# Coefficient of resistivity

1. Sep 20, 2008

### goWlfpack

1. The problem statement, all variables and given/known data

A wire, 50.0 m long and 2.00 mm in diameter is connected to a source with a potential difference of 9.11 V, and the current is found to be 23.45 A. Assume a temperature of 20°C and using Table 17.1, identify the metal out of which the wire is made of.

2. Relevant equations

R=p(l/A)

where R is the resistance p is the coefficient of resistivity, l is the length and A is the area

3. The attempt at a solution

ok since we are solving for p first i found R by the equation V=IR and found R to be .388
next i found the area using the surface area of a cylinder and found that to be .628
next i divided length by .628 and found that to be 79.618
so then i divided .388/79.618 and got 4.8*10^-3
thats the right magnitude but it does match any of my options coefficients.

the problem lets me choose whether the metal is gold, silver, copper, or iron.

2. Sep 20, 2008

### olgranpappy

wrong area. you want the cross-sectional area of the wire

3. Sep 20, 2008

### goWlfpack

ugh ok i got the area by .002*.002*3.14 = .00001256
then i divided 50/.00001256 and got 3980891.72
so then i divided my R .388 by 3980891.72 an got 9.74e-8
not right...

4. Sep 20, 2008

### goWlfpack

plz help i really dont understand why this is still wrong...

5. Sep 20, 2008

### olgranpappy

you still have the wrong number for the area. The *diameter* is 0.002. The *radius* is 0.001.
You used Area=diameter*diameter*Pi, which is wrong.