B field lines of an infinte solenoid?

In summary, the conversation discusses the behavior of magnetic field lines in an infinite solenoid. While the field lines do not technically wrap back around on themselves, they can be thought of as doing so at infinity. This is due to a fundamental rule in magnetism that states all field lines must have a source and sink. Additionally, the conversation touches on the assumptions made when discussing closed contours and the weakening of the return magnetic field as the solenoid becomes longer.
  • #1
cragar
2,552
3
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?
 
Physics news on Phys.org
  • #2
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.
 
  • #3
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.
 
  • #4
thanks for your answers
 
  • #5


You are correct in thinking that the field lines should wrap back around on themselves in an infinite solenoid. However, in this case, the field lines do not actually extend to infinity. In reality, the solenoid has a finite length, and the field lines do wrap around within the solenoid. However, as the solenoid approaches infinity, the field lines become more and more parallel, resulting in a zero field outside of the solenoid. So technically, the field lines do wrap back around within the solenoid, but at such a large distance, the field is negligible. This is a common misconception when dealing with infinite objects in physics.
 

1. What is an infinite solenoid?

An infinite solenoid is a hypothetical object that has an infinitely long, cylindrical shape with tightly wrapped coils of wire. It is often used as a simplified model in physics to study the behavior of magnetic fields.

2. What are B field lines?

B field lines, also known as magnetic field lines, are a visual representation of the direction and strength of the magnetic field around an object. They show the path that a hypothetical north magnetic pole would take if placed in the field.

3. How are B field lines affected by an infinite solenoid?

In an infinite solenoid, the B field lines are parallel and evenly spaced inside the solenoid. They are also tightly packed and uniform, with no divergence or convergence at any point. Outside the solenoid, the B field lines are similar to those of a bar magnet, with a north and south pole at each end.

4. What is the direction of the B field lines in an infinite solenoid?

The direction of the B field lines in an infinite solenoid is always parallel to the axis of the solenoid. This means that they are perpendicular to the surface of the solenoid, creating a uniform magnetic field inside the solenoid.

5. Can the strength of the B field lines be changed in an infinite solenoid?

Yes, the strength of the B field lines in an infinite solenoid can be changed by adjusting the current flowing through the coils of the solenoid. The stronger the current, the stronger the magnetic field and the closer the B field lines will be to each other inside the solenoid.

Similar threads

Replies
22
Views
844
Replies
12
Views
660
Replies
5
Views
1K
  • Classical Physics
Replies
2
Views
1K
Replies
2
Views
4K
Replies
15
Views
3K
  • Classical Physics
Replies
5
Views
1K
Replies
16
Views
204
Back
Top