Calculating Current in a Coil with Given Magnetic Field and Parameters

[endeavor]

"The magnetic field at the center of a 50-turn coil of radius 15 cm is 0.60mT. find the current in the coil."
N=50, r = .15m, B = 6.0 * 10^-4 T, magnetic permeability = 4 * pi * 10^-7.

B = (mag.per. * N * I)/ L
However, I don't have L, which is the length. I do have r, the radius, but it's not in the equation... How do i solve this?

 

[durt]

This is not a solenoid, just a ring of wire. A hoola hoop.

 

[endeavor]

then what does "a 50-turn coil" mean?
my book gives me the equation for finding the magnetic field in the center of a circular loop: B = (mag. per. * I)/(2r). But then how do I factor the "50-turn coil" in? Is this simply superfluous information?

 

[Doc Al]

Think about it: You have the formula for the field at the center of a single circular loop of current-carrying wire, so what would you expect the field to be for 50 of them combined?

 

[endeavor]

50 times the strength I suppose...

well, that does work out:
1/50 B = (mag.per. * I)/(2r)
and solving for I, I get: I = 2.9A, which is the right answer.

Thanks.