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Physics of an electric shock

  1. Mar 11, 2015 #1
    Gruxg has posted a thread on November 4, 2012, titled as
    "If you touch a live wire without touching the ground, do you get an electric shock?".
    I think that this thread would deserve more consideration as it discusses an essential subject but unfortunately it is closed for further replies, so I have to open a separate thread to add some comments.
    All the web pages about electric shock that I found on the net make the very same statement: you receive an electric shock only if you touch two objects with different potentials. (In case of touching a live wire this criterion is met if you are also grounded.)
    However, Gruxg pointed out very cleverly that voltage testers indicate current even if you do not touch anything else but the tester. On the other hand, does anyone really believe that if you touch a live wire on the 10th floor then the building material will act like a perfect conductor through ten levels to the ground? I doubt that this would be the case. I consider it much more likely that the human body itself acts like a capacitor, so an alternating current (AC) can flow through it even if it is not connected to anything else. Of course, if you touch another big object, which doesn't need to be conductive, then it will act as the other plate of the capacitor, hence increasing the effect.
    Then what about those birds standing on transmission lines, which are mentioned everywhere? I think that their case is not the same as touching a live wire in an electrical socket. First, the size of a bird is much smaller than that of a human, so its capacitance is much smaller, too. Second, the resistance of a bird's feet is probably much higher than that of a transmission line. These two things together cause that only a very small portion of the current will flow through the bird's body. But let's suppose that the transmission line is broken (without reaching the ground), meaning that its resistance is practically infinite. Now what will happen if a bird flies on it? Will it get an electric shock or not? This situation is already similar to someone touching a live wire without touching anything else. I guess that the bird will get a shock.
    If I am right (and Gruxg is right) then it would be beneficial to alter the common theory about this subject.
     
  2. jcsd
  3. Mar 11, 2015 #2

    mfb

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    2016 Award

    Staff: Mentor

    Correct.
    Touching something grounded (radiators, every grounded housing of electronics (!), ...) together with the live wire is more dangerous, however.

    How can a transmission line break "between" the two legs of a bird? If it breaks at some other point, it gets switched off and nothing can happen.
    For the AC part, the resistance does not matter much - 50 Hz gives enough time for the current at those small capacitance values.

    Transmission lines are killing some birds, by the way.
     
  4. Mar 11, 2015 #3
    I meant that the transmission line is broken (not between the legs of a bird) and not switched off. If someone touched the wire while standing on the ground then he or she would get an electric shock just like when touching a live wire in the house. But what would happen if a bird flew on the broken wire or a man jumped on it from a distant place? I consider it a different situation from the one when the wire is continuous because in the broken case the current cannot go anywhere else just to the body of the bird (or man). Am I mistaken?
     
  5. Mar 11, 2015 #4
    With AC power, there will be a capacitive current flow to the energized insulated object.
    In the case of EHV 50(60) Hz power lines, and human size objects, these currents can be dangerous.

    Exhibit No.1 :




    Exhibit No.2 :

     
  6. Mar 11, 2015 #5

    mfb

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    As long as the bird does not touch anything else, the situation does not change much. The cable, if touching the ground, will carry some current (it does the same during regular operation), but the resistance of the ground is high - the voltage in the line will still oscillate in the same way as before.
     
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