Static Electricity and Charging

Static electricity occurs when charge accumulates on the surface of an insulating material.

Charging by friction

When two insulators are rubbed together, electrons transfer from one surface to the other.

• The material that loses electrons becomes positively charged.
• The material that gains electrons becomes negatively charged.

Example: Rubbing a polythene rod with a cloth transfers electrons from the cloth to the rod. The rod becomes negative; the cloth becomes positive.

Example: Rubbing an acetate rod with a cloth transfers electrons from the rod to the cloth. The rod becomes positive; the cloth becomes negative.

Only electrons move. Protons are fixed in the nucleus and do not transfer.

Rules of charge interaction

• Like charges repel (positive repels positive; negative repels negative).
• Unlike charges attract (positive attracts negative).

Charged objects attract neutral objects

A charged rod can attract small pieces of paper even though the paper is neutral. This happens because the charge on the rod induces (rearranges) charges in the paper:

• Opposite charges in the paper are pulled closer to the rod.
• Same charges in the paper are pushed further away.
• The closer opposite charges experience a stronger force, so the net effect is attraction.

Why only insulators hold static charge

Insulators have no free electrons, so the charge stays where it is placed. Conductors have free electrons that would spread out and neutralise the charge.

Common errors and how to correct them

• Saying protons move during charging. Only electrons transfer.
• Saying a charged object only attracts charged objects. A charged object attracts ALL nearby objects (charged or neutral) because of induced charge effects.

How to apply this in an exam

Describe the electron transfer (which way, which material gains/loses). State the resulting charges on both objects. For attraction/repulsion, apply the like-repel/unlike-attract rule.