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

Calculus: I can't understand why curl of gradient of a scalar is zero

  1. Sep 5, 2008 #1
    (Sorry, the title should read "...why curl of gradient of a scalar "function" is zero)

    Of course I know how to compute curl, graident, divergence. Algebrically I know curl of gradient of a scalar function is zero.

    But I want to know the reason behind this...and also the reason why gradient of divergence of a vector function is always zero.

    This really makes me feeling bad for a long time. Thanks in advance.
     
  2. jcsd
  3. Sep 5, 2008 #2

    Defennder

    User Avatar
    Homework Helper

    The gradient of a scalar function would always give a conservative vector field. Now think carefully about what curl is. If you've done an E&M course with vector calculus, think back to the time when the textbook (or your course notes) derived [tex]\nabla \times \mathbf{H} = \mathbf{J}[/tex] using Ampere's circuital law. What is the closed path integral of a conservative field?

    I'm wondering about your second question too...
     
  4. Sep 5, 2008 #3

    atyy

    User Avatar
    Science Advisor

    Is this true? In Gauss's law, the divergence of the electric field is to equal an arbitrary charge density. I would be surprised if the gradient of an arbitrary charge density is zero.
     
  5. Sep 5, 2008 #4

    Ben Niehoff

    User Avatar
    Science Advisor
    Gold Member

    [tex]\nabla(\nabla \cdot \vec F) = 0[/tex]

    is certainly false. I think you mean

    [tex]\nabla \cdot (\nabla \times \vec F) = 0[/tex]

    which is true.
     
  6. Sep 5, 2008 #5

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi chingcx! :smile:

    (curl grad f)x = ∂/∂y(∂f/∂z) - ∂/∂z(∂A/∂y) = 0

    Similarly, div curl A = 0
    But (grad div A)x = ∂/∂x(∂Ax/∂x) + ∂/∂x(∂Ay/∂y) + ∂/∂x(∂Ax/∂z) ≠ 0 :smile:
     
  7. Sep 5, 2008 #6

    Defennder

    User Avatar
    Homework Helper

    Hey you're right. Wow I can't believe I didn't even bother thinking about whether it might be correct, as opposed to why it might be correct.
     
  8. Sep 5, 2008 #7
    ya, sorry, I mean divergence of curl of vector function is always zero. Why is that true?
     
  9. Sep 5, 2008 #8
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Calculus: I can't understand why curl of gradient of a scalar is zero
  1. Why can't I do this? (Replies: 2)

  2. Scalar gradient (Replies: 2)

Loading...