Electrical Charge

Electrical charge is a property of matter that causes electrical forces. There are two types: positive and negative. Neutral objects have equal amounts of positive and negative charge.

Attraction and Repulsion

• Like charges repel: positive–positive and negative–negative push apart.
• Unlike charges attract: positive and negative pull together.

Measuring Charge

Charge is measured in coulombs (symbol C). We use the symbol $Q$ for charge. In circuits, current is the rate of flow of charge: $I = \frac{Q}{t}$ (you will meet this more in current).

Making Static Charge by Friction

Rubbing two different insulating materials (for example, a plastic ruler and a dry cloth) can transfer tiny negative particles called electrons from one to the other. The material that gains electrons becomes negatively charged; the one that loses electrons becomes positively charged. In solids, only electrons move; the positive parts stay fixed.

Detecting Charge

• Attraction test: a charged balloon attracts small paper pieces.
• Water test: a charged comb makes a thin stream of water bend.
• Electroscope (simple device): the leaf rises when charge is present.

Conductors and Insulators

Conductors (metals, graphite) have free electrons that can move, so charge flows easily. Insulators (plastic, glass, rubber) hold electrons tightly, so charge does not flow.

Experiment idea: place a material in a simple circuit with a cell and lamp. If the lamp lights, the material is a conductor; if not, it is an insulator.

Electric Fields

An electric field is a region where an electric charge experiences a force. The direction of the field at a point is the direction a small positive test charge would move.

• Around a point charge: lines spread out radially (outwards for positive, inwards for negative).
• Around a charged conducting sphere: like a point charge outside the sphere.
• Between two oppositely charged parallel plates: straight, evenly spaced lines from + to − (uniform field; ignore edge effects).

Common Misconceptions

• Charged objects can attract neutral objects by inducing a small shift of charges inside the neutral object.
• Protons do not move in solids; electrons move.