How to play Particle Gun
- Choose a test and read the particle and the voltage on the card.
- Use q·ΔV = ½·m·v², with q in C and m in kg, or count energy in eV: 1 eV for each e through each volt.
- Type your prediction and press Start. The particle is fired in slow motion; the orange ghost moves at your speed.
- Compare the particle with the ghost, and the energy bars with your orange line.
A potential difference becomes kinetic energy
A charge q that moves through a potential difference ΔV changes its electric potential energy by q·ΔV. When nothing else acts on it, that energy turns into kinetic energy: q·ΔV = ½·m·v². The energy depends only on the charge and the voltage, not on the mass or on the distance between the grids, so it is handy to count it in electronvolts: one electron through one volt gains 1 eV = 1.602 × 10⁻¹⁹ J. The speed does depend on the mass: through the same voltage a proton, 1836 times heavier than an electron, ends up √1836 ≈ 43 times slower. Run backwards, the same rule tells how much voltage it takes to stop a moving charge. Electron guns in old televisions, X-ray tubes, and particle accelerators all work this way.
A focused learning model
The grids are 4.0 cm apart with a uniform field between them and none outside; the particle passes through them without touching. It starts from rest at the first grid, speeds up steadily between the grids, and coasts to a detector 10 cm away; in the stopping test it arrives at 4.0 × 10⁶ m/s and slows down between the grids, and an answer within 2% of the stopping voltage stops it just at the second grid. Gravity is ignored, and every speed stays under 7% of the speed of light, so the model is not relativistic. Each run lasts nanoseconds to hundreds of nanoseconds and is slowed down so it takes about 4 s. Answers within 2% are right.