Permeability

Printed in the Constants group of both the AP Physics 2 and the AP Physics C: Electricity and Magnetism sheets, and the magnetic counterpart of permittivity. As a decimal it is about $1.26 \times 10^{-6}$.

Leave it in the $4\pi$ form when you substitute. On the AP Physics 2 sheet it appears inside exactly one equation, $B = \mu_0 I / (2\pi r)$, and there the $\pi$ cancels against the denominator, turning the whole constant into a factor of $2 \times 10^{-7}$. Converting to a decimal first buys an extra rounding step for nothing. The same cancellation runs through the Biot-Savart law, which arrives as $\mu_0 / 4\pi$ already.

Every equation that builds a magnetic field from a current carries it: $B = \mu_0 I/(2\pi r)$ for a straight wire, $\mu_0 I_{\text{enc}}$ in Ampere's law, $\mu_0 n I$ inside a solenoid.

For matter it is not a table lookup. The AP Physics 2 CED is unusually blunt here: the permeability of matter differs from that of free space, and it is not a constant for a material, varying with factors including temperature, orientation, and the strength of the external field. Density you can look up; this you cannot.

Do not confuse the symbol with the coefficient of friction, which also uses $\mu$, or the two constants with each other. Permittivity is electric and sits under $4\pi$; permeability is magnetic and sits on top of it.