A long, hollow wire with inner radius a and outer radius b carries a uniform current density J. What is the magnetic field as a function of r, the distance from the center of the wire within the wire's material (i. e. what is B(r) in the region a < r < b)?

The magnetic field in the region a < r < b is

Explanation:

If we have the a < r < b. The formula of current is:

Where:

A = area enclosed by the loop.

Itotal = total current in loop.

If we have the Ampere`s law:

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.