B field lines of an infinte solenoid?

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Discussion Overview

The discussion revolves around the magnetic field lines of an infinite solenoid, focusing on the nature of the magnetic field both inside and outside the solenoid. Participants explore theoretical implications and conceptual understandings related to magnetic field behavior in this context.

Discussion Character

  • Conceptual clarification, Debate/contested

Main Points Raised

  • One participant asserts that the magnetic field lines of an infinite solenoid point straight up the solenoid with the field being zero outside, questioning whether field lines must wrap back around themselves.
  • Another participant argues that field lines must wrap around due to a fundamental rule of magnetism stating there are no sources or sinks, suggesting that the concept of an infinite solenoid implies a long enough solenoid for the field inside to be parallel to the axis.
  • A different perspective suggests that while the field lines may appear to wrap back at infinity, as a finite solenoid is extended, the return field outside becomes weaker and spreads out transversely towards infinity.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of magnetic field lines in relation to an infinite solenoid, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

Participants reference assumptions about ideal conditions and the behavior of magnetic fields at infinity, which may not be fully resolved in the discussion.

cragar
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When I have an infinite solenoid , The b field lines point straight up the solenoid and the field is zero outside. But i thought that the field lines have to wrap back around on themselves.
Does this not apply in this example . Or am i crazy?
 
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Good one. It has to rap around. I think when people talk about infinite solenoid, they just mean long enough that the field inside is parallel to the axis of the solenoid, nothing more. There is a more fundamental rule about magnetism that there is no source and sink, all field rap around no matter what. They got to come back.

When they talk about a closed contour surrounding a few turns of the coil, they assume the side perpendicular to the axis of the coil must be small. And they assume ideal situation. If you extend the loop far out enough, you are going to catch some of the return magnetic field.
 
cragar said:
When I have an infinite solenoid , The b field lines point straight up the solenoid and the field is zero outside. But i thought that the field lines have to wrap back around on themselves.

In this case, you can loosely think of them as "wrapping back" at infinity. If you start with a finite solenoid, and make it longer and longer, the "return field" running in the opposite direction outside the solenoid becomes weaker and weaker, and the field lines spread out transversely towards infinity.
 
thanks for your answers
 

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