Lesson 1 of 13 · 7 min

Electric charge, conductors and insulators

NCERT §1.2 and §1.3

Tara runs a plastic comb through her dry hair, holds it over torn bits of paper, and they jump up to meet it. Nothing touched them. What did the comb gain?

The story this chapter follows: Tara's dry winter evening

On a dry December evening Tara combs her hair, and the plastic comb picks up −20 nC. Bits of paper leap up to it, two small balloons she rubbed on her sweater hang apart with +0.5 μC each, and the jumper crackles when she pulls it off. Every idea in this chapter, from one electron to Gauss's law, happens somewhere in that room.

The lesson in notes

In short

Around 600 BC Thales of Miletus noticed that amber rubbed with wool or silk attracts light objects; the word electricity comes from the Greek elektron, meaning amber.

Rubbing experiments show only two kinds of charge: like charges repel and unlike charges attract. The property that tells the two kinds apart is called the polarity of charge.

Benjamin Franklin named the two kinds positive and negative. By convention a glass rod rubbed with silk is positive (the silk is negative), and a plastic rod rubbed with fur is negative (the fur is positive).

Charging means an excess or a deficit of electrons: when glass is rubbed with silk, some electrons pass from the rod to the silk, so the rod is left positive and the silk negative. No charge is created, and only a tiny fraction of the body's electrons move.

A gold-leaf electroscope detects charge: charge given to its metal knob spreads to two thin leaves, which repel and spread apart; the more the divergence, the more the charge.

Conductors (metals, the human and animal body, the earth) contain charges that can move freely; insulators (glass, porcelain, plastic, nylon, wood) do not. Semiconductors lie in between.

Charge given to a conductor spreads over its whole surface, while charge placed on an insulator stays where it was put.

A metal spoon held in the hand shows no charge after rubbing because the charge leaks through the body to the earth; given an insulating handle, it can be charged.