1

Physics, Astronomy & Cosmology / Why do kettles make a whoooarr sound?

« on: 06/07/2023 03:25:31 »

Hi.

   So I went to start making a cup of tea but I couldn't remember if I had switched the kettle on.   I went back out to the kitchen and soon knew because I could hear the kettle making a gentle "whooo / roar" sound.   It's mid-range rather than being high pitched but some people do say the kettle is "fizzing"  (There are little bubbles of gas rising and I think people mistakenly describe what they can see instead of what they can hear).
    Why does it make that sound?   (I mean long before the water has reached the boil).

Best Wishes.

2

Famous Scientists, Doctors and Inventors / Are some scientists unique and only they could have made the discovery?

« on: 16/05/2023 03:38:37 »

Hi.

    This section is rarely used, I thought I'd put something in here.

Is it just time and numbers that are important rather than who you have working on science?
Are there some discoveries and theories that are so extraordinary that only one person could have made them?

    Let's take Newton as one example.  We tend to remember the contributions he made to mathematics such as Newton's laws of motion.  We overlook the fact that a lot of his Philosophi? Naturalis Principia Mathematica contained discussions of what can reasonably be described as some philosophy and theology.   We have selectively taken out only the most relevant and useful bits of his work and only that bit of it tends to be remembered and highly valued.
   If you disregard all the junk that you (the reader) have written over your lifetime and just retain the best bits,  then it's very likely that you have created something profound and well worth preserving for future generations.  Well done and congratulations to you.
    Arguably if we had a million monkeys with typewriters, then all you need to do is disregard the bits of junk in the middle (let's say keep a sentence at a time) and you should be able to find a Shakespearean play in there.

   So, is it just a game of applying time and a large number of scientists to a problem before any problem in science would eventually be resolved?
   You might think of other famous scientists like Einstein and wonder if you just had to have an extraordinary mind to think of such bizarre things like applying a curvature to spacetime but actually it wasn't just Einstein coming up with these ideas.   The mathematician David Hilbert was apparently also developing a theory similar to General relativity.  Einstein knew this and was arguably more driven to complete and publish his work faster.   I don't know how far Hilbert had progressed but it is commonly thought that Hilbert would have got there and if his version of the theory had been published it might have been even more elegant.
     We tend to be aware of mathematicians like Minkowski and Riemann that contributed to Einsteins work but there may well have been others getting involved more regularly but rarely credited.  See https://www.caltech.edu/about/news/not-lone-genius   where a historian looks into the involvement of Einstein's freinds from college (Besso and Grossman) and also Einstein's first wife Mileva Marić Einstein who was a brilliant physicist but at this sort of time that was never going to be accepted or recognised. 

     Albert and Mileva were admitted to the physics-mathematics section of the Polytechnic Institute in Zurich (now ETH) in 1896....
      By the end of their classes in 1900, Mileva and Albert had similar grades (4.7 and 4.6, respectively) except in applied physics where she got the top mark of 5 but he, only 1. She excelled at experimental work while he did not. But at the oral exam, Professor Minkowski gave 11 out of 12 to the four male students but only 5 to Mileva. Only Albert got his degree.

["The Forgotten Life of Einstein's First Wife", Scientific American.
https://blogs.scientificamerican.com/guest-blog/the-forgotten-life-of-einsteins-first-wife/ ]

    We know that Einstein discussed relativity with his wife, there are letters with evidence of that but most of the discussions were verbal rather than being written down.  You should not underestimate how brilliant his wife was.

    ...nobody made it clearer than Albert Einstein himself that they collaborated on special relativity when he wrote to Mileva on 27 March 1901: ?How happy and proud I will be when the two of us together will have brought our work on relative motion to a victorious conclusion.?    

["The Forgotten Life..., Scientific American]

    We aren't sure exactly why Mileva's name didn't appear as a co-author on any of Einstein's work, it is suggested (source:   Radmila Milentijević: Mileva Marić Einstein: Life with Albert Einstein, United World Press, 2015) that given the prevalent bias against women at the time, a publication co-signed with a woman might have carried less weight.
Whatever the situation may have been, Mileva had much bigger problems to worry about then getting her name on a paper:   In 1901 she became pregnant out of wedlock  ..(she) tried to persuade Albert to marry her. She gave birth to a girl named Liserl in January 1902. No one knows what happened to her. She was probably given to adoption.   

