Material that does not transmit magnetic waves. Is there a material that reduces the magnetic field without affecting the magnetic field itself? Separation of space by a superconductor
Consider a conventional bar magnet: magnet 1 rests on the North surface with the pole up. Hanging distance y "role \u003d" presentation "style \u003d" position: relative; "\u003e Y y "role \u003d" presentation "style \u003d" position: relative; "\u003e y "role \u003d" presentation "style \u003d" position: relative; "\u003e Y above it (supported from side to side by a plastic tube) is a second, smaller bar magnet, magnet 2, with the North pole facing down. The magnetic forces between them exceed the force of gravity and keep magnet 2 suspended. Consider some material, material-X, which moves towards the gap between two magnets at an initial velocity. v "role \u003d" presentation "style \u003d" position: relative; "\u003e v v "role \u003d" presentation "style \u003d" position: relative; "\u003e v "role \u003d" presentation "style \u003d" position: relative; "\u003e v ,
Is there a material, material-X, that will reduce the distance y "role \u003d" presentation "style \u003d" position: relative; "\u003e Y y "role \u003d" presentation "style \u003d" position: relative; "\u003e y "role \u003d" presentation "style \u003d" position: relative; "\u003e Y between two magnets, and pass through the gap without changing the speed v "role \u003d" presentation "style \u003d" position: relative; "\u003e v v "role \u003d" presentation "style \u003d" position: relative; "\u003e v "role \u003d" presentation "style \u003d" position: relative; "\u003e v ?
Amateur physicist
such a strange question
Answers
Jojo
The material you are looking for might be a superconductor. These materials have zero current resistance and thus can compensate for penetrating lines of force in the first layers of the material. This phenomenon is called the Meissner effect and is the very definition of a superconducting state.
In your case, plates between two magnets, this will definitely reduce y "role \u003d" presentation "style \u003d" position: relative; "\u003e Y y "role \u003d" presentation "style \u003d" position: relative; "\u003e y "role \u003d" presentation "style \u003d" position: relative; "\u003e Y ,
For speed:
Here, usually the eddy currents induced by the magnetic field lead to a loss of power, defined as:
P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e p P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e = π P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 2 P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e AT P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 2 P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e p P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e d P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 2 P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 2 P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 6 k ρ D P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e , P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e p P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e \u003d P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e π P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 2 P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e В P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e p P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 2 P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e d P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 2 P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e e P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 2 P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e 6 P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e К P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e ρ P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e D P \u003d π 2 B p 2 d 2 f 2 6 k ρ D, "role \u003d" presentation "\u003e,
since, however, the superconductor has zero resistance and thus de facto
ρ \u003d ∞ "role \u003d" presentation "\u003e ρ = ∞ ρ \u003d ∞ "role \u003d" presentation "\u003e ρ \u003d ∞ "role \u003d" presentation "\u003e ρ ρ \u003d ∞ "role \u003d" presentation "\u003e \u003d ρ \u003d ∞ "role \u003d" presentation "\u003e ∞
no kinetic energy should be lost, and thus the speed will remain unchanged.
There is just one problem:
A superconductor can only exist at very low temperatures, so this might not be possible with your machine ... you at least need a liquid nitrogen cooling system to cool it down.
Other than superconductors, I don't see any possible material, because if the material is a conductor, then you always have eddy current losses (thus reducing v "role \u003d" presentation "style \u003d" position: relative; "\u003e v v "role \u003d" presentation "style \u003d" position: relative; "\u003e v "role \u003d" presentation "style \u003d" position: relative; "\u003e v) or the material is not a conductor (then y "role \u003d" presentation "style \u003d" position: relative; "\u003e Y y "role \u003d" presentation "style \u003d" position: relative; "\u003e y "role \u003d" presentation "style \u003d" position: relative; "\u003e Y will not decrease).
adamdport
Can this phenomenon be observed in a car or somewhere in an experiment?
Jojo
The point, however, is that when a superconductor enters a magnetic field, the lines of force are deflected, which will be work-related ... so in fact, entering the area between the two magnets will cost some energy. If the plate leaves the area after, the energy will be won back.
Lupercus
There are materials with very high magnetic permeability, for example, the so-called µ-metal. They are used to make shields that attenuate the Earth's magnetic field in the path of the electron beam in sensitive electro-optical devices.
