How was the mass of a proton determined?

Asked by: David Slochower

Answer

The mass of the proton was determined in a similar way to how the mass of atoms are measured. The particle, whose mass is being determined, is accelerated through an electric field, the particle then passes through a perpendicular magnetic field which deflects the particle (particle must be charged, a proton or an ion for example, for it to deflect). The angle by which it deflects is dependent on the mass of the particle. The mass of the particle can be determined by using the following formula:

Centripetal Force = Force due to magnetic field(B)

(mv²)/r = Bqv

(where m = mass of particle, v = velocity of particle, r= radius of deflected path, B = magnetic field strength, q = charge of particle)
Answered by: Simon Hooks, Physics A-Level Student, Gosport, UK