Jump to content

exchemist

Senior Members
  • Posts

    1959
  • Joined

  • Last visited

  • Days Won

    27

exchemist last won the day on January 19

exchemist had the most liked content!

3 Followers

Profile Information

  • Location
    London
  • Interests
    Rowing, choral singing, walking.
  • College Major/Degree
    Chemistry MA, Oxford
  • Favorite Area of Science
    chemistry
  • Biography
    Trained as a patent agent, then gave it up and worked for Shell, in the lubricants business for 33 years. Widowed, with one teenage son.
  • Occupation
    Retired

Recent Profile Visitors

5279 profile views

exchemist's Achievements

Primate

Primate (9/13)

449

Reputation

  1. The problem is you can't do the Stern-Gerlach experiment with a solid array of atoms. In a solid, the atoms are in a much more complex potential field. Changes to their alignment will depend on interactions with the lattice as well as any external field. You seem to think that there will be radiation from a compass needle oscillating about North position. Maybe that's right. But you are dealing with a conductor, that is with a sea of electrons in the conduction band of a metal.
  2. What you are overlooking is that spontaneous emission has a probability that goes up with the cube of frequency. The atoms in the higher energy alignment states can't just radiate freely. The transition would involve microwave emission and the probability of this occurring spontaneously is very low, because the frequency of the emitted photons would be so low. The atoms in the higher states are trapped there, until something interacts with them to allow them to lose energy.
  3. The passage you quoted is not a classical treatment, but a QM treatment. That's what the wave function integral is about. There is no way for a classical treatment to give you a series of beams, corresponding to discrete orientations. You would get a continuum, since all possible orientations are allowed - or just one spot if the particles had time to orient themselves with the field before exiting it.
  4. Attend a good school (there may not be much you can do about that, I realise), pay attention and ask questions in class, do your homework diligently - and ideally read a bit around the topics for yourself if you are curious. The best students show curiosity - which you seem to have.
  5. @swansontpost explains that. Electrical discharge in a gas laser, or electronic excitation in the solid state, are the two methods he refers to. But don't ask me to get deep into the physics of that. (I'm only a chemist, so what I know comes from quantum chemistry.)
  6. This is all mere assertion on your part and I'm afraid that 150 years of physics and engineering says you are wrong. The fluid analogy - and of course it is only an analogy - is very useful. A temperature gradient for heat is like a gradient on a riverbed for water. Heat is a flow of internal energy and the steeper the thermal gradient the faster it flows. Yes, you are of course right that heat is due to the motion of the molecules of the substance in question, but the way that motion spreads itself out through a body, along temperature gradients, is analogous to a flow of a physical fluid. People of Carnot's time were not morons. They were groping towards the understanding we now have. Caloric was an intelligent idea, even if it proved to be a defective model. My point is that it provided useful insight for those that came later. As to the idea of heat being entirely converted to motion of a piston, it is quite interesting to think about that for a moment. There is no way that heat can give up all its energy to make a piston move. Why? Because molecules don't move all at the same speed. There is a bell-shaped velocity distribution* among the molecules. So at whatever speed the piston moves when molecules hit it, it will be moving faster than some and slower than others. So some motion will always be left, among most of the molecules, after the interaction. Furthermore, in order to exert a force on the piston to make it move, molecules have to rebound from it. So they leave the piston surface still in motion. This is the whole point about heat. It is disordered, random kinetic energy. There is no way to order all the molecules neatly, so that they all move together, at one speed, towards the piston in order to exactly give up all their momentum to push it. That is the reason why it is impossible to convert all heat to work. (This inherently disordered character of molecules in motion is captured in the concept of entropy.) * In fact not quite a bell curve: a Maxwell-Boltzmann distribution: https://en.wikipedia.org/wiki/Maxwell–Boltzmann_distribution
  7. They are all in motion. All the stars in our galaxy, and any associated solar systems, obviously including our own, are revolving about the galactic centre. The same is true for the stars in other galaxies. And the galaxies are in motion relative to one another as well. However, the distances are so vast that, over the lifetime of recorded history, the stars don't seem to humanity to have moved in the sky appreciably, relative to one another.