Since your question brings together two separate parts, I will split it to cover each one separately.
1. Static case : Are the magnetic poles approaching each other when a magnetic shielding plate is installed between them?
Mu-materials do not "kill" the magnetic field between your magnetic poles, but only deflect its direction, directing part of it into the metal shield. This will greatly change the field strength B "role \u003d" presentation "style \u003d" position: relative; "\u003e AT B "role \u003d" presentation "style \u003d" position: relative; "\u003e B "role \u003d" presentation "style \u003d" position: relative; "\u003e B on the surface of the screen, almost suppressing its parallel components. This leads to a decrease in magnetic pressure p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e p \u003d B p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e 2 p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e 8 π p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e μ p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e p p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e \u003d p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e B p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e 2 p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e 8 p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e π p \u003d B 2 8 π μ "role \u003d" presentation "style \u003d" position: relative; "\u003e μ in the immediate vicinity of the screen surface. If this decrease in the magnetic field on the screen would significantly change the magnetic pressure at the location of the magnets, causing them to move? I'm afraid a more detailed calculation is needed here.
2. Plate movement : is it possible that the speed of the shielding plate will not change?
Consider the following very simple and intuitive experiment: Take a copper pipe and hold it upright. Take a small magnet and let it fall into the pipe. The magnet falls: i) slowly and ii) at a uniform speed.
Your geometry can be made similar to the geometry of a falling pipe: consider a column of magnets hovering above each other, that is, with paired poles, NN and SS. Now take a "multi-plate" shield made of parallel sheets held firmly in place at an equal distance from each other (eg a 2D comb). This world simulates several falling pipes in parallel.
If you now hold a column of magnets in a vertical direction and pull a multi-plate with a constant force (analogous to gravity) through them, then you will reach a constant speed regime - by analogy with the experiment with a falling pipe.
This suggests that a column of magnets or, more precisely, their magnetic field acts on copper plates of a viscous medium:
M p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e p l a t e m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e v m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e ˙ m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e = - γ m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e AT m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e V + F m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e n l l m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e m m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e p m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e L m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e T m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e е m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e v m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e ˙ m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e \u003d m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e - m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e γ m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e В m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e v m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e + m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e F m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e p m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e U m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e L m p l a t e v ˙ \u003d - γ B v + F p u l l "role \u003d" presentation "\u003e L
Where γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e γ γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e AT γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e γ γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e В will be the effective coefficient of friction due to the magnetic field disturbed by the presence of the plates. After a while, you will eventually reach a regime in which the frictional force will compensate for your effort, and the speed will remain constant: v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e v \u003d F v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e n l l v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e γ v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e AT v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e v v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e \u003d v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e F v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e p v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e U v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e L v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e L v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e γ v \u003d F p u l l γ B "role \u003d" presentation "style \u003d" position: relative; "\u003e AT ,
If this speed is equal to the speed that you had before you pulled the plates into the magnetic field, it is a matter of how you control the force of gravity. Note : if there is no thrust, the plate will simply be stopped by the magnetic brake effect. Thus, you must pull accordingly if you want to have a constant speed.
Shielding of magnetic fields can be done in two ways:
Shielding with ferromagnetic materials.
Eddy current shielding.
The first method is usually used for screening constant MF and low-frequency fields. The second method provides significant efficiency in shielding high frequency MF. Due to the surface effect, the eddy current density and the intensity of the alternating magnetic field decrease exponentially as we go deeper into the metal:
The rate of decrease in field and current, which is called the equivalent penetration depth.
The smaller the depth of penetration, the greater the current flows in the surface layers of the screen, the greater the reverse MF created by it, displacing the external field of the pickup source from the space occupied by the screen. If the shield is made of a non-magnetic material, then the shielding effect will depend only on the conductivity of the material and the frequency of the shielding field. If the screen is made of a ferromagnetic material, then, all other things being equal, a large emis- sion will be induced in it by an external field. etc. with. due to the greater concentration of magnetic lines of force. With the same material conductivity, eddy currents will increase, which will lead to a shallower penetration depth and a better shielding effect.
When choosing the thickness and material of the screen, one should not proceed from the electrical properties of the material, but be guided by considerations of mechanical strength, weight, stiffness, resistance to corrosion, ease of joining individual parts and making transitional contacts with low resistance between them, ease of soldering, welding, etc.