"The Forgotten Life...", S.A.

   It wasn't a great time for women in science in the 1900's.   Recall, for example, that the Nobel prize committee only wanted to recognise Pierre Curie and Henri Becquerel for the Nobel prize in 1903,  they needed their arm twisted to add Marie Curie's name to that list and it was only the insistence of Pierre Curie that ultimately made it happen.
- - - - - - - - - -

    Anyway, enough of the sad stuff.  I've just recently explained how special relativity could have been discovered by alternative routes if light hadn't been a massless particle, which is partly why this is now on my mind.   It's not just that there were other people involved in developing relativity, there are so many different ways in which relativity would have eventually been discovered.  It was inevitable.

   So, we're back to the original question:   
   Is any scientific discovery such that only one person could have found it?  Is scientific development just about applying numbers of scientists to study the problem and waiting for the problem to be broken with time?

   Looking just on the home front:  If a forum like this one can motivate a few people to study some science, wouldn't it be worth keeping it?   It doesn't matter if it's not discussing the latest developments or hosting the most erudite discussions.  Why are there rumours that the site is closing down and what could be done to maintain it?   

Best Wishes.

Late Editing:  Apologies for not getting all the accents and special symbols like ae to appear.  None of that works anymore.

3

Just Chat! / Where is the "Spiel Chicken"?

« on: 09/05/2023 16:51:36 »

Hi.

    The interface for making replies to forum posts used to have a spell check button.  It seems to have gone.
Why and is it coming back?   Please.
    It wasn't great and often favoured USA spellings but it was something.   You don't realise what you had until it's gone.

Best Wishes.

4

Just Chat! / Quantum puzzle

« on: 17/04/2023 00:51:18 »

Hi.

    Hopefully the poll speaks for itself.   The forum looked quiet this evening and this might provide some discussion or activity.
    There are some answers for this already.   You don't need to go and look for them in textbooks or on the internet but you can if you had a few minutes to spare.  As always, use your own discretion and try to select a sensible and safe website.

Best Wishes.

5

Physics, Astronomy & Cosmology / How many beams or spots emerge from Stern-Gerlach apparatus?

« on: 13/04/2023 16:33:42 »

Hi.

     The scientists Stern and Gerlach developed some equipment that passed Silver atoms (Ag) through a region with a magnetic field.   The important point is the magnetic field, B, has a gradient (let's say in the z-direction).   So a beam of silver atoms moving through it will be deflected according to any magnetic moment it has.  Specifically, μ_z the magnetic component of the z-direction will be important (if its ambiguous later in this post, we are considering z components only).

    This might be familiar to people who have studied some QM.   Here's a Wiki article:  https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment      although it's not the best reference.   (I'll provide more if required - but your own favourite textbook will probably cover it).

    Anyway, the usual result is to obtain just two spots, corresponding to an up or down deflection, on the far side of the apparatus.   From which the usual conclusion is that the electron in the Ag atom had an intrinsic property called spin.   The z-component of this spin can be in only two states (which we'll call "spin up" and "spin down"   or   +  and  -     or  quantitatively the spin z component takes the values +ħ/2   or  -ħ/2 only).    Obviously the Ag atom didn't have just one electron but it's the outermost lone electron that is usually in the 5s orbital which is important.   All the other electrons are assumed to form a reasonably complete cloud with enough symmetry that they contribute a net 0 magnetic moment.   Just to explain why they bothered with Ag atoms instead of using just a simple electron beam - they wanted an overall Neutral charged particle.  A charged beam would have shown deflection just due to the charge moving across a magnetic field which would have dominated over the subtle difference due to spin.

