1

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.

2

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.

3

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 720 times)

Best Wishes.

4

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.

5

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.

6

Radio Show & Podcast Feedback / Feedback on: Question Of The Week in Podcast vs Forum

« on: 23/05/2022 20:19:38 »

Hi.

  Sometimes I wonder if something strange is happening.
There's a short gap (maybe 10 minutes?) between releasing a podcast with the answer and asking the question on the forum.     Sometimes the question precedes the answer but not always.   It's a bit random.

   What seems less random is that putting the question in the forum is almost a  complete waste of everyones time.   @chiralSPO has just spent time creating a well formatted and detailed reply.   The radioshow / podcast people paid it no attention and ultimately it has done nothing more than repeat the podcast.   Even if the person who asked the question does join the forum - which only happens about 1 time in 20 - that reply has wasted their time in addition to Chiral's time.

    I've asked what the point of running these "Questions of the Week" is before,  so I'm not even going to bother asking it again.   All I might do is offer some advice and sympathy to @chiralSPO  -  always look for the podcast first before you spend your own time.   If the podcast isn't there yet, wait 10 minutes.    It was a good answer @chiralSPO  but all you're likely to get is a "thanks" icon at the end of your post from people like me.

Reference:
https://www.thenakedscientists.com/podcasts/question-week/can-acid-remove-springs-potential-energy
   Officially released  23 May 2022.

Best Wishes.

7

Physics, Astronomy & Cosmology / Does a gyroscope still fall in opposition to the direction of the missing force?

« on: 18/05/2022 12:01:08 »

Hi.

    I was looking through some of the old Feynman lectures and came across this section which attempts to explain what makes a gyroscope start to precess instead of falling.   Just to be clear, it does not suggest this is what sustains precession, just what starts it.   You have a situation with a spinning gyro supported on a pedestal at the centre and you are holding the (soon to be) free end stationary with your hand until it is suddenly released.

gyro.JPG (28.45 kB . 583x312 - viewed 635 times)

This is what the lecture states:
     Some people like to say that when one exerts a torque on a gyroscope, it turns and it precesses, and that the torque produces the precession. ........it does not fall under the action of gravity, but moves sidewise instead!  .....( but ).........
    .....The gyro actually does fall, as we would expect. But as soon as it falls, it is then turning, and if this turning were to continue, a torque would be required. In the absence of a torque in this direction, the gyro begins to “fall” in the direction opposite that of the missing force.....

[End of section 20-3,  "Rotation in Space", Feynman lectures.
https://www.feynmanlectures.caltech.edu/I_20.html ]

      I can't be the only one who finds the idea of a "missing force" a bit odd.   To say that things accelerate in the direction of a net force is one thing,   to say that they would accelerate in the opposite direction to a missing force is much harder to live with.
     There's a computer on my desk and it is missing a force to move it left but it does not accelerate to the right.

    Anyway, is this explanation with "missing force" still popular?   At the moment I'm inclined not to recommend that explanation.   Maybe Feynman tried to oversimplify and has ended up with something that isn't all that usefull for understanding?   i.d.k.   I do note that he has quotation marks around "fall" as if he was just making an analogy to objects falling when they are missing a force to support them. 

Best Wishes.

LATE EDITING:  Fixed various spelling errors.   Title changed from "in the direction" to "in opposition to" etc.

8

Physics, Astronomy & Cosmology / How do you move an atom with an atomic force microscope?

« on: 06/05/2022 21:52:08 »

Hi.

    Moving atoms is often done with a STM (Scanning tunneling microscope).   There are several references for that.

Apparently you can also do this with an AFM  (Atomic Force Microscope).   Does anyone know a good reference for that or could briefly explain how it is done?   When I say "how" I also mean "why" it works. 

    Some low-quality references suggest there is a need to oscillate the probe tip to press down on the atom while simultaneously sweeping slightly in the direction you wish the atom to move.   By "sweeping" I mean something like using a dustpan and brush -  lift the tip slightly at the right and move back to the left, move the tip down slightly while moving left to right, repeat until the atom is moved.   "pushing down" seems difficult:  For example, If you have something like a Lennard-Jones potential determining the force between the probe tip and atom, how do you even get a downward force without getting so close to the atom that you are effectively in direct contact?   i.d.k.