It can be seen from the data in the table that for frequencies above 10 MHz, copper and, moreover, silver films with a thickness of about 0.1 mm give a significant screening effect. Therefore, at frequencies above 10 MHz, it is quite permissible to use screens made of foil-coated getinax or fiberglass. At higher frequencies, steel has a greater shielding effect than non-magnetic metals. However, it should be borne in mind that such screens can introduce significant losses into the shielded circuits due to high resistivity and the phenomenon of hysteresis. Therefore, these screens are applicable only in cases where insertion loss can be ignored. Also, for greater efficiency of shielding, the screen must have a lower magnetic resistance than air, then the lines of force of the magnetic field tend to pass along the walls of the screen and in a smaller number penetrate into the space outside the screen. Such a shield is equally suitable for protecting against the effects of a magnetic field and for protecting the external space from the influence of the magnetic field created by the source inside the shield.
There are many grades of steel and permalloy with different values \u200b\u200bof the magnetic permeability, therefore, the value of the penetration depth must be calculated for each material. The calculation is made according to the approximate equation:
1) Protected against external magnetic field
The magnetic lines of force of the external magnetic field (the lines of induction of the magnetic field of interference) will pass mainly through the thickness of the walls of the screen, which has a low magnetic resistance compared to the resistance of the space inside the screen. As a result, the external magnetic field of interference will not affect the operating mode of the electrical circuit.
2) Shielding of own magnetic field
Such shielding is used if the task is to protect external electrical circuits from the influence of the magnetic field created by the coil current. Inductance L, that is, when it is required to practically localize the interference created by inductance L, then such a problem is solved using a magnetic shield, as shown schematically in the figure. Here, almost all lines of force of the field of the inductor will be closed through the thickness of the walls of the screen, without going beyond their limits due to the fact that the magnetic resistance of the screen is much less than the resistance of the surrounding space.
3) Dual screen
In a double magnetic screen, one can imagine that a part of the magnetic lines of force, which will go beyond the thickness of the walls of one screen, will be closed through the thickness of the walls of the second screen. In the same way, one can imagine the action of a double magnetic shield in the localization of magnetic interference created by an element of the electric circuit located inside the first (inner) shield: the bulk of the magnetic lines of force (magnetic scattering lines) will close through the walls of the outer shield. Of course, in double screens, the thickness of the walls and the distance between them must be rationally chosen.
The overall shielding factor reaches the greatest value in those cases when the wall thickness and the gap between the screens increase in proportion to the distance from the center of the screen, and the size of the gap is the geometric mean of the wall thicknesses of the adjacent screens. In this case, the screening factor:
L \u003d 20lg (H / Ne)
Manufacturing double screens in accordance with this recommendation is practically difficult for technological reasons. It is much more expedient to choose the distance between the shells adjacent to the air gap of the screens that is greater than the thickness of the first screen, approximately equal to the distance between the stack of the first screen and the edge of the screened circuit element (for example, an inductive coil). The choice of one or another thickness of the walls of the magnetic shield cannot be made unambiguous. The rational wall thickness is determined. the screen material, the frequency of the interference and the specified screening factor. In doing so, it is useful to consider the following.
1. With an increase in the frequency of interference (frequency of an alternating magnetic field of interference), the magnetic permeability of materials decreases and causes a decrease in the shielding properties of these materials, since as the magnetic permeability decreases, the resistance to the magnetic flux exerted by the shield increases. As a rule, the decrease in magnetic permeability with increasing frequency is most intense in those magnetic materials that have the highest initial magnetic permeability. For example, sheet electrical steel with a low initial magnetic permeability changes little the value of jx with increasing frequency, and permalloy, which has large initial values \u200b\u200bof the magnetic permeability, is very sensitive to an increase in the frequency of the magnetic field; its magnetic permeability drops sharply with frequency.
