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magnetic field reducer?

Discussion in 'Misc Electronics' started by qude, Jun 20, 2005.

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  1. qude

    qude Guest

    Suppose you have a current source with magnetic field
    and you want to reduce it by adding a metal near it
    to induce opposite magnetic field to cancel the one
    from the source (partially). What coil configuration
    must you use? Is this possible?

    Don't reply by telling me to use particular wire
    shielding or bend the source wire. I'm just
    interested in the theoretical rationale of putting
    metal or coil configuration near the magnetic field
    source to create reverse magnetic field to cancel
    a part of it. How much magnetic field do you think
    is reduced (or cancelled)?? This is assuming the metal
    or coil put near it doesn't have any voltage or
    current source itself but just from the induced
    current from the magnetic field of the source.

    qude
     
  2. CWatters

    CWatters Guest

    All electrical circuits have a return path. The best you can do is to use a
    second wire for the return path and to twist it around the signal wire. You
    now have equal currents flowing in opposite directions in close proximity.
    The magnetic fields almost cancel out.
     
  3. qude

    qude Guest

    I'm asking whether putting a separate wire without voltage or
    current source with certain coil configuration can cause
    opposite magnetic field to be produced by the induced current,
    and what is the magnitude of it, Anyone knows?

    qude
     
  4. Dave

    Dave Guest

    yes it can. magnitude depends on configuration and incident field. i.e. a
    plate of aluminum can almost completely cancel a 60hz magnetic field due to
    induced currents.
     
  5. qude

    qude Guest

    Is it always cancellation.. or can you make the configuration such
    that the induced field would add up to the source increasing
    the magnetic field (or does the law of conservation of field
    applies here).

    qude
     
  6. Dave

    Dave Guest

    in antenna design you can configure conductors to increase the field in one
    direction but it always is decreased in some other direction because of
    conservation of energy. i haven't really thought of that for low frequency
    work, but would assume the same would apply, while reducing the field by
    cancellation is one direction it must increase in some other area, minus a
    bit for losses from resistive heating in the material used for the
    shielding.
     
  7. How would you calculate the losses?

    In an earlier question I am asking about using the heating effect?
     
  8. Dave

    Dave Guest

    very carefully. calculating induced currents and losses is not the easiest
    problem, especially in solid plates. it may be a bit easier in wire loops.
    yes, if a current is induced by the magnetic field then there will be some
    loss due to heating in the material.
     
  9. qude

    qude Guest

    But it's not always the case, isn't it. Suppose you turn a connected
    wire in parallel with the current of each in opposite direction.
    There would be cancellation of the magnetic field outside the
    parallel wires and nothing in between them. This means cancellation
    outside indeed occur. Where does the conservation of energy came into
    play here? There is no evidence it is increased elsewhere with
    the same magnitude as the ones outside that is cancelled.

    qude
     
  10. Don Kelly

    Don Kelly Guest

    There seems to be a great deal of confusion involved here. More so in your
    first message.

    Note that inducing a voltage in a parallel wire (and this voltage producing
    a current), the best you will get is that the external field will be the
    same as with the original wire only as all you have is a crude transformer.

    In the case of the wire being physically parallel with a return wire
    carrying the same current (e.g. a lampcord), partial cancellation (never
    full unless the wires can occupy the same point in space) will occur outside
    the wires. However, between the wires, the field will be not be "nothing"
    but will be enhanced as the two currents are then additive with regard to
    the magnetic field. Sketch the field around a current into the page and that
    due to a current out of the page. Bring the sketches together- note the
    relative directions of the fields in different locations. I can give you a
    set of equations for calculation of the field due to a number of parallel
    current carrying wires if you wish.
    As to conservation of energy- there is no problem. The total field energy is
    dependent on the current and the position of the wires. If the position
    changes, then the field changes but there is work input or output to change
    the position In fact, conservation of energy can be used to calculate forces
    in such a situation. (Conservation of energy leads to : change in electrical
    energy in =change in magnetic field energy + change in mechanical energy +
    losses). This can be and is expressed in a number of ways in Electromagnetic
    Energy Conversion texts.
     
  11. qude

    qude Guest


    To avoid confusion.

    This is what I'm describing. A plate of aluminium put over a wire
    carrying 60 hz AC or fluctuating dc. Using the right hand rule,
    the current is clockwise looking from the source at the left. Now
    what I wanna understand is how does the induced current in
    the plate of aluminum (not in contact with it but put on top of
    it at a distance) produce magnetic vector that is in opposite to
    that of the wire. Does this something to do with Lenz law?
    What's the proof that induced current and magnetic field
    in the aluminum plate is opposite to that of the wire... which
    part of the plate the current flows?

