# Electrons direction in cathode ray tube

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.

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:

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?

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.
• 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. – Puk Jul 16 at 19:49