Electric Potential & Equipotential Lines Lab
Drag a test point around two charges and read its potential and field live, then find where moving along an equipotential line takes zero work.
Drag the yellow test point around the two charges. The dashed rings are equipotential lines - moving along one does zero work; crossing them does.
About the Electric Potential & Equipotential Lines Lab
Free electric potential & equipotential lines lab. Drag a test point around two charges and read its potential and field live, then find where moving along an equipotential line takes zero work. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for physics, the electric potential & equipotential lines lab runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Drag a test point around two charges and read its potential and field live, then find where moving along an equipotential line takes zero work. Use it to explore physics ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Electric Potential & Equipotential Lines Lab
- Use the controls to change Charge Q₁, Charge Q₂, Separation between charges. The simulation reacts instantly.
- Press "Reset to defaults", "Lab Report" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Drag the test point along one of the dashed equipotential rings and watch V barely change.
- Find the zero-potential point between two opposite charges.
- Move the test point close to one charge and watch the field arrow grow.
- Try the work-along-an-equipotential challenge.
Key ideas you can learn
- Electric potential from a point charge is V = kQ/r, and potentials from multiple charges simply add (superposition) at any point.
- Equipotential lines connect points of equal potential; moving a charge along one does zero work, since W = q(Vfinal - Vinitial) and the potential does not change.
- The electric field always points perpendicular to equipotential lines, from higher to lower potential, and is stronger where equipotential lines are packed closer together.
- Between two opposite charges of different magnitude, there is a specific point on the line joining them where the potential is exactly zero because their contributions cancel.
Show my work
Challenges
Challenge 1 - work along an equipotential
Drag the test point anywhere, note its potential V₁. Now drag it to a different spot that shows very nearly the same V on the readout (staying on the same equipotential line). The work needed to move a q = 2 nC test charge between two points on an equipotential is W = q(V₂−V₁). Enter the work in nanojoules for your two points (should be close to 0).
Challenge 2 - find the zero-potential point on the axis
Set Q₁ and Q₂ to have opposite signs. On the line joining the two charges, between them, there is a point where V = 0 (contributions cancel: Q₁/d₁ = −Q₂/d₂, i.e. Q₁/d₁ = |Q₂|/d₂). Compute the distance from Q₁ (in meters) to that point, given the current separation and charge magnitudes, using d₁ = separation × |Q₁| / (|Q₁|+|Q₂|)... actually derive it: d₁/(sep−d₁) = |Q₁|/|Q₂|. Enter d₁.
Where this is used in the real world
Engineers map equipotential surfaces to design capacitors, particle accelerators and high-voltage insulation, since components must be shaped to avoid dangerously bunched-up field lines near sharp edges.
Who is this simulation for?
Physics students in middle school, high school and first-year university, teachers who want a quick demonstration for the projector, and anyone revising for exams. It works well for flipped classrooms because students can explore before the lesson.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why does moving along an equipotential line take no work?
Work done on a charge by the electric force is W = q(Vfinal - Vinitial); if both points are on the same equipotential line, Vfinal equals Vinitial, so the work done is exactly zero regardless of the path taken between them.
Why is the electric field always perpendicular to equipotential lines?
If the field had any component along an equipotential line, moving a charge along that line would take nonzero work, which would mean the potential was changing along the line - contradicting the definition of an equipotential.
Is the Electric Potential & Equipotential Lines Lab free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Electric Potential & Equipotential Lines Lab work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.