    There's this reference in Nature but the article is paywalled and I'm not in a rush to buy it.   I might see if I can find it in a library sometime soon.
https://www.nature.com/articles/nnano.2009.347

Best Wishes.

[Spoiler:]

9

Physics, Astronomy & Cosmology / Does charge contribute to mass?

« on: 02/05/2022 17:26:11 »

Hi.

   I've been reading a bit more stuff about classical elctro-magnetic fields.   There is supposed to be energy contained within a E field  (and a B field,  or both together).   An alternative way of describing the same energy can also often be made.  For example if two charges are in close proximity, then you can describe the energy as being stored "in" the separated charges.

   Anyway, I'm once again interested in where this energy really is and how it behaves:

   If a particle has a charge then it creates an electric field.  If that energy is in the particle, then it would probably contribute to the total mass of the particle.    On the other hand if that energy is actually spread out over all space and contained in the electric field then it may not contribute to the mass of the particle.
   Hence, the main question:   Does charge contribute to mass?

Examples:   1.   There seems to be a difference in mass between electrons and neutrinos, although that's complicated because neutrinos aren't exactly uncharged versions of electrons.
     2.   Imagine a small spherical conductor, which we will consider as our "particle".    Rub the thing with some fur to put a charge on it.   Has the mass of the spherical conductor changed?  Will it have more inertia?   Is this change purely because several thousand electrons were added to the spherical conductor and each electron had mass?   Let's try a similar idea -  transfer 1800 electrons to the conductor and remove 1 proton from it    (i.e. the mass of electrons as you would measure it if you separated them, you know - took them off to infinity and then measured the mass there - should match the mass of the proton when similarly separated).   Anyway, the idea being that the overall charge on the spherical conductor should now be   -1801    ( -1800 from the electrons and  -1 by removing a proton),  so it is charged but the separately measured mass of what you've added and taken away shows no overall change.   Would there still be a change in the mass of the spherical conductor?

Best Wishes.


Some references, if you want them.   I'm going to hide them under this spoiler because it helps to keep the main post short:
   

Spoiler: show

 I was getting my information from the textbook  "Electrodynamics" by Griffiths    and also from a lecture series produced by a university.   I've had discussions on other threads about how wrong it is that lectures aren't made freely available - but there we go.   Neither the textbook or the lectures may be freely available to general readers of this forum.   I've found the references below which are freely available on the internet and illustrate the main points:

     https://phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Physics_9C__Electricity_and_Magnetism/2%3A_Electrostatic_Energy/2.1%3A_Potential_Energy_of_Charge_Assembly
    Shows an example of determining the energy required to assemble a configuration of charges  (they consider assembling a sphere of charge Q by progressively bringing more charge from infinity to the sphere located at the origin.    This is described as potential energy that the system has,  i.e. it is energy that the assembled sphere has.   As such you might reasonably expect the sphere to have some mass component that is due to this energy.
   
http://labman.phys.utk.edu/phys222core/modules/m6/field%20energy.html
    Discusses energy stored in the electric and magnetic fields and derive the usual result:
 (Energy density stored in the electric field at a point in space) is proportional to  (the square of the Electric field at that point of space).
   With phrases such as this being used without apology or ambiguity:
  In electrostatics, viewing the energy as being stored in the separated charges or viewing it as being stored in the electric field leads to the same results.  We are allowed to take either point of view....
  In magnetostatics, viewing the energy as being stored in the circulating currents or viewing it as being stored in the magnetic field leads to the same results.  We are allowed to take either point of view....

Best Wishes.

10

Just Chat! / Free lectures

« on: 29/04/2022 20:05:44 »

Hi.

     Why aren't all Universities making their lectures freely available online?

1.    Recently most universities have been dealing with Covid issues and thus recording videos of lectures which are then distributed to the students by ... whatever... a Moodle system for example.... so the students can watch from their own homes.
     So they can't simply say "we don't have this on video", most of them do.  They aren't old and they are of good quality.   Most Universities spent money getting good video technology when they realised they would have to start doing this sort of thing.

2.     Several notable universities have already released some complete lectures on You Tube.   Many of these do form a complete lecture series.  It didn't seem to hurt those universities too much and may even have promoted those institutions.

