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GreenPrint
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Homework Statement
http://img26.imageshack.us/img26/7307/captureghl.png
I have a square that is 5 cm by 5 cm. I have filled it in with graphite from a pencil. The top left corner I have labeled A. The top right corner B. The bottom right hand corner C. The bottom left hand corner D. I take a DMM probe and set it to measure resistance, I put the red probe at A, the black probe at C. I then take a second DMM and set it to measure DC voltage. I place the black probe at C.
At this point I take the red probe and place it around the corners of the squares within the square that are each a one centimeter by one centimeter and measure the voltage. I collect 25 data points.
I observed the following data through measurement. The units are Volts. Please note I labeled the corners in my picture.
http://img577.imageshack.us/img577/382/captureyfu.png
My measured data is obviously not completely accurate, but according to my measured data, I would say that the further away you get from corner C the less the voltage is. This however isn't true along the lines DC and CB. The voltages seem to increase along these lines.
I'm asked to explain my collected data for a lab report.
Homework Equations
R = [itex]\frac{ρl}{A}[/itex]
Where:
R = resistance
ρ = resistivity (in the case of graphite ρ≈1*[itex]10^{-5} Ωm[/itex]
l = length
A = cross sectional area
A = wh
Where:
w = width
h = height (thickness of the graphite layer)
V = IR
Where:
V = voltage
I = current
R = resistance
The Attempt at a Solution
I started off by putting in the formula for resistance into Ohm's law in the hopes that I would be able to explain the observed data.
V = I[itex]\frac{ρl}{A}[/itex]
Now, I guess I just don't understand this. Sense the square is completely filled in, isn't the surface area and length remain constant. I'm not looking at a wire but a completely filled in square. So no matter where I put the red probe within the square to measure the voltage between the red probe and black probe the same resistance exists.
I guess I just don't understand why I'm getting different measurements.
Thanks for any help you can provide!
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