Charging Objects

📋 Teacher notes

• •In ALL of these methods, only ELECTRONS ever move. Protons are locked inside the nucleus and never transfer between objects — every "positive charge" you see is really a region that has LOST electrons, not one that gained protons.
• •Charging by FRICTION: rubbing two different insulators transfers electrons from one to the other. Whichever material holds electrons less strongly ends up positive (lost electrons); the other ends up negative (gained electrons) — equal and opposite charges.
• •Charging by CONDUCTION (contact): the rod and the body must actually TOUCH — this is real physical contact, not just closeness. A negative rod has excess electrons that spread onto the neutral body it touches. A positive rod is short of electrons, so it instead pulls electrons OFF the body it touches. Either way, the body ends up the SAME sign as the rod, and the rod’s own charge is reduced.
• •Charging by INDUCTION: the rod comes very close to the conductor — close enough for its field to act — but must NOT touch it, unlike friction and conduction. A negative rod repels the conductor’s free electrons to the far side; a positive rod attracts them to the near side. Earthing while the rod is still present lets electrons escape (negative rod) or draws extra electrons in (positive rod). Once the earth connection and then the rod are removed (in that order), the conductor is left with a net charge OPPOSITE the inducing rod.
• •A key exam distinction: conduction leaves the object with the SAME sign of charge as the charging body; induction leaves it with the OPPOSITE sign — and induction never involves contact at all.
• •The order of steps matters for induction: the earth connection must be broken BEFORE the charged rod is taken away — if the rod is removed first, the separated charges simply recombine and no net charge is left behind.