Danijel Gorupec

Thinking out loud... At the time, Einstein did not have to feel embarrassed because he didn't know how to reconcile his theory with QM. If one would be developing GR some 40 years later, he would be under some pressure to provide a theory that includes QM. Some guys/girls might abandon their work out of frustration... So maybe, we won't ever have GR as it is now, but we would directly have (eventually) a version given with QM in mind.

@studiot Thanks. I obtained the book (obviously a different edition than yours). Looks fine, will try to read some interesting chapters in my spare time. I am considering electric and magnetic fields because I know them better. The gravitational field might be simpler from the potential energy point of view, but I wouldn't know how to handle its filed energy. Furthermore, the ultimate problem I am considering (what does it mean when we say that electron shifts its energy in magnetic field, like in the Zeeman effect) is related to electric and magnetic fields. @swansont Great that you introduced the capacitor example - it is simple enough that I can make some calculations (see below). But first to answer your 'philosophical' question... Yes, these are two ways of doing calculations. But, imo, not equivalent... The 'field energy' is more precise. Specifically, it precisely defines how the energy is distributed in space (energy density distribution). The 'potential energy' obscures this information. The energy distribution is important for the local energy conservation idea. Imo, if you cannot tell the energy distribution, then the idea of local energy conservation becomes moot. As I take the local energy conservation as a strong requirement, so I think that, at least in principle, we should be able to describe each form of energy by its energy density distribution... It is for this reason that I expect that each forms of potential energy is describable by field energy (or at least by some other way that does not obscure the energy distribution information). As you know from my other thread, I found one example (spin magnetic moment in external magnetic field) where I cannot relate the potential energy change to the field energy change and this itches me. Now the computations... they are striking... We consider a charged plate capacitor. During the experiment, the plates are moved from the starting distance d1 to the ending distance d2. We compute the potential energy change (first line of equations) and field energy change (second line of equations). CASE 1 - constant charge. Everything is clear here and everything fits nicely. The PE change equals the field energy change. CASE 2 - constant voltage. This is interesting. When we look at the system from the potential energy viewpoint, the total energy of the world does not fit! But when we look from the field energy viewpoint, everything fits nicely. Note that in the total energy of the world must include the energy in the idealistic battery that is used to ensure the constant voltage on the capacitor. Note that in the constant-V case the field energy decreases as we are separating the plates. However the energy that is 'pumped' back into the battery is twice the work done... The potential energy viewpoint would give balanced energy only if you avoid looking into the energy change of the battery. Once you peek inside your battery, your energy balance falls apart. So, if I didn't do any major mistake in my reasoning, it seems that the field energy is more precise and more fundamental than the potential energy. If one really, really wants to give a precise answer to the question 'and where is the energy stored' he/she should give the answer from the field energy viewpoint and should avoid the potential energy viewpoint. What do you think?

I am investigating the relation between potential energy and field energy. I think electric and magnetic fields might be simplest to consider. Imagine a simple system of two small charged particles (not elementary particles) separated some distance apart. - we define that the potential energy of this system is U0. - we compute that the energy stored in the electric field is: E0 (note: we computed E0 by integrating the energy density formula for the electric field over the whole space. No need to consider our particles as point charges - say they contain uniformly distributed finite amount of charge in a small finite volume.) We slowly separate the two charged particles some distance farther apart. To do this, we invested some work W. What is now the potential energy U1 of the system? Is it: U1=U0+W? What is now the field energy E1? Does it equal to E1=E0+W? What is the change of the total energy of the system - does it equal to W? My opinion: Yes, the U1=U0+W; yes, the E1=E0+W; and yes, the total energy change equals W. Therefore the change in potential energy and the change in field energy represent the same thing. We should consider either potential energy or the field energy when we compute energy balance. We cannot consider both energy changes as we would be doing double-counting error (in our example, we might compute that the energy difference is 2W instead of W). Let's be stubborn and try the opposite anyway.... let's suppose that potential energy and field energy are two separate things. In this case we could claim: U1=U0+x*W E1=E0+(1-x)*W (where x is a number between 0 and 1) Using these claims the energy will still balance (the energy change is x*W+(1-x)*W=W), but the question would be what is the factor x? Is it x=0.5? Why? So, can we safely say that the change in the potential energy and the change in field energy represent one the same thing? Are there exceptions?

