141–150 of 165 posts
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You are the one regarding them as failures. I had a variety of teachers in school, some were very educated (PhDs), and simply found a calling in teaching. Not everything is about money, you shouldn’t look down on teachers.
Probably related to the old joke: Those who cannot do, teach and those that cannot teach, manage.
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My uncle works for a water treatment company. Medical manufacturers often come to him saying his equipment is off because they open the test port ran the pure water into a clean glass and it tested below specs. He then has to explain that in the 30 seconds between opening the valve and the test the water absorbed enough impurities to fail the spec. The input water was fine.
Another interesting anecdote about water treatment facilities is the situation in Asia. For example, Jakarta has a water treatment facility, but every tourist gets stomach illness there. The reason is that industrial facilities siphon off the treated water, and by the time homes and hotels get any, it's untreated. To give you an idea how bad it is, tourists generally don't drink tap water, but they still get ill from…
Unless the facilities are adding dirty water to the pipe to push pressure back up after they remove it? But even then, unless they routed the pipe from the water treatment facility to the industry first, only the areas downstream from the industry would be affected.
More likely is that the water is not being adequately treated and/or that the infrastructure is not built/maintained and the pipes are leaky.
My guess would be that the treatment is to minimum spec and that there are leaky pipes which allow for contamination. It is hard to maintain pumped water pressure if there are brownouts.
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The basis of the disagreement hinges on your original claim that "compression is a process that is adiabatic, generating no entropy" which doesn't have any valid basis. Nothing about even the definition of compression that you provided above implies that compression is or isn't by definition isentropic (also, adiabatic and isentropic are not the same thing). Are you trying to argue that flow passing through a shock i…
Of course it has a valid basis. The alternative is to say that in any process that reduces the volume of the material, all the heating is due to compression (even though the amount of heating can vary widely in different processes with the same reduction in volume.) The logical error you are committing is saying "if a process includes compression, then the heating is due to compression".
I get your point that the shock contributes to the heating through irreversibility, and agree that has a significant impact on the sensed temperature. If we want to tie your point back to the original post, one could say that "compression and shockwaves" are the cause of re-entry heating to ensure you are explicitly capturing the effect of the shock.
Whether you consider the shock heating to merit explicit mention or whether you see it as a detail associated with the specific means of compression is largely a matter of perspective & practice.
The gasdynamic basis of shock formation is derived by considering what happens when pressure fields are no longer able to smoothly vary as the speed of a body exceeds the speed at which pressure disturbances can be communicated upstream, namely the speed of sound. The pressure jump across the shock is elemental to its derivation. Practitioners commonly view the shock as the means by which compression takes place and pressure fields reconciled. As I noted above, just saying "compression" doesn't imply "isentropic" and so the entropy-generating processes associated with a shock performing compression are viewed, for example, as thermodynamically similar to viscous heating of the fluid in a gas turbine compressor.
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I think the public perception of teachers varies a lot between societies. In breaking bad it was clear that Americans don't think much of a guy who ends up teaching chemistry. If you go to other countries it's very different, eg in Switzerland it seemed totally legit to aspire to teaching. Salary was 120k according to the kid who told me.
My high school was in the US. I think good teachers anywhere are teachers because it is their calling. Breaking Bad is hardly the normative reference of how people should view teachers.
Could you make a movie where being a doctor makes the main character a failure in life? Such a movie would almost certainly have to be a comedy where the character is a failure in spite of being a doctor, not because.
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> Almost everything we think (including me) is probably trash. We just need to accept we are mostly idiotic. I do not like this attitude. Everything is false, because the truth exists only in ideal condition of math. But it doesn't mean that we are all idiotic and know nothing worthy knowing. The trick is to understand the relation between knowledge and the reality, to feel this relation in your guts. It helps to kno…
No - truth doesn’t only exist in an ideal condition of math. I mean, yes maybe literally in some ways, maybe. But not really. Math doesn’t prove the sky is blue on a sunny day. My main point is that of falsifiability. You don’t need math to make a falsifiable argument. However, much of our discourse can’t even meet this basic qualification.
