The following statement is from the book Concepts of Physics Volume 2, by Dr. H.C. Verma, chapter 41 - "Electric Current through Gases", topic "Cathode Rays", page 343:

Cathode rays are emitted normally from the cathode surface. Their direction is independent of the position of anode.

I found the explanation for the first sentence from the section perpendicular emission of the Wikipedia article on Crookes tube. However, I don't understand the second sentence in the quoted statement. It's said that the direction of the cathode rays is independent of the position of anode. Initially, I thought this statement is incorrect as the negatively charged stream, the cathode rays must get deflected towards the positively charged anode. But after seeing the experimental setup of a Crooke's tube (the one with conical flask) it's seems the direction of cathode rays is independent of the position of anode:

Crookes tube - Power off

Image cropped from Crookes tube - Wikipedia

One possible explanation, I came up with is: The negatively charged electrons from the cathode experience strong repulsive force from the cathode, due to which they gain very high speeds. Due to this they are not significantly deflected by the anode. But if this is the case, I doubt why we must have an anode inside the discharge tube and carry-on only with the cathode? So I think this kind of explanation is incorrect.

To put my question in a nutshell:
Why is the direction of cathode rays independent of the position of anode?


The electric field created by the anode is too weak to significantly influence the emission of the electrons from the cathode, which is mainly due to thermal effects, hits by the gas molecules, electron repulsion among themselves, etc.

However, once the electrons are released from the cathode, they do feel the anode field and start accelerating towards the anode. Anode is necessary in order to drive electric current by removing the electrons from the tube. Otherwise, a negative electron cloud would accumulate in the tube and prevent further electron emission.

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  • $\begingroup$ Thank you for your answer. If I understood discharge in a linear tube properly, the field between the cathode and anode is responsible for the acceleration of electrons. If so, in the setup in the question, why wouldn't the electrons move downward. I don't understand how they accelerate in the first place. $\endgroup$ – Guru Vishnu Mar 11 at 13:23
  • $\begingroup$ Not sure I understand the question: you think they should move downward because of the gravity? $\endgroup$ – Vadim Mar 11 at 13:25
  • $\begingroup$ I think they must move downward because of the presence of anode (in the image above) and not because of gravity. But they don't move down in reality. I understand that the effect of gravity on electrons is negligible due to its low mass. $\endgroup$ – Guru Vishnu Mar 11 at 13:26
  • $\begingroup$ They eventually get to the anode, however the acceleration to to the anode is small, compared to the electron velocity when the leave the cathode. $\endgroup$ – Vadim Mar 11 at 13:29
  • $\begingroup$ Fine. If possible, could you tell why the electron must get accelerated towards the left when the electric field is from the anode to cathode (something like a quadrant of a circle)? $\endgroup$ – Guru Vishnu Mar 11 at 13:30

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