# chilehed

Senior Members

159

1. ## Entropy

I'm beginning to see that, although since Boltzman's constant has units of J/K I don't think you mean "forget heat" literally. Alright, I see that I'm actually at Stage 1 (or perhaps that means that I'm finally at Stage 3?). Cripes, I have a headache....I'll be back later.
2. ## Entropy

Yeah, I know that the “entropy is a measure of disorder” statement is highly misleading, it’s only true given a very specific definition of disorder. It has nothing to do with how messy your room is. At least part of my error is in forgetting that the position of the balls is only analogous to the microstates of the molecules, and while the former has nothing to do with heat energy the latter do. And I know that unrestrained expansion is irreversible for a real gas, but I don’t get why it would be for an ideal gas. Nor do I understand whether or not the two-species thought experiment would be classified as a slow expansion. I need to take a graduate-level course.
3. ## Entropy

Somebody once said that there are three stages of understanding entropy:Stage 1 - You haven’t the slightest idea what it is or what it means. Stage 2 - You know what it is and what it means. Stage 3 - You realize that you really don’t know what it is, but you get the right answers.
4. ## Entropy

Let me more fully explain what I'm thinking. I understand that, for the colored balls, there is no difference in entropy between any arrangement of the colors, whether segregated or mixed (there's no heat content to the colors). Also, if you start with them segregated and then shake them into a mixed state, there is no generation of entropy due to the mixing of the colors. There is, of course, some generation of entropy due to the energy expended by your body, viscous drag through the air, and friction losses between the balls themselves, but none due to the mixing of the colors. And in the same way, there is no entropy associated with the case of all of the balls being located in one quadrant of the box vs. them being all spread out (provided the box is of a size that allows the balls to not come in contact with each other). If entropy was generated due to the arrangement of the colors, then you could never unmix the balls by shaking them, even if the number of balls was very small (say, four or six total). But one can take six balls of two colors, and on shaking them find them spontaneously segregated by color given enough trials. Therefore, the arrangement of the colors has nothing to do with either entropy or entropy generation. Same thing if you have ten balls, or a thousand, or a mol. You just need more trials to get there. Extend this to an ideal gas in a rigid, isolated chamber. At time=0 all of the molecules are in one half of the chamber. At a later time they are evenly distributed throughout. There were no intermolecular forces against which work was done during the expansion (it's an ideal gas), the chamber walls are rigid so that no work was done against either an external atmosphere or the box itself, and the system is isolated so that the temperature of each molecule was unchanged. So dS= dQ/T gives a zero result. The process generates no entropy, and is thus reversible. And indeed, the slow compression and expansion of a gas is reversible: all of the work done on the gas is recoverable. If that's the case, then, if the chamber holds two species of ideal gas on opposite sides of the chamber, no entropy is generated when they mix. Thus the mixing of ideal gases is reversible. I've gotta have my head up my rear somewhere on this, right? Like I said, between stage 2 and stage 3: I think I know what entropy is, but I'm beginning to wonder again. At least I do (usually) get the right answers when doing cycle analysis.
5. ## Entropy

OUCH! Tough crowd... You first. In my thought experiemnt, is W a constant? If not, why not?
6. ## Entropy

I'm somewhere between stages 2 and 3 of my experience with entropy and the 2nd Law of Thermo and could use a bit of a sense check. I've been meditating on the fact that there's no entropy difference between colored balls that are all mixed up vs. ones that are separated according to color. When I extended this to the case of an isolated, rigid chamber containing two segregated ideal gases, I concluded that there isn't a change in entropy when the gases mix. Then I got to thinking about Boltzman's discovery of the relationship S = k*ln(W), which seems to confirm my conclusion because in this case W remains constant as the gases mix (and k is, of course, a constant itself). Am I thinking clearly? Or missing something? Thanks.
7. ## help a hotrod fanatic

