chargetomass

Charge-to-mass ratio, often written q/m, is a property of a particle or body defined as its electric charge q divided by its mass m. It governs how the object responds to electric and magnetic fields and thus influences its trajectory in classical and relativistic electrodynamics. The ratio is particularly important for charged particles and ions.

In SI units, q/m has units of coulombs per kilogram (C/kg). For elementary particles, typical values are large due to small rest mass. The electron has q/m ≈ −1.76 × 10^11 C/kg, the proton about +9.58 × 10^7 C/kg, and an alpha particle about +4.82 × 10^7 C/kg. Neutral objects have q = 0, giving q/m = 0; their motion in electric fields is unaffected by charge.

The q/m ratio is central to mass spectrometry and particle accelerators, where charged species are separated by electric or magnetic fields because the radical difference in q/m changes their curvature and deflection. In a uniform magnetic field B, a particle follows a circular orbit with cyclotron frequency ω = qB/m. In plasmas and astrophysical contexts, q/m determines how particles accelerate in electromagnetic fields and respond to radiation pressure, gravity, and collisions.

Measurement of charge-to-mass can be done by directly measuring charge and mass, or by observing motion in known fields and solving for q/m. For elementary particles, q/m is a fundamental constant; for ions it depends on the ionization state and composition.

Related topics include charge, mass, electron, proton, ions, cyclotron frequency, mass spectrometry, and the Lorentz force.