What Are Electric Field Lines?
Imagine you're standing in a field of invisible forces. Every positive charge pushes other positive charges away, and every negative charge pulls them in. If you could release a tiny positive test charge anywhere in space, it would instantly feel a push in some direction — that direction is the electric field at that point.
Now, if you let that test charge move freely, it would trace out a path through space. That path is an electric field line. It's not a real physical line — it's a visual tool, like contour lines on a map, that shows you which way the electric force points at every location.
Field lines are not trajectories of a moving charge (unless the charge starts from rest and no other forces act). They show the direction of force at each point, not the path a charge will take.
The Four Rules, Built from Intuition
1. Field lines start on positive charges and end on negative charges
Think of a positive charge as a source that "emits" field lines outward in all directions. A negative charge is a sink — lines "drain" into it. If you have a single isolated positive charge, its field lines radiate outward to infinity. A single negative charge has lines coming in from infinity.
Why? Because the electric field points away from a positive charge (repelling a test positive charge) and toward a negative charge (attracting it). The line simply follows that direction from start to finish.
2. Field lines never intersect
At any point in space, the electric field has one unique direction. If two field lines crossed, that point would have two different directions for the field — which is impossible. The field can't point both left and right at the same spot.
A common mistake: thinking field lines can "touch" or "meet" at a charge. They don't — they begin or end there, but they don't cross each other even at the charge's location.
3. The density of field lines tells you the field strength
Where field lines are packed closely together, the electric field is strong. Where they are spread far apart, the field is weak. This is a visual convention: we draw more lines per unit area in regions of stronger field.
For a point charge, lines spread out as you move away — the same number of lines passes through larger and larger spheres, so the density drops as 1/r2, exactly matching Coulomb's law.
4. Field lines are perpendicular to the surface of a conductor
When you place a conductor in an electric field, charges inside rearrange until the field inside becomes zero. At the surface, the field must be perpendicular — if it had a component parallel to the surface, charges would keep moving along the surface. So field lines always meet a conductor's surface at a right angle.
Putting It All Together …