Electric Fields

An electric field is the region around a charged object where a force acts on another charged object.

Direction of an electric field

The direction of the electric field at any point is the direction of the force on a small positive test charge placed at that point. Field lines point away from positive charges and toward negative charges.

Field patterns

Point charge (positive): lines radiate outward from the charge in all directions.

Point charge (negative): lines point inward toward the charge from all directions.

Two opposite charges: lines go from the positive charge to the negative charge, curving outward between them.

Two like charges: lines repel, curving away from each other with a neutral point (zero field) between them.

Parallel charged plates: between two parallel plates with opposite charges, the field is uniform. The lines are straight, parallel and equally spaced, running from the positive plate to the negative plate. The field strength is the same everywhere between the plates (except near the edges).

Uniform electric fields

The uniform field between parallel plates is particularly important. A charged particle placed between the plates experiences a constant force:

• A positive charge accelerates toward the negative plate.
• A negative charge accelerates toward the positive plate.
• An uncharged particle is not affected.

Common errors and how to correct them

Drawing field lines in the wrong direction. Field lines go from positive to negative (the direction a positive charge would be pushed).

Saying the field between parallel plates is not uniform. Between parallel plates, the field is uniform (equal strength and direction everywhere, except near the edges). The equally spaced, straight lines represent this.

How to apply this in an exam

Draw field lines with arrows from positive to negative. For parallel plates, draw straight, equally spaced lines. For point charges, draw radial lines. State that field direction is the direction of force on a positive charge.