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Question about mathematical equality

  1. Dec 5, 2014 #1
    Hi there, im reading Chapter 9 of Jackson Classic Electrodynamics 3rd edition, and I don't see why this equality is true, it says "integrating by parts", but I still don't know... any help?

    http://imageshack.com/a/img673/9201/4WYcXs.png [Broken]
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Dec 6, 2014 #2


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    Write [itex] \mathbf J [/itex] as the sum of its Cartesian components and then write the integral as the sum of three integrals of the three Cartesian components. Then integrate by parts each integral, but the [itex] J_x [/itex] w.r.t. to x and so on! If you consider the integrals to be from [itex] -\infty [/itex] to [itex] \infty [/itex] and take [itex] \mathbf J [/itex] to be zero at infinities, you'll get what you want.
  4. Dec 6, 2014 #3
  5. Dec 9, 2014 #4


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    That equality depends on div J=0 (or d\rho/dt=0).
  6. Dec 10, 2014 #5


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    Just my derivation. For an arbitrary constant vector ##\vec{n}## we have
    $$\vec{\nabla} \cdot [(\vec{n} \cdot \vec{x}) \vec{j}]=(\vec{n} \cdot \vec{x}) \vec{\nabla} \cdot \vec{j} + \vec{n} \cdot \vec{j}.$$
    Now integrate this over the whole space and assume that ##\vec{j}## goes to 0 quickly enough at infinity (or most realistically that it has compact support). Then the left-hand side vanishes, because it's a divergence, and thus can be transformed to a surface integral, which vanishes at infinity, if ##\vec{j}## vanishes quickly enough. This implies
    $$\int_{\mathbb{R}^3} \mathrm{d}^3 \vec{x} \; \vec{n} \cdot \vec{j} = -\int_{\mathbb{R}^3} \mathrm{d}^3 \vec{x} \; (\vec{n} \cdot \vec{x} ) \vec{\nabla} \cdot \vec{j}.$$
    Now use for ##\vec{n}## the three basis vectors of a cartesian reference frame, and you see that the vector equation stated by Jackson holds.

    It's independent of whether ##\vec{j}## is a solenoidal field or not. Of course in the case of stationary currents it must be one, but the integral identity is generally valid.
  7. Dec 13, 2014 #6

    Meir Achuz

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    Sorry about that. I was thinking cases where j appears with other variables.
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