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Magnetic fields and Permeability

  1. May 9, 2012 #1
    Basically I just wanted to know how magnetic fields work when a highly Permeable material is placed close to a magnet. I mean normally a magnetic field as far as I know expands out it all directions whether it is from a wire or a coil or a magnet, but what happens when a Permeable material is placed very to close to the source so as to confine the magnetic field within it. As far as I know there is still a magnetic field outside of it, even if it was a super permeable material. The way I have always heard and imagined it work was like this.

    The magnetic field from the coil, wire or magnet expand outward, this field encounters the permeable material, at this point the magnetic field causes a reorientation of the atoms within the material causing them to point in the same direction as the magnetic field from the wire, coil or magnet and thereby increasing the over-all magnetic field strength within the material. Now this magnetized material acts like a magnet and causes more of the external magnetic field (wire , coil or magnet ) to enter it since it would be the path of least reluctance now (less resistance). There will still be a outside field though since not all of the field will be redirected to the permeable material.

    So is that right?

    Also is there still an external magnetic field outside a toroid electromagnet? I know the magnetic field is supposed to be completely inclosed within the core but it would seem there is still a field outside it.



    I guess what I am trying to get too is this, is a magnetic field like an expanding bubble, that's how I have always imagined it. Obviously not exactly a bubble because of the different geometry's of the coils or magnets and this has an effect on the way the field looks but for the most part, I view it as a bubble. Like I view it as a bubble that expands outward at all time's for practically forever until it is so weak it can't be detected at-least, so from my point of view it moves outward irregardless of the material it moves through, it only has an effect on that material like the permeable materials, it increases in strength inside it but outside it does not (actually decreases outside because of the field redirection),( basically more the field is concentrated closer to the source or permeable material) the field outside will be redirected a bit because the material now acts like a magnet itself which changes the way the field moves. I know magnetic field's always come back on themselves too, but from how i see it, the field still moves outward but like a bubble, it is always connected to both poles but still able to forever expand outwards just with increasingly deminished strength, even after it is no loger detectable, it is still there moving outward.

    Also when exactly does a magnetic field become part of a electric field to form a electromagnetic field? does the magnetic field like actually break off from the source , (antenna )

    So, How far off in crazy land am I? Please correct me if needed, which I sure, all of this makes sense and from what I have read and talked about on other forums about magnetic fields, it seems like this is correct.

    You can probably tell that I really need to visualize something to understand it, lol. I even visualize math to understand it, like doing it on paper in my head, lol
     
    Last edited: May 9, 2012
  2. jcsd
  3. May 10, 2012 #2

    mfb

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    The geometry of the magnetic field depends on the geometry of your setup. While material can increase the field inside, the field lines still have to be closed, so they have to return to the other side of the field source somehow.

    There is, but it is small.

    Not really. In the static case (constant current in electromagnets for example), nothing moves.

    If you move relative to a magnetic field, you will see electric fields. In a similar way, if you move relative to electric fields, you will see magnetic fields. These are not two separate concepts. They are both electromagnetic fields.
     
  4. May 10, 2012 #3
    ok so you said that a magnetic field expanding like a bubble is not really true at least for a static field ( contant current like you said). From what I have heard and read is that if you had an electromagnet, your electromagnet is positioned on the earth, you then set up a detector on the moon, this detector can detect even the weakest magnetic fields. Now you turn on the electromagnet and it reaches steady state( just about, never really reaches it) in say 1 us. Now the detector on the moon will still detect that magnetic field after 1 second and if there was a detector on mars then that detector would eventually detect it as well.

    Is that not true?

    Now I understand that a magnetic field and electric field are the same thing viewed differently (barely understand, they say it is, so I believe it is, lol) but why are they so obviously different? Yes they can create one another yet the electric field's never need to come back on the selves and magnet fields do? magnetic fields can create tremendous forces and so can electric fields in certain instance's, but it is much much harder to get an electric field to generate those forces.

    I do have one theory as to when a magnetic field becomes part of a electromagnetic wave or I guess I should say it no longer becomes the dominate force. The idea is that as you said and many other sources have is that magnetic fields also have electric fields and vice versa but for say a electromagnet, the dominate force is the magnetic field because it is so much larger than the electric field but if you lengthen the coil so it is more like an antenna than the electric field starts to become larger (lengthening the coil doesn't really matter to the theory). Now as that magnetic field bubbles out into space, it becomes weaker and weaker until a point where the electric field which is propagating out as well becomes somewhat equal in magnitude to the magnetic field, since the electric field is oscillating and so is the magnetic field, they start to act as a real self propagating electromagnetic wave. one creates the other and vice versa
     
  5. May 10, 2012 #4

    mfb

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    It is correct that changes in the magnetic field propagate with a maximal velocity of c - the detector on mars would need some minutes to see the change. However, unless you try to generate electromagnetic radiation, changes are usually slow compared to light within your setup. A small electromagnet might give some relevant field within ~1m, which is 3ns for light. If you change the current in it within 1ms, this is about 300.000 times longer.

