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Shock when touching an electric fence

  1. Sep 3, 2011 #1
    Hello Everyone,
    If you are grounded and touch an electric fence that is at 10,000V, say, you will experience a shock. What happens if you were to jump in the air when touching the fence? Would you still feel a shock?

    I'm getting confused because even in the air, your body is still at the potential of ground (say 0V) and thus, there will be a potential difference between the fence and your body. So you will get some charge transferred until you are at the same potential as the fence. However, is this charging enough to shock you?

    It seems to me that the shock you feel when you are grounded comes from the sustained current that flows from the fence to ground through your body. Therefore, you wouldn't feel much of a shock if you touched the fence when jumping in the air.

    This is similar to why birds don't get shocked when sitting on power lines. Anyone have better though out answers to these questions?
     
  2. jcsd
  3. Sep 3, 2011 #2
    You have understood it correctly.

    If the fence is at 10000 volts with respect to the ground and you touch it having jumped in the air the only current that will flow is the charge required to charge you (since now you are a capacitor) to the same potential as the fence. Your capacitance is about 100 pF = 10^-10 Farads. A capacitor of capacitance C charged to a voltage V needs a charge of Q given by Q= C*V. Hence Q is about
    10^-6 Coulombs

    A charge Q flowing in time t is a current of Q/t amps. A reasonable time for t is given by the resistance of the connection R by t = 0.6*R*C and a good estimate for R is 200 ohms so t is
    about 1.2 milliseconds hence the current is less than a milliamp for less than a millisecond. Not painful !

    But if you stay on the ground and touch the fence you get all the current it can give as long as you are stupid enough to stay connected.

    The birds you mention have a much lower capacitance than you so the current is trivial for them.

    PS when out in the countryside never take a leak against a bush which may have an electric fence behind it – I did once and it is very defiantly not to be recommended :(
     
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