Magnetic Field Of A Straight Wire

NCERT Class 12, Chapter 4 (Moving Charges and Magnetism) applies Ampere's circuital law to a long straight conductor to obtain the field at distance $r$: $B=\frac{{\mu }_{0}I}{2\pi r}$. The field lines form concentric circles around the wire, their direction given by the right-hand thumb rule. Substituting ${\mu }_0=4\pi \times 10^{-7}$ T·m/A, $I=10$ A, $r=0.05$ m: $B=\frac{(4\pi \times 10^{-7})(10)}{2\pi (0.05)}=\frac{(2\times 10^{-7})(10)}{0.05}=4\times 10^{-5}$ T. The value $2\times 10^{-5}$ T doubles the distance mistakenly. The value $8\times 10^{-5}$ T halves the distance. The value $4\times 10^{-6}$ T is ten times too small from a unit conversion slip. Plausibility check: the field weakens with distance as $1/r$, and a value of order $10^{-5}$ T a few centimetres from a wire carrying $10$ A is physically reasonable, comparable to Earth's magnetic field.