A thin insulated wire forms a plane spiral of N = 100 tight turns carrying a current I = 8 mA. The radii of inside and outside turns (Fig) are equal to a = 50 mm and b = 100 mm. Find:
Text Solution
Verified by ExpertsCHECK THE SOLUTION.
Sol. We consider a circular element of radius r and thickness dr.
∴ The number of turns in the considered element is
dN = 
dr
The magnetic field at its centre due to considered element is
dB = 
∴ B = 
= 
ln 
= 7 μ T
Magnetic moment due to considered element
dP m = No. of turns × current × area
dP m = dN × I × π r 2 ∴ P m = I 
dr
∴ P m = 
= 
(b
2 + ab +a 2 ) = 15 mA.m 2 .
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