Electric Charges

What the section covers

Electrostatics is the study of the forces, fields and potentials produced by charges at rest. Everything in this chapter — Coulomb's law, the electric field, Gauss's law — rests on the two experimental facts established in this section: there are exactly two kinds of charge, and like kinds repel while unlike kinds attract.

The experiments the textbook builds on

• Two glass rods, each rubbed with silk, repel each other. The two pieces of silk also repel each other. But the glass rod and the silk attract.
• Two plastic rods rubbed with cat's fur repel each other, but each attracts the fur.
• The plastic rod attracts the glass rod, and repels the silk with which the glass was rubbed.
• Conclusion: there are only two kinds of electrification, and the property that distinguishes them is called the polarity of charge. Like charges repel, unlike charges attract.
• Bringing the charged glass rod into contact with the silk it was rubbed with destroys the electrification of both — the two kinds of charge nullify each other's effect. Franklin therefore named them positive and negative.

The sign convention

• Charge on a glass rod rubbed with silk, and on cat's fur, is called positive.
• Charge on a plastic rod rubbed with fur, and on silk rubbed with glass, is called negative.
• A body with excess or deficit of charge is said to be electrified or charged; with none, electrically neutral.

Where the charge comes from

• All matter contains charges, exactly balanced in the neutral state. To electrify a body we must add or remove one kind of charge.
• In solids only the loosely bound electrons move. A body that loses electrons becomes positively charged; one that gains electrons becomes negatively charged.
• When glass is rubbed with silk, electrons pass from the glass to the silk: the glass becomes positive and the silk negative. No new charge is created, and the number of electrons transferred is a tiny fraction of those present.
• All the familiar contact forces — the force holding molecules together, adhesion of glue, surface tension — are electrical in origin.

The gold-leaf electroscope (diagram in words)

A vertical metal rod is housed in a box, with a metal knob at the top and two thin gold leaves hanging from its lower end. When a charged object touches the knob, charge flows down the rod onto both leaves. The leaves acquire like charge, repel each other and diverge. The angle of divergence indicates how much charge has been transferred.

Conductors and insulators (NCERT 1.3, folded in here)

• Conductors allow charge to pass easily because they contain charges that are comparatively free to move — electrons in metals, and both positive and negative ions in electrolytes. Metals, human and animal bodies and the earth are conductors.
• Insulators — glass, porcelain, plastic, nylon, wood — offer high resistance to the passage of charge. Semiconductors form a third class, intermediate between the two.
• Charge given to a conductor is redistributed at once over its entire surface; charge put on an insulator stays where it is placed.
• This explains why a plastic comb electrifies on combing dry hair while a metal spoon held in the hand does not: charge on the metal leaks through the body to the ground, both being conductors. A metal rod held by a wooden or plastic handle and rubbed without touching the metal does show charging.

Charging by induction — what a one-mark answer needs

• Bring a positive charge near an uncharged conductor. Free electrons are drawn to the near face, which becomes negatively charged, and the far face is left with an equal positive charge.
• The net charge on the conductor is still zero: charge has been redistributed, not created. This is exactly what CBSE tests when it asks for the net charge on an uncharged conducting sphere with a point charge placed outside it — the answer is zero.
• Because the near face carries the opposite charge and is closer, the net force on the neutral conductor is attractive. That is why a neutral body is always attracted towards a charged one.

Board-standard reasoning to reproduce

• If $X$ (negative) is repelled by $Y$, then $Y$ is negative. If $Z$ is attracted to $Y$, then $Z$ is either positive (unlike charges attract) or neutral (attracted by induction). Both possibilities must be given.
• Repulsion is the sure test of electrification; attraction is not, since it can arise by induction.