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In a CRT, where do the ejected electrons go after they cause fluorescence on the screen, have they lost most of their energy, or do they actually go through the glass?

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3 Answers 3

Typical commercial CRTs for televisions or computer monitors have electron guns that work in the range $5-30\text{ keV}$. Such electrons have a very short penetration depth in solids and will essentially all be stopped in the collimator or the glass of the tube.

See the Particle Physics Data Book chapter on the Passage of Particle Through Matter (PDF link!).

The result is that the screen will develop a net negative charge. This is the cause of the build-up of dust on the screen and of the low level discharge that you can feel if you run your hand over a CRT surface after it has been running for a while.

Left to itself the charge bleeds off. Mostly into the surrounding atmosphere, but also into the bulk of the CRT's case.

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If if bleeds off into the case, then isn't there an eventual pathway for them to be recirculated through re-emission? Does your answer take it as a given that the screen is non-conductive? What voltage would the screen reach from accumulated charge before it finds a pathway away? Would a potential on the screen really not affect the path of the electrons (which I think have to be aimed with high precision)? – Alan Rominger Jun 22 '11 at 4:21
50 to 120 kV is much too much. typical values are 5 to 30 kV. Exeeding this range would imply a lot of Röntgen radiation, not very beloved in living rooms. – Georg Jul 7 '11 at 11:37

CRTs have an "aquadag coating" of fine carbon particles that allow the built-up negative charge to bleed off to ground (or chassis, at least). Wikipedia:

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This coating is outside of the CRT, it does not help to get the electrons back. This coating helps "against" influenced loads, which produce strong dust collection. (Look at a anode ("Plate" for anglosaxons) cable) of a long used CRT. – Georg Jul 7 '11 at 11:43

Most of the beam current eventually flows through the anode. This happens via secondary emission (where a beam electron causes a phosphor molecule to emit an electron towards the anode) and conduction. The inside of standard (non-storage) CRTs is coated with a thin layer of metal (usually aluminum) to increase conductivity. Storage CRTs are made to have higher conductivity to prevent image blurring while in storage mode.

Aluminising the inside of the phosphor is important so that charge does not build up--Any charge accumulation on the phosphor will raise its potential. That reduces the effective acceleration potential of the electrons, which causes the screen to dim.

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