An electric field simulator you can play with: drop positive and negative point charges onto the canvas and watch the field lines and vectors redraw around them instantly. Build a dipole, a line of charges, or a whole cloud, and see how Coulomb's law and superposition shape the field everywhere at once. Field lines stream out of the positives and dive into the negatives — and it all runs on your device.
You are in the Physics lab.
How to read the picture: line direction shows which way a positive test charge is pushed (always + → −), and line density shows the field's strength — the denser the lines, the stronger the field.
What the arrows, the line spacing and the readout are telling you.
Every arrow — and every field line — points the way a tiny positive test charge would be pushed if you let it go there. That means the field streams out of positive charges (orange, a source) and into negative ones (blue, a sink). Switch to Vectors and each grid arrow shows the local direction; switch to Field lines and those same directions join up into smooth curves that always run + → −.
The spacing of the lines encodes the strength. Close to a charge, or between two opposite charges, the lines crowd together and the field is strong; far away they fan out and it weakens. In vector mode the same story is told by arrow length and colour — longer, warmer arrows mark the strong regions. Drag the Field detail slider to draw more or fewer samples without changing the physics.
Add more charges and the field at every point is the vector sum of what each charge contributes on its own — that is superposition. Lines still begin on positives and end on negatives, but now they bend around neighbours: try the Dipole, Two positives and Parallel plates presets and watch the readout update the positive, negative and net-charge counts as the shape of the field changes.
Coulomb's law for every charge, added up everywhere, then drawn as vectors and flowing lines.
Each point charge contributes a field that points along the line to it and falls off as one over distance squared — stronger charges and nearer points feel more. To get the field at any spot, the simulator simply adds up the contributions from every charge as vectors. That sum, the superposition of all the charges, is the field you see.
In vector mode the canvas is covered with a regular grid of sample points, and the field is evaluated at each one. Every sample becomes an arrow pointing along the field there, with its length and colour set by the field's strength on a compressed scale so strong and weak regions stay readable side by side.
In field-line mode the simulator seeds a ring of starting points around each positive charge and walks each one forward in tiny steps, always moving along the local field direction. A line stops when it reaches a negative charge, leaves the canvas, or runs too long — producing the smooth curves that flow from positive to negative.
Teaching or blogging about physics? You can embed this on your own site free — one line of code, no sign-up.
A ready-to-assign lab using the simulation above. Free to use — nothing to install or sign up for, and no student data leaves the browser.
Sketch the field-line patterns for a single charge, a dipole, and two like charges.
Electric field lines point away from positive and toward negative charge, with density showing strength. A point charge’s field weakens as 1/r². For multiple charges the total field is the vector sum (superposition) of each charge’s field.
Educators: link or embed this simulation freely in your LMS or course guide.
Put the Electric Field Simulator on your own page — free, no sign-up, no watermark. It runs entirely in your visitors' browsers. Copy the snippet and paste it into your page's HTML.