Electric field line
= Electric field line
{title2=$d\mathbf r/ds\parallel\mathbf E$}
An oriented curve whose tangent points along the local <electric field>. Where the field is nonzero, choosing arc length $s$ gives $d\mathbf r/ds=\mathbf E/|\mathbf E|$. These curves visualize the direction of the <force> on a positive test charge; they are not generally particle trajectories, since a particle can have inertia and initial velocity. In <electrostatics>, they are normal to <equipotential surfaces> and cannot form a closed loop in a region with a single-valued potential that strictly decreases along the line. A field null can separate families of lines.