Capacitor's dielectric - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: October 31, 2017, 08:01:59 pm
content: Could someone explain, in layman's terms, what happens inside a capacitor ?The way I understand it isWe have two metal plates separated by some dielectric material, even air. We apply a voltage to the two plates from a power source. There is a number of electrons that leave the power source's terminal and travel through the conductors to the capacitor's plate. The same number of electrons move inside the dielectric and towards the other plate, and from there travel to the other source terminal. As far as the source is concerned it saw a number of electrons leaving its one terminal and returning to the other terminal. As far as the capacitor is concerned it grabbed a number of electrons from the source which all accumulated on one plate and part of the dielectric, and gave the same number of electrons from the other part of the dielectric back to the source. So the capacitor now is charged which means there are more electrons on one side of the dielectric than the other, and very slowly they are balancing out but only very slowly so the capacitor takes a long time to discharge internally.The way I also understand this is that there is a movement of electrons inside the dielectric towards which ever direction when external pressure is applied to the plates.Imagine the dielectric to be a thin strip of some material with two plates on either side. I could place an ammeter somewhere in the middle of the material. If I now start charging the capacitor I should see the ammeter registering the flow of electrons ? Just like a resistor?Also, Imagine the same strip of material with two plates on either side. I charge the capacitor say at 10V. Say the material is 10cm long. Would I then measure 1V/cm across the dielectric at every random point I would care to test?

author: Cerebus
author_rank: Super Contributor
timestamp: October 31, 2017, 09:26:23 pm
content: You really need to go and hit a good physics textbook for this, it's too much to cover here. However:Forget dielectrics for a bit. You can make a capacitor with just two parallel plates in a vacuum - that is, with nothing at all between the plates. The basic behaviour of a capacitor is due to the accumulation of charge on the plates of the capacitor. That can bemodifiedby the presence of a dielectric, but the key thing is that a dielectric is not necessary to the creation of a capacitor.I'd suggest a decent physics text book at higher school or 1st year undergraduate level for the details.Or if you prefer, you can sit through the first few hours of Walter Lewin's undergraduate physics electricity and magnetism course here:https://www.youtube.com/playlist?list=PLyQSN7X0ro2314mKyUiOILaOC2hk6Pc3jI strongly suggest that you do not simply skip to the lecture about capacitors but follow through the course from the beginning unless you already properly understand electrostatics, in particular the relationship between voltage, electric fields and charge. Without those key concepts understanding capacitors will be very hard.

author: Fludo
author_rank: Regular Contributor
timestamp: November 01, 2017, 01:24:46 pm
content: The plates of the capacitor are conductors which allow electrons to distribute freely over the surface. When voltage is applied to a capacitor there is a charge build up on the plates that generates an electric field . The dielectric is a special material which becomes polarized by the electric field, but is an insulator to electric current. The molecules of the dielectric are aligned to the electric field of the plates and add to the overall size of that field.In a perfect capacitor there is no current from one plate to the next. Current is transferred through capacitors by magnetic induction, which requires a changing voltage. This is why capacitors are able to block DC while passing AC in filter circuits.There are microscopic holes in the dielectric which allow electrons to jump from one plate to the other and equalize the charges over time. The dielectrics become conductors at higher voltages when the electrons have enough attractive force to tunnel through.