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Does copper or aluminum sheet block magnetic signal?

  1. Mar 27, 2012 #1
    I have a discussion about shielding of noise generated by appliance and florescent lights. I know both Al and Cu are not magnetic material. BUT all noise are time varying meaning it is EM field. Good conductor block E field as loss is very high. How does that work when only the B field can pass through?

    Does the B that pass through create back the E field by Maxwell's equation

    [tex]\nabla \times \vec E =-\frac{\partial \vec B}{\partial t}[/tex]

    So the E field is regenerate again after passing through the good conductor sheet?


    Another question, does the thickness of the copper or aluminum sheet matter on the shielding. If so, why?

    Thanks
    Alan
     
  2. jcsd
  3. Mar 27, 2012 #2

    K^2

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    Perfect conductor completely blocks an EM wave. Magnetic field doesn't "pass through", don't worry about it. In fact, you don't need a solid sheet. You can use chicken wire, so long as the size of the cells is small compared to wave length. Look at the microwave door window for an example of that. It's called a Faraday cage. Look it up.

    Thickness does matter, in principle. At every frequency there is a "skin layer", which is the depth to which the EM wave penetrates. For good conductors the skin layer is very thin, but depends on wavelength. You need less thickness to block high frequency EM than to block the low frequencies.
     
  4. Mar 27, 2012 #3
    Thanks for the reply.

    I did experiment with a coil inside a cage made of aluminum foil. It sense the 60Hz, 120 Hz and even up to 10KHz just as well as if the cage is not there.

    So is this because the frequency is so long the skin depth is very high, so you need a thicker cage to block the low frequency EM noise?

    Also, if the frequency is so low and the wave length is so long, I can use literally chicken wire for shielding the low frequency EM noise?
     
  5. Mar 27, 2012 #4

    K^2

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    Hm, at 10kHz I would expect foil to provide at least some shielding. Strange. But yeah, my first guess would be insufficient thickness. At frequencies that low, it gets a little more complicated. On one hand, any gaps are going to be much smaller than wavelength, but then again, so is the distance from source. At 60Hz, the waves are 5,000km long. I honestly have no experience with shielding at these frequencies.

    I can definitely tell you that whether material is magnetic or not will not make a difference. So you need to concentrate on a good conductor. Aluminum or copper should be ideal for any kind of shielding. But thickness and geometry... Sorry. Not sure.
     
  6. Mar 27, 2012 #5
    Thanks. I sure learn a lot from you tonight, give me a lot of insight to think about.
     
  7. Mar 27, 2012 #6
    When dealing with electromagnetic waves, it is the conductivity of a material that makes it reflective and not its magnetic properties.
     
  8. Mar 27, 2012 #7
    The skin depth represents the depth in a conducting material (aluminum, copper) that attenuates the incident wave to 1/e. See http://en.wikipedia.org/wiki/Skin_effect
    [tex] \delta=\left[ \frac{2\rho}{\omega \mu} \right]^{1/2} [/tex]
    It is proportional to 1/f½, so shielding low ac frequencies is difficult. For very low frequencies and dc magnetic fields, layers of soft iron, followed by a layer of high-mu alloy, works well.
     
  9. Mar 27, 2012 #8

    marcusl

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    Correct, iron is very helpful. Although the skin depth is big at low frequencies, it is possible to shield 60 Hz with a box welded up of annealed 1000 series (high purity high conductivity) aluminum sheets of some 1" thickness. Conductivity of the welds must also be very high to prevent leakage.
     
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