Behaviour of electrons in a teltron tube

[DDesulgon]

If I switch off the anode voltage in a teltron tube (used to determine the charge-to-mass-ratio of the electron), the visible electron beam disappears. Why is this so? The electrons, which were already shot out of the electron gun before, have a high velocity and should continue to move on a circular path with constant velocity? Or are they slowed down so much by collisions with the gas molecules?

 

[tech99]

The tube has a diode electron gun. If the diode-anode is disconnected, no electron beam will emerge. This is because the diode-anode is used to attract and accelerate the electrons from the cathode. With no voltage applied, the electrons fall back to the cathode.
The beam is made visible by a tilted fluorescent screen, and the tube does not use gas to make the beam visible.
Have a look at the circuits in the following:-
https://www.philipharris.co.uk/prod...ructure/teltron-tube-deflection-e/m-/b8a49530

 

[dlgoff]

teltron tube

I remember experimenting with a teltron tube in an advanced lab course at my University.

 

[DDesulgon]

Thanks for the response, tech99, but that's not quite what I meant. I was referring to something like this:
https://en.wikipedia.org/wiki/Teltron_tube#/media/File:Cyclotron_motion_wider_view.jpg

I can see that on this particular picture, there is some sort of metal plate stopping the electrons, but let's assume this wasn't the case. Shouldn't the electrons be visible even if the accelerating voltage was switched off?

 

[rude man]

The electrons are not emitted "at high velocity". They reach their anode velocity by virtue of having been accelerated by the electric field set up by the anode voltage, which you have turned off.

 

[DDesulgon]

The electrons are not emitted "at high velocity". They reach their anode velocity by virtue of having been accelerated by the electric field set up by the anode voltage, which you have turned off.

Of course, I understand that. My question is: Why do these electrons (which have already been accelerated) not keep circling, after the anode voltage was turned off? After they leave the electric field, they move at constant velocity, with the Lorentz force forcing them on a circular path. Why is this not some sort of perpetual motion?

 

[sophiecentaur]

Why do these electrons (which have already been accelerated) not keep circling

The electrons are actually going pretty fast. See the graph on this link to find the sort of speed for a range of eV. The transit time is not many microseconds so you would never spot the effect you are looking for. The fall time of the EHT volts would be at least a few milliseconds after switch off.

 

[rude man]

Of course, I understand that. My question is: Why do these electrons (which have already been accelerated) not keep circling, after the anode voltage was turned off? After they leave the electric field, they move at constant velocity, with the Lorentz force forcing them on a circular path. Why is this not some sort of perpetual motion?

Looks like I'm not sufficiently familiar with a Teltron tube to answer that.

If the E field is gone but the B field persists then you'd get circular motion & no change in speed. If the B field also disappears then you get rectilinear motion from that moment on, also no change in speed. But I'm just telling you what you seemingly already know.

Is it possible the electrons assume a circular path with radius large enough to bang against the side of the tube? That would make them disapoear at a side. At what point does the beam disappear in the tube relative to its sides?

 

[sophiecentaur]

How low is the vacuum? What mean free path? Even 100 loops would be over before you’d know it.

 

[DDesulgon]

We have hydrogen gas with a pressure of about 1 Pa in the tube. Do you think that the electrons then collide very quickly with the gas molecules and are thus slowed down when the anode voltage was switched off?

 

[jtbell]

Do you think that the electrons then collide very quickly with the gas molecules

What makes the electron beam visible?

 

[DDesulgon]

Well, the very same thing of course. I just thought that this only happens to very few electrons (because of the low pressure of the hydrogen gas). But perhaps I was wrong there?
This would also explain the observation that the electron beam becomes weaker "at the end" of its circular path, while it is most clearly visible directly after exiting the anode, wouldn't it?

 

[sophiecentaur]

What makes the electron beam visible?

I now seem to remember the electron beam is fan shaped. The electrons are only seen when they hit a screen-is that right?
EDIT:
I now realize there is yet another form of Textron tube - gas and no screen. So, to see the electron beam you would need a reasonable number of collisions - i.e. taking energy from the beam and reducing its current. In fact, the wiki image shows the circle getting fainter and fainter along its length.

 

[jtbell]

I now realize there is yet another form of Textron tube - gas and no screen.

Right, the Teltron tubes that I used for a long time in our e/m experiment, made the beam visible with a phosphorescent screen.

Based on the picture that the OP posted up-thread, his tube is more like one that I used earlier, in which the beam excited the gas along its path. Our tube used mercury vapor which made the beam "glow" light blue. The OP's tube uses hydrogen which produces a purplish glow that reminds me of our hydrogen discharge tubes for spectroscopy experiments.