Naked Science Forum
General Science => General Science => Topic started by: Zer0 on 31/10/2022 17:57:16
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When a Black Surface is placed under Sunlight, it gets Warmed Up.
How?
P.S. - Try explaining it like you would to a 5 year old.
(Tankz!)
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Hi.
"In simple terms" is what you asked for.
We (human beings) convince young children that Energy is a "thing", and use the word "energy" in phrases much the same way as if it was something like orange juice. Orange juice is a "thing" and a lump of stone is a "thing". We learn there are things and Energy is treated as a thing.
So the children assume it's some kind of thing. As the children get older they start to sort out that some things are less like a physical thing than others.
Mum: Get some sleep !
Child: OK... is it in the cupboard?
Children learn that sleep is not a thing like orange juice is a thing. Orange juice is most definitely a physical thing but sleep is hardly worth putting in the same category, it's a weird thing if it's thing at all.
When you get to school people will still consider Energy as a thing. However, when you study some science it should start to occur to you that Energy is one of those weird things, a psuedo-thing at best. It's not any sort of ordinary physical thing. You can't point to it. You can't find it in the cupboard. However, people say that there is energy "in" some physical things.
Mum: Eat your potatoes, there's plenty of energy in those
Child: Where? When you cut it into little pieces all you find is potato all the way through.
Anyway.... this is taking too long so we'll fast forward a bit....
We (human beings) have made our own problems by convincing ourselves (and misleading our children) to imagine that energy was a "thing" in the first place.
It is certainly not any ordinary sort of physical thing. In all of the particle physics experiments that have ever been done, we have never found a particle of energy. It's not there... a potato really is just potato all the way through, no matter how finely you dice it up, you can't find the energy that was supposed to be in it.
So what is Energy?
It's any one of these, take your pick:
1. An oversimplified concept where it is imagined to be a "thing" that exists in the universe but distributed or spread out in some very specific ways. So that there can be stores of it at certain places and/or it can be contained within certain physical objects.
2. A quantity, just some number which we might call E. It just turns out that many physical systems have a quantity which is conserved (doesn't change with time, or is the same value at the beginning and at the end etc.). These conserved quantities aren't always called "Energy", sometimes there's a conserved quantity called "angular momentum" or "proton number" or something else..... there are many conserved quantities, they're not all the same in every system and they're not all conserved in every system. However a quantity E that behaves like we would expect Energy to behave is almost always there in every physical system. Furthermore, when we combine two systems we have studied so as to construct one new but larger system, then we can usually feed the conserved quantity E1 from the first system into the conserved quantity E2 of the second system and it works quite well. Whenever there is some process to transfer some E1 out of what was sub-system 1 and into sub-system 2, then we do see an appropriate corresponding 1 point for 1 point increase in E2. That is, we do usually end up with a new quantity E3 = E1 + E2 which is conserved. This gives us good reason to believe that E1 from subsystem 1 really was the same kind of thing as E2 in subsystem 2, i.e. that they were both forms of Energy.
3. An approximation or simplification only. Something that usually applies but not always. There does seem to be some systems where what we thought was Energy and should be conserved is NOT actually conserved. Example: The passage of light through an expanding universe where the frequency of the light would decrease over time.
---- This is already too long so I'm going to end here. I'll leave someone else to go through the specific example you mentioned. They might explain it by tacitly assuming that energy is a thing but that is up to them. ----
It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way.
Taken from: Feynmann lecture #4 "Conservation of Energy". Available at this URL:
https://www.feynmanlectures.caltech.edu/I_04.html
This is one of the best introductions to the difficulties of understanding what energy could be (in my opinion). Just bear in mind it's old and we've had a few more revelations about Energy since this.
Best Wishes.
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In simple terms, there isn't a simple explanation or even a simple definition. Energy is one of the quantities that is conserved in classical physics, and very few adults have any idea what that means.
You can introduce the subject to a child by looking at mechanical energy. Winding clockwork, drawing a bow, or lifting a weight with a rope and pulley involves the sensation of work, and then you can use the spring, bow or weight to do something "useful", so you must have put something into the system, which it then releases.That "something" is called energy.
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Mum: Get some sleep !
Child: OK... is it in the cupboard?
lol @ Eternal
I remember from School that Energy is the Input for performing a Task.
Measured in Joules.
Well, from above replies I've gained alot more insight.
Seems like i ended up Biting alot more than i can Chew.
In Essence the OP is Resolved.
Good Job Folks!
P.S. - But what about the second part of the OP?
The Black Surface & Sunlight?
How does that Work?
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Photons arriving from the sun carry this inscrutable "energy" according to E=hf, where f is the frequency and h is planck's constant. These photons are absorbed by the black surface, with minimal reflectance, and give up their "energy" to the surface and thus raise the temperature. On the original question we could also say that energy is the resultant from the decomposition of matter, according to E=mc²
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Paul's 5-year-olds have clearly benefitted from an exceptional nursery education. Never mind the sand box, children, today we will be entertained by Max and Albert, and you can take home the A level physics paper to show your mummies and daddies. Remember to bring your university tuition fees next week: the best spectrophotometer made from a cornflake packet and a toilet roll wins a scholarship to Trinity!
We mere mortals have to resort to James Joule. We covered "work" yesterday by putting manual effort into lifting and pulling, and showed that it can be turned into kinetic energy. So now we are going to convert work into heat by rubbing two sticks together, then use the bicycle dynamo and lamp to convert work into light.
It's pretty obvious to anyone except Aristotle (never trust a philosopher, children - politicians offer you sweeties but philosophers will harm you for nothing) that light travels from a source to a receptor, and since we put work/energy into making light, light must be another form of energy.
