1

Just Chat! / Re: A Short puzzle with dogs.

« on: 14/05/2022 00:14:55 »

Each dog starts running towards the dog immediately anti-clockwise to it.

Apologies for not addressing the problem mathematically, but the experimental scientist knows that whilst small children follow a pursuit curve, dogs are actually better hunters and tend to run in a straight line towards a predicted intercept. Eventually, some children learn the trick and turn into good cricketers or footballers. David Beckham's exceptional ability as a midfielder was being able to make long passes to an intercept so that wingers and strikers could run on to the ball at full speed in a straight line.

Late edit: it's also the art of a fighter controller!

2

New Theories / Re: what is temperature?

« on: 13/05/2022 23:58:56 »

 Also "mesoscopic"?  Did you really mean middle-sized? 

somewhere between microscopic (where quantum effects add significant noise to your measurement) and astronomical (where simultaneity cannot be assumed). Usually refers to manageable things like bricks and cows. 

 

  Temperature is telling you about the average kinetic energy of a particle among a myriad of otherwise identical particles

They don't need to be identical. Porridge is inhomogeneous, but Goldilocks was able to measure its temperature. Temperature is the mean kinetic energy of all the particles in a sample.

3

Physics, Astronomy & Cosmology / Re: Is there a net heat exchange between water and ice at 0 degree C?

« on: 11/05/2022 00:16:26 »

However, it's best not to even try and consider "temperature" to be something that is sensibly defined at such small and local levels

...precisely because, like life, it is not a property of individual particles but of a very large ensemble of particles, i.e. a classical mesoscopic "body".

4

Just Chat! / Re: Free lectures

« on: 09/05/2022 23:28:24 »

Update. Today I attended a free seminar run by the International Organisation for Medical Physics. 3 lecturers presenting the latest hot stuff on radiation epidemiology, for anyone who cared to log in. Over 1000 attendees in (not from) every corner of the world, thanks to Zoom, and the whole show is freely available as a recording for anyone who found the time difference uncomfortable. So if the top-level professional gen is universally accessible at no cost, why not undergraduate teaching?

Clearly there's a difference between presenting your pet research project for an hour once a year and teaching the basic science  three times a week, so someone will need to be paid, but it's definitely time to reduce the number of undergraduate brick institutions and consolidate around a few genuinely universal courses. I can't see many lecturers voluntarily giving up their stipends for the greater good, but there's an opportunity for some really aggressive marketing by an educational supermarket moderated by a small consortium of established universities, to make a substantial killing with a convenience product..

5

New Theories / Re: what is temperature?

« on: 07/05/2022 16:51:41 »

As an example, an object absorbs 2 Joule of energy. 1 Joule is converted to potential energy, and 1 Joule is converted to kinetic energy.
Another object with same mass absorbs 2 Joule of energy. 0 Joule is converted to potential energy, and 2 Joules is converted to kinetic energy.
According to the definition above, the temperature of the object increases corresponding to the increase of kinetic energy. Hence the second object increases its temperature twice as much as the first object.

Correct.

An increase of internal potential energy would correspond to a partial or total change of state within the body.

I encountered this when measuring radiation dose with a calorimeter. Dose is defined as energy aborbed per unit mass, and the principal concern for radiation protection and radiotherapy is the measurement of dose to water. For practical simplicity most primary standard calorimeters use graphite as the absorber because it is mechanically stable and has about a tenth of the specific heat capacity of water so undergoes a larger temperature change (a lethal dose of ionising radiation raises your body temperature by about 0.001 degree - my task was to measure that to ± 10^-6K). One of my colleagues built a water calorimeter - rather less portable device but clearly worth directly measuring the quantity of interest rather than trying to derive it. Problem was that the water calorimeter generally measured about 3% less than the graphite calorimeter, though both were calibrated to  ± 0.01%. I thought the difference was due to "virgin" water forming metastable polymers when irradiated, because the defect gradually decreased with extended irradiation to high doses but later work has revealed all sorts of complex chemistry possible with just H and O atoms and plenty of energetic photons.

