Absolute zero

Zero kelvin sits near $-273$ degrees Celsius. AP Physics 2 arrives at it without ever naming it: the phrase "absolute zero" appears nowhere in the Course and Exam Description. What the CED gives instead is essential knowledge 9.2.A.4, which says a temperature at which an ideal gas has zero pressure can be extrapolated from a graph of pressure as a function of temperature. That intercept is the zero of the kelvin scale.

Two routes to the same point:

• From $PV = nRT$ at fixed $V$ and $n$, pressure is proportional to $T$, so the pressure vanishes as $T$ approaches zero.
• From $K_{\text{avg}} = \frac{3}{2}k_B T$, the average kinetic energy per atom goes to zero with $T$, leaving the atoms nothing to give up.

It is a limit, not a destination. No system can be cooled to 0 K. That unattainability is the third law of thermodynamics, which AP Physics 2 does not cover, so an exam answer should rest on 9.2.A.4 instead: every real gas liquefies long before the line reaches the axis, which is why the CED's verb is "extrapolated".

Absolute zero is also what makes ratios mean anything. Doubling a Celsius reading predicts nothing; doubling an absolute temperature doubles $K_{\text{avg}}$, doubles $U$ for an ideal gas, and doubles the pressure of a gas held at constant volume.