# Calculating the resistance from R = pl/A

1. Jun 5, 2011

### jsmith613

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. Jun 5, 2011

### ideasrule

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.

3. Jun 5, 2011

### jsmith613

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

4. Jun 5, 2011

### jsmith613

seems you are correct - I now get the correct answer
thanks

5. Jun 5, 2011

6. Jun 5, 2011

### jsmith613

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