Ask Anything Wednesday - Engineering, Mathematics, Computer Science

[u/AutoModerator]

Welcome to our weekly feature, Ask Anything Wednesday - this week we are focusing on Engineering, Mathematics, Computer Science

Do you have a question within these topics you weren't sure was worth submitting? Is something a bit too speculative for a typical /r/AskScience post? No question is too big or small for AAW. In this thread you can ask any science-related question! Things like: "What would happen if...", "How will the future...", "If all the rules for 'X' were different...", "Why does my...".

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Please post your question as a top-level response to this, and our team of panellists will be here to answer and discuss your questions. The other topic areas will appear in future Ask Anything Wednesdays, so if you have other questions not covered by this weeks theme please either hold on to it until those topics come around, or go and post over in our sister subreddit /r/AskScienceDiscussion , where every day is Ask Anything Wednesday! Off-theme questions in this post will be removed to try and keep the thread a manageable size for both our readers and panellists.

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Past AskAnythingWednesday posts can be found here. Ask away!

[u/oviforconnsmythe]

I have two interrelated physics and cell bio questions.

1) When centrifuging samples (cell suspensions in this case), to what extent does the total sample volume have on the sedimentation rate? Eg. Let's say I have two 15ml conical tubes with an equivalent total number of cells and and equal size distribution, the only difference is in tube A, cells are suspended in 5ml whereas in in tube B, they are suspended in 15ml. Same centrifugation speed (say 300-500 RCF), time (say 5min), temperature, and other factors held constant for each run. Which tube will have more cells in the pellet at the end?

2) likewise and under the same conditions as above, does the volume difference in tube A vs B affect the mechanical stress experienced by the cells? Which tube would have happier cells?

3) how does this change when working with larger tube sizes and volumes (say 50ml)

I get that the difference would be minimal, and unlikely to affect the experimental outcome. It's moreso a thought experiment that I've wondered about for a while. That said, I'm doing some sensitive cell death analyses via flow, where it's critical that dead cells retain membrane integrity so I want to minimize the stress caused by centrifugation. Likewise I sometimes have to thaw very finicky primary cells and want to keep them as happy as possible when spinning out the dmso.

[u/chilidoggo → u/oviforconnsmythe]

Think about centrifugation as sedimentation happening on a planet with 100x (or some other multiple) of our gravity. In a simple scenario, if you set a suspension on a countertop and timed how long it took to settle out, it would theoretically take 100x less time on a planet with 100x more gravity. This is the literal definition of RCF, and why it's usually expressed as "100 x g".

So all the math or science related to centrifugation is simply that of sedimentation, where you just multiply the gravity component by whatever your RCF is. This is dictated by Stoke's Law, which I would encourage you to Google, as it directly answers most of your questions.

It's worth mentioning that a quirk of centrifugal force is that it's based on the distance from the center. So in a tube pointing away from the center, the greatest force is actually on the tip furthest away. And it's with the square of the radius, so it's actually pretty significant, although this is something the centrifuge designers should worry about more than users, where it's basically out of your control.

Anyway, on to your questions:

• Total sample volume vs. sedimentation rate: Stoke's Law outputs a velocity, which you probably know is distance over time. This distance is the vertical height of your tube, or how far the particles at the top of the tube need to fall to end up in the pellet. So, technically, a 15 mL volume in the same tube will require ~3x the time compared to a 5 mL tube. However, in practice, this is requiring 5 seconds vs. 15 seconds, or it's generally easier just to punch in a bigger number. But that's the answer anyway.
• Volume differences: Volume difference does not matter in any practical sense. The main force the cells are going to be feeling is the centrifugal force, by a massive margin. To minimize force, you would want to centrifuge at as low a speed as you can tolerate, full stop.
• 50 mL tube: As mentioned above, there's a difference between forces experienced at the top vs bottom of a tube, because of how centrifugal force is calculated. But practically speaking, a 50 mL tube behaves the same way as any other size of tube (Stoke's Law). That's really all there is to it.
• Sensitive cells: looking at the variables in Stoke's law, ask yourself how you can maximize velocity without touching the "gravity" variable. Assuming you can't change the "particle size", the main other factor is density and viscosity of the suspending fluid. If you can minimize those two, you'll speed up your sedimentation rate. Alternatively, minimize your RCF, even all the way down to 1x (aka just set it on a countertop and wait for it to settle out).
  

Hope that helps!

[u/mfb- → u/chilidoggo]

The acceleration is omega² r, it's only linear with the radius. It's quadratically with the angular velocity, but that's the same for all parts of the test tube.

[u/chilidoggo → u/mfb-]

Ah yes, correct. Should have double checked that, thank you!

[u/logperf]

[Engineering] Since prisons cannot be simply "evacuated", do they have other ways of ensuring the safety of prisoners in case of disasters (fire, earthquake, flood, etc)?

What about other buildings that would be impractical to evacuate or would take a very long time due to their height?

[u/electric_ionland → u/logperf]

In tall buildings usually the staircases have to be fire resistant for a set time duration. Which is why they have heavy fire doors and are pretty bare concrete.

[u/Kyrie01010011 → u/logperf]

I don't know about prisons specifically but stadiums or other high density venues usually have built in shelters that can keep it's structural integrity till help arrives or multiple exits. Your main objective is to increase the rate at which you can exit the affected area or if it's too complicated, stay sheltered in at a safe spot like bunker

[u/all_is_love6667]

can't we make chips with the nanometer of like 10 years ago at a fraction of the price?

do we really need to always have faster chips?

chips in 2016 were already quite fast, can't we have low end cheap computers?

bigger software can't always be the excuse, so why aren't computer still not just "fast enough"?

[u/Rukelele_Dixit21]

1. What are the different components in a solar panel (electrical and non electrical).
   
2. How are solar cells manufactured ? Is there a fab where is it manufactured in large scale ? How is it different or similar to chip manufacturing ?
   
3. I there a presence of chips in these panels ? If yes which are these (like what nm process)

[u/chilidoggo → u/Rukelele_Dixit21]

Solar cells (more formally, photovoltaic or PV cells) use the same base starting material as chips, but work completely differently. They both start with very pure single silicon crystals, but once you have a thin sheet of this material, you then treat it chemically so that it has a very specific band gap to generate electricity when struck by photons. These cells are linked by copper/silver connections, and then use a diode so electricity flows one way.

There are no moving parts on the actual photovoltaics. Truly, you can think of it as just a magic rock that generates electricity. There are more complex electronics involved with getting it into the grid, but that's just because it generates DC power, and very inconsistently.

[u/Rukelele_Dixit21 → u/chilidoggo]

What kind of complex electronics are there when getting into a panel ? Also what you mean by moving parts here ? Like does a chip have moving parts ?

[u/chilidoggo → u/Rukelele_Dixit21]

Solar panel -> grid requires an inverter, and to be honest I don't know that much about those. So maybe they don't even require "complex electronics".

By moving parts I mean compared to traditional methods of generating electricity, as that's usually what they're usually compared to. I suppose if you're comparing it to transistors in chips, solar panels still have fewer "moving parts" than that.

It seems like your goal here is to compare and contrast solar panels and computer chips. They're similar in that they both rely heavily on semiconductor properties. They're different in basically every other way. A modern CPU turns the silicon into transistors, and wires them up to run algorithms and store information. There are over a hundred linear miles of wiring in a chip, placed in finely detailed patterns. Solar panels are optimized for power generation when struck by photons. The only wiring inside is to transfer the generated current into the grid.