Mass Spectrometer (Sector)
This simulator sketches a classic **sector mass spectrometer** in two spatial dimensions. The first stage is a **velocity selector**: uniform electric and magnetic fields are perpendicular to each other and to the nominal beam direction. For a positive ion moving along +x with velocity v, the electric force qE and magnetic force q(v×B) oppose along the transverse direction when E, B, and v are oriented as in standard textbook diagrams; **straight-line** motion requires **v = E/B** (magnitudes, with consistent sign conventions). Ions with the wrong speed are deflected and do not pass cleanly through the slit. After selection, ions enter a region with **magnetic field only** (idealized, uniform). The Lorentz force bends them into circular arcs with radius **r = mv/(qB)** for perpendicular injection—so at fixed q and selected v, **heavier** isotopes follow **larger** radii and strike a position-sensitive detector at different locations. The animation integrates Newton’s law with the Lorentz force for two species sharing the same q but different masses. Pedagogical simplifications include uniform box-like fields with no fringe effects, no collisions, non-relativistic speeds, and schematic units; real instruments add electric sectors, time-of-flight stages, quadrupoles, and much more elaborate optics.
Who it's for: Undergraduate students learning charged-particle motion, velocity filters, and the basics of mass analysis in chemistry and physics laboratories.
Key terms
- Velocity selector
- Lorentz force
- Cyclotron radius
- Mass-to-charge ratio
- Magnetic sector
- Crossed fields
- Ion trajectory
- Mass spectrometry
How it works
A velocity selector passes ions with v = E/B; a downstream magnet bends them into radii r = mv/(qB), separating species by mass-to-charge ratio.
Frequently asked questions
- Why must the velocities be matched to E/B before the sector?
- The sector radius **r = mv/(qB)** depends on **v** as well as **m**. Without a selector (or another way to fix v), ions of different masses **and** different energies would land on top of each other, confounding the mass measurement. The crossed-field stage enforces a narrow band of speeds.
- Does the simulator include electric-sector focusing or quadrupoles?
- No. Those elements improve beam quality and resolution in real spectrometers. Here only the minimal physics lesson—selector plus uniform B bend—is modeled.
- What happens when I move the v/(E/B) slider away from 1?
- That mimics a **mistuned** selector: ions feel a net transverse force in the crossed-field region, so trajectories curve **before** the magnetic sector, illustrating why matching **v ≈ E/B** matters.
- Are the radii labeled “estimate” exact?
- The readout **r ≈ v/((q/m)B)** is the familiar perpendicular-entry formula in pure **B**. The numerical trajectory also includes the transition from the selector’s combined fields into the sector, so small differences from an ideal semicircle are expected.
More from Electricity & Magnetism
Other simulators in this category — or see all 46.
Resonant Transformer (Tesla-style)
Coupled RLC tanks; tune f_drive near secondary ω₀ to grow V on high‑Q side (linear coupled ODEs).
Wheatstone Bridge
Four arms, G between mid nodes; V_B − V_C and null when R₁R₄ = R₂R₃.
Dipole in Uniform E
τ = −pE sin θ, U = −pE cos θ; damped rotation; optional AC on E.
Ferrofluid (Stylized)
Purple metaball pool, spikes, field-line hints — visual only, not MHD.
Cyclotron (Schematic)
B uniform, oscillating E in gap; spiral growth; ω_c = (q/m)B in sim units.
Bar Magnet & Iron Filings
Drag-and-drop bar magnet on a card of iron-filing rods; each filing aligns with the local two-pole B field.