1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnetism - the proportionality constant question

  1. Jan 5, 2006 #1

    Hi! Can anyone please tell me where this equation was derived from?
    This equation is used to get the magnetic field (B).
    I is current and r is the distance. And I think I understand the Meu. And yet I have no idea where the 2pi came from. I looked in my textbook, internet and asked friends but nobody seems to know.

    Thank you!
    Last edited: Jan 5, 2006
  2. jcsd
  3. Jan 5, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    That is the magnitude of the magnetic field carried by a long wire. I think you meant r instead of y in the equation you displayed. You obtain it by simple application of Ampere's Law and the [itex]2\pi r[/itex] comes from integrating around a circle of radius r (i.e. it's the circumference of the circle).
  4. Jan 5, 2006 #3
    Well, that's the magnetic field from an infinite straight wire, which you can get using Biot-Savart:

    [tex]d\vec{B} = \frac{\mu_0}{4\pi} \frac{I \vec{dl} \times \hat{r}}{r^2}[/tex]

    Biot-Savart is derived from Ampere's Law:

    [tex]\int_C \vec{B} \cdot \vec{dl} = \mu_0 \int_S \vec{J} \cdot \vec{da}[/tex]
  5. Jan 5, 2006 #4
    aha, thank you!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Magnetism - the proportionality constant question
  1. A proportion question (Replies: 27)