# Electrical resistance

lets say i have a block of material, measures a,b,c (length width height) and two wires running in from the top, both with circular cross section, radius r, distance d from the edges

Code:
    ____________________
/  d            d   /|
/<--->O       O<--->/ |
/___________________/  |
|                   |  |
|                   |c |
|                   | /
|___________________|/ b
a

if i were to let electrical current through the block, measure the current and voltage i could find its resistance. But how would i calculate it theoretically if i knew the specific resistance of the material? I have no idea how to do that because i cant imagine how the current would go through the block, i mean through what path.

Or, lets create a more general problem:
lets say i have a space filled with material, measurements infinite (fills up all the space), specific resistance known, and two wires (isolated from the material) (sames as before, circular cross section and radius r) running in from infinity with end points at distance L.

Code:
               <---------->
L
--------------|            |----------------------
--------------|            |----------------------

so what i need to know what the path of the current looks like (its diffuse of course but the further away from the shortest path the less current takes that path)

what i'm actually trying to understand is how electric current moves in human body if i would place electrodes on the skin and apply some voltage, but the geometry of this is nontrivial, which i have not seen before while solvin 'find the resistance of blah blah blah' kind of problems

thanks for any hints

mgb_phys
Homework Helper
Resistance = rho * L / A
rho is the resistivty of the material
L is the distance between the points
A is the cross section area

Born2bwire