Net Charge of Planets and Stars

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Main Question or Discussion Point

The earth is effectively an insulator in space, and the sun continuously beats on the earth with protons, electrons and other particles.

Does the planet earth and all other planets have a net charge, and if so, how can we measure the polarity and extent of that charge?

Turning to the Sun and the stars, do they have a net charge, and if so, how could it be measured?
 

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  • #2
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The earth is effectively an insulator in space, and the sun continuously beats on the earth with protons, electrons and other particles.

Does the planet earth and all other planets have a net charge, and if so, how can we measure the polarity and extent of that charge?

Turning to the Sun and the stars, do they have a net charge, and if so, how could it be measured?

From what i've seen it hasnt been directly mearsured. You'd need to take varying measurements of the E-field produced at varying radi to work out the PD at each point, but I think thats harder to do in practise than it sounds. Its known that neutron stars and "quark stars" possess a spherical electrosphere from their charge, but i'm not sure about planets or the sun. You should beable to see a spherical E-field field surrounding the sun if it had a significant charge, but I dont think we've ever seen that. Theres a few predictions based on electric polarization, basically involving the separation of electrons from their orbits in the gravitational field, but they remain very theoretical at this point, and not many other publications have followed this idea I dont think. They predict ~100 C net charge, which is hardly significant compared to the size of the sun.

This paper gives a good summary (I cant post links yet, just search this paper title) "On the global electrostatic charge of stars" Astronomy and Astrophysics, v.372, p.913-915 (2001)

As was discovered in the nineteen-twenties, a significant electric field exists in the solar corona as well as in the solar interior. This field is a consequence of the tendency of light electrons to segregate from heavier protons in the solar gravitational field. Since the principle is valid for a plasma in every star, the result can be generalized. The presented paper is intended to rehighlight this significant physical property of stars. In particular, we stress that there has to be charge Qr inside a stellar sphere with radius r, which is linearly proportional to mass Mr inside the sphere. Both quantities are related as Qr = 77.043 Mr, if Qr is given in Coulombs and Mr in solar masses. The global stellar electrostatic field is 918 times stronger than the corresponding stellar gravity and compensates for a half of the gravity, when it acts on an electron or proton, respectively. The external electric field has to cause an occurence of electric current and appropriate magnetic field in a highly conductive plasma, when, e.g., the plasma is in a turbulent motion or spirals onto a star in a hot accretion disc.


One thing I read in my physics book the other day, implies that the Earth is pretty much neutral, I'll quote it;

at any moment there are about 2000 thuderstorms taking place on the Earth. In total these are responsible for transferring negative charge down to the ground at 1800A. At this rate the groud should gain -1.6x108 C per day. If this gain in charge was not balanced by a loss of positive, the ground would soon become so charged that lightning strikes would not be possible due to repulsion.

Which implies that the positive out = the negative in, so it will remain largely neutral. Unless theres a delay in this process...
 
  • #3
Borek
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I don't get this thunderstorm argument - these are currents (up and down) that go in the thin atmospheric layer, they shouldn't effect the overall Earth charge, just like electric sparks in the lab don't change electrical neutrality of the building :smile:

Or am I missing something?

Borek
 
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I don't get this thunderstorm argument - these are currents (up and down) that go in the thin atmospheric layer, they shouldn't effect the overall Earth charge, just like electric sparks in the lab don't change electrical neutrality of the building :smile:

Or am I missing something?

No I dont think so, I think thats pretty much it. If there was a net charge, it would probably alternate between positive and negative between each strike. I think that the positive lightning travels upwards (sprites?) and generally negative lightning down. But I cant be sure. There may be a completely different process at work for all I know, and my textbook that I read this from is only A-level grade, so its probably a simplified explanation anyway :)
 
  • #5
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The earth's average electric field at the surface is about 600 V/m. While Borek is right - this doesn't extend to all space, but is canceled by charge in the upper atmosphere, we can use this as an upper limit to the earth's charge imbalance, and it's neutral to about one part in 10^26. That is, one extra (or missing) electron per kilogram of matter.

Most everyday objects are not nearly this neutral.

The extreme neutrality is because of the solar wind. If the earth had a large positive charge, it would preferentially deflect protons and absorb electrons until that charge is canceled out. (And the reverse for a negative charge)
 

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