    Now, just to check I've understood this experiment.   If, for some reason, the outer electron was not in the 5s orbital (corresponding to quantum numbers n=5, l=0) but instead it was in the 5p orbital  (quantum numbers n=5,  l=1),  would we get a different set of spots?    Specifically with l=1  there are now  2l+1 = 3  different magnetic quantum numbers m = -1, 0, +1 that it can have.    These should contribute a Z component to the magnetic dipole.  So overall, there would now seem to be a larger set of magnetic dipole moments the Ag atom can have.

  The angular momentum L_z component due to the magnetic quantum number m should contribute a dipole moment of mμ_B.   Where μ_B  =  eħ/2m_e = a constant called the Bohr magneton.    Meanwhile the spin contributes a magnetic dipole moment of approximately  ±μ_B   (It's approximate because I've taken the Lande factor g ≈ 2   in the relationship between the value of spin and the magnetic dipole moment it creates - details shouldn't be too important).

     Overall we have a magnetic dipole moment in the z-direction given by:
[Contribution from magnetic quantum number, m]   +  [contribution from elctron spin]  =   mμ_B  ±  μ_B     with  m = -1, 0, +1.
    This gives a set of four values:     -1½ ,    - ½ ,    +½,  +1½     (in units of μ_B ).   

So, I would have thought, we get 4 different spots on the far side of the apparatus.    That's the first bit I wouldn't mind getting checked.   Is that right so far?

    Now since the Ag atoms were made into a beam by being heated in a furnace,  I would have thought there was plenty of energy around and quite a few of the outer electrons in the Ag atoms could have been in the more energetic 5p orbital rather than the 5s orbital.   So what I'm puzzled with is why did the experiment actually work so well?    There should be multiple spots on the far side of the apparatus and not just two.   It gets worse the more energetic the outer electron might have been, we could have had something that did look like a continuous smear of atoms running the range from maximum up and down deflections instead of just two clear spots where the atoms pile up on the screen / detector.   To say it another way - we could have obtained the classically predicted result and not the quantised splitting into two beams for which the experiment is famous.

    I don't know.   Just seems that there must have been some reason why the outer electron of the Ag atoms did tend to stay in the 5s orbital.   If anyone can remember their QM or possibly has an idea why a beam of Ag atoms produced from a furnace would tend to keep the outer electron in the 5s orbital, then I'd appreciate some discussion.

Best Wishes.

6

Physics, Astronomy & Cosmology / Forgotten vocabulary - a thing being pulled apart by tidal forces.

« on: 07/04/2023 00:53:05 »

Hi.

   I can't google my result because I just can't recall what it is called.
   Assume you have a small spherical thing which is held together by little more than just mutual gravitational attraction of it's particles.
   that small thing starts to get close to a bigger spherical thing.    Gravitational forces act on the small thing.  There is a certain point (a radius from the big object) where the  small sphere is pulled on the left side too much by the big sphere.   The force of attraction of that left side to the centre of the small sphere is insufficient to cancel that and the small object starts to be torn apart.

   There's a name for this process and I can't remember what it is,  and/or there's a scientist's name for the distance where this happens  -   the   Smith's radius  etc.
   If you remember, please let me know what the names are, thank you.

Best Wishes.

7

Technology / Can you explain this demonstration of a capacitor?

« on: 22/03/2023 05:02:21 »

Hi.

The Tech section doesn't get used as much as the Physics section.  So I've put this in here rather than in the usual Physics section.

Declaration:   This has been set as an exercise by a UK University.

Background:
1.    It's not my homework, just something I saw a month ago, became interested in and still can't explain to my satisfaction.
2.   It was never assessed but just intended as something a student should do in their own time, for their own benefit.   
3.   The general spirit of the exercise is that a student should be able to explain the video to a friend.
   Overall, I would think that a discussion here causes no harm and is perfectly consistent with the spirit of the exercise. Even if next year's students find this thread, it should help not hinder.

Here's the original phrasing of the task:

      This is the video we watched at the start of the live session. You should be able to explain what happens to the current through the circuit and the voltage between the plates at the following times:

a) 0 -> 1m, the plates are connected to a power supply and moved apart from / towards each other;

b) 1m5s -> 1m45s, The capacitor is detached from the power supply, a fixed amount of charge is deposited, and the plates are moved apart from each other;

c) 1m45s -> 2m05s, with the capacitor plates still isolated, a sheet of plexiglass is placed between the plates.

