Answer: B = 0.025 T
Explanation: The formulae for the magnetic field strength of a toroid is given below as
B = uNI/2πr
Where B = strength of magnetic field =?
u = permeability of free space = 1.256×10^-6 mkg/A²s²
I = current flowing through the toroid = 2.1A
r = distance at which magnetic field is created = 1.5cm = 0.015m
N = number of turns = 890
By substituting the parameters, we have that
B = 1.256×10^-6 × 890 × 2.1 /2 ×3.142 × 0.015,
B = 0.002347464 / 0.09426
B = 0.025 T