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?

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

1 Answer 1


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.

enter image description here

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


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