CaptainPanic

Hydrophobic bacteria? (note: this reply was not helpful) Btw, wikipedia has a small webpage about Purell... for those who never heard of the stuff. It says that the active ingredient is simply ethanol (no other additives). That would suggest that the 96% ethanol would work just as fine... or probably even better. Btw, the other one contains IPM. Googling for [iPM solvent] suggests that this IPM is Isopropyl myristate, which is a cosmetic that facilitates absorption through the skin. Doesn't sound like a disinfectant to me. (Of course, a 3 letter abbreviation can have more than 1 meaning).

The picture in that link is an ordinary skate. Not a motorized one. I guess that the tips on that link measure up to the assistance that was given in this and the other thread... with the difference that we're critical about difficult/impossible ideas while this guy makes it all sound easy. (Recharging the battery by coasting? I'm not so sure that should be point #1 priority). What is interesting is that it's true that some motorized skates exist... and they have exactly the looks that I predicted: They're Big. They don't like the cartoon skates at all. They apparently have combustion engines... which is probably a good idea if you want to actually go anywhere. I guess electric engines will work as well... And I'd like to place a few critical notes at the battery... especially on the link that was posted earlier: Here's a picture of a 12 V, 40 Ah battery. Note that it's quite huge (14.5 kg!). To go at a nice speed, and do some acceleration and braking, you really would like to have 750 W of power (1 horsepower). A 12 V, 40 Ah battery can supply 750 W during 38 minutes... which is really quite reasonable. But you must realize that you're skating around with huge skates and 15 kg of battery strapped to your back. I never said it's not possible. I just don't think it will look and behave like in the cartoon... and I want you guys to be realistic before you build anything that doesn't work.

Just click on whatever continent is the place you call home (regardless of where you were born, or where you are right now). What continent do you know the most about? I'd like to find out which continents are represented best on SFN? (I didn't ask for more details because plenty of people haven't filled in their location - meaning they don't want to tell us).

Are you suggesting to make a perpetual motion device out of your rollerskates? You want to generate all the energy for movement in the skate? Please note that it's impossible to do that... impossible. This whole thread isn't making much sense. I haven't read a lot of good ideas... and frankly, I think all people posting here are just kids (<15 yrs) with little engineering background. All people who post here suffer from a number of issues: -A huge underestimation of the power required. -A huge underestimation of the size of both the energy storage (fuel/battery) and engine -A huge underestimation of the complexity of the control system Therefore my message to everybody here is: please stop dreaming of things that you see in cartoons. Find some device that actually works and delivers enough power, and go from there.

Perhaps you don't have to shine so brightly to stand out when you're in a rusty scrapheap? Perhaps we all scored negative points? Perhaps Sayonara³ has tapped into a new, yet undiscovered, source of humor? For those who are clueless about the newsletter. <-- that's the link to it.

Yep: in your hands, there is hardly any fat, and veins are clearly visible. On the arms, just a few centimeters away, there is more fat (and muscle), and the veins are less visible.

I think that the general idea is possible. It's radical, but possible. I have some remarks: 1. The war economy of the 1940's was producing individual fighting units, which needed some support (fuel, ammo and food). This time, you propose to make one giant integrated system. That will require a little more planning and designing. 1-2 years might be a little short to complete the entire project. Don't underestimate the problem of energy storage. 2. People are scared of changes. In WWII, the changes were started by other powers (In a simplified version of the start of WWII, we can say that the Germans, Italians and Japanese were the initial agressors). Now, you ask a population to give up their comfortable and safe lives to settle for a little less luxury, and to develop clean energy. While a really good propaganda machine might be able to pull it off, I don't see it happen. You need an enormous work-force. You need to dedicate a lot of resources and power to build all this clean energy. That means less production of everything else. 3. Hydro power should be maximized. And you really don't plan on having enough windpower. Hydro and wind are currently the cheapest options in many parts of the world (except for fossil fuels). 4. The USA cannot afford any more war expenses on such a scale... unless you scrap the capitalist economy, and go for the plan-economy (Soviet style). But I kinda like the general idea of a war-economy for clean energy. Not because I am a revolutionary or a commie... but simply because it's the only way to get clean energy fast.

What does it say on the bottle?

Spammer. What do you mean by "networking and hardware industry" anyway?

A popular summer university is organized by AEGEE, but it's more cultural than scientific. Still lots of fun though. You'll meet a lot of people with different nationalities. NOTE: this is in EUROPE. I also found a summer school focussed on people about to start university. http://www.perimeterinstitute.ca/Events/Particle_Physics%2C_Cosmology_and_Strings_2009/Summer_School%3A_Exploring_the_Cosmological_Frontiers/ NOTE: This is in Toronto, Canada. If you're not more specific, then the world is a big place.