Hmm... What do I do now - I don't find this answer that much revealing because I feel that terms 'interaction energy' or 'potential energy' are intentionally obscuring (they seem like aggregate terms used when it would be too complex to look into full details). Should I open another thread to clarify the difference between potential/interaction energy and field energy?

I think, if our civilization continues without a reset, we will very soon start to intentionally change ourselves at a rate much faster than natural evolution could do. We will engineer our genetic codes (we will also install non-living implants into our bodies to obtain above-natural capabilities and we will even create self-reproducing machines not based on DNA that will continue to evolve themselves at an 'explosive' rate).... So, ironically, the 'intelligent design' might soon be thought as a mainstream

I believe you introduced the concept of potential energy. Specifically, the potential energy of magnetic dipole in magnetic field. But isn't the idea of potential energy just a shorthand that we use? Isn't it always more precise to deal with the energy stored in the field? That is, I assume that the potential energy change (due to electromagnetic field) can always be represented as the electromagnetic field energy change. The trouble is that I continuously fail to relate the potential energy of our elementary-charged-particle-with-spin to the field energy! If the particle really changes its potential energy, and this is not mirrored in the field energy, should I then accept the the potential energy is something really fundamental (not just a shorthand)? I just cannot imagine the potential energy as a thing on its own. Oh, and yes... you might think that the energy shift of the particle is stored is in the magnetic field, but I don't think so (the energy stored in magnetic field seems to shift in a wrong way - it decreases when the electron shifts its energy up.)

Thanks guys... I am glad that studiot used the term 'energy repository' as I think what bothers me is understanding how the energy is stored during these energy shifts. So could the 'spin' be a possible way to store energy? It would surprise me as the quantum spin is fixed (it can measure only two discrete values of identical magnitude). Here is what I think is the essence of my troubles: A simple atom, with only single electron in a state that only has spin magnetic moment (zero orbital magnetic moment) is immersed into magnetic field. Electron's energy shifts... But there is nothing measurable that changes about this atom. I expect that its orbital shape remains unchanged, and its spin (if measured) remains unchanged. Except, of course, its mass does change (from Einstein) - but is this it, is this mass change all that I should expect to happen with this atom?

I would like to understand better how atomic orbitals behave in (magnetic) fields... After reading about Zeeman effect, I understand that electrons bound in an atom might shift their energies when placed in magnetic field. I ask if this energy shift is also associated with some change in the shape of their orbitals? (I guess, this is equivalent to asking if the probability density function changes). I would expect that orbitals that have some angular momentum (and magnetic moment) do change their shape. I read that atoms near a magnetar star could look needle-shaped... But I don't see how would an orbital with zero angular momentum (and zero magnetic moment) change its shape? On the other hand, if no change in orbital shape (probability density) can be associated with electron energy shift in magnetic field, how do then electrons 'shift' their energies (do they speed-up, become heavier or what)?