6

Just Chat! / Re: Free lectures

« on: 01/05/2022 08:12:34 »

Worth looking at the Open University (UK) which has been delivering high quality distance learning for 50 years, including home science kits and televised lectures (pre-internet!) with a dedicated cadre of "local" tutors - including telephone tutorials for the remotest of students (pre-Zoom!). I was involved in a campaign to establish a medical course which was scotched by the problem of distributing corpses for home dissection, but a number of traditional universities have since experimented with "no dissection" undergraduate courses and the OU itself now offers nursing and health science degrees.

The social element of the OU became somewhat notorious with very intense summer schools in which "mature" students managed to cram a whole year's revision and debauchery into two memorable weeks. Definitely character-forming, and I think I learned some useful mathematics.

7

General Science / Re: Is 2 really prime? If so, why isn't 1?

« on: 29/04/2022 23:03:03 »

All primes are odd
Except 2
Which is odd, because it's even.

8

Technology / Re: Are electric cars responsible for natural gas demand?

« on: 25/04/2022 23:25:32 »

Renault issued press statements reaffirming their vehicles' compliance with all regulations and legislation for the markets in which they operate in 2015.

Note the careful wording. The entire "scandal" is a case of smart-arse legislators being caught by their own cleverness and complaining about it. If you prescribe a very specific acceptability test for a product with a wide range of possible performance, you have no right to complain if the product meets that specification but no other. Here's an old story that gets trotted out at purchaser specification meetings:

Man wants to make a new door frame. This usually consists of about  16 pieces of wood of specific thicknesses, cut to mitre angles. So our hero calculates what he wants and specifies each piece to  ± 0.05 mm and ± 0.05°. Hands specification sheet to a timber merchant who promises to have it ready in 24 hours. Next day he collects the bundle and is horrified to discover it is composed of bits of knotty pine, green ash, plywood, willow, and whatever else the good Lord chooses to call a tree, twisted, crossgrain, sawn, bodged..... Shop assistant produces a ruler, shows that each piece of crap is exactly the size and shape specified, and demands payment.

As he wonders how to complain, a woman walks in and presents a ticket. Assistant hands over a beautiful piece of polished seasoned oak, 8 x 24  x 1 inch, with radiused edges and a neat hole in each corner. Bloke says "what did she order?" Shop assistant says "a seat for her kids' swing".

The moral: tell the manufacturer what you want, not how clever you are.

9

New Theories / Re: what is temperature?

« on: 03/04/2022 23:10:35 »

Is temperature a quantized value?

No.

10

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

« on: 30/03/2022 18:12:22 »

Every scientist I've worked with called me an engineer, and every engineer called me a scientist. The business of medical physics involves me in matters of law and ethics, with a bit of biology so I can understand the problem that the medics want solved with a machine. 

The plane takes me to work and the job pays for the plane. What could be better? A double bass that fits inside an old Cessna, and a weightless bass amplifier!   

Final ambitions 
eradicate Kruger-Dunning syndrome from statutory inspectorates
teach journalists that force, energy, power and strength are not the same thing
teach everyone else that "quantum energy field" is a symptom of bullshit and Heisenberg's Ungenauigkeit is best
      translated as indeterminacy, not uncertainty
to die on stage at the Westport Jazz Festival, just after my solo on "All Blues".

11

Physiology & Medicine / Re: Is sex better in the morning or evening?

« on: 26/03/2022 15:55:20 »

Points taken. I'm inclined to be liberal but this is a public warning to Pseudoscience: please stop behaving like a 13-year-old. 

12

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

« on: 20/03/2022 15:31:24 »

Looks like a perfect solution to me.

13

Just Chat! / Re: Is Nuclear War imminent?

« on: 16/03/2022 16:58:32 »

It is rapidly becoming apparent that the nuclear "deterrent" only deters those who could best help an innocent third party to repel an invader, from doing so.

14

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

« on: 11/03/2022 10:55:22 »

Just bashing the electron into a target is technically simple up to about 15 MeV but not very efficient: you produce a broad spectrum of x-rays, most of which you don't want, and a lot of heat, none of which is useful.