My short answers are 1) Yes there is obviously some maximum, and 2) No, there is certainly cause for concern (but not cause for panic). But I'm more interested in what others think about his question. I agree that there is a maximum. Interestingly, the radiation might not be linear (it's a 4th order function) but many other processes are also not linear. The evaporation of water (vapor pressure of water) is one example of a curve which follows a function that goes up faster than linear (they are often approached by higher order polynomials for a limited range). So, while radiation might be a 4th order function of the temperature, other aspects are also going up strongly. A certain temperature increase might sounds insignificant, but it is significant when processes are not linear.

Libel and Science don't seem to have much in common. http://wordnetweb.princeton.edu/perl/webwn?s=libel I dont' see the problem. Too many people nowadays suggest that the "freedom of speech" is threatened when they're not allowed to discriminate or insult. I find that these people are hijacking the true "freedom of speech" discussion which does not treat the communications between two individuals, but the criticism of the entire system or government.

When you click "sciences", the entire list of sciences shows up. There is something inconsistent there: Let's take chemistry as an example. We have 4 subforums: 1. Chemistry 2. Inorganic Chemistry 3. Organic Chemistry 4. Applied Chemistry In the "sciences" list, they all seem equal. However, when you click "Chemistry", it turns out that the other 3 chemistry forums are sub-forums of the main chemistry forum. This is supported by the main index page. The fact that sciences and the main index treat the "chemistry" forum different seems inconsistent to me. The inconsistency however is supported by the "new posts" (which are mentioned in slightly darker blue on the right side of every page): "Chemistry" has its own threads, it's a subforum on its own. The attachment (pictures) show that on the main index it makes sense that the "new post" of "chemistry" is a post from inorganic chemistry. But on the "sciences" page, it no longer makes sense. On the sciences post, the newest post in "chemistry" becomes invisible whenever somebody makes a post on 1 of the 3 sub forums. The same goes for all other forums which are grouped by category (physics, biology, maths, etc.).

Could it be part of a much larger animal? Are we perhaps looking at a tooth (molar)? I just thought that this remark might expand your horizon a bit. I'm not an expert (not even an amateur) - I hope that was helpful.

As mentioned in the wikipedia article (under "Ion accumulation"), an attempt should be made to relate SI units (charge, mass, mole). The interesting part is that mass will be related to other units, which are also not defined perfectly. There is a certain error in length, therefore using a silicon sphere of certain radius will introduce the error in length (to the power 3, since Volume = Length^3). Using Avogadro's number will only work if we can count individual atoms (using current or some other method). Can't we design an experiment which checks all SI units at once? Then repeating measurements for mass, current, mole, length, time, we can iterate to a perfect definition. I need another cup of coffee to work that out (don't expect a post soon though, our coffee machine is horrible). Iterations in a recycle system are generally a good way to reduce errors. I haven't really worked out why this would not be having the same problem (namely that to define a unit, you need the definition of another unit - so which unit gets defined first?).

You need motivation? Check how much oil is being imported into whatever country you live in. (I know where Ontario is, but I write to a wider audience than GutZ ). The money that goes to countries in the Middle East is enormous. In the Netherlands, at the current price of about 70 dollar / barrel, we pay 10-15 billion euro per year (all that money goes to other countries!). Coal and gas imports aren't included either, so the fact that the Netherlands has gas reserves is irrelevant for that number. Money is a great motivation, and although I think that ecology in general is a fantastic motivation for sustainable energy, it is more important to stop giving money away. In stead, we should invest this enormous amount of money in our own country, or at least make sure that it's being spent usefully. That is also one reason why I dislike bio-fuels from biomass: for many European countries, this would involve an import of biomass. Bad idea.

I think I got it. OTTFFSSENT Hah. I was the first. (amazing that people (ritster73) actually go through all the trouble of making a profile on this forum, only to be the 10th person to post the same answer to this riddle... the fact that that happens puzzles me more than all the riddles together)

Better still, there is always some place on the North Sea that has high tide... Check out this picture. It shows where the high tide is in the North sea. While the earth rotates in 24 hrs, causing high tide twice in those 24 hrs, there is always a place in the North Sea where it is high tide.

I was assuming that p8p3rkut (who started this thread) was working on a homework assignment and might not even know Mr. Boltzmann. Then again, the formula that I posted was also something I didn't learn at high school... so I'm not sure who gave the best homework-reply. Anyway, together we probably gave the most complete answer possible.

