Why a conductive shield doesn't block the magnetic field?

In summary, the conversation discussed the behavior of magnetic fields in shielded wires and how the magnetic field of the inner conductor can penetrate the shield conductor. The boundary conditions for the magnetic field at the surface of a perfect conductor were also mentioned, with the tangential component being Ht = Js surface current density and the normal component being Hn = 0. The question arose as to why the magnetic field of the inner and outer conductors are superposed when calculating the magnetic field in region 3, and why the normal component is still Hn = 0. The conversation also mentioned that this is true for superconductors, but the practical solution for shielding against electromagnetic noise is to use a twisted pair of cables.
  • #1
nabil25
6
0
Hi
While studying the shielded wires, i noticed that the magnetic field of the inner conductor can penetrate the shield conductor (can be calculated in the region 3). However, the boundary condition of the magnetic field at the surface (between dielectric and perfect conductor) of a perfect conductor impose that the tangential component of H (Ht = Js surface current density) and the normal component is Hn = 0. Inside the perfect conductor, we have Ht = 0 and Hn = 0.

So why do we superpose the magnetic field of the inner conductor and the outside conductor when calculating the magnetic field in the region 3 ? Why we still have Hi (inner cond) in the region 3 ?

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  • #2
nabil25 said:
and the normal component is Hn = 0
Where does that come from?

It is true for superconductors - they do provide shielding, but I don't think your question is about those.
 
  • #3
Leaving theory aside, practical signal cabling theory says: "A coaxial cable shields against electrostatic noise. To shield against electromagnetic noise, use a twisted pair."

See https://en.wikipedia.org/wiki/Category_5_cable for a description of a cable that is designed for immunity against electronic noise.
 

1. Why doesn't a conductive shield block a magnetic field completely?

A conductive shield does not block a magnetic field completely because of the phenomenon known as electromagnetic induction. When a magnetic field passes through a conductive material, it induces a current in the material, which in turn creates its own magnetic field. This opposing field weakens the original magnetic field, but does not completely eliminate it.

2. Can a thicker conductive shield block a magnetic field better?

While a thicker conductive shield may reduce the strength of a magnetic field, it will not completely block it. This is because the induced current and opposing magnetic field will still be present, regardless of the thickness of the shield. Additionally, thicker shields may become less effective as the frequency of the magnetic field increases.

3. Why do some materials block magnetic fields better than others?

The ability of a material to block a magnetic field depends on its electrical conductivity. Materials with higher electrical conductivity, such as copper or aluminum, will be more effective at blocking magnetic fields because they can quickly conduct induced currents and create opposing fields. Materials with lower conductivity, such as steel or iron, will not be as effective at blocking magnetic fields.

4. Can a conductive shield be used to block all types of magnetic fields?

No, a conductive shield is not effective at blocking all types of magnetic fields. It is most effective against static or low frequency magnetic fields, but it becomes less effective as the frequency increases. At extremely high frequencies, such as those found in radio waves, a conductive shield may actually enhance the magnetic field rather than blocking it.

5. Is there a way to completely block a magnetic field?

No, there is currently no known way to completely block a magnetic field. However, using a combination of different materials and techniques, it is possible to greatly reduce the strength of a magnetic field. This is often achieved by using multiple layers of conductive shielding, with each layer made of a different material and oriented in different directions.

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