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Electric field angular momentum

  1. Jan 10, 2015 #1
    1. The problem statement, all variables and given/known data
    A straight line with charge density ##\lambda ## is in the middle of a large isolation cylinder, that can rotate around it's axis (line). Moment of inertia for that cylinder per unit of length is ##l## and electric charge density applied on the cylinder is ##\frac{\lambda }{2\pi a}## where ##a## is the radius of the cylinder. At first we have a magnetic field ##B##, which is than slowly turned off. Calculate the angular velocity ##\omega ## after the magnetic filed is turned off. [Feynman paradox]

    2. Relevant equations


    3. The attempt at a solution
    Angluar momentum before $$\Gamma =\varepsilon _0 \int EB dV=\frac{\lambda l B a^2}{2}$$ And after...? Yup, i have i guesstion here. I know that angular momentum should have two terms, one ## J\omega## and the other one that comes from the magnetic filed induced due to the rotation of the charged cylinder. I do have a couple of question concerning that second term.

    To get that second term, I have to calculate the magnetic filed ##B_2## due to the rotation of the cylinder with angular velocity ##\omega ##.
    I have two options but I can't decide which one is right or which one is not. Here they are:
    1)
    Simply $$B=\frac{\mu _0 NI}{l}=\frac{\mu _0}{l}\frac{\lambda l \omega}{2\pi}$$ where I used ##N=1## and ##I=\frac{e\omega }{2\pi}##. I don't really understand this option - but it is the one we wrote at university. My solution differs a bit:
    2) Let's ignore that ##B## and ##j## are vectors for a moment $$\nabla \times B=\mu _0 j$$ $$\nabla \times B=\mu _0 (\sigma 2\pi l a)(r\omega)$$ if ##l## is the length of the cylinder. Integrating the last equation and using stokes law on the LHS leaves me with $$\int B ds =\mu _0 \sigma 2\pi la\omega \int r 2\pi r dr$$ which sadly leads to a different result than option 1). because $$B=\frac{\mu _0\lambda l \omega a^2}{3}$$

    I seriously doubt we got it all wrong at university, so my question here is: Why is my solution wrong? And could somebody explain the option 1.) - especially where this ##I=\frac{e\omega }{2\pi}## comes from. o_O
     
  2. jcsd
  3. Jan 10, 2015 #2
    Blaaaa, ok, nevermind. :D I found my mistake now.

    The charge on the cylinder is only on the surface and not throughout the entire cylinder. This changes $$
    \nabla \times B=\mu _0 (\sigma 2\pi l a)(r\omega)$$ to $$
    \nabla \times B=\mu _0 (\sigma 2\pi l a)(a\omega)$$ and leads to the same result as in option 1.).

    :)) thanks, anyway.
     
  4. Jan 11, 2015 #3
    Ok, I take that back. Something is wroooong :(

    Question: Does one calculate the magnetic field ##B## inside a rotating cylinder with charge density ##\sigma ## ?
     
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