A subatomic particle is observed to follow a curved path as shown. The particle\

Question

A subatomic particle is observed to follow a curved path as shown. The particle's speed is measured to be 2.0*106m/s as it moves along a radius of curvature 42mm. The magnetic field is 1.0T and directed out of the page. What kind of particle is this-- an electron, a positron, a proton, an antiproton, a neutron, an alpha particle, a positively charged pion, or a negatively charged pion? (You should be able to find the masses and charges of these particles on pdg.lbl.gov, or wikepedia).

These are the values I found for masses:

electron mass: 9.1*10-31kg

positron: 9.1*10-31kg

proton: 1.67*10-27kg

antiproton: 1.67*10-27kg

neutron: 1.67*10-27kg

alpha particle: 6.64*10-27kg

positively and negatively charged pion: ?

Explanation / Answer

v = 2*10^6 m/s

r = 42 mm = 0.042 m

B = 1 T

Now, for circular motion,

the magnetic force must provide the necessary centripetal force

So, qvB = mv^2/r

where q = charge of particle

m = mass

So, m = qBr/v

So, m/q = Br/v = 1*0.042/(2*10^6) = 2.1*10^-8 kg/C

Now, mass of alpha particle, m = 6.64*10^-27 kg

charge of alpha particle, q = 3.2*10^-19 C

So, m/q ratio of alpha particle = 6.64*10^-27/(3.2*10^-19) = 2.1*10^-8 kg/C

As its m/q ratio matches with our answer so it has to be an alpha particle <-------- answer

The charge has to be positively charged <-----------answer

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