The current density inside a long, solid, cylindrical wire of radius a = 10 cm is in the direction of the axis and varies linearly with the radial distance r cm from the axis according to J = J0 r / a with J0 =0.5 A/cm2 . Find the magnetic field in μT at r = 0.045 m Ok I know from amperes law that the magnetic field in this case can be derived to be: B=(mew0/2*pi)(I*r/R2) Edit: Wait.. Is that above equation right? That might have been what I did wrong.. When I worked it out I used B= (mew0*I) / (2*pi*r) Was that my mistake? Since it says J = J0 r / a And I know current density is I=J*A where A is area I put those two equations together. Since there is no length of the wire, for Area I just used pi*r^2 J = J0*r(pi*r^2) / a Where r=.045m, a=10cm, and J0 =0.5 A/cm2 Not sure if I have interpreted the question correctly, or if that is the right way to do it.. From there I just put that in as I into the original magnetic field equation, using r on the bottom once again as .045m to get my answer. It was still wrong. Thats my attemp, what did I do wrong? I converted everything to SI units, to convert A/cm^2 to A/m^2 I multiplied it by 10,000. Is that right? Anyone know how to get this? Thanks.