Current Electricity - current flowing through hand

[sharkey1314]

Current Electricity -- current flowing through hand

Homework Statement

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 ]

Homework Equations

Resistance = resistivity(length)/area
V = IR

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 !

 

[tiny-tim]

hi sharkey1314!

Then I multiply this surface area with the resistance of dry hand obtain in i)

but won't the resistance go down if the area of contact increases?

 

[sharkey1314]

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.

 

[tiny-tim]

Just to clarify, what does a potential of 50V means ?

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

ie from your hand to your foot! ​

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.

Yes, but only very slightly …

the drop in potential along a wire is IR, and R for a wire is usually very small!