Now what can we do with light? We can reflect it (with a white surface) or absorb it (with a black surface). So what happens to the energy if we absorb it? In the case of the black surface, most of it turns into heat.
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So the children assume it's some kind of thing. As the children get older they start to sort out that some things are less like a physical thing than others.
Mum: Get some sleep !
Child: OK... is it in the cupboard?
When I hear phrases like "How many calories in that chocolate?" I sometimes think that most adults must think that a calorie is an ingredient, not a unit of measurement.
"How many inches in that string?"
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"How many inches in that string?"
I've mentioned it before, but worth repeating here.
Back in the mists of prehistory I attended the welcoming speech by the Director of the UK National Physical Laboratory to new recruits. He opened thus:
"How long is a piece of string? In law, it is exactly as long as I say it is. Your job is to tell me what to say."
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Indeed Very Nicely Explained Mr Cotter
lol @ vhfpmr
(hope u don't mind me asking what does ur nickname mean?)
Now to get back to the Question...
Not so quick folks, hold on hold on..
I read another OP by Talanum1 @ New Theories.
The user was proposing their hypothesis of the phenomenon.
Was Refuted by views stating that an Electron does Not absorb a Photon.
I didn't Understand, hence Googled & came up with the Law of Conservation of Momentum.
It states if the Electron absorbs the Photon, then with transfer of Energy, even Momentum would be transferred.
That would project the Electron at the Speed of Light.
Hence, Not Possible.
Another view in that OP stated that Electrons just Scatter Photons.
So then how do Photons end up wiggling & jiggling Atoms to vibrate more to raise Heat/Temperature?
Or am i missing the point Completely?
Does Wave Particle Duality needs to be invoked in order to Understand the said Phenomenon?
P.S. - i don't wish to goggle the answer, so could someone please explain, Thanks!
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It states if the Electron absorbs the Photon, then with transfer of Energy, even Momentum would be transferred.
That would project the Electron at the Speed of Light.
No.
https://byjus.com/physics/photon-momentum/ shows you how to calculate the momentum of a photon = h/λ.
If it is all transferred to an electron, you can calculate the subsequent velocity of the electron, knowing its rest mass. It's a lot less than c!
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Was Refuted by views stating that an Electron does Not absorb a Photon.
And electron on its own doesn't (usually) absorb a photon.
And electron in an atom or molecule can.
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Just a reminder to all correspondents:
P.S. - Try explaining it like you would to a 5 year old.
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Hi.
@Zer0 your confusion is perfectly understandable. It's not obvious what is happening.
@alancalverd gave a reasonable response but it might help to phrase it differently.
A photon carries a certain amount of momentum. It is a tiny amount.
The electron would gain that momentum but this does not mean that it gains all the speed of the incoming photon, only the momentum that it carried. There is a significant difference between speed and momentum. You can find this explained in more detail in various places. See, for example, https://www.bbc.co.uk/bitesize/guides/zc9bv9q/revision/1 which is a website designed for GCSE Physics (students aged about 15, which I know is a bit above the 5 year old range originally asked for). A more simplified view would just consider how difficult it is to get a small object to a certain speed compared to a massive object. You can change the speed of a toy car easily just by flicking it with your finger but flicking a full sized car with your finger won't change it's speed much. Mass is important when considering how much momentum a thing has.
The key point is that the electron is massive, so the electron doesn't need to change its speed very much in order to accommodate that tiny bit of extra momentum gained from the photon.
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The next part of your question is a little more difficult to answer and does show that you have considered the situation quite a lot. Temperature is usually considered as a measure of the vibrational energy of particles in a solid and it is not obvious why exciting an electron in one atom of that solid should make a whole set of atoms vibrate a bit more.
So then how do Photons end up wiggling & jiggling Atoms to vibrate more to raise Heat/Temperature?
I don't know, here's my take on it and how I might exlain it to a child (especially if I was having trouble getting them to sleep).
Answer strategy 1: It just does happen. We don't need to worry about the "how". This is the approach that the subject of thermodynamics might take. The internal energy of a solid (the black surface you put under the sun) includes all sorts of different forms of energy. Some of that is vibrational energy of the atoms, some of it is the energy of the electrons in orbits around the atoms. Internal energy of various kinds can contribute to the temperature of the solid. We don't worry about how energy might be passed from one type of internal energy store to another. All we notice is that this does happen, although it might take a little time. Technically, temperature is only well defined and measured when some equilibrium has been reached. This is just a way of saying that we shouldn't really measure temperature until some time has passed and the internal energy has distributed itself over all possible forms or modes of supporting internal energy that it can. In practice the time we have to wait is very short, a thermal equilibrium situation can be reached in microseconds quite often. Just to summarize, a short answer is that we don't care "how", it just does happen that an excited electron can raise the vibrational energy of a whole set of atoms and thus raise the temperature of the solid. This is a good answer not something to be dismissed. It's good enough for thermodynamics which is a pillar of modern science and technology.
Answer strategy 2: We can actually find some explanations if we try hard. They will usually be specific to the type of substance that was receiving the photon. We're going to stay very generic in this section: The distance between the atoms will depend on things like electrostatic attraction between them. If an electron in one atom gains energy then it is moved to a higher energy orbit, i.e. in simple terms we can imagine it is moved further out from the nucleus. This could directly affect their position a little by all sorts of mechanisms. For example it could increase the amount of what is called "screening" that occurs between the two positively charged nuclei etc. We start to get some movement, the beginning of our "jiggle" in the atoms. In more extreme cases, the electrons may be moved to an orbit where chemical bonds can no longer be sustained and then there is a more significant movement of the atoms. Many of these changes are temporary and short lived, so it's not as if you'll notice any chemical changes happening on a macroscopic scale. These transient changes are just fine - we just want a little "jiggle" in the atoms.
