The magnetic field outside a coaxial cable

[Amaelle]

Good day All,
I have a confusion in my mind and i hope to clear it

If we use the Amper law to calculate the magnetic field outside the cable, the value would be 0 because the electric current inside the amperian loop is equal to 0,
but in case we apply the Biot savart law to calculate the magnetic field say in the point B ( in red) i will use the principe of superposition and calulculate the magnetic filed generated by each current separatedly, but in this case because the distance varies, the magnetic field on B will not be 0
Any helps would be highly appreciated!
thanks

 

[NFuller]

You are likely not performing the integration correctly when using the Biot-Savart law. How are you setting up the integral?

 

[Amaelle]

You are likely not performing the integration correctly when using the Biot-Savart law. How are you setting up the integral?

yes you are right , thanks a lot for pointing this problem, my integral was a total mess