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Steady direct current question

  1. May 29, 2010 #1
    I have a question (possibly dumb) if someone could humor me with an explanation .

    I understand that steady currents don't create electromagnetic radiation. If that's true how does a high-voltage steady DC current create interference? For that matter why does a high voltage capacitor create interference if no EM waves are being emitted ? I can understand how it might do it while charging up but not how it would create EM waves when it reaches a steady state.
  2. jcsd
  3. May 29, 2010 #2
    A steady direct current produes an associated magnetic field.

    This interacts with the varying currents in apparatus receiving the interference.

    Don't forget only one thing has to vary to produce interaction.
  4. May 29, 2010 #3
    A steady dc current or magnetic field will not interfere with other circuits. There could be forces created but this is not electrical interference in the usual sense.

    The other possibility is the saturation of magnetic elemets but fields have to be very high to do this.
  5. May 30, 2010 #4
    I think that since indio has two opposing answers he deserves a resolution. Further no one answered the part about the capacitor I have been thinking about that.

    Antiphon was right that in many cases the magnetic field produced by a steady direct current will not cause interference.

    However consider a powerful permanent magnet.
    Hold it still next to the side of a CRT television or computer monitor.
    Note the distortion on the screen.
    Now this same steady magnetic fireld could have been produced by a suitable direct current, exhibiting the same distortion in the picture.
    Now try the same experiment with the magnet near the tuning coils of an old fashioned superheterodyne receiver and listen to the interference detuning.

    That is interference.

    Now to the capacitor.

    A capacitor, once charged, will not admit direct current by itself.
    So an isolated capacitor,charged or not, cannot produce interference.

    As a test an ordinary battery is a very large capacitor.
    Bring it near any apparatus, by itself.
    You will not detect any interference.

    Connect a load to this capacitor and allow it to discharge through this load and you can detect the changing field in suitable equiment.

    Of course this is not steady dc. Bu the point is that there is a load so I wonder if the capacitor you are thinking of is the reservoir capacitor of a power supply>
    You would certianly be able to detect interference from the ripple which also appears at the output as well as the steady dc.
  6. May 30, 2010 #5


    Staff: Mentor

    On the contrary, a battery has negligible capacitance.
  7. May 30, 2010 #6
    The capacitance of a simple 1000 mAhr AAA rechargeable battery is

    [tex]\frac{{1000x{{10}^{ - 3}}x3600}}{{1.5}} = 2400Farads.[/tex]

    Not much really.
  8. May 30, 2010 #7
    That is incorrect. A 1 A-hr 1.5 V battery stores about 5400 joules of energy (E = V*A integrated over time, and assuming a reasonably flat discharge curve). A 1.5 V capacitor with equivalent stored energy would have a capacitance of 4800 farads (E = 0.5*V^2*C).

    This does not imply that the battery has an electric field that remotely resembles such a capacitor, though. Batteries are electrochemical devices for producing electricity from electrically neutral chemical fuels. They are not capacitors, and have very little capacitance. However, it is true that a charged capacitor will not generally produce interference. It is much harder to achieve a high electrostatic field than it is to achieve a high magnetic one, and there's little that would be directly affected by such a field. Charges building up on parts in a CRT could cause "interference" with the electron beam, but like the effects caused by static magnetic fields, that is not what is typically meant by interference.

    Also, about high voltage DC...it's current that causes magnetic fields. Neither continuous high current DC (high static magnetic field) nor a charged high voltage capacitor (high static electric field) will radiate EMI. High current DC will produce spikes of EMI when it is switched on or off, though. High voltage DC may also produce corona discharges or arcing, and since such discharges are not a continuous DC current, they will radiate EM.
  9. May 30, 2010 #8
    Don't you guys here just love missing the wood for the trees?

    Assuming this, assuming that is not the point, nor was it "incorrect", it was just a very approximate estimate barely sufficient for purpose, but sufficient nonetheless.
  10. May 30, 2010 #9
    It was incorrect. It was the wrong equation and the wrong units for the component, it was the wrong component...and an indisputably incorrect claim that one component was another. A battery is not a capacitor, battery capacities are not measured in farads, etc.

    Also...the capacity and capacitance are irrelevant when it comes to the field produced. Equating a battery with a large-value capacitor was not only incorrect, it was misleading. A 1.5 V battery may store far more energy than a small-value 15 kV capacitor (112.5 J per microfarad), but the latter has a far stronger electric field.

    Anyway...something that hasn't been mentioned about high electrical fields: a lot of modern electronics uses MOSFETs and other high input impedance devices. A little leakage or induced charge could build up charge on a gate and falsely switch transistors, interfering with proper operation of the device. It could even build up enough voltage to destroy the gate of a MOSFET. This is commonly caused by one specific sort of high voltage capacitor with about 100 pF of capacitance and 1500 ohms of series resistance, often seen walking around wearing wool or synthetic fibers and inconsiderately touching I/O connections after charging themselves up to several kilovolts...
  11. May 30, 2010 #10
    Instead of making unsubstantiated claims and in particular your own incorrect statements, perhaps you would like to provide your own definition of a capacitor (noun).

    Of course you may prefer to simply address the original question, which would be the polite thing to do, instead of harping on about electrical fields.
  12. May 30, 2010 #11
    What unsubstantiated claims? Point one out.

    What incorrect statements? Point one out.

    A capacitor is a device that reversibly stores energy as a charge difference between electrodes separated by a dielectric, the stored energy depending on the charge difference, the thickness of the dielectric, and the material of the dielectric (specifically, its dielectric constant). I note that it's automatically linked in your post to a detailed definition and description in the PF Library, consult that for further details.

    What is commonly called a "battery" is an electrochemical cell, one which releases chemical energy as electrical power through chemical reactions that occur on the electrodes. Some of them are reversible and superficially resemble a capacitor in operation, but batteries are not capacitors. A 1.5 V battery is certainly not much like a charged high-voltage capacitor.

    The original post specifically asked about interference caused by a high voltage capacitor. Such interference would be in the form of an electrostatic field.
  13. Jun 1, 2010 #12
    Sorry about starting a fight oops..

    What I had in mind though was a high tension line or a parallel plate capacitor running at say 40,000V. I makes sense that it is the strength of the electric field affecting the electronic device's changing current and not EM radiation. If that's the correct way at looking at it.

    it makes me wonder something else though. Does pulsed DC cause EM radiation?
  14. Jun 2, 2010 #13
  15. Jun 2, 2010 #14
    Indio, you didn't start a fight so don't worry your question is perfectly legitimate.

    The point of a capacitor is that the bulk of the field is contained in the space between the 'plates'. A small amount 'escapes' as leakage at the edges.

    EM radiation is generated almost exclusively by the currents in the leads connecting to the plates, which is why very high frequency circuitry uses leadless components.

    A properly detailed description of you question/problem would help resolve it without all the uneccessary spurious posts.
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