# Magnetic field around a wire ?

cragar
Lets say we have an infinite current carrying wire and it is wrapped with a dielectric materiel.
Lets say the dielectric is 1 cm in diameter around the wire. But let's say I want to find the B field when I am outside the dielectric, Could I just use amperes law . And when I am inside the dielectric I would use the H field and then get B by using the magnetic susceptibility of the material . Is this correct .

Gold Member
Off the top of my head I think you can just use the standard magnetic field equation for an infinite wire. The boundary conditions require that the tangential H field be continuous. Since the B field will be described as

$$\mathbf{B}(\rho) = \frac{\mu I}{2\pi\rho} \hat{\phi}$$

Since the fields are tangential to the inhomogeneity they will not be affected since they will automatically have continuous H fields. So this is situation where Ampere's Law will give the correct result.

cragar
Thanks for your response , ok what if i wrapped a superconductor around the wire and cooled it with liquid nitrogen .
And let's say the superconductor goes 1 cm around the wire . Outside the superconductor the B field would be zero , Would amperes law still work?