Taking a Fresh Look at the Physics of Radiometers.
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From: Tim BandTech.com on 19 Jun 2010 07:41 On Jun 17, 4:44 am, Timo Nieminen <t...(a)physics.uq.edu.au> wrote: > On Jun 17, 12:53 am, "Tim BandTech.com" <tttppp...(a)yahoo.com> wrote: > > > > > The question of how the light is heating > > is quite relevant, and rather than looking the other way we should try > > to get to the bottom of it. > > Already mentioned the classical picture of the heating by light as > essentially Ohm's law; augment with a microscopic picture of the > conduction electrons in thermal motion if you wish (their mean thermal > speed, btw, vastly exceeds their mean drift speed due to an applied > field). > > As for absorption of light by atoms, one of the simplest, clearest, > and available treatments is Henderson et al., "How a Photon Is Created > or Absorbed", J. Chem. Ed.: > > http://jchemed.chem.wisc.edu/JCEWWW/Articles/DynaPub/DynaPub.html > > This looks at the dynamics that are usually ignored. While this is one > of the simplest and clearest, I don't think it's very simple or clear, > just more so than alternatives. > > This is only half of the story, as it still doesn't explain how the > energy ends up as heat. The answer to that is collisional de- > excitation. That is, the excited atom (with extra energy after > absorbing a photon) loses that energy, or at least some of it, in a > collision with another atom. After the collision, a good part of this > energy ends up as the overall KE of the atoms involved, rather than as > the energy of the original excited electron, so isn't available for > emission of a photon (of the original energy). > > I think that focussing on the heating by light, especially in detail, > just distracts from the original issue of radiation momentum and > pressure. In particular, the fundamental point is that moving energy > means that you have momentum, even if the energy is being moved by a > wave with no transfer of mass. E.g., by an EM wave, or in quantum > terms, photons of zero rest mass, but also by water waves, acoustic > waves etc, where the material in which the wave propagates doesn't > move from point A to point B. OK Timo. I accept this argument. I'm too tired to argue the finer points. You have done a fine job of upholding the existing theory. While I do see ambiguities in the interpretation, without a clean replacement there is little I can do other than throw pebbles at it. Thanks for your persistence on this thread. I've learned some things from you. - Tim > > This is a key advantage of looking at it in terms of conservation of > energy and momentum, rather than the details of the interaction > between a specific type of wave and a specific type of matter. The > conservation principles are general, and useful whether the moving > energy is wave energy as above, KE of moving matter, thermal energy, > or whatever, and also whether we're talking Newtonian physics or > relativistic physics, or classical or quantum. > > The downside is that it doesn't tell you about those details, even if > you're interested in them. > > -- > Timo
From: Sue... on 19 Jun 2010 15:13 On Jun 19, 7:41 am, "Tim BandTech.com" <tttppp...(a)yahoo.com> wrote: > On Jun 17, 4:44 am, Timo Nieminen <t...(a)physics.uq.edu.au> wrote: > > > > > On Jun 17, 12:53 am, "Tim BandTech.com" <tttppp...(a)yahoo.com> wrote: > > > > The question of how the light is heating > > > is quite relevant, and rather than looking the other way we should try > > > to get to the bottom of it. > > > Already mentioned the classical picture of the heating by light as > > essentially Ohm's law; augment with a microscopic picture of the > > conduction electrons in thermal motion if you wish (their mean thermal > > speed, btw, vastly exceeds their mean drift speed due to an applied > > field). > > > As for absorption of light by atoms, one of the simplest, clearest, > > and available treatments is Henderson et al., "How a Photon Is Created > > or Absorbed", J. Chem. Ed.: > > >http://jchemed.chem.wisc.edu/JCEWWW/Articles/DynaPub/DynaPub.html > > > This looks at the dynamics that are usually ignored. While this is one > > of the simplest and clearest, I don't think it's very simple or clear, > > just more so than alternatives. > > > This is only half of the story, as it still doesn't explain how the > > energy ends up as heat. The answer to that is collisional de- > > excitation. That is, the excited atom (with extra energy after > > absorbing a photon) loses that energy, or at least some of it, in a > > collision with another