/sci/ - Science & Math

>>9612428
Well you see Danny, when some electrons and a physicist love each-other very much....

>>9612428
Heat, light, or an applied voltage can yank electrons out of metal at the small end of the tube. The air has been pumped out so the electrons are not obstructed and move in straight lines.
The fat end of the tube is coated with a material which glows when struck by electrons.
The cross blocks some of the electrons, so you see its "shadow" on the glowing screen.

Electrically charging the cross or waving a magnet near the tube deflects the electrons. This was developed into both the triode amplifier (the basis of all electronics prior to transistors) and the CRT tubes of old-style TV sets.

>>9613101
>Yank electrons out of metal.
Not incorrect, just a bit vague. If you give an electron enough energy, it can escape the electric field (Remember Potential energy = kQq/r where q = -1.6E-19 C). This can be in the form of a short wavelength photon or by "boiling them off" with thermal energy, as you said. The minimum energy required to remove an electron from the surface of a metal is known as the work function.
This metal is the cathode of a high p.d. circuit, and the anode is usually a plate or funnel with a hole in the centre.The liberated electrons accelerate under this high p.d. and most smack into the anode. Some however get through the hole and come out with a high speed, usually at least 5%c. They continue onwards, and this is the cathode ray.
>Air sucked out.
This is true if they are simply hitting a scintillating screen (e.g. CRT TV's, oscilloscopes), but if you can see the ray travelling, it contains a low-density gas that glows when excited like helium or most of the noble gases.
>So that they move in straight lines
They do this anyway. Electrons in cathode ray tubes move so fast (1E+7-2.99E+8 m/s typically) that by the time they cross a 30 cm tube they only fall (under gravity) by about 1E-16 m. This is smaller than a nucleus, so completely negligible.
The "material which glows" is usually some phosphor compound.
Obviously, putting an electric charge or field exerts a force on the electrons and deviates their path.
A magnetic field does something similar, since the electric and magnetic forces are inexplicably linked (they have the same force carrier, the photon. Why is anyone's guess. They do seem quite separate, so it's unintuitive but correct to say that they are the same force.)

>>9614833
They are the same force.
>http://galileo.phys.virginia.edu/classes/252/rel_el_mag.html

I didn't know if OP was aware the tubes contained vacuum (or no more than a trace of gas). If there was anywhere near "normal" pressure (even Martian surface pressure), the electrons wouldn't get very far before being diffused or absorbed.

Otherwise, a most succinct explanation.

>>9612428
I'm baffled that "How does it work?" questions are posed here. The entire Internet is at your fingertips, and yet these are posed on a forum where there is no vetting and no-one is responsible for the answers.

>>9612428
How does www.google.com work?

>>9615623
Good proof, thanks. Had always wondered about that. (At university studying languages; only took physics to a-level, but had and still have a strong interest in wider physics.)
>Low density gas
Yes, I wasn't clear about that. Often a CRT won't contain any gas at all. If it does though, it's usually low density because there's not a lot of it(i.e. Low pressure, low number of molecules.) I made it sound like it was low density because it was helium or neon etc.
About the energy being absorbed, I question that slightly. They are essentially beta minus particles at these speeds, maybe even extremely fast beta minus particles. We know that regular beta minus particles can travel a metre or so in air, so I doubt they'd lose too much energy in a say 30 cm tube. Perhaps they'd be redirected a bit and we'd end up with a fuzzy patch on the screen rather than a dot, but all the energy lost?