## Homework Statement

The blackbody radiation equations of state are E/V=aT^(4) and P=(1/3)(E/V). From the fundamental state derive a quantity involving only P and V that remains constant when this radiation undergoes a reversible adiabatic transformation.

## Homework Equations

Maybe dE=TdS-PdV being the fundamental state?

## The Attempt at a Solution

$$dE = -p \; dV$$