3.      There are obvious benefits of providing freely accessible educational material to the general public.

- - - - - - - - - -
     I think a lot of universities are now taking the idea of getting money from students too seriously.   They hold onto their academic material and consider the student to be effectively buying it from them.  This seems unnecessary, the universities will still be providing the following things only to students that have paid:
    (i)   Small group and/or individual contact with staff,   e.g.   seminars and tutorial sessions.
    (ii)   Physical access to equipment and practical sessions.   For example, a library, a laboratory or a ballet hall with hand rails and mirrors.
    (iii)   The right to have an examination,  to get this marked and ultimately to be awarded with a qualification that is recognised.

     I don't think Universities will lose that many potential students just by releasing their lectures online.  Let's be honest, most students really need the paper qualification.  So why, why oh why, are the universities actually getting in the way of a few people who do just want the information and education?  Why are they trying to keep their lectures as something that must be bought and paid for?
    There should be no or negligible cost to the university to release their videos on to You Tube.   They can even collect the advertisement revenue (if any) if they want to.

Best Wishes.

11

Just Chat! / There's a section with "similar topics" appearing.

« on: 13/04/2022 01:42:46 »

Hi.

   I'm not a very observant person but it's come to my attention that after each thread there's a section that claims to identify "similar topics".   
1.   Is that new or was I really not paying attention?
2.   Does anyone know what criteria are used to identify similar topics?

Here's an example:
    Original post:    "How do you calculate the capacitance of two unusually shaped plates?"   which, naturally enough was about the capacitance of two plates.

    The Similar topics suggested included:
a)   "Can I Use Washing Up Liquid To Wash Hands And Hand Soap To Wash Plates?"
b)   "How long must a prism-shaped corridor be to render a light-source invisible?"

My best guess is that an algorithm is matching some words like "shaped" and "plates" that appeared in the title.

Best Wishes.

LATE EDITING:     ?   It doesn't seem to happen in the "Just Chat" section.    Check out the main sections to see what I mean.

12

Just Chat! / A Short puzzle with dogs.

« on: 12/04/2022 15:20:23 »

Hi.

   Are you bored?  Do you want a short puzzle?  No?  Well here's one anyway.   
It doesn't require much mathematics, I reckon someone who's studied some maths or physics at post 16 years of age should have all they need to solve it.
- - - - - - - - - - - -

Four dogs are in four corners of a square of side length 1. Each dog starts running towards the dog immediately anti-clockwise to it. The dogs start at the same time, they all run at the same speed, and at every moment each dog is running directly towards the neighbouring dog.

During the pursuit, the dogs will run in a spiral before they all meet in the centre. How far does each dog travel before the group collision?

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

Puzzle taken from an article published in the Guardian:   https://www.theguardian.com/science/2019/jun/03/can-you-solve-it-dogs-in-pursuit
   BUT DON'T FOLLOW that link unless you want to see the solution.   

Best Wishes.

13

Physics, Astronomy & Cosmology / How does a red-shift affect a black body spectrum?

« on: 05/04/2022 01:31:29 »

Hi.
 
   Not urgent, just curious and it's too late to grab a pencil and paper.   Someone might already know the answer for this anyway, so I'll just ask.

   How does a red shift (or blue shift) affect the radiation spectrum from a black body? 

Basic Question        Consider a Doppler shift (at relativistic speeds or just Newtonian if you prefer)
    A Black Body which is at rest and at temperature T₁  produces radiation with a known distribution of intensity at different wavelengths  ( a black body spectrum ).
    An observer is in a state of uniform motion with velocity  v≠0  relative to the black body.
    In the rest frame of the observer, is the radiation they receive from the black body still going to have the right distribution to be consistent with a Black body spectrum but just with a different temperature T₂? 

Best Wishes.

14

Just Chat! / What is your main area of interest or expertise?

« on: 30/03/2022 12:50:26 »

Hi.
      There are only about a dozen people who use this forum and half of those are moderators.  What are your main areas of interest or expertise?
     Why am I asking?  Just interested really.   People, especially the moderators, spend hours contributing stuff and answering questions. 

Examples:    Alancalverd  --->  Probably studied practical and experimental physics at University and didn't specialise in Engineering until the last year.  Not shy about declaring their political views.   Has declared experience setting up medical physics equipment (NMR scanners etc.) and flies (aeroplanes).

