If the earth is a bit negatively charged, it is understood that it can take away the positive charges of a positively charged body when grounded. Why does it take away the extra electrons of a negatively charged body when grounded?
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$\begingroup$ Earth is a huge ocean of electrons. Adding or taking a spoonful electrons from this vast sea doesn't bother it . $\endgroup$– user36790Commented Oct 15, 2014 at 7:49
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$\begingroup$ The amount of charge that can be stored on "Earth" depends entirely on the geometry of the second conductor. The planet as a whole has a rather small capacitance relative to infinity, which is on the order of 70uF. A 1C charge would raise the potential of the planet by approx. 14kV. In comparison, we can store many times as much charge in relatively small capacitors at far lower voltages by decreasing the distance between electrodes. $\endgroup$– CuriousOneCommented Oct 15, 2014 at 8:23
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$\begingroup$ What I want to know is why does it not repel electrons from a negatively charged body when in contact with it? $\endgroup$– ThePhysicistCommented Oct 15, 2014 at 8:28
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$\begingroup$ note that positive charges in solids do not move. It is the lack of electrons that creates a positive chanrge. $\endgroup$– anna vCommented Nov 23, 2014 at 7:57
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2 Answers
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If the earth is a bit negatively charged, it is understood that it can take away the positive charges of a positively charged body when grounded. Why does it take away the extra electrons of a negatively charged body when grounded?
This is because of difference in charge density (charge per unit area) between the charged body and charged earth. The charge density is always more in the body than in the earth and hence charges always flow from the body to the earth - be it $+ve$ charge or $-ve$ charge.
The low charge density of earth is because of very high area compared to the area of the body being earthed.
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Earth doesn't suck the electrons. The electrons in the negatively charged body repel each other and push electrons out of the body into Earth until both are at the same potential. It's like two balloons at different pressure connected with a tube and a valve. When you open the valve the lower pressure balloon doesn't want the higher pressure gas. It repels it. But the pressure is too strong and so gas flows from the higher pressure balloon to the lower pressure balloon until both have equal pressure.
The electrons in the negatively charged body are closer to each other than in Earth. When the body is grounded the electron nearest to Earth is repelled more from its closer neighbour electrons in the body than from the further away electrons in Earth, so it moves towards Earth. This flow continues until the forces on the electrons are equal from all sides.