    Thanks guys.

    qude
     
  12. Dave

    Dave Guest

    to aid in figuring this out in your mind replace the plate with a whole
    bunch of little wire loops. as the field changes the db/dt induces a
    current flow in each of the little loops that will oppose the incident
    field. this is a simple case of induced current from a changing magnetic
    field. it gets much more complicated when you realize there are an infinite
    number of these loops and they are all connected to each other when it
    becomes a solid plate, but the small loop model can still be used to give
    good estimates of fields.
     
  13. qude

    qude Guest


    I'm inquiring because I saw a product at a shop which has this
    cooper plate. It is supposed to lower emissions of magnetic
    field from appliances by simply being put near them. I wonder
    how many percentage of magnetic field are suppressed when say
    put directly on top a computer monitor or toaster, any idea?
    It's for those who are very EM sensitive or electrosensitives.

    p6
     
  14. Dave

    Dave Guest

    no, its for those who have more money than brains. at the edges the fields
    wrap around so for a small shield like that it would only reduce the field
    slightly for a small distance on the other side of it. even when shielding
    transformer vaults it requires not only doing the whole wall between the
    transformer and the equipment being protected, but a large part of the
    floor, ceiling and other walls... preferably wrapping the whole room.
     
  15. Don Kelly

    Don Kelly Guest

    Related to Lenz' Law- yes. Related to conservation of energy- also true.
    The aluminum plate is essentially the secondary of a transformer (not a good
    one by any means).
    Look up transformer operation as well as the background for the expression
    Erms =4.44FN(flux maximum) for sinusoidal AC.
     
  16. CWatters

    CWatters Guest

    yeah there are lots of suspect products on the market that claim to "protect
    you from harmful radiation" - but if they worked whole industries would be
    out of business. It can be a bitch preventing EMC escaping from electronic
    devices like computers. I've done my time down a saltmine trying to design
    screening. Extreemly frustrating it can be.
     
  17. qude

    qude Guest

    I checked further. Is it possible the plate of cooper or
    aluminum can be designed so that (by induction) it can produced
    a third waveform, by some kind of circuit. ES or Electrosensitives
    have so called neutralizing frequency and these swedish designers
    created some kind of copper plate in which when you put it near
    any source of magnetic field, it can produce a third waveform
    oscilating at a certain frequency. For example. You are tasked
    to design such circuit that is powered by induction (with power
    requirement even 0.01 mA or less) and oscilating at 20 Hz. How
    would you do that?

    Also is it only magnetic field that can be used for contactless
    induction. How about electric field. Can electric field be used
    for contactless induction too? The former is Faraday law of
    induction, what's the second called (if at all)?

    Thanks.

    qude



    via
    induction,
    works in conjunction with the electric field of the battery and the
    magnetic
    field of the quartz timing coil. Using these two fields, the Teslar
    chip is
    designed to produce a third waveform that pulses at 7-9 times
     
  18. Dave

    Dave Guest

    rfid tags pick up one rf frequency and then use that to power their
    circuitry to respond on a different frequency. anti-theft labels in retail
    stores are even simpler and do a similar job but without the smart data of
    an rfid tag. adding a diode or two to a simple loop of wire you can double
    the frequency. you can also generate subharmonics of the exciting current
    with a properly tuned resonant circuit and a diode or two.
    yes, an electric field can be used by having two plates instead of a coil.
    as the electric field changes it will produce a voltage difference between
    the plates that can be used to power devices. i think some power utilities
    use this for 'unpowered' devices on high voltage lines. many years ago one
    of the 'popular' magazines published a free power motor that ran on the
    clear air field gradient.
     
  19. qude

    qude Guest

    Supposed you are to design it with pure copper foil or plate with
    no diode. Just wire perhaps imbedded in the copper surface. How
    would you do that. Supposed the input frequency is 2 Hz (by
    induction). And you want the output magnetic field to be 15 Hz.

    For example you have a battery, let's say a small calculator battery.
    It should produce some electric field near it. If you put the copper
    tag nearby. What possible induction transfer can occur when it's
    clear you didn't put it in between the plates?

    qude
     
  20. Dave

    Dave Guest

    i would say its not possible without active components. harmonics and
    subharmonics require a non-linear component of some kind like a diode. non
    integer frequency multiplication is much harder and would typically require
    that the incident field be rectified and used to power some kind of active
    signal device like on an rfid tag.
    the electric field from a small battery is very small and static except for
    when you physically move the battery or move the plate around it. but even
    then the change in the field is so small it would even be hard to measure
    without extremely sensitive instruments. also note that for a static field
    source like a battery a current would only be created when it was moving.
     
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