How in the world is current passing through?

[flyingpig]

Homework Statement

The magnetic coils of a tokamak fusion reactor are in the shape of a toroid having an inner radius of 0.700 m and an outer radius of 1.30 m. The toroid has 900 turns of large-diameter wire, each of which carries a current of 14.0 kA.

(a) Find the magnitude of the magnetic field inside the toroid along the inner radius.
___T
(b) Find the magnitude of the magnetic field inside the toroid along the outer radius.
___T

The Attempt at a Solution

(a) I did it the way the textbook wants me to and I got 3.6T, but I don't understand, how could there be a current inside the torus? I mean if I were to enlarge my Amperian loop, I can get a current passing through

(b) Same here, if I were to draw an Amperian loop that is bigger than the torus, wouldn't it be zero? BUt the loop is still valid for it encloses the torus right?

 

[SammyS]

From Wikipedia:

Something like this.

 

[flyingpig]

Where is the Amperian loop in the picture?

 

[Apphysicist]

Right...if you draw an amperian loop (a circle that is coaxial with the torus) that is smaller than the inner radius, you enclose no current, thus no B-field. If you draw it so the radius of the loop is between the inner and outer radii of the torus, you enclose N*I, and r*d(theta) (or your differential line element on the circumference of the amperian loop) is always parallel to the B-field (B (dot) dl = BL = B2*Pi*r. If you draw it larger than the outer radius, you enclose net 0 current, and have net 0 B-field.

 

[SammyS]

A circle within the blue ring itself.