If you really were explaining to a child then explaining screening and chemical bonds will take a while. You might just say the electron is charged and attracts or repels other atoms differently when you move it closer or further away. So changing the orbit of an electron matters, it affects the pull between atoms and the overall distance you will get between atoms.
In some cases it's as simple as realising that after a photon has been absorbed and adjusted an electrons orbit, the electron orbit doesn't stay in the excited state for very long. It can just decay again with another photon being released. For a while then, the first atom (the atom as a whole) had to have some extra momentum (recall that it has just absorbed a photon), so it moved differently for a short while. This might take a while to think about but the idea is that you can't just change the linear momentum (i.e. the motion) of an electron and expect it to stay with the nucleus it is orbiting around. The electron would start to drift away - but that can't happen. The electron and nucleus interact through electrostatic forces, pull on each other and try to hang together, if the electron had a change in its linear motion then the whole atom would. Anyway, the next atom to briefly absorb the photon will have the same sort of brief disturbance of motion etc. The photons can be released in a random direction and could be fired out in the same direction from which they came in. This means that atoms can pass momentum between themselves that can be double the momentum of the photon you find between atoms. Now I have also said "the" photon but there is no reason why a photon of the same energy has to be released as the one that was absorbed (the electron could decay into a middle energy state, if there is one, rather than falling all the way back down to the initial state it was in) - so with the random discharge direction as well, there is now quite a bit of random variation we can get in the momentum passed between atoms.
Anyway.... we can build excuses and explanations to see how it might happen. This is the sort of thing that would be done in the subject of statistical mechanics rather than thermodynamics. It can be done. Make it clear that these are partial explanations, excuses or "plausibility arguments".
For a child, we walk away here and leave them to ponder that. Since there may be others reading and considering what is the best approach to take when explaining to a child then we'll say a bit more. The entire notion of considering atoms, electrons and nuclei as rigid little balls of stuff where a little bit of momentum is passed from one to another in clearly defined separate interactions is very limited. We can spend time finding explanations that make things work based on this model but there's hardly any point. There is probably just the one big wave function describing the behaviour of ALL the particles. As such when a photon is absorbed by an electron at one place in the solid, it has affected the wave function which applies everywhere. (we need to insert a pause here and think about that).
------ pause ----
The wavefunction has changed (even if slightly) everywhere and will evolve with time according to the time dependant Schrodinger equation as usual. In effect, the spread or dispersal of extra "jiggle" in the atoms may have started throughout the whole solid as soon as the photon was absorbed at any one place within it. There does not need to be a "why" or "how", it just does. Quantum mechanical effects are under no obligation to look like separate and sensible interactions between atoms.
Best Wishes.
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A photon carries a certain amount of momentum. It is a tiny amount.
Enter grumpy old pedant, stage left.
I think "imparts" is better than "carries" since mp = 0 but that's being pernickety. It took a while for supposed grownups to understand Einstein's explanation, but I hate lying to children - if they remember the truth, they may eventually understand it.
Don't be coy There is a significant difference between speed and momentum
! Momentum is mass x velocity, and a 5-year-old might just understand or at least remember "multiply".
Photon momentum in the visible spectrum is of the order of 10-27 kg.m/s. The rest mass of an electron is about 10-30 kg, so the subsequent electron velocity would be around 1000 m/s. About the speed of a bullet.
How do photons jiggle electrons? Electromagnetic radiation is just that, and an electron has a charge and a magnetic moment. It's easy to demonstrate magnetostatics and fairly easy to demonstrate magnetoelectrodynamics, so the answer is "why not?"
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Was Refuted by views stating that an Electron does Not absorb a Photon.
And electron on its own doesn't (usually) absorb a photon.
And electron in an atom or molecule can.
Thanks for Pointing that out.
That is Exactly the Mistake i made.
I misinterpreted what you said.
& In my googling keywords search i kept repeating the same mistake.
Rather than searching for " Atom absorbing Photon " i searched " Electron absorbing Photon ".
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Just a reminder to all correspondents:
P.S. - Try explaining it like you would to a 5 year old.
Was a reference to Myself.
If you don't Believe me, check my profile.
Ha ha!
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A Note to Eternal
I Appreciate your Detailed responses.
But i also Feel a sense of Guilt.
Perhaps your Contributions to this Forum are directly proportional with the length of your Grass lawn.
The More you spend time here, the more the Grass grows.
I Hope you are able to maintain a healthy equilibrium between your Online vs Offline worlds.
P.S. - if i were livin anywhere around, i would have cut that damn grass for free for U.
Thanks & Best Wishes!
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Hi.
I Hope you are able to maintain a healthy equilibrium between your Online vs Offline worlds.
Thanks, I'm off grass cutting duties for a while. It's all too wet.
I think "imparts" is better than "carries" since mp = 0
Hmm.... I suppose the principle of conservation of momentum can be suspended until all photons have hit something. Otherwise you have little choice except to say the photons do "have" or do "carry" that momentum.
How do photons jiggle electrons? Electromagnetic radiation is just that, and an electron has a charge and a magnetic moment. It's easy to demonstrate magnetostatics and fairly easy to demonstrate magnetoelectrodynamics, so the answer is "why not?"
I do like that. I don't think many children will argue or find a problem with it and it hadn't really crossed my mind when I was writing. A classical e-m wave can accelerate charges it passes along the way and then just be gone or absorbed (or vice versa - an accelerated charge can generate e-m waves).
