# How does this particle behave in the magnetic field?

I have the above question and I have though about it every way and can't seem to find out. So what I have concluded (ignore the answers on the img) Is that the force is on the z axis because the cross product of the two would be the normal of the plane it lies on which is into or outof the screen.

Thus I am saying that first one is false because it is on the z axis, the finite amount I said true because it leaves the magnetic field when being pushed perpendicularly away. The next one because it IS normal to the velocity. Lastly the y-component is unchanged again because the force points the z direction not the y.

Any reasons to what I am thinking wrong any help would be appreciated.

• Just because a force is always normal to the direction of travel doesn't mean that the object will necessarily travel in a circular orbit. – DumpsterDoofus Mar 14 '14 at 20:54
• @DumpsterDoofus Okay that makes sense actually I was thinking perhaps the circular part would cause me issues I will try it out – capa_matrix Mar 14 '14 at 21:01
• @DumpsterDoofus So according to the rest of my logic using false,true,false,true is still wrong so are there any other flaws I can't see – capa_matrix Mar 14 '14 at 21:03

You can simply decompose the velocity in its $v_x$ and $v_y$ components. The first one will remain unaffected (do a Galileo transformation, if you are not sure, and observe from a frame moving at $v_x(0)$ in the $x+$ direction).
• The force is in the $YZ$ plane, never in $\vec x$
• The particle has a constant $v_x$, so the trajectory cannot be circular.