Electron beam in a magnetic field sim
Fire an electron beam across a deflection tube's screen between a pair of coils, predict with Fleming's left-hand rule (current opposite to the electrons' flow) which way it bends, turn up the coil current to watch it bend, reverse the field, and predict a positive beam as a thought experiment.
Electromagnetism: force on a beam of charged particles · covers the O-Level, IGCSE topic
What the sim shows
Model. Electrons leave the gun at v = √(2eV ÷ m) and enter a uniform field B = (4/5)^(3/2) μ₀NI ÷ R from a Helmholtz pair (N = 500, R = 102 mm, chosen), zero before the screen. The force is F = q v × B, worked in the frame the student sees: Fleming's left-hand rule with the current opposite to the electrons' flow. The path is an arc of radius r = mv ÷ (Be) (beyond the syllabus), traced until it leaves the 10 cm × 8 cm grid. Not relativistic; no gravity; the beam has no width. The positive beam is a thought experiment with the same mass and speed. Nothing runs on its own: every change is a click, a drag or a key.
- The fluorescent screen: the beam glows on it
- Deflection tube on its stand, Helmholtz coils, EHT supply, heater supply, coil supply, reversing switch
Use it in your lessons
- Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
- Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
- Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.
Free guide: 12 practicals as sims, and how to run one in a lesson →
Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.
More physics sims
- Speed of sound by echo simClap in front of a wall, get in time with the echoes and time a run of them with a stopwatch, work out v = 2d ÷ t and see why one echo is too short to time by hand; then time a pulse between two microphones in air, water and steel with v = d ÷ t and compare the three speeds on one graph.
- Stability and toppling simTilt a hinged board with a model bus, a bottle of sand or a wooden cone on it and watch the weight line from G leave the base as it topples; then measure the toppling angle of six wooden blocks on the protractor and plot tan θ against (b ÷ 2) ÷ h.
- States of matter: the particle model simDrag a heater slider to take 50 g of ice to water to steam and back while a magnified box shows its particles go from a vibrating lattice, to sliding past each other, to flying apart and hitting the walls; record the heating curve with its flat parts, then squash sealed syringes of air, water and wax.
- Three forces in equilibrium: the triangle of forces simHang a load from two newton-meters, read the tensions and the angles, then draw the closed triangle of forces to scale and check it against the meters.
- U-tube manometer simJoin a U-tube manometer to the gas tap, a flask squeezed by a syringe or your own breath, read both levels to the nearest mm, find h and work out Δp = ρgh and the gas's pressure, then swap the water for oil or mercury and widen one arm to see what changes.
- Walk the graph: match a motion graph by walking simWalk in front of a motion sensor so your displacement–time, velocity–time, distance–time or speed–time line matches a target, then measure it with the gradient and area tools.