Capacitance and induction

Discussion in 'General Electronics Chat' started by activee, Jan 16, 2014.

Jan 16, 2014
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Sorry for this novice thread but I've just started learning electronic and I've some problems to understand some concepts and I hope you guys can help me to grasp them, hopefully.

1) In my lesson book it says that the density of charges = D = Q/S (S = area).

Well that makes sens.
Then it says :

Q/S = D = E . ε. I'm not sure I get this last part on so many levels.
First of all I don't see why the substance between two charges comes to play a role in the value of a voltage or an electric field. How I see it at the moment is if two charges are bigger then the voltage is bigger. I don't really get that "ε".

2) Capacitance:

My book says the capacitance = C = Q / U . I just don't get it. First of all voltage must relative between two charges. Here we have Q, but which Q is it ? Any of the two Q of the two metallic plates ? Also for me the word capacitance means : If I have a bottle, the capacitance of that bottle is the amount of water I can put in that bottle before the water starts to come out.
I don't see the link between my definition of capacitance and C = Q / U, I just don't see how this formula can give any info on the capacitance of a capacitor since the Q is supposed to change over time.

( 3) In coulomb's law :
F = k . q.q' / d² with k = 1/ 4πε. Well I understand that there must be a proportionality between the two charges' magnitude and the force and an inverse proportionality with the distance. I don't get what the "k" role is there and how it's calculated. )

I'm really grateful if anyone could try to answer my questions because I'm really confused. My third question is the less important of the three for me tho.

2. alfacliff Well-Known Member

Dec 13, 2013
2,449
432
in the first question, it is dielectric constant. dry air at sea level has a constant of 1. other materials have differeng constants. such as the electrolyte in electrolytic caps causes a much higher caacitance than the plate area and spacing. it is usually "K " in formulas. related to refractive index also.

Jan 16, 2014
39
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No, no you got me wrong. Basically the substance between the two plates will interfere with the electric field. I get it. What I don't understand is how it does play a role and what fundamentally is the permeability.

In other words : If I have two charged objects at a distance of each other there will be a force made upon them. This force is the same no matter what is gonna be between them. But the formula says the permeability ε does play a role: that means that substance between the two objects has to add another force in the process to have the final total resulting force.
How does this force appear on an atomic level ?
I hope I make sens.

4. alfacliff Well-Known Member

Dec 13, 2013
2,449
432
the same minerals and chemicals that affect the refractive index also do the same for the permeability, like tantalum and others added to ceramics. basicly permeability = refractive index = dielectric constant. maybe not the same numbers, but the same effects. add tantalum to glass for a higher refractive index, add to caps for higher dielectric constant (more capacitance for same size).

5. #12 Expert

Nov 30, 2010
18,076
9,678
The stuff in the middle does not contribute force or energy. It modifies how many coulombs per surface area are required to produce a volt of difference across a certain space.

Another way to say that is: given the same surface area and spacing, a different dielectric will make the capacitance change.

Manufacturers don't advertise the dielectric constant of their materials, they just find what is required to make a microfarad and label the finished product as a uf. You will almost never need this number in the future. Still, it belongs in the "basics" file in your head. When you need it, you need it desperately.

6. crutschow Expert

Mar 14, 2008
21,339
6,117
You can view the material between the plates as helping to reduce the electric field resistance from one plate to the other. It would be similar to moving the plates closer together with only air (or a vacuum) between the plates. It's rather like the electrons in the material help couple the field between the plates.

The permeability is a measure of how much the material reduces this field resistance between the plates and increases the capacitance.