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Magnetic field

  1. Jun 22, 2009 #1
    Dear all,
    I need a suggestion for my work. I have to measure the variation of some optical features of a material in presence and absence of a magnetic field. Hence, I need to “switch on” the magnetic field in a given instant time, to record the signal an then ““switch off”, or even to change the direction of the magnetic field. Moreover, I need that this “object” is small and handy.
    Do you have any suggestions, a part a small permanent magnet? Probably the solution is banal, but at the moment I am not able to find it.
    Thanks a lot
  2. jcsd
  3. Jun 22, 2009 #2


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    Gold Member

    I'm not sure what "small and handy" is, but remote devices like, TV, car locks, can be reprogramed for use, as you need.
  4. Jun 23, 2009 #3
    Thank you for your hint but probably this is not what I am looking for.
    The “object” that I have to characterize is small, in the order of cm, so probably a proper coil or an electromagnet can be useful, in order to control the magnetic field by the current.
    The problem is that this coil should be small…do you have (or somebody else in this forum) some website to suggest me on this topic?
    Thanks again
  5. Jun 23, 2009 #4
    I can sketch for you a magnet design that you can use.
    First, I need to know the volume you want for your experiment, and the magnetic field you need. The most efficient design is probably an "H" magnet or a "C" magnet rather than a solenoid.
    Second, I need to know how long you want to keep the current on. This is important because the coil can be air-cooled for short pulses, but may need to be water-cooled for dc operation for long periods.
    Third, how do you plan to power this magnet? One possibility is a 12-volt car battery, which is possible if you need less than about 20 amps.
    An example might be an H magnet with a 5-cm gap at 1 tesla will require
    BL = 4 pi x 10-7 NI
    L=0.05 meters
    B=2.5 x 10-5NI
    So 40,000 amp turns for 10,000 Gauss
    A car battery can provide about 100 turns at 20 amps = 2,000 amp turns.
    Bob S
  6. Jun 26, 2009 #5
    Thank you for your precise reply,
    My sample is a squared slim slab with dimensions of 2 x 2 cm2; I need to apply an uniform magnetic field (between 50-500 Oersted) in the direction parallel to the plane of the slab. A “C” magnet could be a proper solution, and the two arms of the “C” could be of 5-10 cm in height. As concern the “H” magnet, I have never heard about it, probably the aim is to obtain more uniform and intense fields?
    I need to keep the current on only for few seconds in a direction and other few seconds in the opposite direction in order to invert the magnetization of the material that I have to investigate.
    Anyway your suggestions have been useful just for now for generating useful discussions with my co-workers, so at any rate thank you very much!

  7. Jun 27, 2009 #6
    Based on your description of the field required, the electromagnet gap should be roughly 5 cm, with a 5 cm diameter poletip. See thumbnail sketch. Using a design goal of 500 Gauss (500 Oersted) would require a total of 2,000 amp-turns. With symmetric coils above and below the gap, this would be 1,000 amp-turns per coil. So about 50 turns at 20 amps would be adequate. Very roughly, the length of each coil would be about 12 meters long, for a total of 24 meters. 16 Ga. wire is about 4 ohms per 1000 ft, or about 0.3 ohms for 24 meters. So the coil would need some series resistance or current regulator to keep the current at 20 amps. The power would be I2R = 400 x 0.3 = 120 watts, so the magnet should not be left on for long periods of time.

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  8. Jul 7, 2009 #7
    Thank you very much!
    Your suggestions will be very useful to me!

  9. Jul 7, 2009 #8
    Hello Dear Friends

    I have a similar kind of issue

    I want to test a material under various magnetic flux values, and i dont have a good background in electrical. i have got a commercial form of electromagnet which i can use. the issue is that
    ==> I am confused that to varry the magnetic flux value between 0 and 0.5 T, whether I need to vary the voltage or the current. And what is the relation of between flux value and other parameters, i.e., if I want to have a flux value of 0.5 T how do i knw that at what voltage and amperes i can have this much of flux value

    ==> secondly i want to vary the flux value(or volts or amps) based on some output from a control algorithm running in the PC(Matlb). Actuallay i want to change the B value based on the responce of the material. So is there any mechanism available or can be designed which could take the input from the PC(Matlab). ...

    i would be gratefull to you if you could help me

    thanks a lot
  10. Jul 20, 2009 #9
    In order to know how many volts and amps you need for 0.5 Tesla, we kneed to know something about the electromagnet design. Secondly, we need to know something about the coil design (# turns and resistance) to determine power supply design.
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