Charging moves charge, never makes it

Charge is not produced. It is separated. When a balloon rubbed on wool comes away at $-3.2$ nC, the wool is at $+3.2$ nC, and the pair still totals zero because it totalled zero before. Nothing about the rubbing manufactured the electrons; the rubbing moved about $2\times10^{10}$ of them across the contact.

Use this as a check on every process, not as a slogan:

1. Draw a box around everything that can exchange charge, including the ground if the system is grounded.
2. Add up the charge inside the box before the process.
3. Add it up after. The two sums must match.

If a story leaves you with a total that changed, the story is wrong, not the law. The commonest place this shows up is an answer where both objects end up with the same sign after they started neutral. That is $-3.2$ nC and $-3.2$ nC out of a system that began at zero, which is $6.4$ nC of charge conjured from nothing.

What moves is always electrons. Protons are locked in nuclei and go nowhere in a solid, so "the rod lost protons" and "the rod gained electrons" are not two descriptions of one event: only the second one happens.

In a conductor, some electrons are free to move through the entire material. Deposit charge anywhere on an isolated conductor and mutual repulsion pushes it apart until it stops rearranging, which puts all of the excess on the outer surface. In an insulator, electrons are bound to their own molecules. Deposit charge on one end of a plastic rod and it sits in that patch, for hours.

Two consequences that get tested constantly:

1. Two identical conducting spheres touched together share equally. Charge flows until neither has a reason to push more across, and identical geometry means an even split. A sphere at $+8$ nC touched to a sphere at $-2$ nC gives $+3$ nC and $+3$ nC, because $(+8 - 2)/2 = +3$. The total was $+6$ nC and it still is.
2. Grounding drains a conductor and barely touches an insulator. Ground a charged metal sphere and its excess charge leaves through the wire. Touch a grounded wire to one point of a charged plastic rod and you neutralize roughly that one patch, because nothing else on the rod can reach the wire.

Before predicting any redistribution, name the material. That single word decides the answer more often than any formula in this unit.

Friction. Two neutral materials are rubbed. One holds electrons more tightly, so it takes them and goes negative while the other goes equally positive. Both objects end up charged, with opposite signs.

Contact, also called conduction. A charged object touches a conductor and they share. The two end with the same sign, and the total is unchanged. This is the only one of the three where the final sign matches the sign of the object you brought in.

Induction. Nothing touches the charged object at all. Hold a negative rod near a neutral conducting sphere: the sphere's free electrons are pushed to the far side, leaving the near side positive. The sphere is polarized, with net charge still zero. Now ground the far side while the rod is still there. The pushed-away electrons leave through the ground. Break the ground connection, then remove the rod, and the sphere is left with a net positive charge, opposite to the rod.

1. Bring the charged object near. Do not touch.
2. Ground the conductor. Charge of the same sign as the nearby object leaves.
3. Remove the ground first, while the charged object is still in place. This is the step people reverse, and reversing it undoes the whole procedure: the electrons simply come back.
4. Now remove the charged object. The remaining charge spreads over the conductor.

Induction with no grounding step is temporary. Take the rod away and the electrons relax back to where they were, leaving the sphere neutral and unpolarized.

A charged rod picks up neutral scraps of paper, and paper is an insulator, so no charge crosses it. The mechanism is polarization on the molecular scale: the rod's field shifts each molecule's electron cloud slightly, so every molecule presents its opposite side to the rod. The charge separation per molecule is tiny and the total charge on the paper stays exactly zero.

Attraction wins anyway for a reason worth stating: the near face is closer, and the force goes as $1/r^2$. Equal amounts of induced positive and negative charge do not give equal forces when one is nearer.

$$F_{\text{near}} = \frac{kq|\delta|}{r_{\text{near}}^2} \ > \ F_{\text{far}} = \frac{kq|\delta|}{r_{\text{far}}^2}.$$

So a charged object attracts a neutral one whatever the sign of the charge, which is why "it was attracted, so it must be oppositely charged" is not a valid deduction. Repulsion is the reliable test: only two objects carrying like charge repel.

In a conductor the same effect is much larger, because entire electrons travel to the near surface instead of a molecule flexing. Same physics, more room to move.