# Heating coil wire resistance and length of wire required

## 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.

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!

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lewando
Homework Helper
Gold Member
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
Homework Helper
Gold Member
Did you use the correct formula for area? Did you use a value for resistivity or conductivity for the symbol "o"?

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!