Adults might find a problem if they try. Most solids aren't metals and don't have electrons sloshing around and roaming free. Zero's original question also did seem to lead on from the assumption that the photon had excited an electron to a new orbit. This is a far more quantum version of things instead of a classical wave model for the e-m radiation.
Best Wishes.
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This brings us to the question of why the surface is black. Annoyingly, it is because the material absorbs visible photons!
So the electronic structure of whatever it is must have the capability of absorbing any and all photons in the range between 1.5 and 3.5 eV. So we aren't talking about quantised energy levels but an effective continuum.
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I suppose the principle of conservation of momentum can be suspended
May I remind my learned friend, with the greatest respect, that this is physics, not politics!
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In the simplest terms, I have always understood energy as the ability to do work.
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Hi.
In the simplest terms, I have always understood energy as the ability to do work.
Yes that's excellent. This is exactly how it's often defined at school level. It will take you a long way.
The remainder of this post is complicated and just for general interest. Don't add it to your answers for school level physics UNLESS it's a long answer question (like an essay) and you're confident you undertsand it.
By the above definition, if you had a bottle that contained some really hot stuff then you have some energy in that bottle. We're going to make that bottle a perfect Dewar flask so that we can carry it around and no heat escapes.
The bottle has energy in it and while we are here on planet earth then we can see this easily. We could open the bottle a little and allow some of that heat to flow from the inside of the bottle to the outside world and while that's happening drive a piston with it. I won't draw the diagrams but you could just let the hot stuff heat some water in a tank and drive an old fashioned steam engine with the steam from the hot water, there's lots of options. The main thing is that it can be done. We can get useful work from the bottle of hot stuff, so it's a store of energy as described in the definition you gave.
Now we're going to travel to another region of the universe, let's assume we can get to the sun. It'll be very hot there but this is theoretical physics not engineering. Now the problem is that the surrounding might be as hot as the stuff you had in the bottle. So the heat just won't flow out of the bottle. Heat only flows from a higher temperature to a lower temperature. Thermodynamics states that you can't get any useful work out of the stuff in that bottle while you are here in this hot place. In fact you'd be a lot better of with a bottle of cold stuff in this place. If you had a bottle of cold stuff then you can get heat to flow from the surroundings into the bottle and extract useful work while that is happening. You see, in the sun, that bottle of hot stuff is useless, it offers you no ability to do useful work. If ytou were a being that lived in the sun then what you want, what you would naturally think of as a valuable commodity is some cold stuff. In the sun, a bottle of cold stuff gives you the ability to do useful work, so by the above definition, the colder something is the better: It has more energy since it has the ability to allow you to perform more useful work while heat is flowing into that cold stuff from your surroundings. You see how backwards this is?
Anyway, if you've understood this, you'll see that "the ability to do work" isn't a property that an energy source can have on its own. It's actually a property of BOTH the fuel substance AND also the environment it is in. So the simple definition of energy as "the ability to do work" falls down slightly. You have to ask does something stop being a form of energy when you move it to a new envronment?
We have a load of physics based on the idea that a hot thing has internal kinetic energy.... there really should be some "energy" in there. However there is no ability to extract useful work from this energy source at all when its in some environments.
Best Wishes.
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And "work" is defined as what? Destroying the British economy with the stroke of a pen, or force x distance? If you are going to use the term at school level, and particularly for a 5 year old, I think you need to get the pupil to do some mechanical work to put the term into context.
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Hi.
If you are going to use the term at school level, and particularly for a 5 year old, I think you need to get the pupil to do some mechanical work to put the term into context.
That does sound a bit like slave labour. We're going to see how soap reduces the friction of a scrubbing brush on this floor... etc.
Best Wishes.
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Of course, mea culpa. You must first carry out a risk assessment then
obtain parental permission to ask the child to kneel
provide appropriate kneeling pads
instruct the child on the correct use thereof
check the child's health record for allergies and intolerances
provide hypoallergenic gloves
review the child's psychiatric profile for claustrophobia
provide goggles and a face mask
provide alternative and equivalent learning ("teaching" implies liability, so we don't do it) facilities for any
claustrophobic children who cannot wear masks and goggles
obtain oxygen, ventolin and sugar in case of asthma or diabetic coma induced by exercise
install an ECG and infant-size defibrillator
assess the environmental hazard of disposal of contaminated water
obtain an Environment Agency dispensation
obtain culturally appropriate brushes and buckets, including at least 10% lefthanded brushes
sensitively identify the non-swimmers to your first-aider (kids can drown in an inch of water)
review the practical housekeeping training records and CPD of the supervising teachers
provide counselling for the kids who come last (every activity turns into a competition - that's childhood)
Clearly it is not practicable to teach physics nowadays, so we have to resort to religion. Say "Energy is the ability to do useful work. Remember these words and repeat them to the examiner or you will rot in Hell for eternity."
"Please, sir, my dad is a priest/politican/philosopher. Does that mean he has no energy?"
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Wow!, Alancalverd, that's some outburst. You are becoming more cynical than me, if that is possible. Seriously, you have some good points here: life is inherently hazardous and efforts to eliminate these hazards will cripple true education, leading to stunted personal development. I hold personal responsibility to be of utmost importance-if I come to grief through my own actions I don't seek someone to blame. The "blame" culture is now so strong that practical science in school has been dumbed down to the point of futility. All off-topic, I apologise.
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My dad used to say that satire should be like a dermatome - cutting just below the surface to expose the rottenness beneath. But now and again it's fun to use a machine gun!
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Hi.
In the simplest terms, I have always understood energy as the ability to do work.
Yes that's excellent. This is exactly how it's often defined at school level. It will take you a long way.