Here's the video:     "MIT Physics Demo -- Adjustable Capacitor with Dielectric",    available on YouTube,  duration ~2 minutes.

- - - - - - - - - - - - - - -

Brief response:

a)    We have      C = εA/d                  [1]
                        V =  Q/C                     [2]
 d increases,  Eqn [1]  =>  C decreases  =>  Eqn [2]  Q must decrease  (because V is held constant) =>  we have a current flow.

    OK... so the Ammeter should show a deflection in the same direction from the central position every time the experimenter turns the handle to move the plates apart.
   BUT....if you look carefully when the plates move apart (at about time 0:27),  the first deflection of the Ammeter during the first twist is to the right.   All the other twists and you have a deflection to the left.    WHY?    i.d.k.   The demonstrator knocked the handle?  I am missing some physics?

   I would blame it on a faulty Ammeter but the Ammeter is actually pretty good in that it manages to show something else that demanded precision from the instrument:   Combining [Eqn 1] and [Eqn 2]  we obtain   Q ~  1/d   meaning that the biggest changes in Q (associated with the bigger currents and deflection) should be seen when the gap was smallest.   (Assuming they turn the handle roughly the same number of rotations and roughly the same speed each time they twist it).   You DO see that in the video clearly enough...... the deflections on the Ammeter are bigger when the plates are close.

   Reverse the behaviour when the plates are moved back together.   That bit seems to give the expected results.   We get deflections to the right only and they become massive, almost to the limit of deflection, when the plates are closest.

----The post is already getting long, so I'll leave discussion of parts b and c for now  ---
Main question from the first part is   Why does the ammeter deflect right on the first twist of the handle when the plates were moved apart?

Best Wishes.

8

Physics, Astronomy & Cosmology / Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 09/03/2023 01:22:57 »

Hi.  I hope everyone is well.

Blackbody radiation is usually considered (theoretically modelled and suitable equations derived) by looking at the radiation that can exist inside a cavity or an "oven".    The interior of the oven can support various modes of radiation within it,  each mode has a particular frequency,   those frequencies ultimately assumed to be caused (generated by) charged particles oscillating at that frequency in the walls of the oven.

For a cuboidal oven you can have modes of radiation within the oven with this general form:

ψ =  A.  Sin (K_x x) . Sin (K_y y) .Sin (K_z z).   

[Equation 1]

Where A = amplitude (a constant)  and  K_x,  K_y and K_z  are also constants  but not quite as arbitrary.    One of the boundary conditions that must be satisfied is that ψ = 0 at the walls of the oven   (since these waves do not continue through the walls and propagate away from the oven).  We really are looking for stationary wave solutions that fit inside the oven.   Hence  only K_x =  p.π / L_x  ,    K_y =  q π/L_y   and  K_z = r π / L_z   for some integers p,q,r   will be allowed  (L_x  being the length of the oven in the x direction etc.)

   That's a typical way in which the blackbody radiation is modelled and you can find more details in most textbooks or online resources.  I won't take up any more of your time here.

    So, taking everything at face value,  it would seem that.... if you have a big oven then you can support e-m radiation at most wavenumbers k or wavelengths λ = 2π/k.   This is just because  π/L_x ,  π/ L_y  etc. will be small numbers and so the permitted wave vectors are separated by only a small amount. 
   For a small oven, however,  the permitted wavenumbers are more widely separated.   You'd still get the overall shape given by the usual blackbody spectrum curve but you may begin to notice that you only have small discrete chunks of that spectrum present.
    I don't know... I'm asking not telling.   I've just presented some of the background so that you can hopefully see it isn't a silly question to ask.

    Taking it to a more extreme situation....  what if you don't even have a cuboidal shaped oven?   Imagine an arbitrary blob with protruding lumps shaped oven.   I haven't spent much time trying the mathematics,  it's just apparent that easy and separable solutions like [equation 1] are out of the question - that's never going to satisfy ψ=0 at the walls.  It would seem that a blob shaped oven would support considerably less modes of radiation within it.

