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A rather basic question on electromagnetic fields

  1. Oct 5, 2003 #1
    Electromagnetic forces are transmitted by photons, according to the widely accepted theory. Is there any way to block these photons, a sort of magnetic shielding? There may be a very common and obvious substance that does this (of which, alas, I am ignorant) or such a thing may be impossible... Does anyone know?
  2. jcsd
  3. Oct 5, 2003 #2


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    What do photons have to do with it?

    You can shield E&M fields with any grounded metallic structure. The wavelength of shielded energy will depend on the size of any openings. If you have a grounded solid copper box there will be no fields inside.

    If you use a grounded metallic screen only wavelengths longer then the opening in the screen will be shielded.
  4. Oct 5, 2003 #3
    So with a superdense grounded metallic screen you can completely block out the effects of a magnet? Is that what lead does? (Heh, sorry I know so little about this)
  5. Oct 6, 2003 #4


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    For a magnet it does not have to be "super dense", did you read my post? It only needs to be a grounded metallic (conducting) enclosure.

    You need density to shield Gamma radiation, but not for anything produced by a magnet. Once again the wavelengths of radiation blocked will be greater then the physical dimensions of the holes in your screen, so if your holes have a 1" side, only EM fields with wavelength greater then 1" will be blocked.

    Gamma rays have a VERY short wave length, only thick slabs of very dense materials have small enough holes to block this radiation.

    Radio waves have length of meters, so even a grounded enclosure of chicken wire will shield most of it.

    Micro waves have wave lengths of a few millimeters, so those small holes in your microwave oven shield microwaves.

    Is this getting any clearer?
  6. Oct 6, 2003 #5
    Integral, this thread sparked a question of my own.

    You say a metal inclosure is a true shield for EM waves. However, a simple cardboard box can block all visible light from entering, thus it is an EM shield just the same(for visable wavelengths anyway). I would asume (haven't actually tested and don't think I need to) that if I were to place a radio inside a closed cardboard box, it could still pick up the radio waves and be able to play.

    Mind explaining the reason behind this?
  7. Oct 6, 2003 #6


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    Uh. Integral. You are wrong on this. A metallic container does not shield DC magnet fields. For that, it needs to be a ferromagnetic or superconducting container.

    So Sikz: No Lead does not shield out the effects of a magnet (unless you are below 7.2 degrees Kelvin).
  8. Oct 7, 2003 #7
    Some experiments for your questions on magnetism may be found at:
    http://www.physics.ncsu.edu/pira/5eandm/5G30.html [Broken]
    To find the suitable magnetic blocker, you need to state the reason for blocking the field.
    (i.e. To create a motor/generator device, to prevent sunburn, to prevent head-aches from radio waves, to levitate an object, to generate H2O2 in a locallized body of water, for operation of a stealth craft, or operation of an mhd submersible, etc., etc.)
    What is your goal in blocking, blanking, or absorbing a magnetic field?)
    Which frequency range (am, fm, microwave) of magnetic radiation, and what magnitude?
    Your question seems to be quite open-ended (on purpose?).
    Last edited by a moderator: May 1, 2017
  9. Oct 7, 2003 #8


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    You are correct, having at one point in time erased a hard drive with a magnet, I should have known better. Constant magnetic fields, such as the earths are not shielded, however, they can be canceled. Using a set of Helmholtz coils an opposing field can be established which exactly cancels the magnetic field in a region inside the coils.

    Since cardboard is not a conductor it will not block Radio frequency EM waves, it does do a very nice job of blocking light, as the light is of the appropriate wavelength to interact with the atomic structure of the box. Now you have entered the world of QM, different rules apply.
    Last edited: Oct 7, 2003
  10. Oct 7, 2003 #9
    To shield the DC you also can use the so called μ-Metal. It costs an eye from the face but is very efficient shielding the magnetic DC.

    It is an alloy which has a very high permeability constant and the magnetic field lines are easily driven in it.
  11. Oct 7, 2003 #10
    A magnet produces a vector field, the magnetic field, at all points in the space around it. It can be defined by measuring the force the field exerts on a moving charged particle, such as an electron. The force (F) is equal to the charge (q) times the speed of the particle times the magnitude of the field (B), or F = q*v x B, where the direction of F is at right angles to both v and B as a result of the cross product. This defines the magnetic field's strength and direction at any point.
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