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I have seen two contradictory descriptions of how electrons and other charged particles travel around magnetic fields; in one, they travel in circles around magnetic field lines, in the other, they travel in coil or spring-like paths around magnetic fields, with that said, which path do they actually travel when it's around a normal permanent magnet?

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  • $\begingroup$ One can setup a system with orthogonal magnetic and electric fields in which the charged particles will actually travel along ballistic (i.e., straight) trajectories in a direction orthogonal to both fields, parallel to the direction defined by $\mathbf{E} \times \mathbf{B}$ (i.e., called the ExB-drift). So no, they do not always travel in helical paths about paths of constant magnetic flux. $\endgroup$ – honeste_vivere Jun 4 '17 at 17:03
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In the case the velocity of the charged particle is perpendicular to the external magnetic field. By this it's not important be this a permanent magnet or an electromagnet. After the electron leaves the magnet it's strength is unchanged. In the cas the velocity is not parallel to the magnetic field the path is a helical.

In both cases looking parallel to the magnetic field the path will be a spiral. This has to do with the emission of photons from the electron. The electrons kinetic energy gets "exhausted" by the photon emission and the electron at the end came to standstill.

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track

If a charge moves into a magnetic field with direction perpendicular to the field, it will follow a circular path. The magnetic force, being perpendicular to the velocity, provides the centripetal force.

If the momentum of the particle and the magnetic field line are perpendicular, then it is a circle. If there exists a momentum component in the direction of the magnetic field line, it is a spiral . Both the circle and the spiral are affected by the ionization energy loss in the medium, the (projected in case of a spiral) circle radius getting smaller as the track lengthens

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  • $\begingroup$ Anna the particle travels in a spiral path. As you know, there is an emission of photons and the particle lose kinetic energy. The velocity decreases. In reality it's not only a spiral path but a chain of tangerine slices along this spiral path due to the discrete photon emission. $\endgroup$ – HolgerFiedler Jun 4 '17 at 19:36
  • $\begingroup$ @HolgerFiedler the loss is very small for the pictures I presume the OP is talking about. It is mainly ionisation losses with interactions in the medium. (in vacuum is a different strory but no pictures can be seen) $\endgroup$ – anna v Jun 5 '17 at 3:35
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When the charged particle enters the magnetic field with it's velocity perpendicular to it then only the charged particle move on perfect circles or on a part of it till it is inside the magnetic field. Now if the initial velocity of the particle is inclined with the magnetic field at some angle then the resultant motion of circular path and straight line will be seen(I.e. a helix or spiral shaped path) because now one component of the initial velocity will be perpendicular to it describing a circle and the other component will be along it so that the force of magnetic field is zero on it and the path of this component will be a straight line! Hope you'll counter the question with this approach!

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