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Voltage and Electromagnetic power

  1. Dec 17, 2009 #1
    Hello Fellows,

    I am trying to build a electromagnet with very little current, but using high voltage of perhaps a circuit from a flash of a camera. Will that generate a strong magnetic field? I only need for a fraction of a sec. Other question is, does high voltage help to strength the magnetic field? I know that current does, but I can't rely on that.
     
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  3. Dec 17, 2009 #2

    cepheid

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    The strength of the magnetic field depends upon the current. If you have very little current, you will have a very weak field. However, a very high voltage will have the effect of driving a large current across the coil.
     
  4. Dec 17, 2009 #3
    So, are you saying that a high voltage will compensate for the low current?
     
  5. Dec 17, 2009 #4

    cepheid

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    NO. I'm saying that a high voltage will produce a high current. WHY do you think that the current would be low?
     
  6. Dec 17, 2009 #5
    Let me put this way... If you have a system with low current and low voltage, would you try to pump the voltage to achieve a strong magnetic field, or would you leave the system alone, since there will be a trade off between current and voltage?
     
  7. Dec 17, 2009 #6

    cepheid

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    Look, a coil is basically a piece of wire, and I think it's pretty reasonable to assume that it has some resistance R, and that if you apply a DC voltage across it, it will draw a current equal to I = V/R, in accordance with Ohm's law. So, as you can see, for a given coil, I is *directly* (i.e. linearly) proportional to V. If you increase V, then I also increases. If you decrease V, then I also decreases. So there is no "tradeoff" between voltage and current. That would imply that they were inversely related, which is not true.
     
  8. Dec 17, 2009 #7
    Thanks Cepheid, but maybe I am not being clear, which I appologize. I am using a transformer to achieve the high voltage, which means the current will decrease even further, right? Like I said, I am using the system of a flash of a camera to achieve that high voltage. If W=VxI then the current should drop if I increase the voltage using a transformer. If that is the case, is there any gain to the magnetic field in pumping up the voltage?
     
  9. Dec 17, 2009 #8

    cepheid

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    As far as I know, there is no gain to the magnetic field. Its strength depends upon the current through the coil alone.
     
  10. Dec 17, 2009 #9
    Thanks Cepheid... you are the best
     
  11. Dec 17, 2009 #10

    Born2bwire

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    I would mention that the flash is designed to give a sharp spike of power from my understanding. If you were to have a large voltage spike to induce your magnet via a coil, then you will probably experience an appreciable reactance due to the coils inductance and Lenz's Law. This may be something to consider when investigating the performance of your design.
     
  12. Dec 18, 2009 #11
    I was about to say, "yes, but only over the long term."

    The larger the inductance, the greater the energy (and time) required to charge it, and discharge it.

    Your tiny voltage is optimized for microsecond disharge. No-go if you're using a traditional LRC circuit.
     
  13. Dec 18, 2009 #12
    What do you mean by "appreciable reactance"? I mean, does it help? Could you formulate more about that?
     
  14. Dec 20, 2009 #13
    First your flash circuit must put out enough power. It can be changed to a magnetic field regardless if it is high voltage or high current.

    There is a second consideration -- total energy. The flash circuit puts out a burst of energy. This will determine if it can drive the field for a long enough time.

    I suspect that you will not have enough energy to do what you want to.

    .
     
  15. Dec 21, 2009 #14
  16. Dec 21, 2009 #15

    sophiecentaur

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    If you really want the maximum current out of your flash capacitor (for the max B field) you would do well to make the circuit resonant - possibly by using a further capacitor.

    In some ways, the use of a capacitor as an energy store is a very good idea - it will supply higher instantaneous current than a conventional lower power supply.
    Needless to say, those flash unit capacitors can give a nasty belt! Take care.
     
  17. Dec 21, 2009 #16
    Interesting point. Would know how to make the circuit resonant? Could you elaborate on that part:"possibly by using a further capacitor". What I mean is do you have a blue print of the circuit that I could use/built?
     
  18. Dec 21, 2009 #17

    sophiecentaur

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    If you have a particular electromagnet and you want a pulse of high magnetic field of a certain period of time then I think the time t ( 1/resonant frequency) will be of the order of root(LC) where L is the inductance of the magnet and C is the capacity. (There's a pi or two involved but that's a detail).
    If you need a much smaller capacitor than the 'lots of' microfarads of the flash unit, then you may need a series capacitor to bring the total capacity to what you need to achieve resonance.
    The actual application would be interesting - what is it?
     
  19. Dec 22, 2009 #18
    It is for a propeler. I got a 400V capacitor from a monitor. I think that should do the trick. I will post my results as I experiment it if you are interested, but that might take a long time since I only work on my project during the weekends. Thanks for you help. You all have a nice holiday.
     
  20. Dec 22, 2009 #19

    sophiecentaur

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    A "propellor"?
    Sounds like you need a motor or a permanent magnet. What would the electromagnet do in a short burst?
     
  21. Dec 22, 2009 #20
    Pardon my english. You are right "propellor"... I have several applications that I want to try on it. Again, nothing too fancy e.g(railgun), by pass the resistence point of a magnetic field of a permanent magnet.
     
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