In: Physics
A proton (charge +e = 1.61x10-19 C and mass m = 1.67x10-27 kg), traveling perpendicular to a magnetic field, experiences the same force as an alpha particle (charge +2e and mass 4m) which is also traveling perpendicular to the same field. The ratio of their speeds, valpha/vproton, is:
A. 0.5
B. 1
C. 2
D. 4
E. 8
The force on the moving charge in the magnetic field is
F = qvBSin θ, where θ is the angle between thedirection of velocity and the magnetic field.
In both cases, θ = 90 degrees, so the force is
F = qvB.
For proton, Fp = evpB
For alpha particle, Fα =2evaB
The two forces are equal, then
evpB = 2evaB
vp = 2va, so the ratio is 2.
C is the correct answer.