Answer didnt check with back of book

[betyoudidntthink]

hey,

I'm not getting the answer matching the back of the book for these questions, can you show me how to do it with your work?

A long wire is known to have a radius greater than 4.0 mm and to carry a current uniformly distributed over its cross section. If the magnitude of the magnetic field is 0.285 mT at a point 4.0 mm from the axis of the wire and 0.200 mT at a point 10 mm from the axis, what is the radius of the wire?

and

A long solenoid (n = 1200 turns/m, radius = 2.0 cm) has a current of a 0.30 A in its winding. A long wire carrying a current of 20 A is parallel to and 1.0 cm from the axis of the solenoid. What is the magnitude of the resulting magnetic field at a point on the axis of the solenoid?

 

[Nick89]

Can you show us what you tried and what answers you got (and how) ?

For the first question, I am assuming you calculated the magnetic field using
\int \vec{B} \vec{dl} = \mu_0 I_{\text{enclosed}}

Did you keep in mind that, at least for the 4mm radius, the enclosed current is NOT the total current?

 

[Nick89]

Finally find the right latex code, sorry :p
\oint \vec{B} \cdot \vec{dl} = \mu_0 I_{\text{enclosed}}

(Why can't I edit my post??)