The remainder of this post is complicated and just for general interest. Don't add it to your answers for school level physics UNLESS it's a long answer question (like an essay) and you're confident you undertsand it.
By the above definition, if you had a bottle that contained some really hot stuff then you have some energy in that bottle. We're going to make that bottle a perfect Dewar flask so that we can carry it around and no heat escapes.
The bottle has energy in it and while we are here on planet earth then we can see this easily. We could open the bottle a little and allow some of that heat to flow from the inside of the bottle to the outside world and while that's happening drive a piston with it. I won't draw the diagrams but you could just let the hot stuff heat some water in a tank and drive an old fashioned steam engine with the steam from the hot water, there's lots of options. The main thing is that it can be done. We can get useful work from the bottle of hot stuff, so it's a store of energy as described in the definition you gave.
Now we're going to travel to another region of the universe, let's assume we can get to the sun. It'll be very hot there but this is theoretical physics not engineering. Now the problem is that the surrounding might be as hot as the stuff you had in the bottle. So the heat just won't flow out of the bottle. Heat only flows from a higher temperature to a lower temperature. Thermodynamics states that you can't get any useful work out of the stuff in that bottle while you are here in this hot place. In fact you'd be a lot better of with a bottle of cold stuff in this place. If you had a bottle of cold stuff then you can get heat to flow from the surroundings into the bottle and extract useful work while that is happening. You see, in the sun, that bottle of hot stuff is useless, it offers you no ability to do useful work. If ytou were a being that lived in the sun then what you want, what you would naturally think of as a valuable commodity is some cold stuff. In the sun, a bottle of cold stuff gives you the ability to do useful work, so by the above definition, the colder something is the better: It has more energy since it has the ability to allow you to perform more useful work while heat is flowing into that cold stuff from your surroundings. You see how backwards this is?
Anyway, if you've understood this, you'll see that "the ability to do work" isn't a property that an energy source can have on its own. It's actually a property of BOTH the fuel substance AND also the environment it is in. So the simple definition of energy as "the ability to do work" falls down slightly. You have to ask does something stop being a form of energy when you move it to a new envronment?
We have a load of physics based on the idea that a hot thing has internal kinetic energy.... there really should be some "energy" in there. However there is no ability to extract useful work from this energy source at all when its in some environments.
Best Wishes.
That's just an engineering problem. If you happen to be stuck in the middle of the sun where the "hot" stuff in the bottle is not any warmer than your surroundings, you simply have to get a well lagged metal bar that reaches far outside the sun then put a radiator on the end of it and let the heat flow along that.
efforts to eliminate these hazards
Is anyone trying to do that?
or are they just trying to reduce unnecessary hazards?
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I do believe there is a move to eliminate all hazards, however impossible this is, in many areas. Even if one stays in bed to avoid external hazards a meteorite might come crashing through the roof. School practical chemistry has been particularly hit. I would not allow students to use, for instance white phosphorus or thallium salts but reducing practical chemistry to mixing vinegar with bread-soda is pathetic and I have heard of this( in aus ). By all means obvious high risk activities should be avoided. Our chemistry teacher did the sodium and potassium in water demo and wanted to obtain rubidium and caesium to show the trend of increased reactivity: he wasn't able to obtain these metals and I doubt if he realised the expense involved. Increased litigation has caused society in general to become increasingly risk averse. Or else i'm just too old!
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The big problem is "ambulance chasing lawyers".
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Absolutely, BC.
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Hi.
That's just an engineering problem. If you happen to be stuck in the middle of the sun where the "hot" stuff in the bottle is not any warmer than your surroundings, you simply have to get a well lagged metal bar that reaches far outside the sun then put a radiator on the end of it and let the heat flow along that.
I like it. I mean it's cheating but I do like it. It took me a moment to think about, so thanks for that. It's better than watching TV for an evening.
Just for the sake of theory, we can dismiss it: What you've said was only possible because you could get to a place that was cooler. We have to assume that you cannot do that. Also we could argue that you have only generated useful work that can be harnessed at the far end (where the radiator was), although engineers are deceitful and they could argue that they will build a piston with a long shaft. Long enough that it will bring the useful work back to the point in the star where the bottle of hot stuff was. As people have mentioned enough times, you can't trust an engineer they're a devious bunch.
Anyway, the idea of not being able to find a cooler place isn't just an unrealistic or "in theory only" condition. The entropy of the universe is increasing. Eventually there will be a uniform temperature everywhere. Under those conditions, your conduction bar or heat pipe doesn't work. Heat won't get out of the bottle into the bar without a small temperature gradient, it won't travel along the bar without a slight temperature gradient and it won't get out at the radiator end either. If the whole universe is at, let's say 10 deg C, then a bottle of stuff at 10 deg C offers you no ability to do any useful work. You can refrigerate or heat up some region of the universe BUT that will take at least as much useful work as that which you can extract later by letting heat flow in or out of the bottle. That's it, end of the line on an entire universe-wide scale, no useful work can be extracted from that bottle of stuff (well just from its natural temperature anyway - you could still try to make it hotter by annihilating some particles within it or something like that but this is beside the point - the thermal energy of the stuff cannot be used to generate useful work and we only need to find one form of energy that fails to be identified by the definition "Energy is the ability to do useful work"). So, with only one more essay worth of writing space, we could re-assert the earlier statement: The ability to do useful work isn't just a property of the substance you are considering as an energy source. It is also a property of the environment it is in.
Obviously none of the above is something I would recommend writing in someone's school work unless they do have a whole essay or two to fit this in.
Best Wishes and thanks again for making me pause to consider another situation.
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Gunpowder, or a watch spring, can do useful work in any environment, surely?