  8. On one point, your comment that greater mass of the molecule is responsible does not sound right to me. CO2 and water molecules have dipoles, (O-C-O being - + - and H-O-H being + - + ) which will couple to the electric vector of radiation at characteristic frequencies, determined by the resonance frequencies of the stretching and bending of their chemical bonds. As a result they both absorb in the IR. By contrast, O2 and N2 have no dipoles and are therefore transparent in the IR. So what happens is sun light of all wavelengths warms the ground, which then re-radiates predominantly in the IR and some of this gets absorbed by water and CO2 instead of being radiated directly out into space. (The molar specific heats of gases are not a function of their molecular weight, but depend on the number of degrees of freedom the molecule has to take up energy. For monatomic gases, which can't rotate and have only 3 translational degrees of freedom, it is 3/2RT. For diatomic gases, which can rotate in 2 dimensions as well, it is 5/2RT.)
  9. Yes. "Stimulated emission" is emission caused by the influence on an excited atom of the electric vector of another photon. The effect is to make the atom emit its own photon, in phase with the one causing the stimulation. What you do is create a "population inversion", a term in statistical mechanics that means you have more atoms in excited states than you would have at thermal equilibrium. That is an unstable, non-equilibrium situation. One emitted photon, passing through this inverted population, will quickly collect more and more other photons as it passes among the excited atoms, leading to a beam of photons all in phase. This in principle will continue until an equilibrium population distribution is regained. In practice mirrors are used, so the photons bounce back and forth, collecting more and more companions with each pass.
  10. Oh, by Metacor do you mean Mercator? As in Mercator's projection, the one used by navigators, as it plots compass bearings as straight lines?
  11. Yes, you make a good point! What Carnot did, as I understand it, was to associate a "work value" with a quantity of caloric, which depended on the temperature of the caloric. So in his analysis, he thought the amount of caloric rejected by a heat engine was the same as the amount absorbed by it from the hot source. Clausius later showed this to be wrong and that work and heat were both energy. However, one benefit of Carnot's way of thinking was the idea that caloric (heat) has a temperature, just as a fluid like water does, and it is that which determines how much work it can do. It also has the benefit of assuming that caloric (heat) has to be rejected from the cycle at the end, i.e. what a heat engine does is allow caloric(heat) to fall through a temperature gradient, like water in a water mill, and thereby get it to do work. So that set the scene for the correct idea that you can't convert all the input heat into work, with no waste heat.
  12. It only seems ridiculous to you, because you don't understand the science behind it. You and I have been down this road before. You are a person who seems incapable of distinguishing between a basic principle of operation and the fiddly incidental details involved in constructing particular machines. In fact, you are now arguing against yourself here. Yes of course there are many incidental factors that go into engine efficiency. That is the whole point of having a theory that tells you what the basic principle is, so that you can set these sources of confusion to one side, determine the theoretical result first - and then look at the confounding factors, as things that cause deviations from the ideal. If you can't do that, you will never see the underlying pattern in anything. You will trap yourself in a medieval world in which all you can do is appeal to God to explain how nature works. To do science, you have to be able to simplify things to their essence, so you can see the pattern, in spite of the complicating factors. As far as Carnot goes, he got it right using the wrong model for what heat is. That can happen. As far as heat flows go, it doesn't affect the result if you think heat is a substance like calorific, or just an energy flow.
  13. The Carnot cycle is a theoretical, ideal thermodynamic cycle, with no losses, that enables the maximum efficiency of any heat engine cycle to be determined. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html All real heat engines are less efficient than this, small ones usually much less, as the incidental losses are large compared to the heat throughput. Any designer or operator of a power plant engine or turbine will be well aware of how far short of Carnot efficiency even such huge machines are in reality. The value of it lies in what it tells you about the gain in efficiency to be had from maximising the temperature difference between hot source and cold sink.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.