Wikipedia:Reference desk/Archives/Science/2021 May 12

= May 12 =

Is Van de Graaff generator invented for funny hair stick out purposes?
Is Van de Graaff generator invented for funny hair stick out purposes? Rizosome (talk) 00:12, 12 May 2021 (UTC)
 * I'm sorry, but I don't think we should be answering your question until you indicate understanding of many past answers we have given you, ranging from speed vs velocity, whether batteries do or do not store electric charge, and many more. Can you respond to some of these past answers we have given you before asking us yet more questions that, honestly, you can likely answer just by looking at the articles that you yourself link to? I do not, for a second, take this question of yours seriously. The second paraphraph, in the very top of the article you have linked to, specifically addresses why it was invented. Read it. Don't waste our time with questions easily answered by a basic read of even the introduction to articles that you clearly know how to find, since you link to them. --OuroborosCobra (talk) 00:32, 12 May 2021 (UTC)
 * Looking at Rizosome's it appears that he's WP:NOTHERE and, even if he was, WP:CIR. I suggest we nip this in the bud. nagualdesign 01:03, 12 May 2021 (UTC)

Is Synchrotron latest Particle accelerator around?
Van de Graaff generator superseded by cyclotron is superseded by Synchrotron. Rizosome (talk) 11:16, 12 May 2021 (UTC)
 * Most modern accelerators are built on the synchrotron model. Wikipedia's List of accelerators in particle physics will help you work out the timeline of development for yourself.  -- Jayron 32 12:47, 12 May 2021 (UTC)

What am I misunderstanding (Heat pumps and Stirling engines)?
I saw an (obviously impossible) suggestion that a heat pump and a sterling engine could be combined to produce a perpetual motion machine. The obvious flaw is that it would take more power to push the heat back through the heat pump than you'd get from using the heat to run the sterling engine. So I thought I'd check some figures. I looked at The heat pump article which shows 5.0 efficiency (Ground source heat pump (GSHP), water at 0 °C heating to 35 C). From this I concluded that a sterling engine could get no more than 1/5th of the energy back, i.e. max 20% efficiency. However the Stirling_engine article says "The thermal efficiency is also comparable (for small engines), ranging from 15% to 30%". At the upper range this would make the perpetual possible - which I know it isn't so what have I misunderstood -- Q Chris (talk) 13:00, 12 May 2021 (UTC)
 * How about plain old fashioned friction losses in whatever mechanism that couples the engine and pump to each other - pulley and belt, gears, etc, none are frictionless. Roger (Dodger67) (talk) 13:10, 12 May 2021 (UTC)
 * No that won't do. It should be impossible to have a theoretical maximum energy output from a closed cycle greater than zero. You can't say "in theory we could have this running for ever and take energy out of it. -- Q Chris (talk) 13:14, 12 May 2021 (UTC)
 * What about heat loss through radiation. Any object above absolute zero in temperature radiates energy away; for most room-temperature objects this is often in the infrared range, but those photons carry energy away from anything, and there's really nothing you can do to stop that.  You can only slow it down.  The second law of thermodynamics is a mean bitch.  Really, you need to treat the first law of thermodynamics axiomatically.  There's lots of good reasons to do so, especially Noether's theorem, which establishes that the first law is an inevitable result of time invariance; if you run an experiment today and make measurements, it should be indistinguishable if you ran the exact same experiment under the exact same conditions next week or next month or a million years from now.  If that is true, then conservation of energy must also be true, and if conservation of energy is true, then any closed system cannot be exergonic without losing internal energy, QED.  -- Jayron 32 13:22, 12 May 2021 (UTC)
 * However it looks like you can have a heat pump with a 500% efficient powered by an Stirling engine with 30% efficiency, giving a 50% energy surplus to the cycle. I know that you can't, and to say that it radiates heat away doesn't really help because then you say you have a closed cycle producing heat from nothing. I am sure there must be a fundamental error in my understanding rather that just not being able to make something work in practice, a 50% surpluss is a lot of energy. -- Q Chris (talk) 14:20, 12 May 2021 (UTC)
 * A 5.0 Coefficient of performance does not mean it is 500% efficient really. That would mean that it could convert 1 joule of work input into 5 joules of work output which is already impossible.  The heat energy the pump is moving from one place to another is external to the system; the COP numbers are basically taking how much internal work the heat pump does to move a given amount of heat energy outside the heat pump from one locale to another.  It's not fundamentally different than asking "How much work do I need to do to move a battery from the shelf to my device" and then taking the energy output of the battery and dividing those two numbers.  That's not really a %efficiency rating.  A %efficiency rating for the heat pump would be "how much work does the heat pump do for a given input of energy into the heat pump".  That number is less than 100%.  That calculation would basically require you to figure out how many kWh of electricity the pump used and then calculated the work that your compressor did over a given period of time.  That number is well less than 100%; obviously as the compressor motor loses heat to the environment exactly like any other motor would.  -- Jayron 32 17:37, 12 May 2021 (UTC)
 * Your question is a very good “sanity check” on the efficiency numbers, and I think your reasoning is correct. The missing piece of information is that the efficiency isn’t a fixed number that’s always the same for the same heat pump or engine. It depends on the reservoir temperatures. A perfect Stirling engine will have the efficiency of the Carnot cycle, η=1-Tc/Th. To make your closed cycle work, you need the Stirling engine to run on the same 0°C and 35°C reservoirs that gave you a heat pump with 5.0 coefficient of performance. But plugging in those temperatures gives η=11.4% —- not high enough for perpetual motion. To get such a high efficiency on the engine, you need a bigger temperature difference. But that will require more power to run the heat pump (lower coefficient of performance). Or you can hook the heat pump and the engine up to different reservoirs at different temperatures —- and now you are aren’t putting the heat back where you got it from. —Amble (talk) 15:54, 12 May 2021 (UTC)
 * From Coefficient of performance, the theoretical limit for heating is COP = Th / (Th - Tc). The ideal Stirling engine has Carnot efficiency η=1-Tc/Th = (Th - Tc) / Th. These are reciprocal to each other. Multiplying them together -- for the same reservoir temperatures -- will always give you unity for an ideal case, or < 1 in reality. --Amble (talk) 17:56, 12 May 2021 (UTC)
 * Thanks that makes a lot of sense - I hadn't realised that efficiency depended on temperature difference. The balancing of the equations means that a heat-pump/Stirling combination becomes like many other "perpetual motion machines", flywheels and pendulums - in theory with no loss they would self-sustain but no energy can be taken out. -- Q Chris (talk) 15:21, 13 May 2021 (UTC)
 * There's the final piece. I was looking for a way to say just that myself; but ended up going off on other tangents.  -- Jayron 32 18:23, 12 May 2021 (UTC)


 * You need 3 reservoirs, not 2 for a heat pump to be powered by a Stirling engine. It is possible to build a house on Pluto and keep the inside warm at a comfortable 20 C, if the outside air temperature is -240 C and the temperature in the ground is -200 C by using a Stirling engine and a heat pump. But it is impossible to keep your house at 20 C if the outside temperature is uniform at 19.999 C in the air and the ground, and you have no access to anything at a different temperature. Count Iblis (talk) 20:43, 12 May 2021 (UTC)