Particles in fields (3D)
About this app
Four devices, two forces: fine beam tube, Wien filter, mass spectrometer and cyclotron as glowing field rooms in 3D. Set U, E, B and the particle, and follow the force and velocity arrows live. The curve |v| shows it: the electric field changes the speed, the magnetic field only the direction. With predictions and exam-style tasks.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
Two forces on charged particles
In an electric field a charged particle feels the force F = qE. It can act along the path and then changes the speed, for example when the particle passes through a voltage U with |q|U = ½mv². In a magnetic field the Lorentz force F = qvB acts. It is perpendicular to the velocity, so it does no work and only changes the direction. This gives the circular path with r = mv/(qB). A magnetic field therefore does not speed a particle up.
What the app shows
Four stations stand on a track of glowing field rooms. In the fine beam tube, U, B and r give the specific charge e/m. The Wien filter only passes v = E/B. The mass spectrometer separates isotopes through r = mv/(qB). In the cyclotron the orbital frequency f = qB/(2πm) stays the same as long as the particle is non-relativistic. The path is integrated numerically and appears as a thread of light that gets brighter with speed. The curve of |v| over time stays level in every magnetic field.
What you can try
Drag the handles in the scene or use the sliders, and watch the arrows for v, the electric force and the Lorentz force. Double B and compare radius and speed. Change E in the Wien filter until the particle hits the aperture. Choose an isotope pair and read the distance of the hits. Detune the frequency in the cyclotron, or switch on the relativistic calculation for the electron. Two predictions and four calculations use the current values from the app.
Frequently asked questions
- Why does a magnetic field not change the speed of a particle?
- The Lorentz force is always perpendicular to the velocity. A force perpendicular to the path does no work, so the kinetic energy stays the same. The size of the velocity does not change, only its direction.
- How does a Wien filter work?
- In a Wien filter an electric and a magnetic field are perpendicular to each other. Only a particle for which the electric force and the Lorentz force are equal flies straight on: qE = qvB, so v = E/B. Charge and mass do not appear in this condition.
- Why does the orbital frequency in a cyclotron not depend on the speed?
- From qvB = mv²/r it follows that r = mv/(qB). The circumference grows in step with the speed, and the period T = 2πm/(qB) no longer contains v. This holds as long as the particle is much slower than light.
- How does the app help with exam preparation?
- It links four typical devices to the same two forces. You set U, E and B yourself and work out tasks on e/m, the Wien filter, the distance in the mass spectrometer and the energy in the cyclotron with values from the app. Each task comes with feedback and a worked solution.
Subject: School physics
More from School physics
- The exposure triangle
- Density and buoyancy
- The Doppler effect
- Additive and subtractive colour mixing
- Franck–Hertz tube (3D)
- Photoelectric effect (3D lab)
- Millikan experiment
- Newton’s cradle and collisions
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