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Calculating the resistance from R = pl/A

by jsmith613
Tags: pl or a, resistance
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jsmith613
#1
Jun5-11, 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)
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ideasrule
#2
Jun5-11, 02:30 PM
HW Helper
ideasrule's Avatar
P: 2,322
I said R = [(1.7*10^-8)*16*0.3)/(pi*0.15/20000)^2)
Putting strands of wire in parallel decreases the total resistance, because the electrons now have many more paths to follow. 16 strands would decrease the resistance by a factor of 16.

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.
jsmith613
#3
Jun5-11, 02:41 PM
P: 614
Quote Quote by ideasrule View Post
Putting strands of wire in parallel decreases the total resistance, because the electrons now have many more paths to follow. 16 strands would decrease the resistance by a factor of 16.

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.
er, sorry abou the pi^2 bad use of brackets

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

jsmith613
#4
Jun5-11, 02:42 PM
P: 614
Calculating the resistance from R = pl/A

seems you are correct - I now get the correct answer
thanks
jsmith613
#5
Jun5-11, 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??
jsmith613
#6
Jun5-11, 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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