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Potential of a body is defined as the amount of work done in bringing that body from infinity up to some point divided by its charge.

The Earth has zero net charge, so the work done in bringing any charge from infinity till the Earth is zero.

So what do statements like, "The potential of this body wrt Earth is +5V" or "This positively charged body has positive potential and on grounding, the charge flows into the ground to make it's potential zero" mean ?

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The potential of this body wrt Earth is +5V

That means to move an electron from the body to the Earth requires an energy of $5$ electron volts.

Strictly speaking setting the potential to infinity is just a choice of gauge, but it's a common choice of gauge so let's go with it. In that case you can conceptually describe the movement of the electron as moving it from the charged body to infinity, then from infinity back to the Earth. The first step takes $+5$eV per electron since that's what its potential of $+5$V means. The second step takes no energy, so the total energy taken to move the electron from the body to the Earth is $+5$eV. Hence the potential difference between them is $5$V.

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  • $\begingroup$ What I don't understand is how is this possible. Since the Earth is uncharged, the work done in moving a unit positive charge from infinity to the Earth is zero hence it's potential should be zero $\endgroup$ – xasthor Aug 15 '17 at 7:44
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    $\begingroup$ @xasthor: yes, that's correct. But it takes energy to pull the electron off the object that has a potential of $+5$V. $\endgroup$ – John Rennie Aug 15 '17 at 7:56

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