# Discuss Stevo's answer to: A proton (mass= 1.67?10-27 kg, charge= 1.6?10-19 C ...

A proton (mass= 1.67?10-27 kg, charge= 1.6?10-19 C) travelling with speed 1?106 m/s enters a region of space containing a uniform magnetic field of 1.2 T.What is the time t required for the proton to ...

The proton is a charged particle and as such is influence by magnetic fields. The stronger the magnetic field B the greater the force imposed on the charged particle. The effect of this field is to deflect the proton from what would be its straight line path. Image an asteroid approaching the Earth. With a certain speed and proximity the asteroid's course will be altered.
The same is true for the proton. Its course is altered by the strength of the field based on the speed of the proton. The size of the field is not mentioned so I would guess you have to think of the field as being a point source and the flux dropping off to the inverse square of distance.
And assuming that the field is perpendicular to the protons velocity.

Its been a long time but I think you need to start from:
(Sum)F = 0(zero)
-qE +qvB = 0
and                v = E / B

work out the Ke of the system using:

0.5mv^2 = eV     or v = sqrt(2eV/m)

E / B = sqrt(2eV/m)

and            e/m = E^2 / 2VB^2

e/m = 1.7588x10^11 C/kg

so you should be able to rearrange to get the time of flight based on the velocity of the proton...??