18.1 Electric fields and field lines

An electric field is a region in which a charge experiences a force; field strength E is force per unit positive charge.

Use a small positive test charge to define direction, then calculate force on any charge with its sign included.

Between parallel charged plates, a positive charge feels force in the field direction and a negative charge opposite it.

The field exists without placing a test charge, and field direction is not always the same as force on a negative charge.

Electric field strength satisfies E=F/q, so the force on charge q is F=qE; direction reverses for negative q.

Use E as a vector and q with sign. In a uniform field, force and acceleration are constant if q and mass are constant.

A −2 μC charge in a 3.0×10⁴ N C⁻¹ field experiences 0.060 N opposite the field direction.

E is not force itself and does not depend on the test charge used to measure it.

Electric field lines point in the direction of force on a positive test charge; closer lines indicate stronger field qualitatively.

Lines start on positive charge and end on negative charge or infinity, do not cross, and are perpendicular to conductor surfaces in electrostatic equilibrium.

Parallel equally spaced lines represent a uniform field between large oppositely charged plates.

A negative charge moves opposite to the arrow direction, and field-line density is a model rather than an exact numerical scale.