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Basic Circuit Q: Why does one get shocked and not the other?

  1. May 7, 2015 #1
    1. The problem statement, all variables and given/known data:

    This is such a basic question that even though it's not a HW problem, I don't know where else to put it. Please see http://i.imgur.com/3O5YrAP.jpg .

    Why does the person on top get shocked and not at the bottom? More specifically, why is there a current through the dirt on top, but not at the bottom? The only difference in the figure is that the top person is touching a point across a resistor where electrons would flow, whereas the bottom person isn't. Why would that somehow make a difference?

    I got an answer from another source but I don't understand their explanation. It is that there is no voltage potential difference on the bottom between the source and the person, but there is one on top. Why would there be a potential difference on top figure but not on the bottom figure? I can see that in the bottom figure, electrons flows out of the source to the ground, then can go through the ground to the person and then back to the resistor and back to the (+) terminal, completing the circuit, so why is there no voltage drop in that sense?

    please see this figure http://imgur.com/L6BpS15

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. May 7, 2015 #2

    Simon Bridge

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    In electric circuit diagrams, anything connected by a line is treated, electrically, as being in the same exact place.
    Everything connected to the ground symbol is also in the same exact place... and that place is also connected to a very large reservoir of charge.
    Also - by convention the reservoir of charge is so large that no current flows in it ever.

    The amount of charge at the hand and foot (bottom figure) are the same - both touching the ground. Electrically the hand and foot are both in the same place. Basically the hand and foot are both touching the negative terminal of the battery.

    In the top figure, the battery is delivering charge to the person's hand and taking it away from the foot.

    The charge flowing through "the dirt" is not relevant to the question. "The earth" is a technical term and does not have to mean that there is a wire buried in some dirt someplace. I think the diagram is just misleading.
     
    Last edited: May 8, 2015
  4. May 8, 2015 #3

    ehild

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    Why would the electrons flow through the body instead of choosing a practically zero path between the earthed point to the person's hand? The human body has got quite a big resistance.
     
  5. May 8, 2015 #4

    ehild

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    The "dirt" is relevant here, as the person stands on the ground, on "dirt" , In real life, the grounding for houses means a big piece of metal dug in the soil. http://en.wikipedia.org/wiki/File:HomeEarthRodAustralia1.jpg
     
  6. May 8, 2015 #5

    cnh1995

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    I think the ground is a common point in the circuit, where -ve terminal and man's feet get connected,making a potential difference to develop across man's body, in diagram 1. So, its like "man in parallel with the load", causing a shock. But in diagram 2, man's hand and feet both are connected to ground i.e. from head to toe, the man is at ground potential (no potential difference). So, there's no shock. The bird is also safe in both the cases as the p.d. across its feet is negligible (almost 0). And I don't understand the issue of dirt or some underground metal box in this case. Because, even when one stands on the floor, current, although very small, passes through the body. A good example can be glowing of tester bulb when plugged in the live terminal of socket board. I think standing on dirt or any metal ground will cause a lethal shock.. Please correct me if I'm wrong..
     
  7. May 8, 2015 #6
    i think i kinda get it now. thanks guys
     
  8. May 8, 2015 #7

    CWatters

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    Identify the possible circuit loops....

    1) In the first one the dangerous loop is shown in red...

    The +ve of the high voltage supply
    His hand
    His foot
    The dirt
    The -ve of the high voltage supply.

    Current flows around the whole loop (including the dirt) because of the high voltage source in the loop.

    2) In the second one the are two possible loops of interest. The green one..

    The -ve of the high voltage supply
    His hand
    His foot
    The dirt
    The -ve of the high voltage supply.

    There is no net voltage in this loop so no current flows around the loop (which includes the dirt).

    3) There is another possible loop in the second circuit shown in red and yellow.

    The +ve of the high voltage supply
    The load resistor
    His hand
    His foot
    The dirt
    The -ve of the high voltage supply.

    However the wire shown in green between his hand and the -ve of the high voltage supply effectively diverts any current that might otherwise flow through the man back to the -ve of the high voltage source. The second circuit is still potentially dangerous. What would happen if this green wire became disconnected?

    No Shock.jpg
     
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