If you will be using your motors and generators near their intended regime (rotational speed, voltage, load), then it is often good enough to just look at the motor/generator nameplate - the efficiency should be stated there. It very much depends on the motor type, class, size... A small asynchronous motor (around 1kW) might be 80-90% efficient. Larger motors have higher efficiency than smaller ones. Technology makes difference too - a permanent magnet brushless DC motor are usually more efficient that equal-power common squirrel-cage asynchronous motor. High-efficency motors can cost significant money. However, if you will be using your motor in wide range of working regimes, just looking at the nameplate efficiency might not be good enough. For more expensive motors you will probably be able to obtain efficiency diagrams from the motor manufacturer. This would be perfect, because calculating it from first principles is difficult - you would need to know everything about motor construction to calculate it properly (in this case, it would be much easier to just measure it than to try the calculation). For qualitative analysis and orientation, you will be able to find various efficiency curves by googling. Some general efficiency notes: - the core loses are usually divided into two parts: hysteresis loses (depends on quality of silicon steel used in your motor - hysteresis loses are roughly proportional to the frequency, keeping other things the same) and eddy-current loses (depends on the resistance of silicon steel and lamination thickness used in your motor - these are roughly proportional to the square of the frequency). In a typical motor, at its nominal speed, the hysteresis and eddy-current loses are about the same. - the copper loses might be about the same as the core loses (in nominal regime). In some types of motor, these can be relatively easy to compute if you know motor current and winding resistance. - mechanical loses are usually divided into bearing loses and (in some constructions) cooling fan loses. Normally are smaller than copper loses and core (iron) loses. - motor efficiency very much depends on the load - the best efficiency is often when your motor gives 70-80% of its nominal load. At low loads, of course, the efficiency drops drastically. - you usually regulate your motor using some circuitry (motor driver). Modern semiconductor drivers (like inverters) can have efficiencies well above 90%. Still, take it in account. Only if you will be designing motors themselves, you will need to dig deeper than simply analyzing motor efficiency curves. Today you would be typically using computers and numerical-analysis software to compute your motor efficiency. Batteries are chemistry and not my area, but of course they have limited efficiency (as anything else). I see that a typical lithium-ion battery might have charge/discharge efficiency of about 80-90%. Of course, if you are not actually charging/discharging your battery, like in the case when you are generating just as much power as you are spending, then this efficiency is not a concern. But you will occasionally charge/discharge your battery (otherwise, you would not even have the battery in the first place). Another source of battery loses is the self-discharge. This means, a battery will lose some percentage of stored energy with time - even if you don't charge/discharge it. I understand that lithium-ion batteries have fairly small self-discharge, so unless you need a long-time storage it is not so much a concern. As everything else, I expect that battery efficiency depend on quality of construction, and this affects the battery price. Gearboxes can be made to have very high efficiency, fairly over 90% - but it again depends on the construction (worm gear is not very efficient, while helical gears can be). Unfortunately, a good gearbox can be surprisingly expensive as it requires high precision. A trivial example: you have an inverter (efficiency 95%) that runs a motor (efficiency 90%) that drives a gearbox (efficiency 95%) that drives a generator (efficiency 90%) that charges a battery over a rectifier (efficiency 97%). Later you discharge the battery (efficiency 85%) to produce electricity for something useful... The fact is that the electrical energy that you obtained from this battery is only about 60% of what you spent in the first place: total_efficiency=0.95*0.90*0.95*0.90*0.98*0.85. I cannot recommend any good source on the Internet - I am sure there must be many, but I just tend google for anything specific and usually find something useful. Hopefully somebody else will provide a good link.

As Bufofrog said, you have no chance with the proposed motor-generator-battery setup. A certain amount of energy will be 'lost' in every step: - your motor has efficiency less than 100% and will 'lose' some energy as it converts electrical energy into mechanical energy - your generator has efficiency less than 100% and will 'lose' some energy as it converts mechanical energy back into electrical energy - your battery has charge/discharge cycle efficiency less than 100% and will give you less energy back than you previously stored into it. Increasing the rotational speed of the generator does not make any essential difference (although the gearing might give you the ability to keep the rotational speed of your motor and your generator at their best-efficiency range, but you will still lose energy. In fact, the gears will introduce additional energy loses, so it might not even decrease the overall loses). BTW, I did it when I was a kid - it did not work

Yep, it does not make sense to split a thread and retain the same title.

What is this thread about? Is it about climate change (as the title suggests) or about plant ability to make sugars (as I understand the first post)?

Thanks for explaining the 74A... yes, as you said, it does not have much meaning as your power supply is limited to 10A About 6 amperes per square millimeter of wire cross-section is usually considered an upper-limit recommendation for coils in commercial products. However my experience is that in experimental setups you can go significantly higher. I guess you should be able to go to 10A per square millimeter without a need for forced cooling (with just some basic common sense regarding passive cooling). But I guess you might want more than 10A per square millimeter (in your case, the wire is about 0.33 square millimeter, which means that with passive cooling you should be able to use about 3.5A of current). So you might want to arrange some sort of forced cooling or use the coil only intermittently. Anyway, this is just a rough estimation. If you decide to use a passive cooling then I would recommend to use as much wire turns as you can. This will allow you to work with a lower amperage for the same effect. If you use a forced cooling, then I am not sure - maybe it does make sense to leave some space between wire turns so that the cooling fluid can pass (but this is also harder to build, so I would go with simple compact coil as neatly and tightly wound as possible)

This is a real-life case with an iron core, and as you seem to be interested in relatively precise calculations, I would suggest using computer software to simulate your magnetic circuit. There are even few free software packages, but I am not sure if these would be good enough for your (I don't know of any free 3D simulator, but maybe 2D can be good enough for you - like 'femm'). If I understood you correctly, only 40 turns of wire can fit inside your core? At 10A this gives 400Ampere-turns - sounds like it might be OK. However I expect cooling problems and your coil might only be energized for a limited time... I don't understand when you say 'around 74A of current' - how did you get this number? (If you find out the 10A is limiting you, you can always use a bit thinner wire and fit more turns into your core - then you will be able to obtain more ampere-turns using your limited power-supply. However heating problems will limit you severely.) Then again, you might be a master of cooling. The superconductor part seems interesting - how do you cool it?

