What is the value of charge of a body that carries 20 excess electrons ?

The elementary charge ... positive on a proton, negative on an electron ... is

               1.6 x 10⁻¹⁹coulomb.

If a body has 20 electrons too many, then its net charge is 20 times
that much ...

             20 x (1.6 x 10⁻¹⁹) = 3.2 x 10⁻¹⁸ coulomb .

And of course, that net charge is negative.

(When written out in all numbers, that charge looks like this:

                       - 0.000 000 000 000 000 003 2 coulomb

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