CRT direction of field in accelerating zone

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Discussion Overview

The discussion centers around the direction of the electric field in a cathode ray tube (CRT) and its relationship with the acceleration of electrons. Participants explore concepts related to electric fields, charge interactions, and the implications of changing distances in the context of the CRT's operation.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes confusion regarding the textbook's claim that the electric field direction is to the left while electrons are accelerated to the right, questioning the implications for field line orientation.
  • Another participant clarifies that the CRT's front acts as a positively charged plate and the electron source as a negatively charged point source, suggesting that the field direction is from front to back (to the left).
  • A third participant acknowledges the misunderstanding that a charge cannot feel its own field, indicating that the field affecting the electrons is external to them.
  • A participant raises a question about how the electric field produces acceleration in moving electrons, referencing the formula F=ma=qE and expressing confusion over the constant value of the electric field given that the distance (r) changes as electrons approach the field.
  • This participant questions how the constant value of E (8.0 X 10^5 N/C) can be valid if the electric field should vary as the electrons move closer to the source.

Areas of Agreement / Disagreement

Participants express confusion and differing interpretations regarding the direction of the electric field and its implications for electron acceleration. There is no consensus on the resolution of these points, and multiple views remain present.

Contextual Notes

Participants highlight assumptions about the nature of electric fields, the behavior of charges, and the implications of changing distances in electric field calculations. The discussion does not resolve these complexities.

Who May Find This Useful

Individuals interested in the principles of electric fields, charge interactions, and the operation of cathode ray tubes may find this discussion relevant.

acherentia
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My textbook says that since the electrons are accelerated to the right in a CRT the field direction is to the left. This leaves me confused, because according to this model the field lines are pointing out from the electrons instead of pointing into the electrons. I thought that field lines are supposed to point into negative charge? Anyone can tell me where I go wrong?
 
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The front of a CRT is essentially a positively charged plate, while the electron source is essentially a negatively charged point source, so the field direction is from front to back, which is to the left in the case you mentioned.
 
I keep forgetting that a charge cannot feel its own field, so obviously the field is not the electrons' field but a field that the electrons are subject to. Thank you.
 
How does the electric field produce acceleration in the moving electrons?

F=ma=qE?

My textbook gives a value for E CRT of 8.0 X 10^5 N/C. Which if this was a constant number, according to my calculations a=1.41 x 10^17 m/s^2.

But I also know that E=kQ/r^2. Since r is changing as the electrons are moving closer and closer into the field, so r is getting smaller and smaller, shouldn't the value of E be constantly changing.

If the value of E is constantly changing as electrons move closer to it, how did they come up with the number 8.0 X 10^5 N/C for E?
 

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