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## Homework Statement

Calculate the length l of an an iron-free cylindrical coil if it has 9400 turns (N), a thickness of 0.6cm and an inductance L = 68 mH. Take magnetic field constant [tex]\mu[/tex]

_{o}= 4[tex]\pi[/tex] x 10

^{-7}Wb/Am

(a) l = 63mm

(b) l = 54mm

(c) l = 46mm

(d) l = 36mm

## Homework Equations

L = N

^{2}[tex]\mu[/tex]

_{o}[tex]\mu[/tex]

_{r}A / l

## The Attempt at a Solution

0.068 = (9400)

^{2}x ( 4[tex]\pi[/tex] x 10

^{-7}) [tex]\mu[/tex]

_{r}A / l

how to calculate if [tex]\mu[/tex]

_{r}and A are not given??

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