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The Earth carries a negative electric charge of roughly 500 thousand Coulombs. Does that mean the Earth is negatively charged?

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By definition, a thing with negative electric charge is negatively charged, so the question as asked is answered "yes, tautologically."

Of course, this is referring to the charge of the planet sans atmosphere. The atmosphere has a nearly equal opposite charge, and the charge separation between the ground and the ionosphere is what enables things like lightning. See Wikipedia or one of its references.1

The Earth+atmosphere system is more neutral,2 since any imbalance would tend to cancel over time without something driving it, and other than the solar wind there isn't much out there to pull positive or negative charges preferentially away from the planet as a whole.

1 Unfortunately that in turn references lecture notes that have since disappeared. If anyone knows of online notes on this, feel free to link them.

2 See the related question What is the net charge of the Earth?

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The net charge of naked earth it about $-10^5 \; \mathrm{C}$, but if we include the charge of its nearest atmosphere it would be $-1 \; \mathrm{C}$.

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    $\begingroup$ Minor comment to the post (v4): In the future please link to abstract pages rather than pdf files, e.g., vixra.org/abs/1008.0071 $\endgroup$
    – Qmechanic ♦
    Commented May 8, 2014 at 19:08
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You are partly right.The Earth surface is negatively charged. According to the charge-neutrality principle, the electric charge of the whole Earth is ZERO. The reason why the surface of the Earth is negatively charged remains to be clarified. It probably because at the inner core of the Earth, the temperature and pressure is so high that the atoms there are ionized. So the inner part of earth is posstively charged. Therefore, its surface is negatively charged.

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    $\begingroup$ An ionized plasma is just as likely to be neutral as anything else in the universe - moreso, in face, as any nonzero conductivity will tend to reduce charge separation. $\endgroup$
    – user10851
    Commented Dec 29, 2013 at 9:47
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    $\begingroup$ Citing bulk charge neutrality is dissatisfying since charge deposition from the solar wind and a solar photoelectric effect could continuously pump charge onto or off of Earth, bringing the Earth's total electrical charge to some equilibrium. It's recently been found that Venus has a large net electrical charge sufficient to electrostatically lift oxygen from it's ionosphere end expel it from the planet. space.com/… $\endgroup$
    – Schroeder
    Commented Jul 28, 2016 at 16:54

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