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Can someone explain this to me?

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jonathand1000

New Member
Hi,
Just re-reading some basic stuff in books, and came across this passage (about a capacitor charging):

"If [x happens], a positive potential is applied to the upper plate of the capacitor; this causes the orbits of the electrons in the molecules of the dialectric to become eliptical so that they orbit nearer to the upper (positive) plate of the capacitor. The movement of electrons in the dielectric repels electrons away from the upper plate and into the external circuit; the movement of these electrons constitutes current flow in the circuit"

OK, the bit I don't understand is the bit about the eliptical electron orbits. If electrons are flowing into the 'upper' plate of a capacitor, surely this would *repel* electron orbits in the dielectric? I thought that the reason a capacitor stopped charging and current then flowed 'over the top of it' was to do with the balance betwen repulsion of electrons already stored on the positive plate and emf.

Actually, it would really help if someone could explain to me what an electrical flux is. That would be great if you could. I have never really got the hang of what flux is except that one is established in the dielectric when voltage is applied over a capacitor.

Thanks alot in advance,
Jon
 

bmcculla

New Member
You're forgetting that electrons have a negative charge. So if there is a positive potential on the upper plate it means that electrons have been pulled away from that plate by the whatever is causing the potential- leaving a positive charge.

The electron in the dialectric are attracted to the positive charge on the upper plate and repelled by the negative charge on the lower plate. I wouldn't worry about the eliptical orbits(probably a simplification from what quantum mechanics says will really happen). The important thing you need to know about dialectrics is that each molecule gets polarized by the charges on the plates. So each molecule has a small - charge on the +plate side and a small + charge on the -Plate side. This polarization allows more total charge to be stored in the capacitor.

The reason charge stops flowing into a capacitor is that the potential between the charges on the + plate and negative plate grow as more electrons are moved from the + to - plate. The potential opposes the potential causing the charging until there is no potential difference between the plates (Vapplied - Vcap = 0).

Flux is the flow of something through an imaginary surface. In this case its the electric field lines passing through an imaginary surface between the plates. Flux is a just a mathematical construct that us used to measure the electric field not a physical quantity itself.

Hope this helps
Brent
 

jonathand1000

New Member
Hi,
thanx alot...I think I was just getting confused by the positive voltage in respect to conventional/actual flow... :( :oops: Haha, I would *never* think an electron had positive charge!!! I was actually reading this stuff in order to expand my knowledge of it a bit further into quantum mechanics, just getting back an absolutely firm basis before going into detail.

Oh, and thanx very much foir the explanation of flux tho, I've been reading this term for ages but never really understanding it. Haha, one book I have helpfully types 'electronic flux' in italics in the middle of a sentance, obviously meaning you to glean lots of information from the different typeface... :roll:

Cheers,
Jon
 
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