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This is incorrect. Creating a charge gradient in a system increases its mass-energy. In this situation the potential energy is dramatically larger than the rest mass of the precharged person.

Edit in response to your edit: what it shows is that it would take an extraordinary amount of energy to cause the change.

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Isn't that what I wrote? That separating the charge would be equivalent to adding potential energy basically adding to mass of your 70kg body so that it no longer is 70kg?

He's saying that you have to include the energy you put into the system to remove 1% of the electrons in your E=mc^2 calculation.

Yes, it takes over 70kg of "mass-energy" (e.g. mass converted to energy in nuclear fission) to remove 1% of all electrons in a person weighing 70 kg. That is not, in itself, contradictory.

Depending on how relativistically inclined you are, this may affect what you consider the weight of the person. But this doesn't really matter for the thought experiment.

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