The concept is straightforward, but the execution will be quite challenging. Why not just turn the scoop around and have the entrance at the rear of the hood? You'll have plenty of cool air from a good high pressure zone, and by sealing the air cleaner housing to the underside of the hood you'll keep out the underhood air. It sure would be a lot simpler and cheaper than what you're proposing, and would work at least as well. Then you could spend the saved time, cash and aggravation on the nitrous kit you've been wanting (or the cam or headers or gears or whatever it is ). Don't get me wrong, it's a cool idea. But very difficult to execute well. A buddy of mine had a Maverick with a 351W and a toploader. Nothing fancy, just a 600 CFM vacuum secondary Holly, slightly longer/higher cam, 4-to-1 headers and 3.89 rear gears. It was a lot of fun, but would have had a lot better response with a 465 CFM carb. And if I was building a car today I'd definitely try to set it up with a modern closed-loop fuel/spark controller.
8. ## space shuttle

Unfortunately, there is no such thing as an engineering no-brainer.
9. ## help a hotrod fanatic

As I recall the old Plymouth (?) "Air Grabber" scoop doors used a mechanical linkage tied to the throttle actuator. The scoop opened as you depressed the gas pedal. Wouldn't be a good idea here, the mass of the scoop would be so high and the wind loads so great that you'd be at risk of high force required to open the throttle and/or a stuck throttle at high speeds. If you were clever you could make an electronic controller tied to vehicle speed, but you'd need to include some way to get an electronic speed signal. Big job. It'll also be tough to make the thing and install it so that it doesn't look like a piece of junk. Hope you're really good with sheet metal and paint. For performance the best place to have the air inlet is at the high-pressure zones at the very front of the vehicle (in the grill or below the bumper) or at the base of the windshield. On my '68 Torino I had planned to run a duct through the sheet metal into the cowel where the wiper mechanism goes, but never got around to it.
10. ## trebuchets

Does anyone else here besides Pseudoswallo and me think that trebuchets are cool? Pseudo, I've only made one, a real lousy thing cobbled out of whatever scraps I could find, with a 5-foot bamboo arm and a bent nail for an axle. But I managed to pitch a large softball into the third yard down, and the neighbors all thought I was nuts. Mission accomplished. I have fantasies about making something better and larger, and dragging it about the neighborhood on hallowe'en dressed like a medeaval corpse collector, pitching fake heads down the street and hollering "bring out your dead".
11. ## Last Tank of Gas

You obviously have WAAAY too much time on your hands, which means that you should build a huge trebuchet and see how far you can throw it.
12. ## Electrostatics question

It won't work, it's an inherently unstable arrangement. With charged shells, the electrons go to the outside surface, so you'll have a negatively charged OD on the inner shell and a positively charged ID on the outer one. If you have shells of atomically perfect form and exactly on center with each other you can theoretically have a resultant attraction of zero between them, but the slightest imperfection in form or location will cause a nonuniform concentration of electrons on the shells. This will result in a non-zero resultant force and the shells will move toward each other.
13. ## Cotton candy machine...

If you used the right kind of plastic I'm sure it would. But it certainly WOULD produce one seriously annoyed cotton-candy salesman!

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15. ## Structural and Civil Engineers

I think that they mostly make targets...
16. ## Biofuel / Emissions Analysis

No. The exhaust sensors output voltage as a function of the partial pressure of the oxygen in the exhaust gas. They can't detect anything else.
17. ## Engine Efficiency

If by "besides replacing it with a smaller one" you mean that you want to keep the same basic engine and just do stuff too it, then you've boxed yourself into a bad position. The engine's going to displace the same volume, and the engineering compromises won't let you get both reduced power and increased thermal efficiency, especially if you're interested in things like drivability and the like. If reduced displacement is indeed on the table, then I'd head in the direction of a small displacement turbocharged diesel cycle. You'll have lower pumping losses and motoring torque, higher thermal efficiency due to the compression ratio increase, and you'll be getting useful work from the otherwise wasted exhaust heat. Beyond that, just do all of the details well: piston and bore geometry, valve timing, fuel and spark calibration, lube system pressure and viscosity, materials selection, etc, etc.
18. ## Are hydrogen engines more efficient than gasoline engines?

