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Lorentz force using an electron beam deflection tube

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LAN_403

Physikfan

Sun Jun 25 2017, 04:02PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

I'm planning demonstration experiments on the Lorentz force

F = q (E + v x B)

F force on the charge
q charge
E electric field strength
v speed â€‹â€‹of the charge
B magnetic flux density

with an electron beam deflection tube:

P1190673400x399

The tube is pear-shaped, on the right is the electron source with a heated cathode.
An electric field can be applied to the electrons by means of two capacitor plates,
which are installed in the tube and are located above and below a fluorescent white screen which
can make visible the path of the electrons.
The electric field is thus perpendicular to the direction of propagation (and thus also to the velocity) of the electrons.

The magnetic field of two Helmholtz coils (the two black circular rings in the image) will provide the magnetic deflection of the electrons. This magnetic field is also perpendicular to the propagation direction of the electrons.

Please, has someone already done similar experiments and shares with me his experiences?

Regards

Physikfan

Dr. Slack

Mon Jun 26 2017, 04:49AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I did that in 1972, as part of my Physics A-level, with the school apparatus. IIRC there were no surprises, it all checked out with what should happen within instrument reading uncertainty. My memory is not good enough to tell you what sort of order of accelerating voltage we used, whether it was 10v or 1000v. We also went outside the lab notes and poked bar magnets at it, with the expected results.

Old CRT oscilloscopes use the electrostatic version of this, while old tube TVs use the magnetic version, the difference being cost, bandwidth, and how well it fits into other aspects of the instrument like the cosmetic premium put on tube shortness (ie reciprocal length).

Some old oscilloscopes had a Helmholtz coil arrangement coaxial with the tube, which was a 'trace rotate' magnet, so that the trace could be lined up properly with the graticule/grid marked on the face of the tube. Reasons for misalignment were a) the tube electrode placement tolerance b) a replaceable graticule positioning error and c) an unexpected axial magnetic field.

Back at the end of the 70s, I worked on the first, or one of the first, instruments with a digital graphical display, the TF2370 spectrum analyser. It used a cheap magnetically deflected CRT, and a collosal 4kbit display memory (not RAM, it was sequential access, 4 1k MOS ICs with a dual clocked CCD bucket-brigade shift register (we nearly went with tried and trusted acoustic quartz delay lines, but saw the way the world was going and opted for the then super expensive ICs)). Because we could, we wrote the graticule on the display, instant end to alignment problems. Fast forward to another job a few years later and I was working on MRI scanners. They had a problem that the high external field from the superconducting magnets meant that the trace on spectrum analysers in the lab shifted excessively. I recommended the 2370, and they got one in and played with it. Within a few metres of the 0.15T magnet (very early MRI!), any misalignment with the cylindrical axis sent the display off the face of the tube, but keeping it in alignment only rotated it, and of course the trace and graticule together. As we advanced down the axis as far as we could, the trace rotated a whole revolution, and then a bit more, to much amusement of all concerned. It was almost as much fun as when we got a forklift truck stuck to the magnet, and had to wind the field down (a long and painstaking job) to free it.

johnf

Mon Jun 26 2017, 09:10AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Why is this here
Senior high school
or Physics 101 for those who missed the high school lesson

Probably in this internet enabled age this sort of thing is distant history
and has been replaced by learning HTML

Sulaiman

Mon Jun 26 2017, 12:14PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

A large part of this forum is dedicated to 1900's technologies such as Tesla Coils, Electromagnetic Waves, Telecommunications ....

Personally, I like old physics fundamentals demonstrations - more please !

There is in my opinion a HUGE difference between reading, and doing.
Hands-on experience gives me a 'feel' for things
whereas reading (or being taught by a poorly read teacher) can introduce misunderstandings.

Keep it up Physikfan, you have at least one follower :)

EDIT: One of the greatest losses to my hometown of Birmingham was the loss of the science and industry museum

Physikfan

Tue Jun 27 2017, 10:00AM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Dr. Slack, johnf and Sulaiman

Thank you very much for your answers and comments.

Ad johnf:

"Senior high school or Physics 101 for those who missed the high school lesson"

You are right, the Lorentz force is a historic experiment and may be explained in the high school or in an undergraduate physics lab.
But it makes a difference, whether one has to demonstrate experiments in front of students within a very limited time, or you can perform experiments with pleasure without time pressure, topics in which I was always interested in.

Especially the Lorentz force is very important in electrodynamics:
motors, particle accelerators, magnetic field-dependent resistors, Hall sensors, interactions of elementary particles from space with the magnetic field of the earth, loudspeakers, microphones, mass spectrometers, etc.

Just basic experiments, if you carry them out by yourself, always lead you back to the wonderful world of experimental physics.
Also at the training of students in the lab, you really can see, how hard it is actually to deepen the existing interest of the students and to put their physics knowledge on a solid basis.
Once they will arrive at school, they should be able to build, demonstrate, and explain experiments on physics.

Ad Sulaiman:
"A large part of this forum is dedicated to 1900's technologies such as Tesla Coils, Electromagnetic Waves, Telecommunications ....
Personally, I like old physics fundamentals demonstrations - more please !
There is in my opinion a HUGE difference between reading, and doing.
Hands-on experience gives me a 'feel' for things
whereas reading (or being taught by a poorly read teacher) can introduce misunderstandings."

Thank you very much for your support for posting physics demonstrations in this forum.

State of the art experimental physics has its place in scientific journals,
basic physics experiments with some didactic background are published in didactic journals,
there is also a scientific community on historic physics apparatus.

In my opinion this forum is a wonderful mixture of members with different backgrounds, from interested amateurs up to professionals.

Up to now from all members I could learn a lot.

Best regards

Physikfan

Sulaiman

Tue Jun 27 2017, 03:27PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

P.S. on offer at the moment ...

Physikfan

Tue Jun 27 2017, 08:00PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Sulaiman

Thanks for your hints.
A modern version of my electron deflection tube is also here:

radiotech

Wed Jun 28 2017, 02:59PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Goodness, at the cost of that tube, I shudder remembering some of clutz stunts
dropping things past classes.

Here is a tube that cost about a dollar.

1498661946 2463 FT180049 Mass Of Who Know What Experiment

Physikfan

Thu Jun 29 2017, 07:38AM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Here are links to experiments with this electron deflection tube:

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