2. In magnetic materials exposed to a high-frequency magnetic field of interference, the surface effect is noticeably manifested, ie, the displacement of the magnetic flux to the surface of the walls of the screen, causing an increase in the magnetic resistance of the screen. Under such conditions it seems that it is almost useless to increase the thickness of the walls of the screen beyond those values \u200b\u200bthat are occupied by the magnetic flux at a given frequency. This conclusion is incorrect, because an increase in the wall thickness leads to a decrease in the magnetic resistance of the screen, even in the presence of a surface effect. At the same time, the change in magnetic permeability should also be taken into account. Since the phenomenon of the surface effect in magnetic materials usually begins to manifest itself more noticeably than a decrease in the magnetic permeability in the low-frequency region, the influence of both factors on the choice of the screen wall thickness will be different at different frequency ranges of magnetic interference. As a rule, the decrease in the shielding properties with increasing interference frequency is more pronounced in shields made of materials with a high initial magnetic permeability. The above features of magnetic materials provide a basis for recommendations on the choice of materials and wall thickness of magnetic shields. These recommendations can be summarized as follows:
A) screens made of ordinary electrical (transformer) steel, having a low initial magnetic permeability, can be used, if necessary, to provide small screening factors (Ke 10); Such screens provide an almost constant screening factor in a fairly wide frequency band, up to several tens of kilohertz; the thickness of such screens depends on the frequency of the interference, and the lower the frequency, the thicker the screen is required; for example, at a frequency of the magnetic field of interference of 50-100 Hz, the thickness of the walls of the screen should be approximately equal to 2 mm; if an increase in the shielding coefficient or a large screen thickness is required, then it is advisable to use several shielding layers (double or triple shields) of a smaller thickness;
B) screens made of magnetic materials with a high initial permeability (for example, permalloy) are advisable to use if it is necessary to provide a large screening coefficient (Ke\u003e 10) in a relatively narrow frequency band, and the thickness of each shell of the magnetic screen is impractical to choose more than 0.3-0.4 mm; the shielding effect of such screens begins to drop noticeably at frequencies above several hundred or thousand hertz, depending on the initial permeability of these materials.
Everything said above about magnetic shields is true for weak magnetic fields of interference. If the screen is located near powerful sources of interference and magnetic fluxes with high magnetic induction appear in it, then, as is known, it is necessary to take into account the change in the magnetic dynamic permeability depending on the induction; it is also necessary to take into account the loss in the thickness of the screen. In practice, such strong sources of magnetic fields of interference, in which one would have to reckon with their effect on screens, are not encountered, with the exception of some special cases that do not provide for amateur radio practice and normal operating conditions for radio engineering devices of widespread use.
Test
1. With magnetic shielding, the shield should:
1) Have a lower magnetic resistance than air
2) have magnetic resistance equal to air
3) have a higher magnetic resistance than air
2. When shielding the magnetic field Grounding of the shield:
1) Does not affect the shielding efficiency
2) Increases the efficiency of magnetic shielding
3) Reduces the effectiveness of magnetic shielding
3. At low frequencies (<100кГц) эффективность магнитного экранирования зависит от:
a) Thickness of the screen, b) Magnetic permeability of the material, c) Distances between the screen and other magnetic circuits.
1) Only a and b are true
2) Only b and c are true
3) Only a and b are true
4) All options are correct
4. Magnetic shielding at low frequencies uses:
1) Copper
2) Aluminum
3) Permalloy.
5. Magnetic shielding at high frequencies uses:
1) Iron
2) Permalloy
3) Copper
6. At high frequencies (\u003e 100 kHz), the effectiveness of magnetic shielding does not depend on:
1) Screen thickness
2) Magnetic permeability of material
3) Distances between the shield and other magnetic circuits.
Used literature:
2. Semenenko, V. A. Information security / V. A. Semenenko - Moscow, 2008.
3. Yarochkin, V. I. Information security / V. I. Yarochkin - Moscow, 2000.
4. Demirchan, KS Theoretical foundations of electrical engineering Volume III / KS Demirchan S.-P, 2003.
Two methods are used to shield the magnetic field:
Bypass method;
Screen magnetic field method.
Let's take a closer look at each of these methods.
The method of shunting the magnetic field with a screen.
The method of shunting the magnetic field with a screen is used to protect against a constant and slowly changing alternating magnetic field. Screens are made of ferromagnetic materials with high relative magnetic permeability (steel, permalloy). In the presence of a screen, the lines of magnetic induction pass mainly along its walls (Figure 8.15), which have a low magnetic resistance compared to the air space inside the screen. The quality of the shielding depends on the magnetic permeability of the shield and the resistance of the magnetic circuit, i.e. the thicker the screen and the fewer seams, joints running across the direction of the magnetic induction lines, the shielding efficiency will be higher.
