One form of this quantity defined for electromagnetic radiation is given by the time-averaged Poynting vector over the speed of light.
Both van der Waals processes and the Stark effect are examples of spectral line "broadening" named for this physical quantity, a phenomenon more generally occurring due to collisions of atoms with neighboring particles. Thermodynamically, this quantity is equal to the negative volume derivative of Helmholtz free energy at constant temperature. The Legendre transform of internal energy to give enthalpy involves the addition of this quantity times volume because it and volume are (*) conjugate variables. One form of this quantity defined for electromagnetic radiation is given by the time-averaged Poynting vector over the speed of light. This quantity is modified by the "a" factor over molar volume squared to account for molecular interactions in the van der Waals equation. For 10 points, name this quantity equal to nRT over V for ideal gases and measured in atmospheres or Pascals.
Both van der Waals processes and the Stark effect are examples of spectral line "broadening" named for this physical quantity, a phenomenon more generally occurring due to collisions of atoms with neighboring particles.
pressure [prompt on P]