# Trajectory of charged particle moving in a magnetic field

by BogMonkey
Tags: charged, field, magnetic, moving, particle, trajectory
 P: 76 In this video here are those pink green the magnetic field lines? http://www.youtube.com/watch?v=a2_wUDBl-g8 If so where does the proton come from? If I'm not mistaken a charged particle in motion will only be influenced by the magnetic field if its not moving parallel to the magnetic field lines but in this video the proton appears to be moving in the direction of the magnetic field lines its only the centripetal motion thats perpendicular to the green lines. Does that mean the proton flies into the magnetic field then gets stuck in that circular trajectory? If so whats causing it to move in the direction of the magnetic field lines? For example in this diagram Would that black arrow be the direction that the charged particle comes in or are the pink arrows the direction of the particles velocity before it entered the magnetic field?
P: 1,057
 Quote by BogMonkey If I'm not mistaken a charged particle in motion will only be influenced by the magnetic field if its not moving parallel to the magnetic field lines but in this video the proton appears to be moving in the direction of the magnetic field lines its only the centripetal motion thats perpendicular to the green lines.
You are partly right. There is no influence parallel to the magnetic field. No influence means no force. By Newtons first law this means the particle will keep a constant velocity in this direction. So basically the particle is moving in that the direction, because nothing stops it.

 Quote by BogMonkey Does that mean the proton flies into the magnetic field then gets stuck in that circular trajectory?
Yes, in a constant magnetic field, a proton will exhibit helical motion.

 Quote by BogMonkey If so whats causing it to move in the direction of the magnetic field lines?
Just inertia.

 Quote by BogMonkey Would that black arrow be the direction that the charged particle comes in or are the green arrows the direction of the particles velocity before it entered the magnetic field?
The green arrows are the path of the particle in the magnetic field. I suppose the particle came from outside the sketch where there is no magnetic field. Not sure what the black arrow is for.
P: 76
Damn I meant to say pink lines in the diagram not green lines. What I was really asking is if the pink lines are the magnetic field lines. I see that the green circular lines are the trajectory of the particle but I'm wondering if the particles initial velocity upon entering the magnetic field is parallel or perpendicular to those pink lines. Or would it be that blue line denoted v?

 Quote by Gerenuk Just inertia.
Is this inertia always in the direction of the magnetic field lines?

P: 1,057

## Trajectory of charged particle moving in a magnetic field

The pink lines are the magnetic field. The particles initial velocity is along the green lines. So it's basically somehow diagonal. If it were parallel to the magnetic field, then it would go on a straight line. It it were perpendicular to the magnetic field then, it would go in circles at a fixed position. The blue lines are the current velocity of the particles.

Basically two things happen:
The vector component of the velocity along the magnetic field stay unaffected. The vector component perpendicular to the magnetic field turns around so that the particle circles around magnetic field lines.
 P: 76 Ah right that explains it. Thanks a lot.

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