Answer:
[tex]5.0\cdot 10^{-12}T[/tex]
Explanation:
We can think the axons as current-carrying wires
The strength of the magnetic field produced by a current-carrying wire is
[tex]B=\frac{\mu_0 I}{2\pi r}[/tex]
where
[tex]\mu_0[/tex] is the vacuum permeability
I is the current
r is the distance from the wire
In this problem we have
[tex]I=0.040 \mu A = 0.04\cdot 10^{-6} A[/tex]
r = 1.6 mm = 0.0016 m
So the strength of the magnetic field is
[tex]B=\frac{(4\pi \cdot 10^{-7}H/m)(0.04\cdot 10^{-6} A)}{2\pi (0.0016 m)}=5.0\cdot 10^{-12}T[/tex]
