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Electrical Resistivity Equation 
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#1
Feb2005, 07:43 AM

P: 25

Is the correct equation that relates to the electrical resistivity of a conducting material to its resistance:
p = RS/l p = is the electrical resistivity R = is the resistance of the material S = is the crosssectional area l = is the length of the specimen if I'm using the right formula, would my answer to this question be correct? A potential difference of 1.5V exists across the ends of a copper wire of length 2.0m and uniform radius 0.40mm. Calculate the current in the wire. I've been given the resistivity of copper as 1.7 x 10^4 ohms.meter [Answer] p = RS/l = (1.7 x 10^4) x 0.40mm / 2.0m = 0.0007 x 0.0004 / 2.0 = 0.00000014 corecto? 


#2
Feb2005, 08:05 AM

Mentor
P: 41,325

First, you need to calculate the resistance (R) of that piece of copper wire, not the resistivity ([itex]\rho[/itex], which is a fixed property of copper). You are given the resistivity, not the resistance. Rearrange that formula as so: [tex]R = \rho L / S[/tex] Once you find the resistance, then you can use Ohm's law to find the current. 


#3
Feb2005, 09:44 AM

P: 25

Thanks Doc!
Using the rearranged formula I've calculated: R = pl/S R = 0.0007 x 2.0 / 0.0004 R = 0.0014 / 0.0004 R = 3.5 Ohms Using Ohms Law I = V/R I = 1.5V (as stated in the question) / 3.5 ohms I = 0.42857.. I = 0.43 


#4
Feb2005, 10:24 AM

Emeritus
Sci Advisor
PF Gold
P: 11,155

Electrical Resistivity Equation
You are given the radius of the wire, not its crosssectional area.



#5
Feb2005, 01:09 PM

P: 25

Thanks Gokul! I've really got to start reading questions more thoroughly.. I'm losing marks like this all of the time



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