Heating coil wire resistance and length of wire required

[servin247365]

Homework Statement

A portion of Nichrome wire of radius 2.50 mm is to be used
in winding a heating coil. If the coil must draw a current of
9.25 A when a voltage of 120 V is applied across its ends,
find (a) the required resistance of the coil and (b) the
length of wire you must use to wind the coil.

Homework Equations

RI = V
R = I/oA

The Attempt at a Solution

(a) RI = V
R = V/I = 120V/9.25A = 12.973Ohms

(b)
R = l/oA
l = RoA = (12.973Ohms)(.000001ohms*m)(.0025m*3.14*2) = 2.036761e-7m

The answer to the second part of the equation is 255m and I can't figure out what I did wrong...

Thanks!

 

[lewando]

Your units aren't working out well--equation for resistance seems odd.

Try R=ρl/A

ρ, resistivity in Ωm
l, length in m
A, cross-sectional area in m².

 

[TSny]

Did you use the correct formula for area? Did you use a value for resistivity or conductivity for the symbol "o"?

 

[servin247365]

Ah OK, that worked. I am not sure why the other formula from the book didn't show that but maybe I missed something... Thanks!