The method of displacing the magnetic field by the screen.
The method of displacing the magnetic field by the screen is used to shield alternating high-frequency magnetic fields. In this case, screens made of non-magnetic metals are used. The shielding is based on the phenomenon of induction. This is where induction is useful.
We put a copper cylinder in the path of a uniform alternating magnetic field (Figure 8.16, a). Variable EDs will be excited in it, which, in turn, will create variable induction eddy currents (Foucault currents). The magnetic field of these currents (Figure 8.16, b) will be closed; inside the cylinder it will be directed towards the exciting field, and outside it - in the same direction as the exciting field. The resulting field (Figure 8.16, c) turns out to be weakened at the cylinder and strengthened outside it, i.e. the field is displaced from the space occupied by the cylinder, which is its shielding effect, which will be the more effective, the lower the electrical resistance of the cylinder, i.e. the greater the eddy currents flowing through it.
Due to the surface effect ("skin effect"), the eddy current density and the intensity of the alternating magnetic field decrease exponentially as it goes deeper into the metal
, (8.5)
where 
(8.6)
Is an indicator of the decrease in field and current, which is called equivalent depth of penetration.
Here is the relative magnetic permeability of the material;
- magnetic permeability of vacuum equal to 1.25 * 10 8 gn * cm -1;
- specific resistance of the material, Ohm * cm;
- frequency Hz.
It is convenient to characterize the shielding effect of eddy currents by the value of the equivalent penetration depth. The smaller x 0, the greater the magnetic field created by them, displacing the external field of the pickup source from the space occupied by the screen.
For a non-magnetic material in the formula (8.6) \u003d 1, the screening effect is determined only by and. And if the screen is made of ferromagnetic material?
If equal, the effect will be better, since\u003e 1 (50..100) and x 0 will be less.
So, x 0 is a criterion for the screening effect of eddy currents. It is of interest to estimate how many times the current density and magnetic field strength become less at a depth of x 0 in comparison with the surface. To do this, substitute x \u003d x 0 into formula (8.5), then
whence it is seen that at a depth of x 0, the current density and the magnetic field strength decrease by a factor of e, i.e. to a value of 1 / 2.72, which is 0.37 of the density and tension on the surface. Since the field weakening is only 2.72 times at depth x 0 insufficient to characterize the shielding material, then they use two more values \u200b\u200bof the penetration depth x 0.1 and x 0.01, which characterize the drop in current density and field voltage by 10 and 100 times from their values \u200b\u200bon the surface.
Let us express the values \u200b\u200bx 0.1 and x 0.01 through the value x 0, for this, on the basis of expression (8.5), we compose the equation
AND 
,
deciding which we get
x 0.1 \u003d x 0 ln10 \u003d 2.3x 0; (8.7)
x 0.01 \u003d x 0 ln100 \u003d 4.6x 0
Based on formulas (8.6) and (8.7), the values \u200b\u200bof the penetration depths are given in the literature for various shielding materials. For the sake of clarity, we will also present the same data in the form of table 8.1.
It can be seen from the table that for all high frequencies, starting from the medium wave range, a screen made of any metal with a thickness of 0.5 ... 1.5 mm is very effective. When choosing the thickness and material of the screen, one should not proceed from the electrical properties of the material, but be guided by considerations of mechanical strength, rigidity, corrosion resistance, convenience of joining individual parts and the implementation of transitional contacts between them with low resistance, ease of soldering, welding, etc.
From the data in the table it follows that for frequencies above 10 MHz, a film of copper and, moreover, of silver with a thickness of less than 0.1 mm gives a significant screening effect... Therefore, at frequencies above 10 MHz, it is quite permissible to use screens made of foil-clad getinax or other insulating material with a copper or silver coating applied to it.
Steel can be used as shields, just remember that due to the high resistivity and the phenomenon of hysteresis, the steel shield can introduce significant losses into the shielding circuits.