   So,  does the spectrum of the interior of an oven actually show any changes when you alter the shape and size of that interior space?   For example, does the spectrum seem to become more discrete and less continuous with very small interior spaces?   Does a cube shaped oven have a better and cleaner match to a blackbody radiation curve than the interior of a kidney bean shaped oven?

   None of this is urgent,  I'm of the opinion that the usual way blackbody radiation is presented in the textbooks (very roughly as the above) is just far too simplified.   I am pretty sure that the shape of an oven does not affect the radiation spectrum, I'm just not sure I could explain why.

Best Wishes.

9

Just Chat! / Advertisements on the forum

« on: 02/03/2023 02:27:40 »

Hi.

   So there seem to be more adverts on the web page than there used to be.   I don't mean people putting SPAM into the forum disguised as a proper forum post,  just plain adverts in their own little sections of the web page.

    It's a nuisance and detracts from the website  BUT .... OK - We can see why that might be needed.   Web sites do have a cost to run and maintain.

   Have you (administrative staff) tried being a bit more selective about the adverts you allow in the middle of the web page?   Would you consider doing it?   I suspect you have, so the examples section below is probably irrelevant but I'm just including it to explain why the question is worth asking.

   Examples of what you could do:   You have some affiliation with Cambridge University,   run adverts for courses at Cambridge.
   Blacklist (prevent) car advertisements for fossil fuel vehicles,  allow electric vehicles, bicycles, hydrogen fuel vehicles maybe.
   Identify a few organisations you would not mind being associated with,  e.g.  National Science Museum,  CERN, other educational establishments maybe.    Run their adverts for staff recruitment, courses being offered, special events  etc.

   - - - - - - - -

Identify key moments when the user should be able to concentrate on the genuine forum content and ensure there are no adverts at those times.   Examples:   When clicking on "view the most recent forum posts" make sure there are no adverts stuck on the bottom of the page that could be misleading or accidentally clicked on as if they were recent posts.    Similarly, when replying to a post, adverts can get in the way of scrolling through the previous posts and trying to extract quotes.

Overall:   By allowing adverts, your revenue may have gone up.   However, your user-friendliness has gone down.   Have you kept statistics about the number of new users you are now attracting and/or the number of existing users you are retaining?    The forum is less attractive now.

Best Wishes.     

10

Just Chat! / Merry Christmas

« on: 25/12/2022 00:56:53 »

Merry Christmas everyone.

   
  {Image from Wikimedia - should be copyright free, more or less}

Stay cheerful and we'll see you in the new year.

11

Physics, Astronomy & Cosmology / Is there a substance where entropy decreases at higher temperature?

« on: 14/12/2022 23:44:10 »

Hi.
   I apologize for the title, it had to be very compressed to fit in the space available.   Hopefully it makes sense.

Typically when you transfer heat to a substance, its temperature increases and also the entropy of that substance (or the entropy of the system which is just that substance) increases.   

Are there any counter-examples?

    Well, yes, I think so.   An ideal gas, starting from an initial state with pressure and volume P_i and V_i can be subjected to compression (to a volume V_f  < V_i ) and then some surplus heat can be removed while maintaining the volume at V_f.   It should be possible to reach a higher final temperature but lower final entropy.
    You can determine this by looking at the Sackur-Tetrode equation for the entropy of an ideal gas.   (Reference: http://hyperphysics.phy-astr.gsu.edu/hbase/Therm/entropgas.html  ).   I can show the mathematics if required.

     There are probably other examples but you can hopefully see that the situation or the combination of changes you make, can be as important as the substance you choose.
    Now if we insist that all changes are restricted to maintaining a constant volume, we have the question that I was really hoping to ask:   Is there a substance where the temperature can be increased but the entropy of that substance decreases and all changes are done at constant volume?
   (It doesn't seem possible to me, using a thermodynamic definition of entropy,  just because  C_v = specific heat capacity at constant volume is always positive for every substance I know about.   I'm asking because I don't know for certain if there is a substance with C_v < 0.    Also, using a statistical mechanics definition of entropy you don't seem to be asking for too much:   If there was a substance where you can increase the total energy content from E_i  to  E_f    but the number of microstates availble for total energy E_f is less than the number of microstates available for total energy E_i    -  then you would seem to be on to something.)