Having the requisite number of limbs and eyes, I have the ability to play cricket. But I'd be more of a liability than an asset at County level. I think we need to distinguish between capacity and realisability.
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Gunpowder, or a watch spring, can do useful work in any environment, surely?
Seriously?
You think they would work in the middle of the sun? We have to assume that you cannot do that.
No, we don't.
We just need the universe to cool down until it is cooler.
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Also we could argue that you have only generated useful work that can be harnessed at the far end (where the radiator was),
No
I plan to install a perfectly lagged, very good conductor out to the radiator so the end of it that is in the sun is cold.
Then I can run a steam engine using the temperature difference between the hot thing thing and the cold sink.
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Eventually there will be a uniform temperature everywhere.
Not quite everywhere
Remember, we were provided in the first instance with some "hot stuff" in a perfectly insulated container.
if you had a bottle that contained some really hot stuff then you have some energy in that bottle. We're going to make that bottle a perfect Dewar flask so that we can carry it around and no heat escapes.
I think "imparts" is better than "carries" since mp = 0
I think carries is the right word because the momentum starts from one place and ends up in another place.
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Quote from: alancalverd on Today at 09:30:57
Gunpowder, or a watch spring, can do useful work in any environment, surely?
Seriously?
You think they would work in the middle of the sun?
OK, delete "useful". Fact is that neither depends on a cold sink. We have discussed elsewhere what happens when you dissolcve a compressed spring in acid, and the same applies here. The gunpowder, or the watch spring, would locally increase the temperature of the sun.
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Hi.
Not quite everywhere
Remember, we were provided in the first instance with some "hot stuff" in a perfectly insulated container.
No, we're not having any of that. Post # 31 said...
If the whole universe is at, let's say 10 deg C, then a bottle of stuff at 10 deg C offers you no ability to do any useful work.
The hot stuff was deliberately replaced with stuff at 10 deg C. That is still "hot" in comparison to absolute zero (0 k) specifically it should have thermal energy due to its temperature. We need only find ONE example where the definition of energy as the ability to do work fails. This is the example we are choosing. A bottle of stuff at 10 deg C offers no ability to do work when the whole universe is at 10 deg C.
Best Wishes.
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If the whole universe is at, let's say 10 deg C
We just need the universe to cool down until it is cooler.
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Hi.
We just need the universe to cool down until it is cooler.
It won't cool down. It's all at 10 deg C. It is not in contact with anything cooler.
You'd need an expanding universe to stand any chance of the temperature continuing to fall (things like molecules slow down in an expanding universe and settle to a static co-moving co-ordinate position).
Before you say, "we'll allow an expanding universe because it is", if you allowed yourself an expanding universe then there are other examples we can find where something doesn't give the right ability to do work.
In an expanding universe there isn't the time symmetry we want for Noether's theorem, so it's all a bit of a moot point worrying about what "Energy" is. It isn't a conserved quantity and we can find examples where something gave us the ability to do a lot of useful work at t=early time but almost no no ability to do useful work at t = later time. Changing to a different time is just changing the "environment" (in the wider sense).
Best Wishes.
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In an expanding universe there isn't the time symmetry we want for Noether's theorem,
I'm pretty sure there is except at the moment of the big bang.
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That is still "hot" in comparison to absolute zero (0 k) specifically it should have thermal energy due to its temperature.
Er, no. It has temperature because of its thermal energy.
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It isn't a conserved quantity
That is the only definition of energy. Quantities conserved in classical mechanics include energy and momentum.
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Hi.
I'm pretty sure there is except at the moment of the big bang.
1. Even that would technically be enough. We need only one example for the original statement which was many posts ago. Take your energy source back to that environment (the big bang) and it has a different ability to do useful work.
2. There actually isn't time symmetry at most times. For example, electromagnetic waves drop to lower frequencies whenever they travel through expanding space. What we thought was Energy E = hf is just gone because f is lower.
This article gets a mention in the forum every now and again but it's been a while, so I'll put the link in here:
https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
That (...being a conserved quantity...) is the only definition of energy.
Yes, I think that was the main thrust of the discussion.
Er, no. It has temperature because of its thermal energy.
OK. Cause and effect are a little arbitrary. We can develop thermodynamics and state variables like Temperature and Pressure without needing to connect it to thermal energy or microscopic statistical mechanics. When we do connect them, there is a relationship but no reason why one must be the cause of the other rather than the other way round (as far as I can see anyway). However, if you want it that way round, that's OK.
Best Wishes.
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I think heat, or call it thermal energy is a poor choice for the original question, "what is energy". We know from thermodynamics that at most between zero and some fraction of the total may be converted to useful work. Electrical or mechanical energy can, in principle, be 100% converted.
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Arbitrary? We can add or remove energy from a body, using e.g. electrical power, impact, friction, boiling, radiation, or chemistry. We do not have a source or sink of temperature. I think this distinguishes between cause and effect!
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Hi.
I think heat, or call it thermal energy is a poor choice for the original question, "what is energy". We know from thermodynamics that at most between zero and some fraction of the total may be converted to useful work.
Yes. Although I'd argue that's why it's a very good choice.
The original old school definition was something like this:
Scientists define energy as the ability to do work....
[Taken from US Energy Information Administration, https://www.eia.gov/energyexplained/what-is-energy/ ]
..energy, in physics, (is) the capacity for doing work.
[Encyclopedia Brittannica, https://www.britannica.com/science/energy ]
By your statement, we can just throw that definition away or else teach them that thermal energy is NOT energy, it's something else.
Arbitrary? (about thermal energy causing temperature)..
OK. It probably is more sensible that way round. In the original sentence where the issue appeared, we only needed the idea that the temperature was above 0, so there is thermal energy there.
