# Where do electrons come from in thermionic emission?

Let we suppose to have a HV Tube or CRT . The filament is connected to the secondary of the transformer used to supply the filament.

At time $t=0$ the wires and cathode are neutrally charged. Then when we switch the power on, the filament begins to heat and free electrons are being evaporated from the cathode. Now I would come to conclusion that if the electron has left the metal there should remain a hole and the cathode would begin to be more and more positively charged, but this is not happening.

Are those electrons replaced? If yes, then from where?

Or if they aren't emitted at all, are they generated?

It might be clear from the picture that, electrons emmited by cathode never strikes the anode. The anode is used for accelerating the electron, not to recombine the electron.
Further question is: Where the electrons in anode come from? For accelerating the electron beam, energy is used proportional to the kinetic energy of electrons, but how the current flows if the electons never strike the anode?

• First off, its left not leaved, secondly electron flow is what allows for current. You switch on the current and electrons leave the tube and is being replenished by the power source. – Horus Sep 21 '15 at 13:17
• @Horus, he may not be a native English speaker. Instead of crapping on grammatical errors, why didn't you just suggest an edit? – Sean Sep 21 '15 at 13:42
• @Sean Merely pointing it out is as good as telling them to edit is it not? Also telling him to edit without telling him his mistake is unhelpful. My comment may seem mean in retrospect but it was not intended. – Horus Sep 21 '15 at 13:45

Here is an image with the conducting layer all around the glass from the inside:

and here is the circuitry (from a different link):

As you see the circuit closes with the conducting layer. The power supply provides the energy to keep the cathode negative and the anodes positive. In this diagram the heating of the cathode comes from a different power supply. The electron beam part of the circuit current

Here is another paper

It should be mentioned that the screen has a plate of some metal (often aluminum) underneath the coating of phosphors; This plate is given a very strong positive charge, on the magnitude of several thousand volts. This positive charge pulls the electrons strongly toward the screen.

and of course, close the circuit with the power supply.

With the above in mind,

Now I would come to conclusion that if the electron has left the metal there should remain a hole and the cathode would begin to be more and more positively charged, but this is not happening.

Are those electrons replaced? If yes, then from where?

because the power supply through the circuit replaces the charge

Or if they aren't emitted at all, are they generated?

Of course they are thermally emitted. If there were no thermally emitted beam of electrons the circuit would represent an elaborate capacitor circuit, no current would be flowing.

To keep the electrons flowing, you need to connect the anode and cathode of the tube to a suitable circuit. If the cathode is at a sufficiently negative voltage compared with the anode, then the electrons being emitted by the cathode will be constantly replaced with new ones.

• This is true if electrons hit the anode, like in the triode, penthode. In CRT tube or electron gun the electrons leave the the tube, the anode is a ring called Wehnelt cylinder. Anyway, forget the cathode and anode, we have a tube and this tube is generating an electron beam, its a particle generator. I want to know how this electrons are born in the tube. – Marko Buršič Sep 21 '15 at 14:35
• The electrons in a CRT don't leave the tube. They hit the back of the screen. The screen is coated in a conductive material which acts as another anode, and this allows the electrons to be collected and recycled. – Simon B Sep 21 '15 at 15:10
• THe coating just transforms the kinetic energy of the electrons to the visible light. Without coating the electron would pass trough. The coating is not connected, do not confuse this coating with graphite coated coil in TV CRT. – Marko Buršič Sep 22 '15 at 9:12