Best Wishes.

[Spoiler:]

12

Physics, Astronomy & Cosmology / Why does free vortex flow have this velocity field?

« on: 22/11/2022 03:50:42 »

Hi.

   Anyone know of a good explanation / derivation of the free vortex velocity formula?

   V(r) =  k/r      ;     with V(r) = velocity of the fluid at radius r (and it will be tangential);  k = constant ;   r = radial co-ordinate.

   Just to formalise,  using   cylindrical co-ordinates (r, θ, z)   a free vortex around the z-axis  can be defined to be a fluid flow where the velocity field is given by:

          

   Animated image from Wikimedia.

That's all some texts do, just define it that way.   However some try and go a bit further and explain why this sort of velocity field would be reasonable from some elementary assumptions about what is happening.   What I'd like to see is a good derivation of that formula.

   I've been Googling the thing for a while now and not found much that is reliable.

This post is already long and the main thing is for me to ask you something - I would like a good derivation from some elementary assumptions.   I'll put one weak argument under a spoiler, you can see it if you want.  The post is not about me telling you stuff.   It's just that if you do find a derivation based on angular momentum, pause for a moment and see if it really is reasonable.   I'm always happy for any discussion.

Spoiler: show

A weak derivation can be presented as follows:
    You've got your fluid swirling around the z-axis to start with (you may have needed to drive it around with something to start this, that doesn't matter).   For the free vortex we consider the situation where the fluid has now been left for a while, so it is not driven and there is no net torque applied to the fluid.   The flow of fluid has adjusted and settled down to a "free vortex" instead of a "forced vortex" fluid flow.

    Most of the fluid moves in circular rings around the z-axis (that's either an assumption or you could physically go and observe it to check it's reasonable).   Then we consider the angular momentum of a particle of that fluid at radial position r from the z-axis   (so let's say we're using cylindrical co-ordinates   (r, θ, z) :    That particle of fluid has mass m and angular momentum  mrv  around the z-axis .   Specifically we can set   L = mr v(r) where v(r) is the (size of) the tangential velocity of the particle  (velocity in the direction).  We can argue that v = v(r) = function of r only because of the symmetry of the situation and that the entire velocity is (mainly) tangential from an earlier assumption.
     Typically, the particle just has velocity tangentially so it never moves towards or away from the z-axis BUT assume that for some reason (in the real world there's lots of slightly imperfect flows and random events), that particle actually does move towards or away from the z-axis a little bit, so that it's now at radial position R ≠ r .  It will have to acquire a new tangential velocity v(R) - if it didn't it will be bashed into by other particles at that radial position until it does acquire the same general velocity v(R).   (You could add an assumption here, we're only interested in smooth flow solutions, so the particle must acquire the velocity v(R) in some smooth way because we can't have any of this turbulent rubbish with particles crashing into each other).
    Then it has a new angular momentum   mR v(R)    with  R ≠r.   For a "free vortex" fluid flow we assume there is nothing driving the fluid around, specifically there is nothing giving an overall torque to the fluid.   So conservation of angular momentum should apply and we have   mR v(R)  =  mr v(r) = L = constant.   Specifically we have  mr v(r) = constant = L,  irrespective of the value of r.     Re-arrange this and you get   v(r) =  L/(mr)  =   k/r    for some constant k.

   -----
It's a weak argument / derivation of the velocity field but still the best I've been able to come up with by blending other arguments together.   It's weak for many reasons but one of the main ones is that the lack of an externally applied torque to the fluid as whole does not prevent an individual particle from getting some torque.  The particle can receive some torque from one ring of fluid provided that ring receives the same (but directionally opposite) amount of torque.   At best, the argument is just that if v(r) = k/r  then  no particle needs any torque when it moves inward or outward.   The conservation of angular momentum does nothing to prevent a particle actually getting some internally sourced torque if you wanted it and thus changing it's angular momentum as it moves to a new radius.

