# [SPM Physics Electronic] Why the 'X' position is there?

### Question

(c) (i) A set of magnet is placed across the deflection tube as in Diagram 2.2.
Satu set magnet diletakkan merentasi tiub pemesongan seperti dalam Rajah 2.2.

In Diagram 2.2, mark â€śXâ€ť the position of the cathode ray when it hits the fluorescent screen.
Pada Rajah 2.2, tandakan â€śXâ€ť kedudukan sinar katod apabila ia menghetam skrin berpendaflor.

Hi Farhaini,
Basically, a cathode ray is a beam of electrons emitted from the cathode of a vacuum tube.

Under normal circumstances when there is no electric field and magnetic field, the electron beam is in a straight, undeviated path.
Here is a simplified diagram to help you visualize it better:

So the front view would be a dot in the centre.

But for this question, there is a magnetic field applied.
When there is a moving charged particle (electron in this case) in a magnetic field, we know that there will be a magnetic force that acts on the charged particle.
So we have to use Flemingâ€™s Left Hand Rule to find the direction of this force.

We know that direction of the magnetic field is from North to South,
so the field lines look something like this:

So B, your index finger is pointing downwards.

We also know that the electrons are coming out of the page, or you can imagine the electrons are coming out of your laptop (or mobile phone) and directed towards you

But for Flemingâ€™s Left Hand Rule, we cannot directly use the direction of electron, but we have to use the direction of current. And we know that direction of electron opposes the direction of current.

Your middle finger should be pointing horizontally towards the computer screen or mobile phone screen, and it should be perpendicular to your index finger which is pointing downwards.

You will realize that your thumb is pointing towards the left, which means that there is a magnetic force pushing the electrons towards the left, hence the â€śXâ€ť mark is to the left of O. (I hope my drawing is understandable haha )

Hope this helps!

1 Like

I completely understand nowâ€¦ Thank you very much!