Electrical Charge and Electric Fields

[Physicist3]

Hi,

Am I correct in thinking that for a positively charged spherical electrode, the charge is distributed radially from the entire surface of the sphere, and as such, the electric field is distributed in this way too? If an earthed electrode is then placed nearby, a high voltage would be required to cause breakdown as the electric field strength would be quite low owing to the large area over which the charge is distributed?

 

[mfb]

What do you mean with "is distributed radially from the entire surface of the sphere"?

An electrode nearby breaks the spherical symmetry.

a high voltage would be required to cause breakdown as the electric field strength would be quite low owing to the large area over which the charge is distributed?

Low relative to what?

 

[Physicist3]

Hi, basically, I would like to know why the electric field strength between two spheres would be lower than for two parallel plates if the electrode distance was the same and the applied voltage was also the Same? Is this because the concentration of charge between the two spheres would be lower than for the plates? That is the bit I would like some help with

 

[zoki85]

Hi, basically, I would like to know why the electric field strength between two spheres would be lower than for two parallel plates if the electrode distance was the same and the applied voltage was also the Same? Is this because the concentration of charge between the two spheres would be lower than for the plates? That is the bit I would like some help with

E-field between two charged sphere's isn't uniform (E-field between two infinitely big parallel plates is).
Uniform E-field gives the highest electrical strenght to the gap.

 

[Physicist3]

E-field between two charged sphere's isn't uniform (E-field between two infinitely big parallel plates is).
Uniform E-field gives the highest electrical strenght to the gap.

Thanks for the reply. So because the electric field between two spheres is not uniform between two spheres, a higher voltage would be required to achieve the same EFS as for two plated where the EF is distributed uniformly between opposing faces (providing electrode distance is the same). Also, do pin electrodes require an even lower voltage because the EF is concentrated towards the ends of the pins?

 

[zoki85]

So because the electric field between two spheres is not uniform between two spheres, a higher voltage would be required to achieve the same EFS as for two plated where the EF is distributed uniformly between opposing faces (providing electrode distance is the same). Also, do pin electrodes require an even lower voltage because the EF is concentrated towards the ends of the pins?

Yes!

 

[Physicist3]

Yes!

Thank you for your help! :)

 

[jim hardy]

Thanks for the reply. So because the electric field between two spheres is not uniform between two spheres, a higher voltage would be required to achieve the same EFS as for two plated where the EF is distributed uniformly between opposing faces

same EFS where ?

 

[meBigGuy]

I don't understand why the breakdown would be different. It depends on the gap at the closest point, not the nearby physical configuration. The field is based on the potential difference between two points.

From http://home.earthlink.net/~jimlux/hv/bruce.htm
A comment made by Naidu, et.al, in connection with their discussion of uniform field gaps is that there is no significant difference in breakdown voltage between sphere gaps and the various uniform field gaps, given the fairly large uncertainty in all the measurements. In practice, breakdown measurements are made with spherical electrodes, because small changes in the surface of a uniform field electrode can cause field irregularities which in turn dramatically affects the breakdown voltages. Also, a deviation from parallelism of the opposing electrode faces can cause significant deviations from a large uniform field area. Spheres are easier to make and keep smooth, even though there isn't a "nice" analytical solution to the field between the electrodes. Furthermore, if there is a misalignment between the spheres, the region between the spheres is still geometrically the same.

Also, see
http://home.earthlink.net/~jimlux/hv/sphgap.htm
http://home.earthlink.net/~jimlux/hv/spherev.htm

 

[Jeff Rosenbury]

The OP was correct in his original assertion that large spheres are better than small spheres which in turn are better than points to distribute charge and thus prevent insulation breakdown.

Spreading the charge helps. This is why we often see multiple conductors on a single phase on transmission lines. The multiple conductors spread the charge over the geometric mean radius of the conductors. This adds to the capacitance and reduces coronal discharge.

When we place a charge near a conductor it does form an image of the opposite polarity in the conductor by drawing charge carriers (which for simplicity we'll assume are electrons) in the conductor nearer the placed charge. This will affect the resulting electric fields. But I think this is a second order effect. (It might be a third or fourth, I don't know for sure.) All the first order pictures I've seen are like Jim's.

 

[meBigGuy]

I think you are wrong. Corona discharge is not in any way related to breakdown voltage between two charged highly conductive points.

Please read what I posted. Read what the people who use breakdown to measure high voltages have to say on the subject.

Jim's picture shows that the breakdown will be in the straight line between the points. If you had flat surfaces, the breakdown potential would be the same (assuming no corner effects).

 

[Jeff Rosenbury]

I think you are wrong. Corona discharge is not in any way related to breakdown voltage between two charged highly conductive points.

Please read what I posted. Read what the people who use breakdown to measure high voltages have to say on the subject.

Jim's picture shows that the breakdown will be in the straight line between the points. If you had flat surfaces, the breakdown potential would be the same (assuming no corner effects).

It is my understanding that coronal discharge happens when the voltage is high enough to ionize the air around an object, but not high enough to ionize a complete path between the object and ground/another electrode.

Looking at Jim's figure, it is clear the field strength is highest near the surface and drops off with distance. So it is easy to see that breakdown near the surface could occur while not further from the surface. To me, that's what coronal discharge is.

As such it is related to, but not the same as a breakdown voltage between two charged highly conductive points.

One notable difference is that coronal discharge is a cold plasma caused solely by electrical action, while a breakdown path quickly heats and can sustain itself after the voltage drops (as long as the current keeps the heat up anyway.).

 

[nsaspook]

In this video of mine you can see (if you slow it down to 1/4 speed and look closely) around the annotation box the coronal discharge that builds then the electrical breakdown between a connection tab incorrectly attached outside the Corona ring and the process repeating.

Sound effects added.

 

[meBigGuy]

Sorry, I sort of miss spoke. Obviously coronal discharge is part of the breakdown process. The subject area of reducing coronal discharge into the atmosphere and the breakdown voltage between two plates are two different aspects of the science involved.