Electromagnetism - Find length of coil

[Deathfish]

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 $$\mu$$_o = 4$$\pi$$ x 10^-7 Wb/Am

(a) l = 63mm
(b) l = 54mm
(c) l = 46mm
(d) l = 36mm

Homework Equations

L = N² $$\mu$$_o $$\mu$$_r A / l

The Attempt at a Solution

0.068 = (9400)² x ( 4$$\pi$$ x 10^-7) $$\mu$$_r A / l

how to calculate if $$\mu$$_r and A are not given??

 

[Zryn]

Given that the core is iron-free, what sort of material would you expect to find there? This material has a very nice relative permeability for calculations!

If you have a (circular cross section of a) piece of wire that is 0.6cm thick (diameter), what is the (cross sectional) Area of this wire?

*NB: Beware of units, as 'l' is in meters, 'A' is in meters squared and 'L' is in Henry's.