    It's not just moderators who use the site but mentioning others is unreasonable.    However, if you're happy and well aware of the security and safety issues about putting info down on the web, then write your piece.   There's nothing wrong with staying quiet.  Let's cut down on the amount of stuff before it gets out of hand:  If you can't read it in 1 minute it's too long and let's say you're only a regular if you've made 40 posts.

Best Wishes.

15

Physics, Astronomy & Cosmology / How do you calaculate the capacitance of two unsually shaped plates?

« on: 20/03/2022 13:24:57 »

Hi.

DECLARATION:   This is a homework question from a Physics book.   It's not my homework, I'm trying to help someone else but wouldn't want to advise them badly.   The minor issue is that I never studied electronics beyond school and I'm just going about this with some concepts of electrostatics.

    I'm looking for ideas on how to calculate the capacitance of the following object:

Two conductive plates of size   2cm x 1cm    ( x-axis length   by   y-axis length)    but there's a step in the thickness (z-axis measurement) of the plates at the location  x-axis = 1 cm.   The plates are separated with an air gap of  1mm   at one end   and   3mm at the other end.   See the diagram below.   Note the different units of measurement:   Lengths in centimetres but gaps in millimetres.

This is what I was going to suggest they did:
  One approach is to cut the diagram in half, consider it as two different capacitors each capacitor being a pair of conventional rectangular plates   1cm x 1cm.   
  The first capacitor having a gap of 1mm and the second having a gap of 3mm.    Use the usual estimate for parallel plates:   Capacitance, C =  ε . A/d.     Then find the total capacitance as usual   C  = C₁  + C₂ .     These (conceptually cut apart) capacitors are at the same voltage since the plates were a conductive material, so the conceptually separated capacitors are effectively connected in parallel .   
    Is that going to be a reasonable approximation, or does someone else have a better idea?
    This is a 5 mark question, so really shouldn't take more than 10 minutes under any circumstances.

Best Wishes.

16

Physics, Astronomy & Cosmology / Why does luminosity vary predictably in Cepheid variable stars?

« on: 13/03/2022 01:04:59 »

Hi.

    I'm looking for references to explain why the Luminosity of a Cepheid has the relationship it has with the period of pulsation.

   Wikipedia has plenty of talk about the data and the fact there is such a relationship but only limited waffle about the why.

   Some on-line reference would be great, although I think I've got a very old copy of   "An introduction to modern astrophysics", Carroll and Ostlie where I can find it.   I'll have a glance at that if no one has a better suggestion.

.... Actually I've had 10 minutes to look through Carroll and Ostlie before finishing writing this post.   They seem to be explaining the Luminosity change by temperature variations.... oh and pages of stuff... and  Helium ionisation gets a mention...  and more pages of stuff...

   Well,  before I spend hours ploughing through Carroll and Ostlie, I'd still be keen to seek alternative references.

Best Wishes.

17

Physics, Astronomy & Cosmology / How do you determine if a galaxy has S0 morphology?

« on: 08/03/2022 23:03:45 »

Hi.

    How do you determine if a galaxy has S0 morphology (but is seen head on) and not E0 morphology (seen from most angles)?   
    Presumably, it's not in our power to spin the galaxy around and check if you're just seeing it from certain angle.
   
My best guess:   S0 or    "Lenticular" galaxies tend to have bright central bulges but E0 galaxies less so.    Anyway, I don't know and would be grateful for anyone's gudiance or opinion.

Best Wishes.

18

Chemistry / How well understood is the Chemistry of the trans-uranic elements?

« on: 27/02/2022 23:47:34 »

Hi.

The Chemistry section never seems to get a lot of questions.   So let's just ask a brief question:

Is the Chemistry of the trans-uranic elements  well understood?     Are they just too unstable and haven't been studied much?

For example, Carbon based Chemistry is fairly complicated and people make a living studying the chemistry just of Carbon.    (I suppose you do throw in a bit of  H,  O  and maybe some N   but by and large Organic Chemisty is just about C).   You have all the usual arguments about how the chemistry is diverse enough to support life.  By comparsion,  Silicon, doesn't seem to support anything like as many molecular structures and would be a poor choice of something to base life on.   There's nothing much in the first few periods on the table that seems to support the diversity of chemistry that carbon can.

