A wire of circular cross-section carries current density that is not uniform but varies with distance from the center as j(r) = A r / R, for radius r in the range 0 < r < R. Here, A is a constant with units amperes per square meter, and the radius of the wire is R = 8.1 mm. Find an expression for the magnetic field outside of the wire, for r > R.

i believe the answer is a hope it and is correct

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.