This image below show the movement of an electron in a magnetic field, it's taken from the video [Histoire des sciences] La découverte de l'électron explaining the experience of J.J. Thompson when he discovered the electron.

                    Cathode ray tube

I don't understand why electrons are going the way indicated the red dotted line. Below is how I understand this image with the right-hand rule:

Cathode ray tube (edited)

Because the force $\vec{F}$ is oriented toward the top, a positively charged ion (or cation) should go toward the top as shown on the picture. But here, it's an electron, a negatively charged particle, so it should be going toward the bottom. So why is it going toward the top? What's wrong in my understanding?

Also, is it correct to mark a plus (+) and a minus (-) on the magnet?


1 Answer 1


The force on a particle with charge $q$ and velocity $\vec{v}$ due to a magnetic field $\vec{B}$ is given by $\vec{F} = q(\vec{v} \times \vec{B})$. For an electron, $q$ is of course negative. For the geometry in the figures you posted, $\vec{v} \times \vec{B}$ points downward, and hence the force is upward.

In terms of the vectors in the second figure, I suppose $\vec{I}$ indicates the direction of current flow, which is in the opposite direction of electron flow. The right hand rule says that the force is in the direction of $\vec{I}\times\vec{B}$, which again is upward, as you can deduce by aligning your thumb with $\vec{B}$ and your middle finger with $\vec{I}$, whereby your index finger will point upward. For a positively charged particle, $\vec{I}$, and hence $\vec{F}$, would have the opposite orientation.

Marking a whole magnet plus or minus doesn't make much sense. Magnets have two poles: north and south. Outside the magnet, $\vec{B}$ tends to point from north to south. In this case, the magnet you labeled with a plus likely has its north pole facing the tube, while the other magnet has its south pole facing the tube. The two cylinders in the figure could also be (and I suppose usually are in practice) two different poles of the same magnet.

  • $\begingroup$ Sorry, I still don't see what's wrong in my understanding... $\endgroup$
    – DevonDahon
    Jul 16, 2019 at 15:05
  • $\begingroup$ With $\vec{I}$ oriented as shown in your second figure, you will find by applying the right hand rule that $\vec{F}$ is upward. But this direction of $\vec{I}$ assumes that the particles are negatively charged: otherwise $\vec{I}$ would point in the opposite direction and $\vec{F}$ would be downward. $\endgroup$
    – Puk
    Jul 16, 2019 at 19:49

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