Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B field of a wire carrying AC current

  1. Jul 6, 2011 #1
    Hi. New to the forums so bear with me.

    I am trying to calculate the magnetic field at a distance r from a wire carrying a 495Hz signal at 8Vpp and current 0.8mA. Initially I tried ampere's law, but fell into the trap of using the un-modified form (without maxwell's correction). I know that the field changes with the frequency, but I am unsure of exactly how they tie together, and I cant seem to find the relationship between dE/dt and frequency.

    Any ideas?
  2. jcsd
  3. Jul 6, 2011 #2
    you still would use amperes law . your current will be a function of time I(t)
    You would only use maxwells correction if you had free charge and if it was changing as a function of time. Like charging up a capacitor. And i dont think you have any free charge.
  4. Jul 6, 2011 #3
    Sorry, it's not that simple. When you have AC currents, you have changing fields, which means that the electric and magnetic fields are coupled. If the distance r you are interested in is very close to the wire compared to the wavelength of the signal, then you you can treat the problem as pseudo-static and just use Ampere's law. Otherwise you have to solve all of Maxwell's equations simultaneously because the fields are coupled. The best way to do this is to transform Maxwell's equations into non-homogenous wave equations and solve using Green functions. See http://faculty.uml.edu/cbaird/95.658%282011%29/Lecture6.pdf" [Broken], for example.
    Last edited by a moderator: May 5, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook