Electric Fields

An electric field is the region around a charged object where another charge would feel a force. Think of it like an invisible wind that pushes charges. The field shows where and how strongly the push or pull would act.

Charges and charging

• There are two types of charge: positive $+$ and negative $-$. Like charges repel; unlike charges attract.
• Charge is measured in coulombs $C$. We use the symbol $Q$ for charge.
• Charging by friction (e.g., rubbing a balloon on hair) transfers electrons. Only electrons move; gaining electrons makes an object negative, losing them makes it positive.
• Conductors (metals) let electrons move easily; insulators (plastic, rubber) do not.
• Detecting charge: a charged rod attracts small pieces of paper, or makes the leaves of an electroscope separate.

What is an electric field?

Definition: a region where an electric charge experiences a force. The direction of the electric field at a point is the direction of the force on a small positive test charge placed at that point.

Field lines and their rules

• Arrows show direction: away from $+$, towards $-$.
• Closer lines mean a stronger field.
• Field lines never cross.
• They start on positive charges and end on negative charges (or fade away at infinity).

Typical electric field patterns

Around a point charge

Positive point charge: lines spread out evenly in all directions (radial, pointing out). Negative point charge: lines point inwards. A charged conducting sphere has the same pattern outside the sphere.

Between two oppositely charged parallel plates

Lines are straight, parallel, and evenly spaced from the positive plate to the negative plate. This means the field is uniform (same strength and direction) between the plates. Edge effects are not needed here.

Forces in fields

A charge placed in an electric field feels a push or pull along the field lines. Neutral objects can still be attracted because the field shifts their electrons slightly (induced charge).

Common misconceptions

• Field lines are models we draw; they are not real threads in space.
• Field direction is defined using a positive test charge, even though in solids electrons actually move.
• Static electricity is not magnetism; electric fields act on charges, not magnets.