# Thermionic emission

I have a pretty basic question regarding the beam of electrons as a result of thermionic emission.

In an electron gun, the emitted electrons from the cathode become incident at a point on the other end of the tube, as they travel through a gap allocated in the anode.

I understand that the motion of the electrons from the cathode to the anode is as a result of the electric field, and as a result, the electrons are accelerated towards the positively charged anode.

1. Is this attraction, due the the electric field, the electrostatic attraction and repulsion, from the anode and cathode respectively?

After some electrons pass through the hole in the anode, they are no longer in the electric field, and so as a result, travel at constant velocity, towards the wall of the tube.

1. What i don't understand is why the electrons don't decelerate, as after they have passed the anode, would they not be attracted to it, due to it being positively charged,and electrons being positively charged?

2. Finally, is thermionic emission an explanation of how 'cathode rays' are produced? If not, how are cathode rays produced/ are there any other ways the elections can be emitted from the cathode?

Cheers.

• Why can't the 'anode' be at ground potential? – Jon Custer Mar 30 '16 at 0:18

## 2 Answers

1. Yes.
2. This is the cool thing about the parallel plate arrangement of electrodes in an electron gun. The field is strong and (fairly) uniform in the the space between the plates but much less outside of the plates. In the ideal case of a pair of infinitely large plates with an equal and opposite uniform charge distribution, the field outside the plates would be zero. If you have a point charge the electric field strength decreases proportionally to the distance from it squared. If you have an infinitely long line of charge the electric field strength decreases proportionally to the distance (not distance squared). But, if you have an infinite plane of charge the electric field strength it produces is the same - independent of the distance to the plane. Now, say you put your positive plate on the bottom it will produce an electric field directed upwards above it (away from the positive plate). If you then put a negatively charged plate above the positive plate it will produce an upward field below it (towards the negative plate). These two fields are both in the same direction and so result in a strong field. But, the negative plates field above itself is downwards (still towards it) and the field there from the positive plate is upwards (still away from it). So in the region above the negative plate (or below the positive plate) the fields from each are in opposite directions and, since the distance from the plates does not affect their strength, they cancel out completely. Obviously in an electron gun the anode and cathode are not infinitely large but even with finite plates the field "outside" of the anode is much, much smaller than on the field between the plates so the electron is not pulled back. Having written all this and then looked at some images of electron guns I realize the the version I described is a simplified one with a hot filament between two planar electrodes. The geometry of practical electron guns is a bit different.
3. Thermionic emission gets the electrons out of the metal filament but I don't think you can really call the electrons a cathode ray until you accelerate them. A regular incandescent light bulb will produce thermionic emission but the emitted electrons are just attracted back to filament. There are cold cathode devices that emit electrons by means other than thermionic emission.

I take it that the scenario in mind here is a gridlike anode, with a solid anode beyond it.

It is when the electric potential is constant beyond the first anode that the electrons are not decelerated by it. The constancy of the potential between them is a consequence of the being held at the same potential as each other of them. In a sense, the entire region that the electron is now travelling in is the anode.

Also it's necessary that the electrons have some heat in order to pass an anode made of filaments, or an otherwise perforated anode. If an electron started off with zero velocity of it's own, it would 'slide' down the very floor of an eleectric-potential 'valley' terminating in the conducting substance of the anode. Insofar as it has some thermal motion, it can 'skip-up' the sides of that valley & therefore miss the substance of the anode. Insofar as the (energy) distance it must 'skip up' the side is a small fraction of the thermal energy, the proportion of electrons that pass the anode will approach the proportion of the anode presenting a gap, ie that is empty of substance. What I'm talking about here is more a grid really.