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Calculating the resistance from R = pl/A 
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#1
Jun511, 02:15 PM

P: 614

1. The problem statement, all variables and given/known data
A laboratory lead consists of 16 strands of fine copper wire twisted together. Each strand is 30 cm long with a diameter of 0.15 mm. Calculate the potential difference across the lead when it is carrying a current of 2.0 A. (The resistivity of copper = 1.7 × 10^8 Ohm*m) 2. Relevant equations What I would do is R = pl/A and then multiply by 2 BUT I can't find R 3. The attempt at a solution (the answer is. apparantly, 0.036 V but I cant see how) I said R = [(1.7*10^8)*16*0.3)/(pi*0.15/20000)^2) 


#2
Jun511, 02:30 PM

HW Helper
P: 2,323

Also, why are you dividing 0.15 by 20,000? Why is pi being squared? The area of a circle of pi*r^2, not (pi*r)^2. 


#3
Jun511, 02:41 PM

P: 614

it was meant to be pi * (0.15/2000)^2 again sorry, Question: it is still ONE strand of wire, just twisted, are you sure total R decreases 


#4
Jun511, 02:42 PM

P: 614

Calculating the resistance from R = pl/A
seems you are correct  I now get the correct answer
thanks 


#5
Jun511, 02:49 PM

P: 614

http://www.edexcel.com/migrationdocu...e_20090521.pdf
BUT if you look at page 19 then we multiply by 8 not divide by 8 why?? 


#6
Jun511, 02:59 PM

P: 614

oh, don't worry I get it
first example 16 seperate strands second example one strand with 8 branches its ok 


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