B field lines of an infinte solenoid?

[cragar]

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?

 

[yungman]

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.

 

[jtbell]

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.

 

[cragar]

thanks for your answers