   
Best Wishes.

13

Just Chat! / HELP! LATEX isn't working.

« on: 17/11/2022 19:00:00 »

Hi.

    How do I get the symbol for an inexact differential?    I want a   symbol    d     with a bar through it.
Example:   this is       and   this is      which is "h bar".    I  want  a  "d bar"   please.

Google is telling me various options.    Here's an extract from Wikipedia....

In LaTeX the command "\rlap{\textrm{d}}{\bar{\phantom{w}}}" is an approximation or simply "\dj" for a dyet character, which needs the T1 encoding. 

   None of that works,   I'm guessing it's because the implementation of LATEX on this forum isn't set to use the T1 encoding.

     Another option is to define a symbol with code like this:

Code: [Select]

\def\dbar{{\mathchar’26\mkern-12mu d}}   and then you can call that code with  \dbar   whenever you need it.   That doesn't work because the LATEX implementation on the forum doesn't seem to tolerate any such special routines being defined.

Help!   If you can, please.

Best Wishes.

14

Just Chat! / Should we do more on the hot days to save the universe?

« on: 16/11/2022 02:12:39 »

Hi.

   So how much does the entropy of the universe change just when a roof tile falls off a roof that was 10 metres above the ground?   Specifically, the final state of our universe is taken to be when the tile has hit the floor and all energy has been dissipated in the usual ways.   The starting state was where the tile was at rest 10 meters up and let's say it was perched on the edge ready to fall off with the slightest bump or vibration - a vibration we can ignore for our purposes of energy considerations.    Other than the tile, there weren't any other significant changes in the universe over that time we need to consider  (I know that's unlikely - I'm just interested in the entropy change due ONLY to the tile falling because there's precisely nothing we can do about the entropy changes due to some far away astronomical phenomena but roof tiles, that's a different thing - we CAN make sure we don't do any roof tiling at certain times of the year).

    Anyway, this sort of thing gets mentioned in a few textbooks on Thermodynamics.    The change isn't a reversible process but we can reach the final state from the initial state through some series of slow and reversible changes without much trouble.   All we need to do is make sure we do have the same initial and final states.   So at the end we have a static tile on the floor and some heat released into the surroundings.   In the beginning we had a static tile in the air BUT it had potential energy  mgh =  10mg.   We can devise various reversible processes where we slowly lower the tile down and pass all the energy  mgh   to a heat reservoir.   For simplicity the environment locally to the tile,  let's say within a couple of miles of the tile can be taken as a suitable heat reservoir.  We can pass all the heat to that and the temperature of it doesn't really change.   That's also good enough for our requirement that the end state has the heat in the surroundings,  it's more or less all in the first couple of miles of the tile.   So we get a rough estimate for the change in entropy of the universe  =  ΔQ_rev / T   =   mgh / T.

   Now the thing is,  T appears in that formula and we took T to be the temperature of our local heat reservoir  (the local surroundings of the tile).   Does this mean that the entropy change of the universe is higher when the tile falls on a cold day and lower on a hot day?
   Should we save the universe from ultimate heat death by insisting that builders only undertake roof repairs on a hot day?   More generally, since every process we undertake ultimately delivers some heat to the environment, should we just restrict all major activities to the hot days?
   Anyway, first things first,  the change of entropy for the local area does seem to be dependant on the ambient temperature of the day.  That much I can go along with.  However, does this really hold out if you extend out to the entire universe and leave enough time for the heat to fully dissipate out to everywhere?   Is the entropy change of the universe really less if the tile falls on a day where the temperature local to the tile was high?
    (I don't know....  I've just been re-reading an old textbook and I never really stopped to question it carefully when I was younger).

Best Wishes.

15

Physics, Astronomy & Cosmology / Is there an experiment that shows the oscillation in the E field of light?

« on: 12/11/2022 20:00:33 »

Hi.

Does anyone know of an experiment that shows the oscillation in either the electric or magnetic field (or both) when a light ray travels through some place?