    That is quite a fundamental insight and given by the Maxwell equations and special relativity (SR). For low velocities, they look like two different things. But they are just two different ways how an electromagnetic field can look like at v=0.
    If you change the current in your electromagnet quick enough, you get a significant voltage between its ends (inductance of the magnet), and get electric and magnetic fields at the same time.
     
  6. May 10, 2012 #5
    What exactly did you mean by

    did you just mean the frequency of the electromagnet generating the field?


    What happens if the frequency of the Electromagnet is on the order of nanoseconds or even picoseconds. I have read that if the frequency of pulsing is faster than the time it would take for the magnetic field to collapse, than that magnetic field which didn't collapse in time is released away as Electromagnet radiation. Is that correct?
     
  7. May 11, 2012 #6

    mfb

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    The frequency of the current (changes) in the electromagnet.

    You won't get a significant amplitude with a real electromagnet and this frequency. However, you would get fast-changing electromagnetic fields everywhere, even within the individual turns in the coil. And you would probably get significant electromagnetic radiation (depends on the geometry).
     
  8. Jun 4, 2012 #7
    I believe you meant period of oscillation when you were talking about nanoseconds and pico seconds. if the T (period) is >= 1 ns then we have GHZ frequencies, and if T>= 1ps then we have THZ frequencies.

    an electromagnetic is simply an inductor. for frequencies that high, unless your electromagnet was extremely small, you would almost certainly be causing the waveforms to propagate off the wires and into the air, thus causing EM radiation at the same F as the signal through the wire. and in that case, both the wires connecting the inductor and the inductor itself would be acting as antennas. The wavelength of a 1GHZ signal is 1 meter, and it gets drastically smaller as you increase frequency since wavelength=(speed of light)/frequency or λ=c/F

    something to think about is skin effect and transmission line effects. the higher the F, the smaller the skin depth for an AC current passing through a wire. also, for any sort of AC waveform of any frequency, if the wire is long enough, there will be transmission line effects present. what this means is that if both the frequency and its transmission line are long enough, the waveform will begin to propagate off the wire and into the air, thus causing energy loses and other issues. the fields from waveform can also cause wave reflections back to the source and whatnot if they are not impedance matched.

    there is a very simple formula used to determine weather or not the waveform through the line will have any noticeable propagation off the line, aka its used to determine weather or not to take transmission line effects into account. if the length of the line divided by the wavelength ≤ 0.01 then you probably dont need to worry.

    so for 60Hz transmission line aka the power station up in the mountians to your house
    the λ=c/f ⇔ (3*108 m/s)/(60hz) = 5,000,000m

    and if our T-line is 100 km long, or 105 meters long, then we have:
    l/λ= 105/5*106 = 0.02. this is close, but not so close that we'd need to account for any transmission effect.

    but lets look at your case: a 2.5GHZ (0.4ns) signal passing through a wire - anything like that in the real world probably wouldnt have a very long line - probably 1 meter at best.
    for 2.5GHZ, λ=c/f = 0.12 meters

    will we have T-line effects? l/λ= (1m)/(0.12m)= 8.333 in this case you definitely must take these effects into account. and you can bet that that 2.5 GHZ wave will be radiating off those wires.

    hope that helps. I still have much to learn about waves and propagation myself.
     
  9. Jun 4, 2012 #8
    about permeability.

    yes anytime a permeable material is placed in a B field it will contain and concentrate the flux through it, because it does provide a path of lower reluctance. However the material in question can only contain a finite amount of flux. magnetic materials have two important properties, one is their μ (permeability) and the other is their maximum allowable flux, which is called their saturation magnetization, or Bsat or just their max flux density.

    ferrites - which are those black ceramics often made of up Mn and Zn or Ni and Zn and iron3oxide - are often used for high frequency inductors and transformers, and also as cheap permanent magnets. these usually have high permeabilities (μ>10,000) and low flux densities (Bsat≤0.5 Tesla) they can only contain so much flux until they saturate, and then as the B field increases beyond the Bmax, it will only increase as if it was just passing through air. (0)

    though it is interesting, most magnets used to hold cabinets closed use a crappy ferrite sandwiched between two steel plates. if you take this apart, you'll see that neither of the plates retain much magnetic field, and you'll also see that the ferrite itself is a rather weak magnet when you stick it on your refrigerator. until you put it back together and notice how much the field strength has increased due to the presencese of the steel. steels have a permeability of around ≥1000 and Bmax at 1.5T.

    reading in depth about how transformers work will give you some good insight into the relationships between μ and B as well as the hysteresis loop works when you compared the applied magnetic field intensity H (which is directly proportional to the current in the windings) affects the magnetic flux density (B field) of the inside of the coil.

    to answer your question about toriods. Yes since no toriod is perfect there WILL be an external field. if you were trying to measure it, like if you were checking a switch mode power supply for external magnetic fields, you would probably check it and measure it with whats known as a "sniffer probe" which is like an oscilloscope probe but with a hall effect sensor attached.

    Also, when you have a magnetic field. as long as it is static, meaning the current used to create it is a steady current, there will be no external E field created from the B field.

    however if the B field changes at all, there will be an E field produced with the same frequency as the change in B. or if you start moving a conductive object near the static or moving B field, there will be an electric field happening inside the conductor due to the eddy currents that the B field induces.
     
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