1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Magnetic Field in an Infinitely Long Copper Rod

  1. Nov 26, 2011 #1
    1. The problem statement, all variables and given/known data

    A long, solid, copper rod has a circular cross-section of diameter 10.0 cm. The rod has a current of 5.0 A, uniformly distributed across its cross-section. Calculate the magnetic field strength at:

    a) the centre of the rod
    b) 2.5 cm from the centre
    c) 5.0 cm from the centre
    d) 7.5 cm from the centre

    2. Relevant equations

    This is supposed to be done somehow using B = μ/2∏ * I/r

    I don't have any calculus knowledge.

    3. The attempt at a solution

    I've tried many ways, but nothing has been consistent. I assume that I need to figure out the value of I at each different point of the copper rod, which I've tried doing through all this finicky work with area and all this stuff, but I truly don't understand what I'm doing.
  2. jcsd
  3. Nov 27, 2011 #2
    You stated that,
    That's going to be very problematic, and a damper on your solving equations here;
    You'll have to consider http://en.wikipedia.org/wiki/Ampere%27s_Law#Integral_form.
    Also, remember that you're told that the current is distributed evenly across the cross section, or surface area, in other words, I' = (I/A)*A'(r), where A' is the local area, or, to be precise,
    (I/R^2)*r^2, r<=R, where r is the distance from the center, and R=D/2;
    So you ought to get, B*2*Pi*r=mu_0*(I/R^2)*r^2.
    Master calculus, it's imperative,
    Good luck,
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook