Video: Eg17S1-Physics-Q29B



Video Transcript

State one effect of stopping the action of the electric and magnetic fields in a cathode ray tube as an electron beam passes.

Well, before we talk about the effect of stopping the action of these fields, let’s talk about why they’re there in the first place. If we looked at some of the major components of a cathode ray tube, those would include the cathode, which is the source of electrons which helped to form the beam. Then the anode, which attracts the electrons through an opposite electrical polarity. And then once the electrons are up to speed, the beam only needs to be directed, whether up or down left or right.

To make adjustments in the vertical direction, an electric or magnetic field is formed in between parallel plates. The purpose of this field is in a precise and controllable way to be able to vertically redirect the beam of electrons. And then, similarly, there are a pair of parallel plates with an electric or magnetic field between them, which are designed for horizontal direction.

From our vantage point, that would affect whether this beam moves into or out of the screen. When the beam emerges from the horizontal and vertical directors, it’s ready to move ahead in a straight line from where it’s moving onto the screen. This screen is the only part of the whole apparatus that the viewers actually see.

It’s through careful adjustment of the electric and magnetic fields responsible for vertical and horizontal redirection of the beam that the beam is able to impact the screen at different locations and indeed scan across the screen to create an image. And this all happens very quickly of course so that our eye seems to be seeing a continuously moving image.

As we mentioned, there’s a physical driver allowing us to horizontally and vertically redirect this electron beam. Those causes of redirection are electric and magnetic fields that exist between these pairs of parallel plates. Based on the direction and magnitude of those fields, the electron beam is redirected accordingly.

But what happens, our question wonders, if we were to stop the action of those fields, if we were to turn them off completely? Well, in that case, there would no longer be any reason for the beam of electrons to be diverted off of its original path. It would make no adjustments in the vertical or horizontal direction. It would just continue on straight through unaffected. And then it would hit the very center of the screen.

We can write this as one effect of stopping the action of the magnetic and electric fields. That the electron beam passes straight along and it strikes the screen at the midpoint of the screen.

Another way to think of this is as viewers on the other side of this CRT screen. To our eye, what would this look like if the magnetic and electric fields were turned off? We would just see a very bright spot at the center of the screen that wouldn’t move. Another way then of stating this effect is to say that a luminous spot appears at the middle of the screen and no image is formed. These are two different, yet equivalent, ways of saying what would happen if we turned off the electric and magnetic fields in a cathode ray tube.

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