Electric field lines
Also called Field line diagram, Electric field line
Electric field lines are a simplified diagram of an electric field: continuous curves leaving positive charge and arriving at negative charge, with the field tangent to the line and the spacing showing relative strength. They give directions and comparisons, never numbers.
The AP CEDs treat the line diagram as the second-best representation, and the wording matters. EK 10.3.A.3.ii says electric field maps use vectors to depict the magnitude and direction of the field at many locations. EK 10.3.A.3.iii then says electric field line diagrams are simplified models of those maps, and can be used to determine the relative magnitude and direction of the field at any position in the diagram.
So the vector map carries magnitudes and the line diagram carries comparisons. "Relative" is doing real work in that sentence.
Reading one.
- The field at a point is tangent to the line through it, pointing along the arrowhead.
- Lines run away from positive charge and toward negative charge, which is the direction rule from EK 10.3.A.2.ii.
- Where lines crowd together the field is stronger. Where they cannot cross, it is because one point cannot hold two field directions.
What the lines have to do at boundaries. They meet a conductor's surface at right angles, since EK 10.3.B.1.i puts the field perpendicular there, and there are none inside the conducting material of a conductor in electrostatic equilibrium. They also cross every equipotential surface at right angles and always point toward lower potential (EK 10.5.B.2.ii and EK 10.5.B.2.iii).
For an actual value, leave the diagram: use per source and add the vectors, as the field and potential guide does.