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Lightning: AC or DC?

  1. Apr 10, 2005 #1
    I've been having problems trying to come up with an answer for this seemingly simple question. I would think that lightning would be one short jolt of direct current; the current flowing in only one direction for a brief duration. However, I've been told that it is AC current by a teacher of mine. When trying to find the answer online, so far I've found two different answers. One from a PhD who says it is direct current, and another from a PhD who say's its 'neither.' If true, how can it be neither? Everyone 'knows' there are only two types of electricity right? I don't understand how such a common phenomenon such as lightning can be so misunderstood. This should be an easy question! If someone can shed some light on this, it would be much appreciated :smile:
     
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  3. Apr 10, 2005 #2

    chroot

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    It's DC. Static charges build up in the cloud and on the ground, producing a large potential difference. This potential difference is then "resolved" via a spark, through which large numbers of electrons travel in one direction.

    - Warren
     
  4. Apr 10, 2005 #3

    Pengwuino

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    Well i think the big problem is the assumption that "Everyone 'knows' there are only two types of electricity right?". I think DC and AC are just man-made words/phrases that arent necessarily laws of nature. Im pretty sure you can have a large pulse of electricity which wouldnt be defined as AC or DC. I'd side with the 'neither' situation... but i dunno, need more opinions.
     
  5. Apr 10, 2005 #4

    chroot

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    Of course, Pengwuino, it depends on your definition of DC. If your definition is simply "electrons flowing in only one direction," then lightning is DC. If your definition is "the current between two points with a constant potential difference," then it's not DC. (The potential difference decreases as the bolt carries current from one place to another.)

    - Warren
     
  6. Apr 10, 2005 #5

    Pengwuino

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    Well my knowledge of electricity is limited so i dunno :P
     
  7. Apr 10, 2005 #6
    AC is a human invention. It is DC of course.
     
  8. Apr 15, 2005 #7
    strictly speaking there is no such thing as DC current in the nature. Because DC means not a fixed direction, but a fixed magnitude versus time. Everything is changing, so no current is absolutely stable, even in a superconducting ring. (somebody will perturb it eventually)

    Of course, if we have a time interval of interest, we can assume that current
    is DC if any change are negligible during this interval. So lightning is AC within 1 seconds, but may be considered a DC for the faster processes.
     
  9. Apr 15, 2005 #8

    SGT

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    Wrong! DC means that the current does not change sign during time. A current whose magnitude does not change during time is a constant current.
    If you submit an AC voltage to a rectifier, the result is a DC voltage, even if it varies over time. If you want this DC voltage to become constant, you must use a filter in order to eliminate the time varying components.
    Of course, no filter will be able to really provide a constant voltage. There will always be a ripple, but we can discard this as irrelevant to most applications.
     
  10. Apr 15, 2005 #9
    if you put the "AC" current throu the rectifier, you will have the "AC" current still. The negative component will be small but nonzero.
    From the technical point of view you may need to distinguish the current(voltage) sign, but if something can transmit "AC", it will transmit pseudo-DC with oscillating component as well (but it may suppress bias). So, if you are sitting on the other side you will not able to tell if you have "AC" or pseudo-DC source. For example, it does matter if you have the electrolitic capasitor before the usual one or after that. In first case pseudo-DC is OK, in the second case you can destroy it.

    Now, if you have "AC" power supply with the period 10,000 years, it will be as good as DC source with the stability 0.01% per year.

    One may say this is just a terminology, but let me remind that EM waves are produced by electro/magnetic oscillations, and lightning produces EM waves for sure.
     
    Last edited: Apr 15, 2005
  11. Apr 16, 2005 #10

    krab

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    It is not useful to try to categorize as AC or DC. These are convenient categories for man-made electricity. One would like to know whether the current is "constant". But "constant" is a relative term. Lightning is not a constant current on the scale of human activities, even if it is only one pulse, since it only lasts a fraction of a second. But lightning is commonly multiple pulses, up to 25 in the space of half a second. See here: http://www.apttvss.com/white/white-lightning.asp
     
  12. Apr 16, 2005 #11

    Ivan Seeking

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    I agree. To call lightning DC is a bit like saying that AC is DC for a half of a cycle. I guess it could be considered a time-varying DC pulse, but you also have the flow of both [real] positive and negative charges - the electrons and the ionized air molecules. In fact, as the strike forms, the step leader, which is made of postive charge, travels down to the ground in jumps of 50 meters ,with 50 microsecond pauses between each jump, and eventually completes the circuit along which the electrons travel up.

    Edit: btw, the polarity of this can all be reversed at times. There are even places where this is common.
     
    Last edited: Apr 16, 2005
  13. Apr 22, 2005 #12
    i agree to one of you saying it is DC as DC is currnet moving at the same direction at all time while AC is current moving at directions over time.
     
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