At higher energies you use various forms of magnetic wiggler or undulator. Remember a photon is emitted when an electron accelerates, so if you keep the speed constant but bend the track, you will generate photons tangential to the bend. A good wiggler or undulator will give you a very intense beam of near-monochromatic x-rays.

15

Just Chat! / Re: Is there a universal moral standard?

« on: 06/03/2022 14:43:00 »

Choosing whom to kill would raise some moral dilemmas. Merely ridding the world of priests, politicians and philosophers would make life more peaceful but wouldn't have much impact on sustainability. You might then start on pensioners, but we have a limited life span anyway, and have contributed to the public good through our taxes and investments, so we should be allowed to enjoy our pensions. Companies that rashly introduced early retirement to save money on the short term often found that, within a year or two, everyone who knew anything useful or had acquired a significant skill, had disappeared. 

But every baby is a net consumer for about the next 20 years, without having contributed anything. So a baby not born is a significant exchequer saving and improvement in the future quality of life for those already here.

Killing people takes effort and organisation, and doing it on a big scale can pose problems disposing of the bodies. Not making babies is the perfect "do nothing" option, with no waste product.

16

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

« on: 24/02/2022 17:32:42 »

If you (alancalverd) get a moment, is there a useful link to some information about how the latest high energy gamma rays are actually generated?   X-rays are easy enough... just accelerate some electrons into a tungsten target inside an x-ray tube - but can you really push those frequencies up higher to the gamma range just by increasing the velocity of those electrons?  Are your machines producing medical gamma rays directly from nuclear decay these days?   Even if they are produced directly from nuclear decay, where are you (we) going to get even higher frequency gamma rays from after that?

Sorry I missed the question earlier!
Nuclear decay isn't terribly useful for tele-radiotherapy applications. By the time you have enough  gamma flux to be useful, you have a very large radionuclide source that you can't switch off, can't modify the spectrum, and its output is continually decreasing and probably being contaminated with decay product gammas and betas. AFAIK all existing 1.3 MeV cobalt teletherapy units (apart from a few exotic "gamma knife" systems) are being phased out.

Linear accelerators are very useful up to about 15 MeV, producing a large controllable flux of electrons which we do indeed bash into a target to make x-rays (same as gammas, but man-made rather than of nuclear origin).

Betatrons can produce small fluxes of 300 MeV electrons and x-rays which are useful for industrial radiography, and synchrotrons currently go up to 15 TeV (CERN LHC source) but I'm unaware of any routine medical use of  photons above 15 MeV.

17

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

« on: 21/02/2022 22:33:59 »

At the lower end, there is a limit to the energy of a photon we can observe at Earth's surface,

True-ish of cosmic radiation, but we've been generating bucketloads of 15 MeV photons for radiotherapy for as long as I have been in the business, and AFAIK work is proceeding on a high-brightness 6 GeV source in China.

18

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

« on: 21/02/2022 11:05:41 »

The Planck length has no physical significance apart from being, theoretically,  the shortest length of an object that can be measured with a photon. That doesn't imply the converse that you can't have a photon with a shorter wavelength.

19

The Environment / Re: Is hydrogen a better fuel source for the environment?

« on: 15/02/2022 23:20:24 »

That is 3 times the drag for the same distance at 3 times the speed, that is very convenient.

No, it is 9 times the drag force, over the same distance. Energy = force x distance.

20

Plant Sciences, Zoology & Evolution / Re: Why does an earthworm look the same on both ends?

« on: 27/01/2022 22:22:21 »

Relax, chaps! The story was amusing but sadly has fallen among pedants.

It's easy to criticise Cronin for being unobservant, but at the same time she should be applauded for entertaining at least one real kid and exercising the minds of three boys who never grew up (every scientist is a Peter Pan at heart) in this forum.

And full marks to said real kid for asking a penetrating question, plus a star for Dad's effort: the bilateral near-symmetry  of animals gave Alan Turing much food for thought.

IIRC earthworms always tunnel forwards but make a vertical U-turn to bring their heads to the surface to eat.

Meanwhile I heard a great line from a woman interviewed in a serious (Radio 4, anyway) program about twins. She said "When I get dressed  to go out I think "I could be really beautiful", then I look at my sister and realise I couldn't.