Indeed. As an electrical engineer, I imagined current intensity to be vector-like my whole life - a thing that runs through one-dimensional wires. So I reacted with disbelief when first read that generally the current intensity is considered a scalar. It took me some time of more careful thinking to accept this fact.

Do I assume correctly that you are talking about current intensity, not current density (when you just say 'current', I am not sure)? Current intensity is not defined for a point (also not for a line between two points). Current intensity, as it is mostly understood, is referred to a surface - it is a flow of charge through a surface in a unit of time. Basically, when you talk about current you should also mention (or assume) the surface through which this current flows to make its value meaningful. Generally, direction does make sense when referred to a point (vector values are often like that). However the value of current intensity is usually not understood as a vector value - it is taken as a scalar. So, this would be correct for current density, but not for current intensity (I am not sure which one Swansont actually meant, but I assume the question was about current intensity) The current intensity is an integral value and so does not have a finite value for a point and does not have a direction.

That wording made me laugh :)... Did you ever try copywriting - I think you might have a talent. Few less-practical ideas of my own: - I sometimes dream of a small USB device that I can connect to my laptop and use to torture it. I would love to torture my laptop sometimes. Is it only me? - A small scale producer can put QR code to packages of its chicken breast, pork, beef steak. This should lead me to a web page where I could see the animal (being born, having play, suckling, growing up etc.... and a short description of its character). Well, I would even like to see the tree my toothpick is made from.

Good point! I didn't think about it. You might be well right... As with the first point, I cannot tell for sure without some numerical computation. But I guess you are most likely right.

I would expect so. I must admit I have difficulties imagining how exactly would a 'swirl' of the current look around the magnet moving through mercury. A computer simulation might come good. Of course. Of course. all currents, including eddy currents, generate magnetic field.... That is, when computing total magnetic field, you will have to include contribution from eddy currents. Yes (in classical electrodynamics these are from different principle, but the effect is the same)

The outcome of some viral diseases might be affected by the exposure viral load, and indeed, lower exposure viral load could lead to milder symptoms. But even if you deal with such a disease, you cannot use limited viral exposure as a form of vaccination. The important quality of vaccine is that it creates immunity, but does not create spreadable disease.... In your case, however, such 'vaccinated' persons would spread the disease and your method might make things worse.

New Zealand probably did it right. On the other hand, my country (Croatia) also has a good statistics, but I suspect we only did it by locking people inside their homes. I don't see improvements in infrastructure and I don't see why yoyo effect would not happen to us... So I guess it can depend from country to country.

Personal satisfaction that some people feel when they are building something with their own hands; includes the satisfaction of learning in the process. All other reasons being of lesser importance, imo. So if you don't feel excited with the idea of building a telescope on your own, it is perfectly reasonable to just buy one. The price is almost never a valid reason. If you don't have enough money, finding a paid job would be a less work/time-intensive way to obtain the telescope. The only exception would be if you want to work on something so specific that it is not possible to find a proper tool on the market. Then you will build one even if you don't enjoy doing it.

New Zealand is one striking example of a whole class of countries that claim (rightfully) success in fighting the virus. At least they won the initial battle (as I doubt the war is over). I think, the question is if during that time they managed to establish effective work procedures, organize tracking teams, educate population, increase hospital capacity... If they did, then the initial strong reaction was a good investment. If not, then I they are again at the beginning.... I mean, they cannot remain an island in the ocean forever

This is what proponents of this idea are pointing out - allegedly, the BCG vaccination is still obligatory in eastern Europe for all children, while is not obligatory in western europe (Portugal being an exception). I didn't check these claims... If UK still has a large portion of population BCG vaccinated, as you suggest, then the idea just falls apart.

If this was already discussed on SFN, please direct me... Here in eastern europe, there is some speculation that the reason why eastern europe seems less affected by covid-19 (in comparison to the western europe) could be due to previous vaccination against tuberculosis (the BCG vaccine). This seems far fetched to me, but what is your opinion?