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Magnetism and electricity question

Discussion in 'Electronic Basics' started by Don Bruder, May 31, 2006.

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  1. Don Bruder

    Don Bruder Guest

    Simple (seeming...) question that I can't find a simple, understandable
    answer to. Everything I've managed to encounter so far has left me more
    confused.

    So let's try asking a group of humans:

    When speaking of electromagnets such as a winding in a motor, or the
    voice-coil of a speaker, and all other things being equal, which gives a
    stronger magnetic field - higher volts, or higher amps?
     
  2. Bob Eld

    Bob Eld Guest

    Amps.....The magnetizing force H = .4*Pi * N * I /lm . In cgs units, H is in
    Oersteds, N is the number of conductor turns, I is the current in Amps, lm
    is the magnetic path length in cm.

    Of course, voltage creates the flow of current by acting on the resistance
    or impedance by ohms law so you can't have Amps without Volts, but it is the
    current that creates the magnet.
     
  3. Dorian

    Dorian Guest

    Just to help you get a feel for the symmetry in electronics there are two
    type of fields utilized in electronics.



    ELECTROSTATIC - Field strength varies with voltage and some other factors.



    It is possible to have an electrostatic field without current such as
    between the plates of a charged capacitor (actually a tiny leakage current
    will usually flow).



    Using a mechanical system as an analogy this is similar to a force with no
    movement such as pushing on an object that won't move.



    MAGNETIC - Field strength varies with current and some other factors.



    It is possible to have a magnetic field without voltage as in the case of a
    superconducting magnet.



    Using a mechanical system as an analogy this is similar to movement with no
    force such as an object moving on a frictionless surface or in space.



    As far as magnetic fields are concerned I once maintained a few MRI units.
    To "ramp up" or activate the superconducting magnet we connected it to a
    variable power supply capable of providing 40 volts at 400 amps and slowly
    brought the current up to 360 amps. Once the magnet was brought up to 360
    amps a superconducting "short" was activated to allow the current to
    circulate only in the magnet. The power supply was then slowly brought back
    down to 0 amps again and turned off. The 360 amps would continue to
    circulate in the magnet indefinitely without any voltage.



    In electronics we typically deal with systems the have both voltage and
    current and therefore consume power. Remember that power is used or produced
    in a system only if both voltage and current are present. P = V x I so if
    either V or I are zero then no power is consumed or produced.





    Dorian
     
  4. Tim Williams

    Tim Williams Guest

    Volts have nothing to do with magnetic fields- okay, you can't have current
    without voltage, err in most cases anyways, so you usually need *some*
    voltage to get it moving. But magnetic field is always proportional to
    current.

    Where (AC) voltage comes in is because XL = 2*pi*F*L and I = V / XL.
    Transformers, and well anything operating on Faraday's laws of induction,
    depends on a *change* in magnetic field to produce an electric potential.

    To answer your question, volts alone will not produce a magnetic field, so
    by default the answer is amps.

    Tim
     
  5. Don Bruder

    Don Bruder Guest

    Bingo... Just what I needed.

    Thanks.
     
  6. Guest

    For a practical electromagnet running at maximum limits, the magnetic
    field strength is set by the core material and core shape, and by the
    wattage thermal dissipation, and by the grams of copper in the coil
    winding ...*not* by the current.

    If you try to increase the current, you'll exceed the maximum wattage
    unless you replace your coil with one which has fewer turns of thicker
    wire, and runs at lower voltage and higher amps. But with fewer
    turns, the total amp*turns remains the same as before! So any
    attempt to increase only the current is useless.

    To make a stronger field, you can add a core, or change its shape.
    Or you can crank up the wattage after installing a heat sink. Or
    you can add more copper to the coil (replacing the coil with thicker
    wire of the same no. of turns, or just adding more turns of the
    same gauge already there.) Then change the drive voltage to
    get back to max wattage.

    Example: the number of turns are irrelevant because if max wattage
    is fixed at 10 watts, then 1 amp and 10 volts needs 10 ohms of wire,
    which might require 100 turns of thin wire. Or 100 amps at 0.1 volt
    would require one single turn of very thick wire to maintain 10 watts
    max. Amp-turns is 100 in both cases, so changing the number of
    turns can only re-program the volts-amps ratio of the power supply.
    It doesn't get you a stronger field.

    All this just means that bigger, heavier electromagnets give stronger
    fields. And water-cooled electromagnets give stronger fields. But
    we already knew that.


    ((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
    William J. Beaty Research Engineer
    UW Chem Dept, Bagley Hall RM74
    Box 351700, Seattle, WA 98195-1700
    ph425-222-5066 http//staff.washington.edu/wbeaty/
     
  7. Jasen Betts

    Jasen Betts Guest

    All other things being equal the best ansewer is "Yes".

    Bye.
    Jasen
     
  8. Don Bruder

    Don Bruder Guest

    Boy, now there's a useful (for absolutely nothing) answer...

    You seem to be good at those lately, Jaysen. Have you been practicing or
    something?
     

  9. Well, he did say that it was an "An-sewer", didn't he?


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  10. Don Bruder

    Don Bruder Guest

    I'll grant you that point. (though I hadn't noticed it previously)
     
  11. Magnetic field is due to the relative motion of charge and an observer.
    There is no such stuff as "magnetic stuff". All E/M efects are due to
    photon momentum exchange between charges. So...

    How do we get charge moving? Apply an accelerating potential, i.e. a
    voltage. It is thus the *voltage* or technically, an electric field,
    that actually "makes" the charge move, i.e produces a current, by way
    of:

    F=qE=d(mv)/dt

    Force is charge times Electric field (V/d), which is equal to the rate
    of change of momentum.

    So... to get a bigger magnetic field, i.e. more charge flowing, you need
    a bigger voltage. Of course, this voltage always originates from other
    charge.

    For a loudspeaker, the basic design principle is that the output is
    proportional to applied voltage, not current. The impedance of a speaker
    is all over the place with frequency, causing wide variations in current
    over frequency, yet the sound output response can be reasonable flat.

    This can be summed up by the notion that everthing is controled by way
    of charge -> voltage -> current.

    Kevin Aylward B.Sc.

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "There are none more ignorant and useless,than they that seek answers
    on their knees, with their eyes closed"
     
  12. Ahmmmm.

    Kevin Aylward B.Sc.

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "There are none more ignorant and useless,than they that seek answers
    on their knees, with their eyes closed"
     
  13. Well, it's good to know that you see yourself as the rest of the world
    sees you.

    Cheers!
    Bobo
     
  14. ehsjr

    ehsjr Guest

    Whichever gives you higher ampere*turns.
    Ed
     
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