Kelvin (K)

Write $300$ K, never $300$ degrees K. The scale is absolute rather than relative, so it has no degrees to be counted in. The AP Physics 2 table of information prints the symbol as a bare K, right next to degree Celsius, which does keep its degree sign.

A kelvin and a Celsius degree are the same size. Only the zero moves, by about $273$. Two consequences that pull opposite ways:

• A temperature is never the same number on both scales. $27$ degrees Celsius is $300$ K.
• A temperature difference is always the same number on both. A rise of $25$ K is a rise of $25$ degrees Celsius, which is why $Q = mc\Delta T$ tolerates either scale while $PV = nRT$ does not.

Where only kelvin works: $PV = nRT = Nk_BT$, $K_{\text{avg}} = \tfrac{3}{2}k_BT$, $U = \tfrac{3}{2}nRT$, and over in modern physics $\lambda_{\text{max}} = b/T$ and $P = A\sigma T^4$. Each of those makes a physical quantity proportional to $T$, and none can survive a negative temperature or a zero parked at the freezing point of water. Feeding $27$ into the gas law instead of $300$ is an error of about a factor of eleven, and nothing in the algebra objects.

The conversion is not printed. The AP Physics 2 table of information lists kelvin and degree Celsius as unit symbols and gives no relation between them anywhere on its three appendix pages. Absolute zero near $-273$ degrees Celsius is a fact you supply.

Kelvin appears on one of the four AP unit-symbol tables, the AP Physics 2 one. Neither Physics C course covers thermodynamics.