Filtration
Filtering is the main means of attenuating constructive interference created in the power supply and switching circuits of DC and AC power systems. Suppression filters designed for this purpose can reduce conducted interference from both external and internal sources. The filtration efficiency is determined by the insertion loss of the filter:
dB,
The following basic requirements are imposed on the filter:
Ensuring the specified efficiency S in the required frequency range (taking into account the internal resistance and the load of the electrical circuit);
Limiting the permissible drop in DC or AC voltage across the filter at maximum load current;
Ensuring acceptable non-linear distortion of the supply voltage, which determines the requirements for the linearity of the filter;
Design requirements - shielding efficiency, minimum overall dimensions and weight, ensuring normal thermal conditions, resistance to mechanical and climatic influences, manufacturability of the structure, etc .;
Filter elements should be selected taking into account the rated currents and voltages of the electrical circuit, as well as the surges in voltages and currents caused by them, caused by instability of the electrical regime and transients.
Capacitors. They are used as independent noise suppression elements and as parallel filter links. Structurally, interference suppression capacitors are divided into:
Double-pole type K50-6, K52-1B, ETO, K53-1A;
Support type KO, KO-E, KDO;
Checkpoints non-coaxial type K73-21;
Bushing coaxial type KTP-44, K10-44, K73-18, K53-17;
Condensing units;
The main characteristic of a noise suppression capacitor is the frequency dependence of its impedance. To attenuate interference in the frequency range up to about 10 MHz, you can use two-pole capacitors, taking into account the small length of their leads. Reference noise suppression capacitors are used up to frequencies of 30-50 MHz. Balanced feed-through capacitors are used in a two-wire circuit up to frequencies of the order of 100 MHz. Feed-through capacitors operate in a wide frequency range up to about 1000 MHz.
Inductive elements... They are used as independent elements of noise suppression and as successive links of noise suppression filters. Special types of chokes are structurally most common:
Coiled on a ferromagnetic core;
Coil-free.
The main characteristic of an interference suppression choke is the dependence of its impedance on frequency. At low frequencies it is recommended to use magnetodielectric cores of grades PP90 and PP250, made on the basis of m-permaloy. To suppress interference in the circuits of equipment with currents up to 3A, it is recommended to use HF chokes of the DM type, at high rated currents - chokes of the D200 series.
Filters. Ceramic pass-through filters of B7, B14, B23 types are designed to suppress noise in DC, pulsating and alternating current circuits in the frequency range from 10 MHz to 10 GHz. The designs of such filters are shown in Figure 8.17.
The attenuation introduced by filters B7, B14, B23 in the frequency range 10..100 MHz increases approximately from 20..30 to 50..60 dB and in the frequency range above 100 MHz exceeds 50 dB.
Ceramic in-line filters of the B23B type are built on the basis of ceramic disk capacitors and turnless ferromagnetic chokes (Figure 8.18).
Turn-less chokes are a tubular ferromagnetic core made of ferrite grade 50 VCh-2, dressed on a through-feed output. The inductance of the choke is 0.08 ... 0.13 μH. The filter housing is made of UV-61 ceramic material with high mechanical strength. The body is metallized with a layer of silver to ensure a low transition resistance between the outer plate of the capacitor and the grounding threaded bush, with which the filter is fastened. The capacitor along the outer perimeter is soldered to the filter housing, and along the inner perimeter - to the through-hole. The filter is sealed by filling the ends of the body with a compound.
For B23B filters:
nominal filter capacities - from 0.01 to 6.8 μF,
rated voltage 50 and 250V,
rated current up to 20A,
Filter dimensions:
L \u003d 25mm, D \u003d 12mm
The attenuation introduced by the B23B filters in the frequency range from 10 kHz to 10 MHz increases approximately from 30..50 to 60..70 dB and in the frequency range above 10 MHz exceeds 70 dB.
For on-board power plants, it is promising to use special noise suppressing wires with ferron fillers having high magnetic permeability and high specific losses. Thus, for wires of the PPE brand, the insertion loss in the frequency range of 1 ... 1000 MHz increases from 6 to 128 dB / m.
The known design of multi-pin connectors, in which one U-shaped noise suppression filter is installed on each contact.
Overall dimensions of the built-in filter:
length 9.5 mm,
diameter 3.2 mm.
The filter insertion loss in a 50-ohm circuit is 20 dB at 10 MHz and up to 80 dB at 100 MHz.
Filtration of power circuits of digital RES.
Impulse noise in the power buses arising in the process of switching digital integrated circuits (DIC), as well as penetrating externally, can lead to malfunctions in the operation of digital information processing devices.
