I What is the Abstract Definition of Energy?

[jbriggs444]

Essentially 'energy' is a form of disturbance in an otherwise flat/silent medium.

A brick on the top of a wall (*) has potential energy without involving any waves or medium.

(*) More pedantically, the brick+earth+wall system has and maintains this potential energy.

 

[CarlM]

I suppose my brain associates it with the object because the Earth isn't going to go flying toward the 5 kg object.

Yes it does. You just do not see it that way.

 

[Drakkith]

Anything in the universe emits some form of radiation- this means that at the point the thing exists there are vibrations/waves/whatever you want to call them. 'Energy' is these waves. The higher temperature a thing has, the more energy it contains so more waves are produced/emitted. When an object stops emitting/having waves, that object ceases to exist as there is no energy to sustain its existence.

That's not quite correct. Energy has a specific definition in physics, though it is a bit abstract and can take different forms. What you've written is not how energy is defined. Energy isn't required to sustain the existence of objects, though it may be required to sustain a specific state of an object.
Give this article a read: https://en.wikipedia.org/wiki/Energy

Note that the precise definition is a mathematical one. All of the other text is simply an attempt to explain an abstract concept non-mathematically.

 

[sophiecentaur]

I used to tell the (14 yr old) kids that "Energy is what makes things happen. Without energy being used, nothing happens". Then I would sit back and wait for the class to come up with things they though proved me wrong (always good fun for kids). I managed to come up with an answer that showed what I meant. Except "OK Sir, what about dying?" The BS level used to up a bit at that point.

 

[member 563992]

What about symmetry breaking? I'm not sure if there's anything more fundamental to it but I always go back to the vacuum ground state. We also say there that it's the lowest form of energy however it seems a bit "funny" if you'd try not to keep entropy into discussion. What's bothering me is the spontaneous symmetry breaking with which I guess everyone is familiar with this data: http://www.fnal.gov/pub/today/images/images08/iSGTW050908.jpg

But I may be talking non sense in the end as I'm a complete ignorant on the mathematics of all of this anyways. For some reason I'd love to see beyond the CMB (gravitational waves) and hoping for an answer over there.

 

[gleem]

I have mentioned this in other threads, There is something called the "Flame Challenge" instituted by Alan Alda to promote the communication and understanding of science. This year the challenge is to explain "What is Energy" to an 11 year old. The award for the best answer is announced June 3. The award is judged by 5th and 6th graders from around the world. The finalists and their responses are posted at http://www.aldakavlilearningcenter.org/practice/flame-challenge/past-challenges/energy three are videos and three are written.

What do you think do you think there is a real winner?

 

[nasu]

You mean is the winner "really" a winner? :)

 

[gleem]

As far as we are concerned.

 

[nasu]

If the winner is decided by the 5-6 graders themselves, how do we know that the result of the "best explanation" is an understanding of the real concept rather than some misunderstanding induced by a simple, attractive, funny and wrong "model"?

 

[gleem]

But if a correct explanation is not appreciated what good is it. Your trying to explain concepts at an 11 yr old level. This thread shows some difficultly with trying to explain energy to a more mature person.

 

[Orodruin]

But if a correct explanation is not appreciated what good is it. Your trying to explain concepts at an 11 yr old level.

If the 11-year-old thinks it is a brilliant explanation and gets the completely wrong idea, what good is it?

 

[gleem]

Have you read the six finalist presentations? All seem qualified to give an explanation. I think the take away from this exercise is that you find what the kids are receptive to and then fine tune it to be scientifically sound if need be. After all the winner's explanation is not automatically incorporated into any curriculum. An who know what is currently being taught at this level.

