Why Does It Keep Turning? A 3D DC Motor Virtual Lab Built on the Correct Physics

Every physics teacher owns some version of this bench model: two chunky ferrite magnets painted red and blue, a rectangular coil on a vertical axle, a split-ring commutator with brushes, and a DC power supply. Switch it to 8 V, give the coil a nudge, and it whirs into life. It is the canonical demonstration of the motor effect — and also one of the hardest to explain, because everything that matters is either invisible (the field, the current, the forces) or too small and too fast to see (the commutator doing its trick twice per revolution).

Our new DC Motor 3D Virtual Lab recreates this exact apparatus in an orbitable 3D scene — frame, pole magnets, coil, brushes, power supply, trailing red and black leads and all — and makes every hidden mechanism visible and adjustable.

Designed on the correct physics, not an animation loop

The important design decision is that nothing in this simulation is choreographed. The coil does not rotate because a script says "rotate". It rotates because, each frame, the model computes the actual physics:

• The force is a real cross product. On each vertical side of the coil, the force is evaluated live as F = (I × B) L, using the coil's instantaneous orientation. The two magenta arrows always form a couple — equal, opposite, tangential — and they flip exactly when the commutator reverses the current, and at no other time.
• The commutator is modelled, not faked. The split ring is drawn as two bright, contrasting segments (orange and green) with real gaps, geometrically aligned so that the gaps pass under the brushes at the very moment the coil reaches the vertical. When a brush sits over a gap, the current genuinely reads zero, the yellow current arrows go dark, the force arrows vanish — and the coil keeps turning on momentum alone.
• The dead spot is real. Just like the physical model, if the coil comes to rest at the commutation position, no current can flow and the motor will not self-start. A "Give a Push" button supplies the same flick of the wrist your students use at the bench.
• Back-EMF emerges naturally. The coil current is computed as I = (V − ε_back)/R, with the back-EMF proportional to the angular speed. Set 8 V and watch the ammeter reading fall as the motor spins up — the motor is also a generator, and the simulation quietly teaches Lenz's law before the word is even mentioned.

Because the physics is honest, the classic prediction tests all work: reverse the polarity and the spin reverses; swap the magnets and it reverses again; do both and it stubbornly turns the same way. Students can interrogate the model with Fleming's left-hand rule at any frozen instant and the arrows will agree.

A commutator you can widen — the teaching feature I am most excited about

The simulation adds one control no bench model can offer: a Commutator Gap Angle slider, from 5° to 90°. Drag it wider and the split in the ring visibly grows — and so does the electrical dead zone, in exact lockstep. At 10° the motor barely notices the gaps. At 90° it spends half of every revolution coasting with no current at all, plainly carried through by rotational inertia, and at low EMF it can even get trapped in the dead zone until you push it. In two minutes of play, students discover for themselves both why the commutator must switch the current and why real commutator gaps are kept small. The live readout narrates the whole story: "orange segment → + brush", then "brush over the gap (dead spot!)", then "green segment → + brush" — the current reversal, caught in the act.

Built for classrooms

Like its sibling, the Homopolar Motor 3D Explorer, this lab runs entirely from a single folder with a fully localised three.js library — no CDN, no internet dependency, perfect for school laptops with no Wi-Fi. The page ends with the concepts illustrated, five check-your-understanding questions with click-to-reveal answers (from applying the left-hand rule, to slip rings versus split rings, to why the current falls as the motor speeds up), so it works as a self-contained lesson or a homework task.

Try it: Open the DC Motor 3D Virtual Lab, download the complete ZIP package, or browse the Electromagnetism collection at Open Source Physics @ Singapore.

Made by lookang, created using Claude Fable 5. For more interactive resources, visit iwant2study.org.