Trace of cathode ray in changing magnetic field.

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Discussion Overview

The discussion revolves around the behavior of cathode rays in the presence of magnetic fields, particularly focusing on their trajectory when entering uniform and varying magnetic fields. Participants explore theoretical implications and potential outcomes related to the interaction of cathode rays with magnetic fields, including their paths on a phosphor screen and the effects of alternating current in coils.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that cathode rays follow a helical pattern in a uniform magnetic field, leading to a single point on a phosphor screen if the beam intersects a plane.
  • There is a suggestion that if the magnetic field strength changes, the path traced by the cathode ray could form a spiral pattern, with the helix radius inversely proportional to the magnetic field strength.
  • One participant questions how electrons behave when approaching a coil energized with alternating current, proposing that they might form a current loop with a diameter dependent on the magnetic field strength.
  • Another participant argues that if the magnetic field is parallel to the beam, the electrons would not be affected, prompting further inquiry into the effects of a varying magnetic field.
  • Concerns are raised about the implications of Faraday's law, suggesting that a time-varying magnetic field should induce an electric field that could affect the motion of the electrons.

Areas of Agreement / Disagreement

The discussion contains multiple competing views regarding the behavior of cathode rays in varying magnetic fields, with no consensus reached on the effects of a non-uniform magnetic field or the behavior of electrons in relation to an energized coil.

Contextual Notes

Participants express uncertainty about the specific conditions under which the cathode rays interact with the magnetic fields, including the assumptions about the orientation and strength of the fields, as well as the distance from the coil.

Samson4
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I know that cathode rays follow a helical pattern if they enter a uniform magnetic field at an angle less than 90 degrees. This leaves me with two questions.
1. If the cathode ray described above, hit a phosphor coated screen, would it show only 1 spot or a circle?

2. If the magnetic field was constantly changing in strength, would the spot trace a path of spiral, concentric circles or some other shape?
 
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1. The beam follows a helical pattern indeed, so the intersection with a plane is always just one single point (ignoring the beam size).

2. Helix radius is proportional to 1/B . All other things remaining the same you get the spiral. (screen ##\bot## Helix axis)
 
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Thank you so much.
 
If the cathode ray is aimed through a coil; and the coil is energized with ac, how will the electrons behave as they approach the varying magnetic field generated by the coil?

I think the electrons will form a current loop with increasing diameter as it approaches the coil. I also think the diameter of the loop will depend on the strength of the magnetic field. The direction of spin will correlate with the polarity of the field. Is this all wrong?
 
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If you aim properly, the magnetic field is parallel with the beam, so nothing happens to the electrons :smile: .

If you mean something else, perhaps you can post a drawing of the situation ?
 
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Oh no, these electrons are killing me. That's exactly what I thought until I "learned" more and convinced myself otherwise. :H

How come the varying magnetic field won't effect the electron beam? I thought a uniform time-varying magnetic field induces an electric field by Faraday's law. If an electron at rest will spiral in a time varying field, why won't one moving towards the field also spiral? On another view of the same situation; if the coil is far enough away or small enough, wouldn't the electrons experience a force in the direction of the nonuniform magnetic field?

If I'm not making since I will make a drawing after work. I really appreciate your help.
 

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