 

[RonL]

I am probably at the 11 year old level in many ways, but still I try... If I have a flywheel spinning and it has a peg extending out of the rim, I hit it with a bat and the impact increases the speed by 10 rpm, the energy is transferred at impact, but the speed increases at a later time, where does the energy reside between impact and final speed ?
I'm not a strong advocate of mind over matter, but some things are being developed that make me set up and take notice

 

[nasu]

The speed increase during the impact and not at a later time. It is a good idea to get the facts right before looking for an explanation. :)

 

[nasu]

But if a correct explanation is not appreciated what good is it. Your trying to explain concepts at an 11 yr old level. This thread shows some difficultly with trying to explain energy to a more mature person.

I did not say that is wrong for a good explanation to be appreciated. The problem is that the two do not necessarily go together. Appreciation is not a guarantee of correctness. A good explanation will have both. If you look at Feynman's "explanations", he never makes up stuff that is not true or is irrelevant and still succeeds in being attractive to the public.
Probably would be a good idea for the clips to go through sort of peer review process (just for physics content, not artistic level). And then be judged for attractiveness.
I am not saying that the clips linked have some problem, I understand that we discuss the general idea, right?

 

[Victor Ray Rutledge]

I'll give my best shot at describing what energy is:

Anything in the universe emits some form of radiation- this means that at the point the thing exists there are vibrations/waves/whatever you want to call them. 'Energy' is these waves. The higher temperature a thing has, the more energy it contains so more waves are produced/emitted. When an object stops emitting/having waves, that object ceases to exist as there is no energy to sustain its existence.

Essentially 'energy' is a form of disturbance in an otherwise flat/silent medium.

Feel free to correct me if I sound too pseudo-sciencey.

Then the 'cold death', of the Universe, represents an empty void, since the structure of the time/space continuum appears to be contingent on its contents, and without contents, there is no universe.

 

[ZapperZ]

We have had question like this many times before, even one recently like this:

https://www.physicsforums.com/threads/what-actually-is-energy.905850/

I even had compiled a list of numerous threads on this very topic (anyone else remembers that?), but now I can't find that post anymore.

My response to this remains that same. Find something that you THINK you know, and ask yourself, "What is that [insert subject here]?" Look at your answer and see if you think what you describe is REALLY what it is.

Zz.

 

[FactChecker]

I can't help but comment that I think the definition of energy as "the ability to do work" is absolute genius. When I think about all the dissimilar ways that energy presents itself, as steam pressure, light, heat, altitude, velocity, compressed spring, etc., etc., etc., it amazes me that it has been consolidated into a useful concept and definition. That may make it difficult for a person to appreciate, but it should be looked at with great admiration rather than criticism.

 

[Jimmy]

We have had question like this many times before, even one recently like this:

https://www.physicsforums.com/threads/what-actually-is-energy.905850/

I even had compiled a list of numerous threads on this very topic (anyone else remembers that?), but now I can't find that post anymore.

It was an old blog post that doesn't exist anymore. I did find it on the Wayback Machine:
https://web.archive.org/web/20120217062315/https://www.physicsforums.com/blog.php?b=3203

Actually, if you're expecting an answer to that question, you'll be sort of disappointed. I won't answer it here.

But after coming across ANOTHER thread asking "What is Energy?", I kinda sigh and then wondered if this is the question most often asked on PF. It might be. I did a quick search and found these thread asking for the same thing:

https://www.physicsforums.com/showthread.php?t=507345
https://www.physicsforums.com/showthread.php?t=508051
https://www.physicsforums.com/showthread.php?t=498497
https://www.physicsforums.com/showthread.php?t=153105
https://www.physicsforums.com/showthread.php?t=445072
https://www.physicsforums.com/showthread.php?t=359856
https://www.physicsforums.com/showthread.php?t=316894
https://www.physicsforums.com/showthread.php?t=314845
https://www.physicsforums.com/showthread.php?t=275297
https://www.physicsforums.com/showthread.php?t=247178
https://www.physicsforums.com/showthread.php?t=232810
https://www.physicsforums.com/showthread.php?t=209185
https://www.physicsforums.com/showthread.php?t=180809
https://www.physicsforums.com/showthread.php?t=174712
https://www.physicsforums.com/showthread.php?t=161069
https://www.physicsforums.com/showthread.php?t=141360
https://www.physicsforums.com/showthread.php?t=130490
https://www.physicsforums.com/showthread.php?t=85718
https://www.physicsforums.com/showthread.php?t=97845
https://www.physicsforums.com/showthread.php?t=75227
https://www.physicsforums.com/showthread.php?t=69954
https://www.physicsforums.com/showthread.php?t=61450

