Demonstrating Electric Charges

We often see static electricity in everyday life, like a balloon sticking to a wall. This happens because objects can have electric charge. There are two types: positive and negative. Like charges repel, unlike charges attract. Charge is measured in coulombs, written as $Q$ in units of C (but everyday static charges are tiny fractions of a coulomb).

Making Charge by Friction

Rubbing two different insulators can produce charge. Example: rub a plastic (polythene) rod with a wool cloth. Electrons move from one material to the other. Only electrons move; protons stay in the atoms. The object that gains electrons becomes negatively charged; the one that loses electrons becomes positively charged. The two objects get equal and opposite charges.

• Insulators (plastic, glass) hold charge because electrons cannot move easily.
• Conductors (metals) let electrons move, so charge spreads out and often escapes to Earth unless insulated.

Detecting Charge (Simple Experiments)

• Paper test: bring a charged rod near small paper pieces; they jump up (attraction).
• Pith-ball electroscope: a light ball on a thread is attracted or repelled. If the rod and ball have the same type of charge, the ball moves away (repel). If opposite, it moves toward.
• Gold-leaf electroscope: touch or bring a charged object near the metal cap; the leaf rises. A larger rise means more charge. Earthing (touching the metal case or using a wire to a tap) discharges it.

Attraction, Repulsion, and Induced Charge

A charged object can attract a neutral object. This happens by induction: the charged object pulls opposite charges slightly closer inside the neutral object, so the nearer side is attracted more strongly. Like charges always repel; unlike charges attract.

Electric Fields (Idea Only)

An electric field is the region around a charge where another charge feels a force. Field lines show direction a positive test charge would move: out of positive, into negative.

• Point charge or charged sphere: radial lines.
• Between two oppositely charged parallel plates: straight, evenly spaced lines (uniform field).

Common Misconceptions

• Rubbing does not create charge from nothing; it transfers electrons between materials.
• A neutral object can still be attracted (by induction).
• Metals usually do not keep static charge unless insulated from Earth.