Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Current Electricity - current flowing through hand

  1. Sep 7, 2011 #1
    Current Electricity -- current flowing through hand

    1. The problem statement, all variables and given/known data

    i)For a layer dry skin 1mm thick, determine the resistance of a 1 cm^2 area of skin.
    Resistivity of dry skin : 3 x 10^4 ohms m

    [3.0 x 10^5 ohms ]
    ii)A person, who is well-earthed, accidentally grabs a wire of diameter 0.4 cm at a potential of 50V. His hand makes contact with the whole circumference of the wire over a distance of 9cm.
    The average thickness of skin of his hand is 1mm. Estimate the current flowing through him.

    [1.89 mA ]
    2. Relevant equations

    Resistance = resistivity(length)/area
    V = IR

    3. The attempt at a solution

    For i) , I managed to solve it.
    Using the equation directly.

    For ii), I couldn't get the answer.

    First, I calculate the surface area of the cylinder ( wire ) where it is in contact with the wire.

    Surface area = pi . diameter . length
    = pi . (0.4 x 10^-2) . (9 x 10^-2)
    = 0.0011309 m^2
    Then I multiply this surface area with the resistance of dry hand obtain in i)

    Total resistance = surface area x resistance of dry skin 1 mm thick, area of 1m^2
    = 0.0011309 x 3.0 x 10^5

    Next, I used V = IR. Sub in total resistance and P.D of 50 V
    The answer I got was 0.147 A. Totally off.

    Thanks for the time !
     
  2. jcsd
  3. Sep 7, 2011 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    hi sharkey1314! :smile:
    but won't the resistance go down if the area of contact increases? :wink:
     
  4. Sep 7, 2011 #3
    Re: Current Electricity -- current flowing through hand

    Thanks a lot. I re-used the equation for resistance to find the new resistance of the hand. Then, after using V = IR, I managed to get the answer =D

    Just to clarify, what does a potential of 50V means ? And does the potential changes along the length of the wire. i.e. the start and end point will have different potential ? If so, does the answer change since potential varies.
     
  5. Sep 7, 2011 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    it means the potential difference between the wire and the ground …

    ie from your hand to your foot! :biggrin:
    Yes, but only very slightly …

    the drop in potential along a wire is IR, and R for a wire is usually very small!! :smile:
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook