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2019-09-18, 14:35:26
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Author Topic: Tossing Ideas Around  (Read 2379 times)

Group: Tinkerer
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Posts: 3095
tExB=qr
I have a couple of questions regarding capacitors:

1. If "displacement current" crosses from "one plate to the other", why does the other plate have to be connected to the other side of the circuit?

2. If I measure the voltage between either plate and ground, what voltage reading do I get?
   
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@Grumpy
Quote
1. If "displacement current" crosses from "one plate to the other", why does the other plate have to be connected to the other side of the circuit?

I don't know, if you were me and I was me and I was you who was me and we were them then why does a south african striped Koala fly up in a south by north western eastern?

Regards
AC


---------------------------
"Great minds discuss ideas; average minds discuss events; small minds discuss people." - Eleanor Roosevelt.

Be careful when you blindly follow the Masses... sometimes the "M" is silent.
   
Group: Guest
Quote
If "displacement current" crosses from "one plate to the other", why does the other plate have to be connected to the other side of the circuit?

Because there would be no displacement current if the other plate was not connected.

Quote
If I measure the voltage between either plate and ground, what voltage reading do I get?

I'm not sure that I understand the question, but if the other plate is not connected, then an ideal voltmeter (infinite impedance) would measure the same voltage on both plates.
   
Group: Guest
...
1. If "displacement current" crosses from "one plate to the other", why does the other plate have to be connected to the other side of the circuit?

You must have an electric field between the plates, therefore a potential difference. The electric field slightly shifts the electron couds from their normal position around the positive charges of atomic nuclei of the dielectric. This is a real motion of charges. The electron clouds being no more exactly centered around the positive charges, each atom constitutes an electric dipole and the displacement current represents this relocation of charges. If the dielectric is the vacuum, the electron/positron pairs that are created at random due to the quantum fluctuation, play the same role as the atomic charges of the dielectric. This explains why the permittivity ε0 is not null in vacuum, allowing also displacement currents.

Quote
2. If I measure the voltage between either plate and ground, what voltage reading do I get?

This voltage is not defined. It doesn't depend on the charge of the capacitor. It is as if you had additional real capacitors connected between each plate and the ground. You have a circuit, the voltage depends on the voltage sources that can exist along this circuit, and these additional capacities depend on the distance between each plate and the ground, on the geometry of the conductive surfaces facing one another, and on all other conductors that are near the capacitor and can be more or less connected to the ground and disturb the electric field. In practice, the magnitude is on the order of pF.

It is to be noted that there is strictly no relation between the value of the capacitor which is the capacity between plates, and the capacity between each plate and the ground or the conductors in the surrounding. It is like several capacitors in series: each one must be calculated independently. In practice, all generators and electronic circuits present a capacity relative to the ground, so it is extremly difficult to modelize a real circuit when these capacities are not negligible, which is always the case at high frequencies, due to the fact that even some tenth of pF present a significant impedance at these frequencies.

   

Group: Tinkerer
Hero Member
*****

Posts: 3095
tExB=qr
1. If "displacement current" crosses from "one plate to the other", why does the other plate have to be connected to the other side of the circuit?

I don't have time for lengthy explanation, but I don't think that displacement current is necessary to describe a capacitor.  Turns out that there are other theories that do not use it at all.  I'm preferring waves over particles too. I asked this because I was having a hard time accepting displacement current.

2. If I measure the voltage between either plate and ground, what voltage reading do I get?

I have found that you can use a device called a vibrating capacitor probe to determine the potential of object relative to ground without contact.  The negative plate of a capacitor really does have a negative potential relative to ground.
   
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