Static electricity · ⁨静电⁩

Sparks and sticky balloons

• Rub a balloon on your hair and it sticks to the wall; lightning is the same effect, in giant form.
• Both come from static electricity 静电 — charge 电荷 that has built up and is not moving.
• It all comes down to tiny electrons 电子.

Electric charge

• There are two kinds of charge: positive and negative.
• The rule is like magnetic poles: same charges repel, different charges attract.
• Charge is measured in coulombs 库仑 (C).

Charging by friction

• Rub two different materials together and electrons (tiny negative particles) move from one to the other.
• The material that gains electrons becomes negative; the one that loses them becomes positive.
• Only the negative electrons move — the positive charge stays put.
• Example: a plastic rod rubbed with a cloth becomes negative (it gained electrons); the cloth is left positive.

Conductors 导体 and insulators 绝缘体

• A conductor (copper and other metals) lets charge flow easily, because it has free electrons.
• An insulator (plastic, rubber, glass) does not — its electrons cannot move freely.
• Test it: put the material in a circuit with a lamp and a battery — the lamp lights only for a conductor.

Electric fields 电场 and ions 离子

• An electric field is a region where a charge feels a force.
• Its direction at a point is the direction of the force on a positive charge.
• Field patterns: lines point away from + and toward −; between parallel plates the lines are straight and evenly spaced.
• An atom that loses electrons becomes a positive ion; one that gains electrons becomes a negative ion.

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