Find the magnetic dipole moment of the spiral of total number of turns N, carrying current I having inner and outer radii v and b respectively

Explanation

Let us take a thin coil of thickness dr. Then the number of turns of the coil is
`dN=N/(b-a).dr` the dipole moment of the coil is
`dN=(dN)(i)(A)=((Ndr)/(b-a))(i)(pir^(2))=(piNi)/(b-a)int_(a)^(b)r^(2)dr`
`M=(piiN)/3(a^(2)+ab+b^(2))`