nick c wrote: ↑Thu Dec 17, 2020 3:22 pm
Hi Higgsy,
Your position for the equivalence (or that it doesn't make any difference) between an ideal gas and plasma is in my view wrong.
You are entitled to your view, but your view is not well informed. By the way, I didn't say that it doesn't make any difference, so please don't put words in my mouth. What I did say was
"No-one is saying that the ideal gas laws give precise results, but they do give an excellent first approximation to the overall picture which then has to be refined with electrostatic pressure, heat convection, radiation pressure and transport considerations etc etc." Do you understand that that is different from "it doesn't make any difference". It makes a difference, but not enough of a difference to disqualify an ideal gas treatment to a reasonable approximation.
Ideal gas is not plasma.
Indeed, a plasma is not an ideal gas, nor is any real gas an ideal gas. The clue is in the adjective. If a gas has to be an ideal gas to be treated as one, then the concept of an ideal gas would be altogether useless.
If that distinction wasn't important than why did Eddington specify ideal gas as an assumption of his model?
Because he understood that an ideal gas sufficiently closely approximated the plasma so far as thermodynamic properties such as pressure and temperature go, and the behaviour of an ideal gas is well understood. So it is a reasonable first step. But you write as though no further work has ever been done on the Sun since Eddington proposed the model, that it has never been refined to take into account the difference between the plasma and the ideal gas behaviour, and that the model has not been tested empirically. You also write as though you think solar physicists are stupid, and the idea that the fact of the gas being fully ionised might affect the precision of the model has never occurred to them.
If it does not matter whether it is an ideal gas or a plasma, since they are both beasts of burden, then why don't you begin your "revised" Eddington model by dropping the assumption of an ideal gas and change it to a plasma? and begin your modeling from there?
Because it would end up in the same place as the current model which does take the difference between a fully ionised gas and an ideal gas into account, as well as taking into account many, many other things like radiation pressure, convection and other mass flows and so on, which are not covered by the ideal gas laws.
How can a cosmic plasma cloud, dominated by electric forces, be made to collapse under the force of gravity?
Well that is an entirely different question, but in what way is the star forming region of a cosmic plasma "dominated by electric forces", seeing as such a plasma is neutral at all scales above the Debye length, which in this case is about 10m? I can tell you how to calculate the Debye length, but I don't get the impression that quantifying things comes high on your agenda. And while we're talking about the Debye length, you might be interested to learn that in the core of the Sun it is of the order of 10^-11m
Finally, the polytropic model of the Sun, which is what you are talking about, was proposed long before Eddington - by Lane and Ritter in the 1870s and refined by Emden in the early 1900s. Eddington represented another step along the way, and after him, a vast quantity of work has been done by an army of astrophysicists. Some things that Eddington proposed have been shown to be wrong (the details are rather arcane, at least at the level of this conversation), but there is no prospect that the line of argument you have embarked on can be successful.