Yeah, it doesn't. So it is not a truth, but a belief. It is pretty strong belief, so strong that it almost indistinguishable from the truth. But nevertheless it is just a belief.
> My main point is that of falsifiability. You don’t need math to make a falsifiable argument. However, much of our discourse can’t even meet this basic qualification.
Even falsifiability is a too rigid property for some discourses. My main point is that attempts to find a silver bullet to shoot "bad beliefs" are no-go. Knowledge couldn't be bad or good, true or false. It is the way knowledge applied could be good or bad.
For example, even "the sky is blue on a sunny day" could be misapplied. Take the sky of a Moon or a Mars for example, which are not blue in a sunny day. Or let us imagine atmosphere of Earth with a lot of dust after collision of Earth and an asteroid: on such Earth sky could be orange, or gray or of some other color. I mean that even "true" statements have limits of their truthiness.
As an opposite example, a "false" statement could be applied in a "good" way. Earth is flat? Oh, yes, it is convenient to think so when dealing with a relatively small area of the Earth surface. Or when communicating with people, who believe that Earth is flat. You need only to remember the limits of this belief, to not misapply it accidentally.
In math the situation a little different. Math statements also could go wrong when assumptions change (the sum of angles of a triangle could be not equal to PI, if triangle drawn not on a plane but on some other surface), but mathematicians make sure to strictly state all the assumptions as axioms and definitions. It is possible in ideal conditions, but not possible in real ones.
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Not the OP, but a fourth grade substitute teacher made the claim that we always saw the same side of the moon because it didn't rotate. I don't know what got to me the most: that she didn't understand, that my attempt to explain it didn't go over, or that the other kids in the class didn't get it either. In any case, I've been working on improving my explanations ever since.
And your beef is that it does rotate, at a rate of one rotation per revolution? Sounds kind of pedantic to me, and confusing (certainly to kids).
Doesn’t seem like a small point to me.
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Of course it has a valid basis. The alternative is to say that in any process that reduces the volume of the material, all the heating is due to compression (even though the amount of heating can vary widely in different processes with the same reduction in volume.) The logical error you are committing is saying "if a process includes compression, then the heating is due to compression".
The semantic error you're making is redefining the general term compression as the more specific "isentropic compression". The process by which the compression takes place is not prescribed just by the term "compression" as you did originally. Just saying "compression" specifies nothing about the process by which it happens. Compression can take place (theoretically) through an isentropic process, or a non-isentropic…
Because entropy was added, the gas is now hotter than initially. But it is not compressed. The heating wasn't due to compression, it was due to dissipation at the shock. That dissipative process is separate from compression, and you are confusing the two.
The incoherence of your position should have been clear to you. You seem to think that compression, as a cause of heating, doesn't even have a well defined amount of heating. Your explanation isn't even wrong, it's undefined.
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I have never seen compression to be considered inherently reversible, certainly not in the aerospace or hypersonics/reentry communities. I assume you come from a different background? Care to explain your second argument? Even when the density reaches a limit, the change in pressure over the shock does not, so you can pump in energy arbitrarily (of course your gas would dissociate at some point). The total pressure l…
Let's look up a definition. "Compression: decrease in volume of any object or substance resulting from applied stress." Compression refers to a reduction in volume, not an increase in pressure. This is finite for gas crossing a shock, even as the mach number goes to infinity.
What I say is that even when dV approaches a constant, the pressure still rises, so the product also rises.
[edit: removed snark]
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Compression due to a shock is not isentropic. Parent is incorrect. This is basic gasdynamics. An accessible reference: Modern Compressible flow, by Anderson.
I didn't say compression at a shock was isentropic. I was saying the claim that compression at the shock could not be the cause of (all) the heating, unless you simply define heating at a shock as being due to compression. Instead, I think you should separate out the heating there into two parts: one due to isentropic compression, and one due to injection of additional entropy by dissipation at the shock.
The second part of your argument is well defined and makes sense, and no-one argued that.
But explain to me again how this is now 'like friction'?