart.

12. Aug 15, 2014

### phinds

That's fair enough. I was thinking more of traditional caps in an electronics circuit.

13. Aug 15, 2014

### AlephZero

Dielectric breakdown in a liquid or gas is usually "self-healing" when the voltage source is removed, unless it starts a self-sustaining chemical reaction, e.g. the fluid catches fire. Breakdown of a solid dielectric often causes some irreversible change, e.g. cracking of the solid material.

14. Aug 16, 2014

### Staff: Mentor

Consider parallel plates separated by a vacuum. Because the plates are in such close proximity, if an electron is added to one plate its electric field line "reaches across" the gap and repels exactly one electron off the other plate. So to an outside observer, it may appear that the electron has jumped the gap, but nothing could be further from the truth. Only field lines span the gap, electrons themselves don't jump across.

The presence of a material in the gap changes things somewhat, having the effect of magnifying the capacitance by its material constant $\epsilon$r.