Thanks, the parts in italics were the answer for me. I feel a bit silly for needing this long to see the light (pun intended - probably the most unpopular joke with people working in optics). Until now I was focusing (pun intended again) my attention on the wrong side of the lens: I sort of assumed that parallel light was available. And of course, it can be approximated by placing a source really far away... but that obviously reduces the intensity of the light. Thanks for all the explanations and patience. The world makes sense again

If your smoke is perfectly mixed with the air, then you won't see the flow of air... because every bit of air contains the same amount of smoke. Did you make a wind tunnel? I know the following idea is a bit disgusting, but how about simple cigarette smoke? You can create small amounts, to show an air flow without filling the entire experiment with smoke. It's very simple to do, and should be easy to repeat. In addition, this smoke is produced continuously. Please note that cigarette smoke is dangerous to your health. Other types of smoke also have a chance of being unhealthy. If you're under the legal age for smoking, ask whoever is your supervisor to assist and advise you about this. The reason why I don't advise a chemical reaction for smoke is that they usually become really hot: that also creates an impressive amount of turbulence... which will ruin your experiment.

Hmm... I still would like to see a better explanation... but hopefully we're getting close I'm feeling a bit stubborn because I keep asking, but I honestly still don't understand why a 2nd object cannot become warmer than another object that radiates photons (IR, light, whatever radiation), when you use a lens. The radiation originating from any object includes, as was already mentioned, pretty much all wavelengths. Let's assume a perfect black body (see: graph), which does radiate all wavelengths. I still don't see why with a perfect lens you cannot concentrate the light so much that it will heat up an object to a temperature higher than the source of the photons. Just focus it on a really really tiny spot. The flux of photons should become so great that a higher temperature can be achieved? I understand perfectly that the energy balance will not be breaking the 2nd law of thermodynamics. The total energy of all photons originating from the source of photons will always be greater than on the "really really tiny spot". But why wouldn't it reach a higher temperature? That does not necessarily go against the 2nd law (look at heat pumps and your fridge: it's possible). I think that for determining the temperature that is achieved somewhere we must look at the flux (photons / receiving surface area) and the wavelength. This flux can be concentrated immensely using lenses.

I'm afraid that I didn't understand your 2nd paragraph. Why can't you concentrate light down to an arbitrary small spot using a lens? I mean, that's what lenses do, right? They either concentrate light, or diverge... wikipedia lists all the options here. I know that capturing all the radiation is impossible in practice, which is why I suggested to look at a simple theoretical (fictional) case. I agree that B cannot be hotter than A, but I still haven't understood exactly where my reasoning goes wrong.

Mostly for practical reasons - I didn't have didn't look for other values. First of all, I used not "just" a European number, but I used southern Spain, 250 W/m2. That's not Alaska in latitude, but more like Oklahoma and Kansas (for latitude) and New Mexico (for the type of weather). I did a quick check on insolation in New Mexico (which we can hopefully agree is one of the sunnier places of the USA?). Taking the high value (August): 6.9 kWh/m2/day = 24840000 J/m2/day = 287.5 W/m2. I used a value of 250 W/m2 for the year-round average. I therefore conclude that it's reasonable. I know they included a tilt, so this conclusion isn't 100% waterproof, but I hope you can accept that the error is less than 100%. In addition, I was discussing issues that will not be solved by 100% more sunlight. I attempted to show that more fundamental problems are found in the estimates of algae production. Please note that I don't think that algae are a bad idea. I am just stating that the production is often overestimated by incorrect extrapolations.

Assumptions For the sake of the discussion, let's assume the perfect IR lens to exist, and to work at 100% efficiency (meaning no absorption and only transmission (as in: allow the IR to pass through)). Let's also assume two bodies: A and B. A is significantly larger than B. A is hotter than B at t = 0. The lens is designed and positioned such that it will focus all IR from A, and focus that on B. It therefore focuses IR from a large area on a smaller area. And at the same time, of course, IR from B will diverge and hit A. We're also assuming that (somehow) no heat is lost to anywhere else (all IR emitted from A or B will pass through the lens). Reasoning The lens will now catch more IR from A than from B. This will cause B to heat up. This in turn increases the IR radiation back from B to A. Radiation (or better: IR-flux) will only be equal in both directions if a similar amount of IR originates from B as from A. That is only possible if the temperature on B is higher than A. Problem Now, I also see that this goes straight against all I learned about thermodynamics. But I fail to find the flaw in the reasoning. There should be something fundamentally wrong here (not just the fact that a perfect IR lens does not exist, or that perfect insulation does not exist).