Your YouTube Adventures
- Thread starter matthewr
- Start date
Stunsworth
pfm Member
David Hepworth and Mark Ellen. Basically two middle aged blokes and a guest talking about music. I enjoy it a lot...
https://www.youtube.com/user/TheWordMagazine
https://www.youtube.com/user/TheWordMagazine
gavreid
Pretty Words...
Rotation is very counter-intuitive, think gyroscopes and bike wheels. Try holding a spinning bike wheel by its threads and then try to move it around from the vertical. The effect here comes about from the object being asymmetrical so it has different properties in each of the 3 axes, x,y, and z, which one has the lowest 'energy', if you like, is important. It's not the same, but if you have a cone on a surface it is stable standing on it's base and lying on its side (maybe it will roll) but standing on its point it could be, but is unstable with respect to the smallest of perturbations and will fall over - but you can stand a spinning cone on it's tip all day long. So there are three different 'energy states' (how fast the thing rotates in each axis) and it happens that two are stable but one is not. It's been understood in terms of spinning gas molecules for decades (that's what radio telescopes observe,) it has nothing much to do with those russians.
gavreid
Pretty Words...
@Tony L recognise this? This is the guy who worked out the maths, Lev Landau - no not @droodzilla 
This picture is from his prison cell in Lubyanka - he was released because he would be useful to Stalin in inventing a Russian nuclear bomb. He also set out the modern way of describing quantum mechanics.
This picture is from his prison cell in Lubyanka - he was released because he would be useful to Stalin in inventing a Russian nuclear bomb. He also set out the modern way of describing quantum mechanics.
droodzilla
pfm Member
@Tony L recognise this? This is the guy who worked out the maths, Lev Landau - no not @droodzilla
Haha, nice one! Total genius, of course - one of the most important physicists of the 20th century.
One of the more bizarre Landau-related developments in recent years is this:
https://www.theguardian.com/film/20...had-absolute-freedom-until-the-kgb-grabbed-me
His life and times are now the inspiration for a gargatuan, Truman Show style experiment, sections of which will be released as movies. It sounds completely bonkers (and therefore a must-see, in my view).
My interest in him relates to his work on phase transitions (e.g. water to steam, but also more esoteric stuff like the onset of superconductivity as a material is cooled) He developed a very general framework for thinking about such phenomena, which were not well understood until well into the 20th century. One of the "Wow!" moments in my undergraduate degree was a lecture about Landau's theory of phrase transitions which, despite its power and generality, can be explained using GCSE level mathematics (a bit like special relativity relying on nothing more complicated than Pythagoras).
gavreid
Pretty Words...
droodzilla
pfm Member
One of my regular bugbears is not quite "getting" the concept of chemical potential. I mean, I can give you a definition, but I don't have much of an intuitive feel for what it represents. I sometimes wonder if a chemist might understand it better, having been introduced to the idea earlier. Same goes for "osmotic pressure".I taught thermodynamics for nearly 20 years, but only to chemists
gavreid
Pretty Words...
One of my regular bugbears is not quite "getting" the concept of chemical potential. I mean, I can give you a definition, but I don't have much of an intuitive feel for what it represents. I sometimes wonder if a chemist might understand it better, having been introduced to the idea earlier. Same goes for "osmotic pressure".
Yeah, it's a daft expression, it was taken from the idea of electrical potential and the flow of electrons. The Gibbs energy (which chemists usually care about i.e. open systems) changes if the pressure or temperature changes (ignore that), but also when the amount of each substance changes. The amount of each substance changes, trivially, by how much is put in, but during the course of a reaction or physical process. The chemical potentials are just the contributions of each species to the total Gibbs energy.
Osmotic pressure is cool. It's really the same idea - a solution of something in a solvent isn't at equilibrium with respect to a pure solvent, the Gibbs energies of the systems are different. To achieve equilbrium you must apply a pressure to the solution i.e change the chemical potential of the 'solvent' to match that of the pure liquid, so that's the exact opposite of the osmotic pressure
Mongeddavid
pfm Member
droodzilla
pfm Member
Thanks, that's as clear an explanation as I've ever read. And not a partial derivative in sight!Yeah, it's a daft expression, it was taken from the idea of electrical potential and the flow of electrons. The Gibbs energy (which chemists usually care about i.e. open systems) changes if the pressure or temperature changes (ignore that), but also when the amount of each substance changes. The amount of each substance changes, trivially, by how much is put in, but during the course of a reaction or physical process. The chemical potentials are just the contributions of each species to the total Gibbs energy.
Osmotic pressure is cool. It's really the same idea - a solution of something in a solvent isn't at equilibrium with respect to a pure solvent, the Gibbs energies of the systems are different. To achieve equilbrium you must apply a pressure to the solution i.e change the chemical potential of the 'solvent' to match that of the pure liquid, so that's the exact opposite of the osmotic pressure
