field lines

Meaning: Field lines are a picture of a field. The tangent to an electric field line at a point gives the direction of the force on a positive test charge there. Lines leave positive charges and end on negative charges, and closer spacing means a stronger field. Lines do not cross, since the field has one direction at each point. Magnetic field lines instead form closed loops, running outside a magnet from north to south.

Equation: E = F/q, where E is the electric field (N/C), F is the force on the test charge (N), and q is the test charge (C). For a point charge, E = kQ/r², where k = 8.99 × 10⁹ N·m²/C², Q is the source charge (C), and r is the distance (m).

On the MCAT: A passage shows a diagram of point charges or charged plates. You find where the field is strongest (where lines are closest), give the force direction on a proton or an electron, or relate lines to equipotential surfaces, which meet field lines at right angles.

Common confusion: Field lines are not paths that charges travel. The force is tangent to the line, but a charge released from rest in a curved field need not follow the line. Also, an electron is pushed opposite to the line direction, because F = qE and q is negative.