Revise: Electricity
Electric charge, Coulomb's law, the atomic origin of electricity, conductors, insulators and semiconductors, and how objects become electrified.
Like charges repel; unlike charges attract.
A charged balloon sticks to a wall by induction.
Q = ne. Charging = electron transfer, never creation.
5×10¹² electrons → Q = 8×10⁻⁷ C.
Conductor: free electrons. Insulator: bound electrons.
Copper core (conductor) + plastic coating (insulator).
Friction, conduction, induction — three ways to charge an object.
Lightning: friction charges storm clouds.
A neutral object has equal positive and negative charge.
Two negative charges always repel.
Electric force is far stronger than gravity for the same-scale objects.
+4μC & +5μC, 0.2m apart → F = 4.5 N.
Atoms: positive nucleus (protons, neutrons), negative electrons around it.
Q = −3.2×10⁻¹⁸ C → 20 excess electrons.
Semiconductors have controllable conductivity — basis of electronics.
Silicon chips power every computer and phone.
Induction needs no contact; conduction needs direct contact.
A nearby charged rod redistributes charge on a metal sphere.