So, what about some of the really heavy elements?  How well do we know their chemistry?   Could there be a heavy element with Chemisry as diverse as carbon?

Also, what about this metal and non-metal stepping line anyway?   Is it certain that all heavy elements behave like metals?   On what basis do we decide something is a metal (other than just being on one side of the stepping line or the other).     I'm mainly asking because I'm suspicious the first few answers will be that elements on the left of the stepping line don't support covalent bonds very well.  How much of this is well grounded in theory as opposed to being speculation based on apparent patterns in the periodic table?

Summary:   How well do we know the chemistry of the heavy elements?   Could any trans-uranic elements have chemistry as complicated as Carbon?   

Best Wishes.

19

Just Chat! / Is there a maximum frequency for a gamma ray?

« on: 21/02/2022 04:39:49 »

Hi.

This is the Just Chat section, so this is all informal.   It's just a quiet evening and this has been worrying me:
 
Is there a maximum frequency for a photon?    (I might call this thing a gamma ray elsewhere in this post).

   There's a few reasons why you might think the frequency of EM radiation is limited,  here's a few to start with:

   1. Perhaps the wavelength can't meaningfully be less than 1 Planck length.
   2. There should be some sort of energy density in space associated with a photon.   If you keep pushing up the frequency then you have enough energy density to create a micro black hole.
   3.  Perhaps EM radiation is always ultimately generated as a result of charged particles like electrons moving and they can't be made to oscillate arbitrarily fast.
   4.   Quantum stuff instead of classical:   Maybe photons always arise as a result of electrons changing orbitals around an atom or nucleons re-arranging themselves in a similar way inside the nucleus.   Even the largest of jumps  (from say n= ∞ to n=1) is still a finite energy change and there are only a handfull of different sub-atomic particles and  only a finite number of ways of putting them together to make something with these energy states.
   5.   Some other reason.

Anyway, that's the first question:   Is there an upper limit on the frequency of a gamma ray?
If you think that there could be (I'm about 71%  inclined that way), then the follow up question has to be......
What if you change reference frames?   Give yourself a boost to another frame and watch for the Doppler shift   What's broken? 

Best Wishes to everyone.

20

Physics, Astronomy & Cosmology / How did Astronomers measure the size of the Milky Way Galaxy?

« on: 16/02/2022 17:51:08 »

Hi.

    I'm a bit lazy, so instead of using Google, I thought I'd ask for some pointers here.

How did Astronomers measure the size and general shape of the Milky Way Galaxy?

1.  Parallax will help with some close stars.  Earth orbits around the sun in 1 year and you get some parallax... I know that bit.

2.   I believe they were also using Cepheid variables to estimate some these distances... I know that bit.

However the following issues seem to confound the exercise:
1.    What if the galaxy had an outer edge that is just extremely deficient in Cepheids?   Do you just not measure the distance to that bit or just not include any of those stars in the galaxy?  Is it just that Cepheids are sufficiently common that was unlikely to happen?
2.    Where and for what reason do you decide the galaxy has ended?    If you get a better set of telescopes and start finding more Cepheids, why or how do you decide they aren't in our galaxy?    Is it just that there is a clear border, some big void where stars aren't found and it's just convenient to say the galaxy ends there?
3.    Dark matter is the new confounding problem.   There seems to be some in a halo around our galaxy but, by definition, you can't see it.  Annoyingly it seems to extend a bit further out beyond the visible stars in our galaxy.  None the less it's presumably important for contributing to the how and why this galaxy formed originally.  It's also presumably gravitationally bound by the mass in this galaxy and stays with it like most of the other things that we decide are in this galaxy system.   We probably should include some of that dark matter halo as being something that is in this galaxy rather than some other galaxy.  How do we now extend the definition or size estimates for the galaxy to include some of that dark matter?

    That's it....  so another way of stating the question would be something like this:   How is the border or edge of a galaxy decided upon?  How do you know a galaxy has ended?  For what reason, both historically and with the benefit of our modern understanding, would you say a galaxy ends here -->      rather then     here --------->     ?

I'd be grateful for any answers or references.
Best Wishes.