# When a coil is wrapped around a ring of non magnetic material, is the path length (l) the circumference or the diameter of the ring?

I'm doing some practice questions on magnetic fields:

A coil of 300 turns is wound uniformly on a ring of non-magnetic material. The ring has a mean circumference of 40cm and a uniform cross-sectional area of 4cm^2. If the current in the coil is 5A, calculate (a) the magnetic field strength. (b) the flux density and (c) the total magnetic flux in the ring.

For (a) I have rearranged to get Magnetic field strength = (Number of coils * Current) / length. In this instance is the length the mean circumference or the diameter of the ring?

• 'be' a bit of magnetic flux. Travel round the ring. How far do you travel? Commented Feb 3, 2020 at 15:04

The length in question is the path along which the magnetic field travels. Given that you have described a circular solenoid (unless I'm mistaken) then the mean circumference is the length you need to use in your equation when calculating magnetic field strength, H.

The dotted line marked B (flux density) travels along the mean path length = $$\2\pi r\$$