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Everything posted by kevinalm

  1. The effect is negligable for modern permanent magnetic materials (such as NIB). For older materials it is a serious concern. Alnico as an example, iirc.
  2. You (the op) have a slight misunderstanding of what the Poynting vector is. S is a vector that describes the density of power (in watts /m^2) passing through a point and in which direction. Because light travels at a constant speed c, S is also a measure of energy density (joule/m^3) which as swansont implies can be volume integrated so you can "see" how energy moves and is conserved as it does so.
  3. If it is intended to be pulsed (by capacitive discharge perhaps?) then you would definately _not_ want to use any iron or ferromagnetic core. You need to minimize self inductance so as to maximize peak current. Fewer turns of heavier guage is the usual method.
  4. Interesting. Didn't know about water injection on ww2 aircraft. Makes sense though. Same problem as kerosene tractors. Predetonation. My dad actually ran some of those old tractors as a youth. They had three tanks, a small gasoline tank, a water tank and a kerosene tank. They started on gasoline. Then when they warmed up enough you switched over to kerosene which was much cheaper at the time. And then you fed water to smooth out the "pinging". Actually, he said you fed water to stop the engine from "clattering like a ........". There are young people on the board so I'll let your imagination fill in the blank.
  5. Eye relief. A single lens is impracticle for more than 30x or so. Although you can with some difficulty get to ~150x, but the specimen is very close to the lens and the lens is nearly touching your eye.
  6. Ah yes, the old burning water thing, aka the urban myth that wouldn't die. It even spills over into an almost fanatical belief that hydrogen fuel is the savation of the planet among certain groups, and of course this brown's gas nonsense. The hilarious part is there is a small (a very tiny) grain of truth like in most urban legends. Once upon a time in America, when horses were being replaced by tractors all across the country, the petroleum companies had a little problem. The infrastructure to refine and deliver all that gasoline the tractors needed didn't yet exist. They could produce and deliver plenty of kerosene, as that was already in use for oil lamps, home heating, cooking and such. Problem solved. Now with some modification an internal combustion engine will run fine on kerosene. You have to design it to have the carburator and intake manifold at a higher temperature so the kerosene will volitalise. And you have to find a way to raise the octane rating of kerosene, which is pretty bad. So, a water injection system was devised, with its own tank and injection pump. For something like every 15 or 20 gallon of kerosene you 'burned' a gallon or so of water. Smoothed the engines out nicely and also eliminated carbon buildup. Yes, there were engines that 'burned' water, but not as fuel.
  7. The op has a point, albeit a trivial one. The statement that all objects fall at the same speed assumes that the object(s) in question have a negligable mass compared to the earth so that the earth can be assumed not to fall toward the object(s). I don't think the op realizes that the discrepency is so small for comman objects (say a hammer and a feather) that no experiment could even theoretically distingish between them. (Pretty sure you'd run afoul of quantum uncertainty.)
  8. The 12 guage remark was for 5614, who seems to be a little confused. As to the question of how the inertia/momentum of the firing pin can resist the explosion, think of it this way. Suppose you have 2 hydralic cylinders, A and B, connected to each other by a line that lets oil flow back and forth between the two. The piston of B has 100 times the area of A. You press in on B with 100 lb. To counter that force, you only have to push 1 lb. on A.
  9. The chamber that surrounds the cartridge actually fits the casing to very close tolerances. When the round goes off the entire case is presses very hard against the "breach", this surrounding chamber. This includes the thin skin of the primer, except for the rather small hole where the pin passes through the breach to strike the primer. It is the breach that resists the pressure, not the casing. So what about the firing pin hole? Well, as I said it's small, .050" or maybe a little larger. And it's filled by a firing pin that is moving toward the primer at the moment of detonation. So the momentum/inertia of the firing pin itself prevents the primer from rupturing at the point of impact. And by the way, equal and opposite reaction absolutely applies. We call that the recoil of the firearm. Ever fire a 12 guage shotgun?
  10. Yeah, The Core had so many really _dumb_ science errors. I got the dvd off the bargain rack at Wallmart. I like to have a couple of beers and watch it just to heckle it. Even the basic premise of the movie, that catastrophe would result if the Earth's magnetic field quit. It's done that many times in the geologic record. Weakened, strengthened, gone to zero, reversed... no correlation with mass extinctions, climate shifts, etc.
  11. My nomination for worst science in a movie has to go to "The Core". To many problems to list but my favorite is obtaining power to drive the ship from the ambiant heat of its surroundings. Apparently, the writers never heard of Carnot or the laws of thermodynamics.
  12. I was just going by a _very_ dim memory that for metals n is rather high, compared to say glass at n~1.5. Naturally in most circumstances this would be "swamped out" by L C considerations of the geometry, but I seem to recall that it can be a hard limit in critical circuit design (cpu's and the like).
  13. Shouldn't that be v=c/n? Also, I was under the impression that for most metals n~10, thus v~0.1c. If I'm way off please correct me. >>edit v in this context is the wave propagation velocity _not_ the drift velocity of course.
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