Best Wishes.
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We just need the universe to cool down until it is cooler.
You'd need an expanding universe to stand any chance of the temperature continuing to fall (things like molecules slow down in an expanding universe and settle to a static co-moving co-ordinate position).
Before you say, "we'll allow an expanding universe because it is", if you allowed yourself an expanding universe then there are other examples we can find where something doesn't give the right ability to do work.
Been thinking about the bottle thing. I have a universe that is homogeneous, all equal temp and pressure. I have an insulated bottle full of gas at the same pressure as outside, and want to extract energy from it. I wait 20 billion years for the universe to cool and then extract energy from warmth of the bottle and from the pressure therein as well. Where did that come from?
Well for one thing, energy is not conserved in any non-static metric, and an expanding metric isn't a static one. So thermal energy tends to cool, moving rocks tend to approach zero speed, light redshifts into oblivion.
The bottle seems to represent the energy needed. In an expanding metric, the bottle has kinetic energy due to the sides of the bottle moving towards each other relative to that metric. The rigidity also provides a steady force doing work on the bottle as it resists the expansion of its sides that it would do if it were just free bits. That force•distance is work, and we eventually harvest that work when we open the bottle billions of years from now.
The original old school definition was something like this:
Scientists define energy as the ability to do work....
[Taken from US Energy Information Administration, https://www.eia.gov/energyexplained/what-is-energy/ ]
..energy, in physics, (is) the capacity for doing work.
[Encyclopedia Brittannica, https://www.britannica.com/science/energy ]
I don't think this is a well thought out definition. It certainly doesn't meet the definition of the kind of energy that is conserved in a closed system in a static metric like an inertial coordinate system. I have hot on one side, and cold on the other. I let them mix and the energy vanishes, violating said energy conservation laws.
It also directly contradicts the definition of entropy, which, if I google it, yields:
1. PHYSICS
a thermodynamic quantity representing the unavailability of a system's thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system.
So if I substitute say Brittannica's funny definition, one gets "a thermodynamic quantity representing the unavailability of a system's thermal capacity for doing work for conversion into mechanical work", which, if you boil it down, says entropy is the portion of capacity for doing work that has no capacity to do work.
No, find a better definition than Brittannica please, one that they're talking about when they say that energy is conserved.
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I have hot on one side, and cold on the other. I let them mix and the energy vanishes,
If I have a liter of water at 300K (room temperature) and another liter of water at 350K (from the hot tap) the total thermal energy is 650 kcal.
If I mix them I will end up with 2 liters of water at 325K (nice for shaving) with a total thermal energy of ....er....650 kcal.
Or does my handbasin not obey the laws of physics?
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Halc, the energy in your bottle after 10 billion years was always there, but unavailable to do work due to the lack of a cold sink or pressure differential. That is the problem with heat energy, it may or may not be able to perform work, depending on it's environment.
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Hi.
I like the idea of a force acting on the gas from the sides of the bottle as the universe expands. You (Halc) have obviously been thinking about this BUT we're not going to let you explain away the extra useful work that easily...
Been thinking about the bottle thing. I have a universe that is homogeneous, all equal temp and pressure. I have an insulated bottle full of gas at the same pressure as outside, and want to extract energy from it. I wait 20 billion years for the universe to cool and then extract energy from warmth of the bottle and from the pressure therein as well. Where did that come from?
You're being very human or scientist and trying to cling to the idea that energy should have been conserved and so the ability to extract more useful work might have come from somewhere. The walls of the container might be responsible for giving the energy we can extract from the presssure of the gas (compared to the external universe) but that is all.
Do this:
Assume the universe has increased in size by a factor k over the billion years or so. So the bottle should really be k times bigger but it isn't. OK... that shows the work available from the pressure MIGHT be due to work put in from the walls BUT we don't have to allow that to contribute to the useful work we can extract. Instead we can just waste that bit of work availability if we want.
Perform a "Joule expansion" of the gas in the bottle.
(https://upload.wikimedia.org/wikipedia/commons/thumb/1/10/JouleExpansion.svg/240px-JouleExpansion.svg.png)
We have a gas in one small portion of a container as shown in the diagram. We just pull out the wall between the two halves of the container. The walls of the container are insulated so that no energy flows in or out. What we have is an isothermal change but it's unlike any conventional isothermal expansion you can plot on a PV diagram. There was no well defined pressure or volume until after a while when the gas particles fill the available space. The thing just disappears off the PV diagram and re-appears later with a new Pressure and Volume. This is the textbook example of an irreversible expansion. This is expansion without extracting any useful work at all.
Anyway, hopefully you can see where I'm going with this. Perform a Joule expansion on your bottle of gas to get it to a volume that is the factor of k bigger, i.e. as if the bottle had expanded with the universe. We've just undone all the extra work that was put into the bottle of stuff by the walls in one go. However, the expanded gas hasn't lost all ability to give us useful work. The temperature of the gas has not changed. It is still hotter than the rest of the universe and you can still get useful work out of that.
So that's it... you can't explain that the ability to get some useful work out of the bottle (a billion years later) was due to the force exerted by the walls of the bottle moved through a distance as the universe expanded. At most, that gives you some extra useful work you could extract just due to the pressure, it does nothing to explain away the temperature difference that would exist a billion years later and from which you can now extract useful work.
The situation, I think is very much as you described later:
...energy is not conserved in any non-static metric, and an expanding metric isn't a static one.
and we could easily say the same about "useful work". The capacity of a thing (lets say an energy source) to provide useful work changes with time.
I don't think this is a well thought out definition.
