How to play Electron Beam
- Choose a test and read the plates, the field, and the beam on the card.
- Use E = V ÷ d, a = q·E ÷ m, t = L ÷ v₀, and y = ½·a·t², in metres, seconds, and coulombs.
- Type your prediction and press Start. The particle is fired in slow motion; your prediction is the orange dashed path.
- Compare the glowing path with your orange one, inside the plates and on the right panel.
A uniform field bends a beam
Two parallel plates with a voltage V across them, d apart, make a uniform field between them, E = V ÷ d, pointing from the + plate to the − plate: one volt per metre is one newton per coulomb. A charge q there feels the same force q·E everywhere, so a particle fired straight between the plates moves like a ball thrown sideways: its speed along the tube stays v₀, while across the tube it accelerates steadily, a = q·E ÷ m. Its path is a parabola, y = ½·a·(x ÷ v₀)², and once it leaves the plates it goes straight. An electron is so light that a modest field gives it an acceleration of about 10¹⁵ m/s², and gravity doesn't matter at all. A proton with the same charge size bends the other way and about 1836 times less. Old television tubes, oscilloscopes, and inkjet printers steer beams this way.
A focused learning model
The field is uniform between the plates and zero outside them, with no fringe. The particle enters along the axis, keeps its speed along the tube, and accelerates steadily across it, so its height is a parabola in the plates and a straight line after them; one that reaches a plate stops on it at that exact moment. Gravity is ignored: it is about 10¹⁴ times weaker than the electric force here. In every test the speed stays under 9% of the speed of light, so the model is not relativistic. Each run lasts a few nanoseconds and is slowed down so it takes about 4 s. Answers within 2% are right.