That's a subtly different question. The heating value of a stoichiometric mixture of vaporized gasoline is about 16% greater than an identical volume of a mixture of hydrogen. Even if you jack the compression ratio up to take advantage of the higher octane rating of hydrogen you won't be able to get an 16% improvement in thermal efficiency, so right there you'll get less distance down the road using hydrogen. Add to that the fact that you'll only be using a part of the total heat value of the fuel if you strip the hydrogen from gasoline, and you'll go less distance still. The only way to make up for the loss of heating value is to have a very, very efficient hydrogen reaction, which means you don't use a combustion engine. The safety risks of compressed hydrogen are way overblown (no pun intended). Certainly you don't want the tanks to rupture, but they're pretty tough. As for leaks, the flammability limits for hydrogen are very narrow (so it's easy for the mixture to be either too lean or too rich to burn) and the stuff is so volatile that it dissipates very quickly.
19. ## Are hydrogen engines more efficient than gasoline engines?

A friend of mine worked on fuel-cell vehicles for a while. Their system used an on-board reformer that scavenged the hydrogen from gasoline for use in the fuel cell, and burned the carbon as the heat source to operate the reformer. The emissions were CO2 from the reformer and H2O from the fuel cell. My understanding is that the largest inefficiency in combustion engines is due to uncontrolled electron sharing during the reaction. Fuel cells don't suffer this loss because once an electon is free it's sent through the circuit to be used. So even though none of the heat from the oxidation of the carbon got used to move the vehicle, the high efficiency of the hyrdogen side of the process was more than enough to make up for it. It was promising, but the company went belly-up due to lack of investment capital. If you're asking about a comparison between two internal combusion engines, one fueled by gasoline and the other by hydrogen, then hydrogen wins hands down (assuming you can optimise both for compression and fuel/spark calibration). The single biggest driver for efficiency in an IC engine is compression ratio, which is limited by the octane rating of the fuel. Hydrogen has an octane rating around 140, which means you can raise the compression ratio a lot higher than on a gas engine. And since H2 is gaseous, a lot of the mixture distribution problems you can encounter with liquid fuels just go away.
20. ## Producing electricity with kinetic energy

The efficiency of tidal generators is limited because the working fluid moves at a low velocity. The same would hold for a system using the motion of a bouy riding on ocean waves. They're both expensive on a cost per kWh basis, but in some locations it may be worth it to have a system that doesn't require a high temperature source.
21. ## the arrow pointing at me says,"I'm with stupid"

Power = Force x Velocity. You know the force, you know the velocity it's acting at, that gives you the power. Use an appropriate conversion factor to express it in watts, and don't forget to account for inefficiencies in the process of turning it into useful work. And it's not a stupid question. This is cool: I was wondering once how much power the Saturn V rockets produced. As I recall they generated 5,000,000 lbf of thrust and first-stage separation occured at around 3,000 mph, which calculated to about 90 BILLION horsepower!!
22. ## Automotive engine for an electric generator?

Didn't the early automotive cruise controls use a mechanical control system?
23. ## Einstein was WRONG!!!!! And EVIL!!!

Wow, Fred Hutchinson sure knows how to speak long and eloquently about something he obviously knows nothing about. A classic case of an 8-cylinder mouth being run by a 1-cylinder education. If I was younger it would almost make me ashamed to admit I'm a Christian.
24. ## Do you believe in miracles?

As I said, "Certainly in a given case the safe attitude is to refrain from concluding that a miracle occurred unless every possible natural cause is positively eliminated; this would protect you from being taken in by charlatans..." Note also that my objection was to the statement "miracles are impossible", not to the statement "miracles do not occur". They are not identical statements.
25. ## Do you believe in miracles?

It seems to me that in order to definitively conclude that miracles are impossible one must first prove that all causes are observable (which is another way to say "the scientific method can explain every real event"). Given that one cannot prove that all causes are observable, I am unwilling to reject out of hand the possibility of real miracles. The question then becomes "what evidence is there that miracles have occured?" Certainly in a given case the safe attitude is to refrain from concluding that a miracle occurred unless every possible natural cause is positively eliminated; this would protect you from being taken in by charlatans. But at the same time one must guard against the opposite assumption: that no amount of evidence is sufficient to support a conclusion that a miracle occured.
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