Electric Field Lines
Field lines traced from the computed field rather than sketched by hand. They leave positive charges, land on negative ones, crowd together where the field is strong and never cross. Drag the test charge around and flip its sign to see it pushed along the lines or straight against them.
How to use this simulation
- Press One charge for the radial pattern of an isolated charge
- Press Two unlike or Two like to see lines join up or push apart
- Press Parallel plates for the uniform field between charged plates
- Flip the test charge sign and drag it to compare the force directions
What to look for
- Lines are densest close to the charges, where the field is strongest
- Two unlike charges have lines running from one straight to the other
- Two like charges have a point between them where no line passes and the field vanishes
- Between parallel plates the lines are straight, parallel and evenly spaced — a uniform field
- Reversing the test charge reverses the force without changing its size
The physics behind it
A field line is a curve whose tangent gives the direction of E at every point, and the density of lines in a region is proportional to the field strength there. Lines begin on positive charge and end on negative charge, which is why an isolated charge has lines running to infinity. They can never cross, because the field at a point has one direction only. A positive test charge feels F = qE along the local line; a negative one feels the same magnitude directly against it.