Charged particles in a uniform electric field – electron gun and deflection tube
PhysicsElectric & Magnetic FieldsAges 16–17
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Sign in to playAn electron gun accelerates electrons, protons or alpha particles through a potential difference Vₐ (|q|Vₐ = ½mv²); parallel plates with voltage V and separation d then give a uniform field E = V/d that bends the beam into a parabola. Readouts show the speed, time between the plates, exit angle, spot position on the screen and kinetic energy in eV. Add a magnetic field B to balance the electric force (v = E/B) and find q/m as Thomson did.
Lesson: Motion of charged particles in a uniform electric field; electron gun; specific charge q/m
What it shows
Between parallel plates the electric field is uniform, E = V/d, so a charged particle feels a constant force qE, just as a ball feels constant gravity. A particle entering at right angles to the field keeps a constant speed along the plates while accelerating across them, so its path is a parabola. In the electron gun the field does work qVₐ, which becomes kinetic energy ½mv². The deflection y = VL²/(4dVₐ) contains neither q nor m, so electrons, protons and alpha particles land the same distance from the centre, on opposite sides for opposite charges. A magnetic field can balance the electric force and reveal q/m.
How to use
Choose a Particle, then drag Accelerating voltage, Deflecting voltage, Plate separation, Plate length and Plate-to-screen distance. Releasing a slider fires a new shot; earlier shots stay grey for comparison until you press Clear traces. Read the speed, deflection, angle and energy below the picture. Raise the Magnetic field B and fine-tune it with − and + until the spot returns to the centre, then read q/m.
Parameters you can change
- Particle Electron, Proton, Alpha particle (α)
- Accelerating voltage Vₐ 200–5000 V
- Deflecting voltage V across the plates -1500–1500 V
- Plate separation d 1–8 cm
- Plate length L 2–15 cm
- Plate-to-screen distance D 5–30 cm
- Magnetic field B between the plates (into the page) 0–5 mT
Questions to explore
- Why do an electron and a proton accelerated through the same voltage land the same distance from the screen centre?
- What happens to the spot's deflection if you double the accelerating voltage, and why?
- When the electric and magnetic forces balance, why must the particle's speed equal E/B?