To reduce the level of noise in power buses, circuit design methods are used:
Reducing the inductance of the "power" buses, taking into account the mutual magnetic coupling of the forward and return conductors;
Reduction of the lengths of the sections of the "power supply" buses, which are common for currents for various ICS;
Slowing down the edges of impulse currents in the "power" buses using noise suppression capacitors;
Rational topology of power circuits on a printed circuit board.
An increase in the cross-section of the conductors leads to a decrease in the intrinsic inductance of the buses, and also reduces their active resistance. The latter is especially important in the case of the ground bus, which is the return conductor for signal circuits. Therefore, in multilayer printed circuit boards, it is desirable to make the "power" buses in the form of conducting planes located in adjacent layers (Figure 8.19).
Hinged power rails used in printed circuit assemblies on digital ICs have larger transverse dimensions in comparison with buses made in the form of printed conductors, and therefore, lower inductance and resistance. Additional benefits of outboard power rails are:
Simplified routing of signal circuits;
Increasing the rigidity of the PCB by creating additional ribs that act as limiters that protect ICs with mounted ERE from mechanical damage during installation and adjustment of the product (Figure 8.20).
High manufacturability distinguishes "power" busbars, manufactured by a printed method and mounted vertically on the PCB (Figure 6.12c).
Known designs of hinged tires installed under the IC body, which are located on the board in rows (Figure 8.22).
The considered designs of the "power" buses also provide a large linear capacity, which leads to a decrease in the wave impedance of the "power" line and, consequently, a decrease in the level of impulse noise.
The wiring of the IC power supply to the PCB should be carried out not in series (Figure 8.23a), but in parallel (Figure 8.23b)
It is necessary to use the power wiring in the form of closed loops (Figure 8.23c). This design approaches in its electrical parameters to the solid planes of the supply. To protect against the influence of an external interference-carrying magnetic field, an external closed loop should be provided along the perimeter of the PCB.
Earthing
The grounding system is an electrical circuit that has the property of maintaining a minimum potential, which is the reference level in a particular product. The grounding system in the ES should provide signal and power return circuits, protect people and equipment from faults in the power supply circuits, and remove static charges.
The following basic requirements are imposed on grounding systems:
1) minimizing the overall impedance of the ground bus;
2) the absence of closed ground loops, sensitive to the effects of magnetic fields.
An ES requires at least three separate ground circuits:
For signal circuits with low currents and voltages;
For power circuits with a high level of power consumption (power supplies, ES output stages, etc.)
For chassis circuits (chassis, panels, screens and plating).
Electric circuits in the ES are grounded in the following ways: at one point and at several points closest to the ground reference point (Figure 8.24)
Accordingly, grounding systems can be called single-point and multi-point.
The greatest level of interference occurs in a single-point grounding system with a common serially connected "ground" bus (Figure 8.24 a).
The further away the grounding point is, the higher its potential. It should not be used for circuits with a large spread in power consumption, since powerful FUs create large return ground currents, which can affect small-signal FUs. If necessary, the most critical FU should be connected as close to the ground reference point as possible.
The multi-point grounding system (Figure 8.24 c) should be used for high-frequency circuits (f≥10 MHz), connecting the FU RES at the points closest to the ground reference point.
For sensitive circuits, a floating ground circuit is used (Figure 8.25). Such a grounding system requires complete isolation of the circuit from the case (high resistance and low capacitance), otherwise it is ineffective. Solar cells or batteries can be used as power sources for the circuits, and signals must enter and leave the circuit through transformers or optocouplers.
An example of the implementation of the considered grounding principles for a nine-track digital tape drive is shown in Figure 8.26.
There are the following ground buses: three signal, one power and one frame. The most susceptible analog FUs (nine sense amplifiers) are grounded using two separate ground rails. Nine recording amplifiers operating at signal levels greater than the sense amplifiers, as well as control ICs and data product interface circuits, are connected to the third signal line “ground”. The three DC motors and their control circuits, relays and solenoids are connected to the power ground rail. The most sensitive drive shaft motor control circuit is connected closest to the ground reference point. The frame bus "ground" is used to connect the frame and the casing. Signal, power, and frame ground buses are connected together at a single point at the secondary power supply. It should be noted the expediency of drawing up structural wiring diagrams in the design of radio electronic devices.