I agree. In fairness, there are some GCSE physics syllabus' (syllabi ?) which are taught in UK schools which have been trying to move away from that old definition. However, most of them are still advocating that energy is thought of as some quasi-material thing rather than just being an abstract conserved quantity that appears in some models we use in physics.
(I get the impression that most of the contributors to this post agree with that. Energy is a complicated thing and a single sentence from encyclopedia Britannica isn't going to cover it).
@paul cotter, your statement seems OK. Except that there probably is some small portion of extra work available due to the pressure, which we can let Halc have. The energy due to temperature, we're not letting him have that. As you say, that was always in the stuff not given to the stuff by the walls of the bottle.
Or does my handbasin not obey the laws of physics?
That's OK @alancalverd. You might be a few posts behind. Your handbasin obeys the laws of modern physics just not the old school stuff. Unless it's an old school basin, they were great things with a tap that was cold and another that was also cold. They realy knew how to conserve energy in the old days.
Best Wishes.
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If you look at my wash basin (preferably before I've shaved) you can see that energy is always conserved but the efficiency of extracting it depends on the source temperature if the sink is not at absolute zero.
Remember Carnot: η = 1 - Tc/Th
That is as "old school" as it gets - at least "O" level 1960, if not James Watt. If there is any inconsistency with whatever they teach in school today, someone needs to inform everyone who designs builds or uses engines today, or educate a few teachers: physics is about making mathematical models of what happens, not finding pointless ways of confusing students.
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To return to the original question, I propose the following: energy is the capacity to do work with the limitation that in the case of thermal energy some or all( worst case ) will not be able to do useful work. This is the reality because thermal energy manifests as random particle motion and hence has a degraded potential for work.
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Not according to Carnot or Watt. Kinetic energy is kinetic energy. If you have one last atom whizzing around at random in your elastic cylinder it will eventually collide with the piston and make the train move - as long as the temperature outside is lower!. Physics is relentless.
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Ok, but to get full use of your thermal energy you would need an infinite 0°k sink.
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Absolutely true!
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If I have a liter of water at 300K (room temperature) and another liter of water at 350K (from the hot tap) the total thermal energy is 650 kcal.
If I mix them I will end up with 2 liters of water at 325K (nice for shaving) with a total thermal energy of ....er....650 kcal.
Or does my handbasin not obey the laws of physics?
That is the problem with heat energy, it may or may not be able to perform work, depending on it's environment.
You both make my point, which is that the laws of physics uses a different definition of energy than the one Britannica uses. The dictionary one isn't conserved in a closed inertial system. The energy you're both taking about is.
Ok, but to get full use of your thermal energy you would need an infinite 0°k sink.
Similarly, to get full use of something's kinetic energy, one requires an infinite mass against which to react.
Energy seems to be an abstract concept, not actually a thing.
What's the kinetic energy of a rock? Well that's entirely a frame dependent question, so kinetic energy isn't something that just exists, it is simply a computed relation relative to an arbitrary frame. That kinetic energy can only be harvested for work if there is other mass with a relative velocity.
Potential energy has similar issues. What's the PE of the moon? I suppose again that it is a relation with another object. It has less PE in relation to the JWST and more in relation to the Earth. What PE does it have in isolation? There's actually and answer to that as well but they can only be negative. Does an object having negative potential energy mean that work cannot be extraced from its PE? Of course not. There are all sorts of power generating stations harvesting useful work from the potential energy of say water by simply making it even more negative.
So again, what's the PE of the moon? The answer could be answered if there was a place to put it that was infinitely far from other masses, but of course there is no such place.
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Please remind me again of the dictionary definition of energy and that defined by the laws of physics.
To the best of my knowledge mgh = ½mv2 = msT in all classical physics texts, and the units of work (F.Δx) are the same as those of energy.
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Similarly, to get full use of something's kinetic energy, one requires an infinite mass against which to react.
No. That would give you maximum transfer of kinetic energy from one object to another. Since the infinite object wouldn't move, you could hardly describe it as useful work!
Energy seems to be an abstract concept, not actually a thing.
Absolutely, which is why modern primary teaching is wrong. It is a conserved quantity in classical physics, and until you understand the notion of conservation, it is meaningless. It is no more a "thing" than speed.
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Hi.
I don't know where to start and I'm sure I will miss some comments. Sorry - but rest assured that I'm grateful for any time anyone has put in here.
It is a conserved quantity in classical physics, and until you understand the notion of conservation, it is meaningless. It is no more a "thing" than speed.
Energy seems to be an abstract concept, not actually a thing.
I propose the following: energy is the capacity to do work with the limitation that ...(sometimes it isn't)....
So what is Energy?
.....
2. A quantity, just some number which we might call E. It just turns out that many physical systems have a quantity which is conserved...
I think we're all in agreement that "Energy" is an abstract concept and not some quasi-material thing.
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physics is about making mathematical models of what happens, not finding pointless ways of confusing students.
OK but that doesn't have to hold for online science forums, surely.
Obviously the aim is not to confuse students, which is why many of the early posts had comments like this:
The remainder of this post is complicated and just for general interest. Don't add it to your answers for school level physics....
However, it can't be a bad thing to have a place where some people can discuss stuff and occasionally point out that some of the models or ideas that were taught in schools and presented as science aren't really all that simple.
Best Wishes.
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Just another thought.
Die stamping is a means of transferring all the kinetic energy of the ram into the work required to deform the blank into a coin or whatever you are making. There may be some losses as noise, but the principle is clear. Much the same applies to splitting a log with an axe. Hitting a bird with a shotgun pellet does a lot of useful work in destroying its vital organs and if you are really clever the residual kinetic energy usefully makes it drop where you want it to.