Phew! There's a few more, but they were going back to 2003 and older, and at that time, PF didn't have that strict (yet) of a rule. So you do run into a lot of very speculative and crackpottish responses.

Also interestingly enough, a lot of the threads run afoul of the rules (in the later years) and often ended up being locked. So a "What is energy?" thread does not meet a very pleasant ending at times.

Zz.

 

[sophiecentaur]

but the speed increases at a later time, where does the energy reside between impact and final speed ?

First of all, have you actually observed this happening the way you describe?
If you examined what went on a bit more closely (slo mo camera) you would have seen the impact, in which the bat and the peg were in contact for a finite time. During the contact, there was some distortion of both, which temporarily stored some elastic potential energy. During contact, the batter could also have been pushing against the resistance of the peg. The flywheel would have gradually (over the contact time) accelerated and the bat would have slowed down. Once the contact ended there would be no further acceleration - if you assume the flywheel was rigid. If there were spokes involved the hub could accelerate after the contact time (catching up with the rim and the rim might have accelerated a bit as the peg straightened.
But, once contact had finished, there would be no more energy transferred to the wheel. There can have been no 'magic' store of energy; it must have all been in the strain of deformation of the bat and wheel.

 

[RonL]

First of all, have you actually observed this happening the way you describe?
If you examined what went on a bit more closely (slo mo camera) you would have seen the impact, in which the bat and the peg were in contact for a finite time. During the contact, there was some distortion of both, which temporarily stored some elastic potential energy. During contact, the batter could also have been pushing against the resistance of the peg. The flywheel would have gradually (over the contact time) accelerated and the bat would have slowed down. Once the contact ended there would be no further acceleration - if you assume the flywheel was rigid. If there were spokes involved the hub could accelerate after the contact time (catching up with the rim and the rim might have accelerated a bit as the peg straightened.
But, once contact had finished, there would be no more energy transferred to the wheel. There can have been no 'magic' store of energy; it must have all been in the strain of deformation of the bat and wheel.

The flywheel gaining speed at impact is what I'm blocked at, if the speed is 100 rpm and at impact the effect is an increase of 10 rpm, so in the space of maybe one degree of rotation the wheel is now slowing from an elevated speed of 110 rpm.
It just has my brain a little twisted, but I'll see it eventually :)

 

[Drakkith]

The flywheel gaining speed at impact is what I'm blocked at, if the speed is 100 rpm and at impact the effect is an increase of 10 rpm, so in the space of maybe one degree of rotation the wheel is now slowing from an elevated speed of 110 rpm.
It just has my brain a little twisted, but I'll see it eventually :)

What's the problem? If you apply a force over some time, there will be an acceleration and the angular velocity will increase. This is just basic mechanics.

 

[nasu]

The flywheel gaining speed at impact is what I'm blocked at, if the speed is 100 rpm and at impact the effect is an increase of 10 rpm, so in the space of maybe one degree of rotation the wheel is now slowing from an elevated speed of 110 rpm.
It just has my brain a little twisted, but I'll see it eventually :)

You really mean "slowing"?

How about a billiard ball being accelerated from 0 to about 10 m/s over a distance of the order of 1 mm, when hit with the cue?
Collisions are usually characterized by large changes in speeds over relatively small distances. And this is because the forces involved during collision are quite large but they act during a short time interval.

