Question:
A long straight wire carries a current of 35 A. What is the magnitude of the field B at a point 20 cm from the wire?
Solution:
Current in the wire, I = 35 A
Distance of a point from the wire, r = 20 cm = 0.2 m
Magnitude of the magnetic field at this point is given as:
$B=\frac{\mu_{0}}{4 \pi} \frac{2 I}{r}$
Where,
$\mu_{0}=$ Permeability of free space $=4 \pi \times 10^{-7} \mathrm{~T} \mathrm{~m} \mathrm{~A}^{-1}$
$B=\frac{4 \pi \times 10^{-7} \times 2 \times 35}{4 \pi \times 0.2}$
$=3.5 \times 10^{-5} \mathrm{~T}$
Hence, the magnitude of the magnetic field at a point $20 \mathrm{~cm}$ from the wire is $3.5 \times 10^{-5} \mathrm{~T}$.