electric field lines
Summary
Electric field lines are a picture of an electric field: the tangent to a line shows the force direction on a positive test charge, and closer lines mean a stronger field. They leave positive charges and enter negative ones, and where the field is nonzero they do not cross.
Detail
Meaning: Electric field lines are a drawing of the electric field. The tangent to a line at a point gives the direction of the force on a positive test charge, and closer spacing means a stronger field. Lines leave positive charges and enter negative charges, or run off to infinity.
Equation: E = F/q = kQ/r², where E is field strength (N/C), F is the force on the test charge (N), q is the test charge (C), k is Coulomb's constant (8.99 × 10⁹ N·m²/C²), Q is the source point charge (C), and r is the distance from it (m).
On the MCAT: A passage shows a two-charge or parallel-plate diagram. You find the sign of each charge from the arrow directions, rank field strength by line spacing, or give the force direction on a proton or electron. Evenly spaced parallel lines mean a uniform field. Field lines meet equipotential surfaces at right angles.
Common confusion: Lines show the direction of force, not the path of a moving charge; a released charge with inertia can leave a curved line. An electron feels a force opposite to the line direction. Field lines do not cross where the field is nonzero, because the field has only one direction at each point.
Sources
- University Physics
- Fundamentals of Physics
- Physics for Scientists and Engineers
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.