A plane spiral has N=100 tight turns carrying I=8 mA, with inner radius a=50 mm and outer radius b=100 mm. Find the magnetic induction at the centre in μT.

Question

G1 Physics · Accepted Answer

For tightly spaced turns, dN=N dr/(b-a). A ring of radius r contributes dB=μ₀I dN/(2r). Thus B=μ₀IN/[2(b-a)]·ln(b/a). Substitution gives about 7 μT.