2
$\begingroup$

Using my world changing invention and my massive intellect I have created the perfect defensive tool for myself and any minions valued friends I deem might need one.

The tool is a super suit that doesn't actually exist until it's needed. The suit is worn as a bracelet, and when activated the bracelet will take all the air (and clothing) in it's immediate vicinity and start to transmute it (in a lossless way, including all binding/rest energies) into the metallic components of the suit, rapidly expanding outwards over the user's body. The growth rate of the suit over the person is exponential as more air becomes available to transmute. Once the suit is fully complete the wearer will be practically impervious to all known weaponry, not to mention super strong, super fast and equipped with fully operational life support.

The question is this: Given that the materials that make up the suit are roughly equivalent to 20 kg of iron: How much air is required to make the suit, and will this cause an appreciable gust of wind if the suit is applied at maximum speed at sea level? For bonus points work out what the maximum speed is.

For the sake of simplicity clothing doesn't add any appreciable energy to the equation (assume everybody is buck naked).

A quick note: While the premise of the question isn't hard, equations/workings are needed for this one, hence the 'hard-science' tag rather than 'science based'.

$\endgroup$

This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

  • 2
    $\begingroup$ I'm having a tough time with the hard-science on this one. I can make you Iron by fusing the oxygen in the air. This will take about 110,000 cubic meters of air. But you'd have a much bigger problem, specifically the petajoule of energy I just released, or about 1 Tsar Bomba. Thats going to displace a lot more air than the 'suction.' I vote this as not-answerable with the current tag. $\endgroup$ – kingledion Mar 23 '17 at 14:53
  • $\begingroup$ @kingledion: The device in question can make use of all available energy, including binding, in order to make other matter, hence the note about hard science over science based. I'll edit that in to make it explicit without following the first link. $\endgroup$ – Joe Bloggs Mar 23 '17 at 14:55
  • $\begingroup$ What's hard science about this? 20 kg of energy is 20 kg of energy, and you say you can use it all without any significant energy loss. All you are asking is how much "wind" will be caused by about 20 cubic meters of air suddenly disappearing indoors/outdoors. $\endgroup$ – AlexP Mar 23 '17 at 23:24
  • $\begingroup$ @AlexP: Did you read the note at the bottom of the question? $\endgroup$ – Joe Bloggs Mar 24 '17 at 8:02
  • $\begingroup$ @JoeBloggs: Equation: the energy of 20 kg of costume = the energy of 20 kg air. 20 kg of air is a cube with a side of about 2.7 m. Suppose that you consume that air in a second, and you are outdoors, you will have a gentle gentle very local "wind" peaking at about 1.35 m/s (about 5 km/h). Indoors you have do deal with the pressure differential between the inside of the room and the outside; the pressure differential depends on the size of the room. In a small-ish 60 cubic meter room you will use about one third of the air. In a larger 200 cubic meter room you will use about one tenth. $\endgroup$ – AlexP Mar 24 '17 at 9:14
2
$\begingroup$

Assuming by "lossless transmutation" all we care about is mass.

Dry air has a density of 1.2 kg/m$^3$ at stp. If your armor weighs 20kg it will consume 16m$^3$ of air in it's transmutation.

This would create a noticeable wind.

$\endgroup$
  • $\begingroup$ And if activated inside a room you are looking at potentially spectacular effects -- windows and some/most walls are going to break under the pressure differential. $\endgroup$ – AlexP Mar 23 '17 at 23:21
1
$\begingroup$

A quick disclaimer: I know that I don't know enough about either chemistry or fluid dynamics (hence why I asked the question), so if you spot a mistake please correct it.

Assumptions: The air is at STP and is composed entirely of elemental nitrogen. I'm also assuming that the molecular binding energy is 0, since it represents such a small number compared to the others.

The total amount of energy present in either material is going to be:

Erest+Enuclear binding = Etotal

where nuclear and molecular are both binding energies. Let's just take a first order approximation of these energies to check if the binding energy is actually important. Since we're doing first order approximation I only care about the first significant digit and the number of zeroes, so for the suit:

Erest-iron = 20kg * (3*108)2 = 1018 joules

Enuclear binding-iron = 10-12 j * 300 mol * 6*1023 = 2*1014 joules

OK. Not really close enough to care about that extra energy. But is the difference between iron and nitrogen's binding energy enough to care about? This could be a bit tricky, since the total binding energy of the air depends on how many nitrogen atoms the suit has to absorb, which is dependant upon the difference in binding energies between iron and nitrogen, but let's just assume to start with that it's transmuting 20kg of nitrogen too (I'm making my approximation numbers more precise here).

Enuclear binding-iron = 1.408*10-12 j * 358 mol * 6*1023 = 3.024*1014

Enuclear binding-nitrogen = 1.233*10-12 j * 1427 mol * 6*1023 = 1.055*1015

Ediff = 7.526*10^14 joules

Putting that in context the difference in binding energy between 20kg of nitrogen and 20kg of iron is 0.07% of the total energy.

We need very slightly less air than straight rest energy conversion would suggest, but not really enough to be noticeable. We can assume that we need 20kg of nitrogen which translates to:

20 kg/1.25 kg/m3 = 16 m3

of nitrogen gas (as sphenning noted in his answer).

Now onto whether the suit can actually absorb that much air in any reasonable length of time. I'm assuming that the suit acts as a perfect pipe into a perfect vaccuum. While the size of the suit will change dynamically I'm just going to look at the two extreme cases of bracelet suit and full body suit. I'm doing this to simplify things, since my flow dynamics is weak.

The flow of air into the suit will almost instantly reach it's peak velocity (the speed of sound). It will also reach it's peak mass transfer velocity. At that point it is 'choked' and more air won't be forced into the bracelet any faster. We can calculate the volumetric flow, therefore, as

Area of suit * speed of sound

Assuming the bracelet represents 0.1 m2 of area this means we can in theory shift 34.3 m3 of air per second, or that our suit can be fully deployed using nothing but the bracelet in about half a second. If the full suit is 1.8m2 then it can get the requisite air (to build itself all over again) in 0.025 seconds.

Would this classify as wind?

Absolutely.

It would also have some unintended side effects. Firstly: The noise would be immense. Secondly: Nearby objects would be thrown about (or potentially broken) by the shockwave of low pressure air moving outwards from the suit. Thirdly: once the suit is done deploying there will still be an awful lot of air moving in towards it, rushing to replace the removed air. This new air would then find itself rushing headlong not into a vacuum but instead into a solid super-suit. It would rebound and head back out as a second shockwave of air that would manifest as a very loud bang.

In essence: This suit wouldn't just cause wind. It would also cause a thunderclap.

$\endgroup$
  • $\begingroup$ It sounds like you might want to make the conversion rate tweakable, especially if you're wanting to keep these suits secret. Some sort of stealth mode, where you can hide in a corner and slowly absorb the air might be valuable, as well as your super-fast-local-environment-destroying mode. $\endgroup$ – Callum Bradbury Mar 24 '17 at 13:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.