    For a radio wave, we can just put a metal rod (an antennae) in the path of the wave and show that an electrical signal is generated in the antenna.   We can argue that this is the result of forces from the electric field,   F = Ee and also the magnetic field (which is moving relative to the static charges in the rod, giving us some v>0 in the usual formula)  F = e (v x B) ).  There is plenty of equipment, like a good multimeter, that will directly show we get an electrical signal when it's at these sorts of radio frequencies.    Can we do anything like this with e-m radiation in the visible spectrum?   Is there an antenna that picks up light (rather than reflects it) and equipment fast enough to respond and show we have a rapidly oscillating electrical signal?

    It doesn't have to be an antenna.  Is there perhaps some experiment that has been done with a line of charges and then a laser beam was fired along the path of that line of charges?  The laser being used to ensure you get a nice ray of light which is all in phase.  (Hopefully causing some rapid oscillation in those charges that we could measure).   
 
    Anyway, is there any experiment that you know of that directly (or as directly as possible) shows the oscillation in the electric (or magnetic) field for e-m radiation when the frequency is up in the visible band (or higher)?
Is there some ultra-fine probes that we can attach to a multi-meter and just stick them into space and directly measure the oscillating E field as the light ray passes through?

Best Wishes.

16

Just Chat! / It's too quiet: I'm seeking a recommendation for a thread to read.

« on: 10/07/2022 00:29:45 »

Hi.

It's been too quiet for days now.
    I don't spend a lot of time looking over some of the sections in the forum and it would be worth broadening my horizons a bit.    Would someone be able to suggest an interesting forum thread in one of these sections:

1.   Technology.
2.   Geek Section.
3.   Geology, Paleontology, Archaeology.

     The thread shouldn't be more than 2 years old.  I'll just have a read through it to pass some time.   Please don't recommend a thread that is just a personal view of things, it should discuss something that could be found in the mainstream literature.

Thanks and Best Wishes.

17

Just Chat! / FAULT on the website?

« on: 30/06/2022 13:42:03 »

Hi.

The spell check feature seems to be freezing up and timing out.   Is that just me and my computer?

Best Wishes.

LATE EDITING:    It seems random or intermittent.   If you're persistant it will work.

18

Just Chat! / Test of the Poll system

« on: 22/06/2022 22:43:25 »

Hi.

This is a test poll.
Setting one up is simple.....
Instead of selecting   "New Topic" select   "New Poll".


New Poll.JPG (102.27 kB . 1335x750 - viewed 1725 times)

Best Wishes.

19

Just Chat! / Titles

« on: 04/06/2022 18:59:44 »

Hi.

   Can I make a protest,  request, strong suggestion or something like that?

As you post more often, your "title" changes.   It seems I'm about to become a  "Naked Science Forum King!"

I don't want to be a king!    and I don't want an exclamation mark after my name !!!!   Who came up with these titles?
Apparently the one above this again makes you a god.   I'm fairly sure that's not in your power to bestow upon someone.

What happens if you delete 100 posts you've made, would that work?

Is it time to let the users take part in a name changing survey or competition?   Let's have some sensible titles such as......
Newbie;    Member;    Jaded member;    Seen it all member;   Still alive member.       

Best Wishes.

20

Chemistry / Help with quoted enthalpy values, please.

« on: 04/06/2022 12:14:21 »

Hi.

Please can I also ask a minor question?

Reactions are often given a ΔH value in KJ/mol.     Now if the reaction is something like this:

3A₂  + 3B₂    →   2A₃B₃                 ΔH = -100 KJ/mol.

With A , B  =   arbitrary atoms.

Then what is that ΔH  value actually telling us?    There's nothing in that reaction that has a stoichiometric value of 1.    Is there actually 100 KJ of heat released for every  3 moles of  A₂  consumed?    Or equivalently 100 KJ  for every  2  moles of A₃B₃ produced?    Or are you supposed to  divide by 3 and say   there was  33.3 KJ of heat released per 1 mole of A₂ consumed,  or   50 KJ per 1 mol of A₃B₃ produced?      Anyway... you get the idea...   the quoted enthalpy value is supposed to be per mole  of what exactly?

Thanks and Best Wishes.