The paper is passed near a pool of liquid ink with the opposite charge.

If a cloud comes near an opposite charge, an enormous spark may result.

When a cloud comes near the earth, an opposite charge is induced on the earth's surface.

Bringing opposite electric charges near each other lowers the potential energy of a system.

These are then attracted to each other by virtue of their opposite charges.

In other words, how do you know they are of opposite charge?

It has the same mass but opposite charge of its normal matter counterparts.

Therefore the opposite charges induced on the inside and the outside must be equal in size.

A positron is nothing but an electron with opposite charge, true?

Fundamentally, it is the mirror image of matter but with an opposite electrical charge.