Minimum velocity for a charged particle to go through all the magnetic field's zone i´m trying to do this magnetic field exercise, where the particle goes through the next following scheme:

Where the particle´s electric charge is q = 1.6x10-6 C the particle´s mass is 3.4x10-27 Kg and the magnetic field is B = 10^-3 (-k) T and the magnetic field´s width is 2 meters
However i´m kinda lost on the first question that is asking me to calculate the minimum velocity for the particle that is needed for the particle to go through all the magnetic field´s zone, any help would be highly appreciated! thanks a lot in advance cheers!
 A: The particle will perform circular motion:
$$\frac{mv^2}{R}=qvB$$
$$R=\frac{p}{qB}$$
For the particle to go through the region, need
$$R>2\text{m}$$
Hence
$$p>(2\text{m})qB$$
A: When charged particles are in a constant magnetic field, like the problem you have, they are constantly turning to the right or the left. To find out which way, and how fast, calculate the Lorentz Force $$ \vec F = q(\vec v \times \vec B). $$ It looks like your particle is a proton, so it will always be moving to the left while it is in the B field. 
Anyway, starting from this equation, it can be shown that $$ r_{gyroradius} = \frac{mv}{qB} $$ so the faster your particle moves, or the more massive it is, the larger the radius of the circular motion will be, but the more charge it has, or the stronger the magnetic field, the smaller the radius will be. Check these relationships with your intuition to make sure it makes sense. 
So the find the velocity you are looking for, set the minium radius equal to the length of the region of the B field.
