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Return stroke

  1. Apr 25, 2012 #1
    Hi,
    I have a question about lightning return strokes. It makes sense when I think about the return stroke on its own - as in lightning from the ground to the cloud but I'm a bit confused when I think about it as a reaction towards lightning.
    Why does it occur? Doesn't lightning equalize the potential and if so - why would there be another action? Why is the potential difference suddenly in the other direction?
    It should take more energy than simply equalizing the potential so what am I not considering?
    Thanks,
    Michelle
     
  2. jcsd
  3. Apr 27, 2012 #2
    Bumping, is there a good book I can read on the subject?
     
  4. Apr 27, 2012 #3

    Bobbywhy

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    "A flash of lightning can consist of a number of individual strokes, each following the same path from cloud to ground. The plasma is recreated over and over again in the same channel. The dark intervals between these strokes are typically in the
    range of tens of milliseconds. This explains why the human eye perceives lightning as “flickering.” A lightning flash typically contains three to five strokes, although the observed number of strokes ranges from one to twenty-six.

    Each downward stroke is composed of a downward-moving “leader,” followed by an upward-moving “return-stroke.” The leader creates a conducting path between the cloud and ground and distributes negative charge from the cloud along that
    path; the return stroke follows the same path moving from the ground toward the cloud, distributing positive charge to neutralize the negative leader charge.

    The electrical current in a lightning flash varies from stroke to stroke as well as during each stroke. The return-stroke current rises rapidly to an initial peak of tens of thousands of amperes. That initial current pulse may be followed by a
    current of hundreds of amperes lasting for tens of thousandths of a second. The high return-stroke current rapidly heats the channel to a peak temperature near or above 50,000ºF, increasing the pressure in the channel to ten or more times
    normal atmospheric pressure. This makes the channel produce the intense light that we see and makes it expand, producing a shock wave that eventually becomes the thunder sound wave we hear in the distance."
    www.plasmacoalition.org/plasma_writeups/lightning.pdf

    Excellent diagrams and technical descriptions of cloud charge transfer, the stepped leader, upward-moving discharges, the return stroke, and the dart leader:
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ligseq.html

    Here is the Wiki page dedicated to the person known as “Mister Lightning”:
    http://en.wikipedia.org/wiki/Martin_A._Uman

    And here is his little paperback book, the best possible book for lightning lovers to own…I am one! The cost is eleven bucks!
    https://www.amazon.com/Lightning-Dover-Books-Physics-Martin/dp/0486645754
     
  5. Apr 27, 2012 #4
    Thanks Bobby,
    I read a little about the subject and I'll do some more reading.
    I still feel like there's a part of the puzzle missing since it seems like more charge is transfered than needed and then some of the charge is returned. Most physics laws strive for minimal energy usage for things that occur naturally. What got me interested in the subject is that either it isn't minimalistic or I just don't understand it.
    Maybe this is caused by the fact that although the potential charge at one point is a certain number, by the time it is discharged, the difference has changed since the cloud can be subject to an outer source of charge.
     
  6. Apr 27, 2012 #5

    A.T.

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    It's not. It is always in the same direction. The charge doesn't flow in the reverse direction during the return stroke. It is called "return" because the section of increased current propagates upwards. But the direction of the charge flow the same as during streamer formation.

    When you have a column of balls in a vertical pipe, which glow only when they move, and you open the lower end of the pipe, the balls will fall downwards, but the glow will propagate upwards.
     
  7. Apr 27, 2012 #6

    Bobbywhy

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    One observation: if in a lab experiment you had a charged sphere and discharged it with a grounded wire most of the charge would be "removed" from that sphere.

    When a cloud "discharges" during a stroke it does remove lots of charge, but as you mentioned, neighbouring clouds don't get discharged so there is still lots of difference of potential between the cloud and earth. That's why there can be multiple strokes.
     
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