I'm looking at this page for the compressible Euler equations.
To me it seems, in the 1-dimensional case, there should be 3 unknowns: density, velocity, and pressure. This is because the energy $E$ is a function of density and velocity, as the internal energy $e$ seems like it should be constant, because the flow is adiabatic. Additionally, it is stated
The equations above thus represent conservation of mass, momentum, and energy. Mass density, Flow velocity and pressure are the so-called physical variables, while mass density, momentum density and total energy density are the so-called conserved variables.
So three unknowns, right?
However, later in the page, it is stated that
there are thus N+2 equations and N+3 unknowns
So there should be 3 equations and 4 unknowns. Thus an equation of state, relating pressure, energy, density, and velocity, is invoked, allowing us to solve for 4 unknowns with 4 equations.
However, before it appeared to me that there were only 3 unknowns. Is it that the internal energy $e$ is nonconstant, which gives us the fourth unknown, or is it something else?