atom. After the collision, a good part of this > > energy ends up as the overall KE of the atoms involved, rather than as > > the energy of the original excited electron, so isn't available for > > emission of a photon (of the original energy). > > > I think that focussing on the heating by light, especially in detail, > > just distracts from the original issue of radiation momentum and > > pressure. In particular, the fundamental point is that moving energy > > means that you have momentum, even if the energy is being moved by a > > wave with no transfer of mass. E.g., by an EM wave, or in quantum > > terms, photons of zero rest mass, but also by water waves, acoustic > > waves etc, where the material in which the wave propagates doesn't > > move from point A to point B. > > OK Timo. I accept this argument. I'm too tired to argue the finer > points. You have done a fine job of upholding the existing theory. > > While I do see ambiguities in the interpretation, without a clean > replacement there is little I can do other than throw pebbles at it. > > Thanks for your persistence on this thread. I've learned some things > from you. > > - Tim Hi Tim, Have you read any of links or the 2010 experiment on this page? http://en.wikipedia.org/wiki/Abraham-Minkowski_controversy Some recent experiments may decide the issue and It seems related to how light's momentum was handled (or mishandled) in this thread. Sue... > > > > > This is a key advantage of looking at it in terms of conservation of > > energy and momentum, rather than the details of the interaction > > between a specific type of wave and a specific type of matter. The > > conservation principles are general, and useful whether the moving > > energy is wave energy as above, KE of moving matter, thermal energy, > > or whatever, and also whether we're talking Newtonian physics or > > relativistic physics, or classical or quantum. > > > The downside is that it doesn't tell you about those details, even if > > you're interested in them. > > > -- > > Timo
From: Timo Nieminen on 21 Jun 2010 04:20 On Jun 19, 9:41 pm, "Tim BandTech.com" <tttppp...(a)yahoo.com> wrote: > > Thanks for your persistence on this thread. I've learned some things > from you. If you still want to dig for some nuggets in the angular momentum/ energy calculation, they're still there. The one that's of more interest to you, I think, is the 5Hz photon. E=hf vs angular momentum = hbar. Note the units! -- Timo
From: NoEinstein on 23 Jun 2010 13:03 On Jun 12, 9:30 pm, Sam Wormley <sworml...(a)gmail.com> wrote: > Dear Sam: Wrong! The momentum (of a unit weight) varies due to velocity change, only. The correct formula is F (force in pounds) = v / 32.174 (m). NoEinstein > > On 6/12/10 7:36 PM, NoEinstein wrote: > > > Dear Timo: Momentum is: The increase or decrease in the impact force > > Nope. Momentum is conserved in closed systems. Momentum can be > changed by force. See Newton's second law: F = dp/dt
From: NoEinstein on 23 Jun 2010 13:17
On Jun 13, 7:12 pm, Timo Nieminen <t...(a)physics.uq.edu.au> wrote: > Dear Timo: Solar sails are non-science. Photons don't push, they induce "pull" (gravity). Follow my threads to recent posts and replies which explain why. NoEinstein > > On Jun 13, 8:11 pm, "Sue..." <suzysewns...(a)yahoo.com.au> wrote: > > > On Jun 12, 4:46 pm, Timo Nieminen <t...(a)physics.uq.edu.au> wrote: > > > Not to leave out the really conclusive ones: look at atom trapping, > > > where the effects of the background gas is catastrophic to the > > > trapping. > > > Heating of the interstellar media would not seem helpful > > for holding a solar-sail in place. That is not a catastrophe > > if the intent was propulsion. > > The collisions can be rather beneficial for the sail, anyway. Ride > that solar wind! > > > Wiki has the Nichols Radiometer grouped with solar sails. If > > the device fits better with tweezers, showing how and why > > could be a useful contribution. > > You can divide the radiation forces into reflection, refraction, and > absorption forces. All still Newton 2, so no fundamental difference. > The traditional division in tweezers is into "scattering" forces and > gradient forces, which is of some practical value, good for Lorentz > force calculations, but bad for Newton 2 calculations. > > You could divide devices into far-field and near-field devices, > tweezers/sails/Nichols vs electric motors/railguns etc., radiation (in > the far-field sense) vs induction. > > Or devices into EM forces dominate vs other forces dominate vs all > forces matter. > > What the best taxonomy is depends on the purpose. |