 

[sophiecentaur]

The flywheel gaining speed at impact is what I'm blocked at,

OK - not intuitive.
Take it one step at a time. It's easier to think in terms of a linear impact. A railway locomotive traveling at 60km/hour with a thick steel plate on the front, running into a soft squidgy beach ball. You can imagine the contact involving the ball squashing 'till it's almost flat and then bouncing off in front of the train. The impact would last perhaps 0.1s . You have also probably seen pictures and high speed film of golf balls (harder and denser); they go pretty flat during contact. But you don't see that in a game of golf. Now take a steel ball bearing, hit by the train. The steel is very hard and will deform only a small amount until the force is great enough to push the ball away. That process need only take less than 0.1ms but it still happens. In every case, once the collision is over there's no more force acting so no transfer of Energy.
Ones mind has a very narrow range of "what's reasonable". That's why Engineers and Scientists use numbers and measurements.

 

[RonL]

I have seen the golf ball deformation and I am back on track, just not seeing in the correct time frame appears to have been my blunder. I'll go back to what I understand better, a shot of compressed air driving a sliding vane of an air motor, a maximum pressure applied and then energy applied through expansion.
Thanks to each that replied

 

[Mark Harder]

Hi, Abstracted Universes, and welcome to PF!
To me, "energy is the capacity to do work" is actually both intuitive and abstract...

Not quite. Heat is a form of energy, but not all heat can be used to do work. Some just leaks away. In the thermodynamic statement of the 1st law, the change in the internal energy potential of a system is the sum of heat absorbed by the system minus any work done by the system. If a change in internal energy was just the work done, there could be no exchange of heat with the system's environment. But this is true only in special, so-called 'adiabatic', systems. To take one example, there's no other place with which the universe can exchange heat, the universe is an adiabatic system. But creating a practical system that does work on its environment (or vice-versa) yet remains thermally isolated from that environment is a task that can only be approximated, by slowing the exchange of heat to the point where it may be neglected. Thermal energy always plays a role in real-world processes and cannot be simply ignored.

 

[ProfChuck]

Is there a difference between what something does and what something is? Most (if not all) scientific theories, definitions and descriptions are behavioral in nature. In the case of energy we know a lot about what energy does. We know how it can be transformed from one state to another, how it can be stored and released, how it can be focused or diffused and so on. But all of this is behavioral description. Can we truly understand anything in terms of its underlying existential nature? Behavior can be observed and measured and models can be constructed that allow us to predict future outcomes. When these models work can we say we understand the underlying mechanisms or only that we appreciate some of their observable properties?. It seems to me that to ask "what is energy" reveals an attempt to transform behavioral properties into existential identity. I don't think science is equipped to do that.

 

[ZapperZ]

Is there a difference between what something does and what something is? Most (if not all) scientific theories, definitions and descriptions are behavioral in nature. In the case of energy we know a lot about what energy does. We know how it can be transformed from one state to another, how it can be stored and released, how it can be focused or diffused and so on. But all of this is behavioral description. Can we truly understand anything in terms of its underlying existential nature? Behavior can be observed and measured and models can be constructed that allow us to predict future outcomes. When these models work can we say we understand the underlying mechanisms or only that we appreciate some of their observable properties?. It seems to me that to ask "what is energy" reveals an attempt to transform behavioral properties into existential identity. I don't think science is equipped to do that.

It doesn't, and I think most people do not realize that the things that they THINK they know about, such as an apple that I used in my response to the thread that I linked earlier, are also described by a series of its properties and behavior.

So asking for "what is energy?" and wanting an answer that is not a description of its properties is a bit puzzling. It is as if there is another way to answer the question "What is X?" without giving a series of description of the property of X.

It isn't just in Science.

Zz.

 

[Dadface]

If it moves or if it can make things move it has energy.

 

[member 563992]

It is as if there is another way to answer the question "What is X?" without giving a series of description of the property of X.

Oh. That sounds a bit of/like Wittgenstein.