What Is the True Nature of Energy?

[Schrodinator]

I understand that. It is a matter of exchanging words for words. Still the question remains as to what is energy? The question remains because the capacity to do work fits everything except possibly zero point energy. The point is that when one asks what is force or what is energy, one is asking what is cause? Capacity is another trade off word for cause. So, what is capacity?

James

Capacity is not a trade of, it is a measure of volume or an amount, it means what it means. In this example it means the quantity with which an entity is bestowed which enables work to be done.

This definition fits zero point energy to btw.

And what you are asking is unknown, the laws of nature are only reducible so far. Might as well say God done it though if you are looking for a why instead of how question that goes further than the smallest iota. God is the only first cause without a cause ok, happy.

Why does F=ma?

I don't know it just seems to be the way it works.

Why?

I don't know?

Why?

I don't know.

Ad nauseum.

 

[Dale]

Energy is force times distance.

No it isn't. Provide a credible reference or stop saying something that you know is wrong.

You have been provided the definition of energy many times with several credible references. It is becoming obvious that you have a hidden agenda and are not interested in anything else.

 

[Schrodinator]

Much as I hate using the W word. I did say what W was earlier. Wor, er woor oh you know what I mean,it's that W word not energy that equals force x distance.

W = \int_C \mathbf{F} \cdot \mathrm{d} \mathbf{s}

This integral has a limit of the speed of light.

In physics, mechanical work is the amount of energy transferred by a force acting through a distance in the direction of the force. Like energy, it is a scalar quantity, with SI units of joules.

Like being the operative word, work and energy not being the same thing exactly.

Energy = mass x speed of light^2
E=mc^2

Is what energy is in maths jargon.

or e=hf if you are quantumizing it up.

 

[superg33k]

If a body gives off the energy L in the form of radiation, its mass diminishes by L/c².

Einstiens paper
http://www.fourmilab.ch/etexts/einstein/E_mc2/www/

And the mass diminished in this quote is equal to the relativistic mass of the photons. The conservation of relativistic mass is the conservation of evergy (of course the conservation of rest mass isn't true).

Everytime you increase an objects energy (possibly giving it kinetic energy), you increase its mass. Everytime you increase its mass, you increase its energy. To measure its mass, you are measuring its energy. They are equivilent, they are the same thing: Energy is mass.

BTW, when someone disagree's with me please let me know which bits you disagree with.

And I'd evem go further into quantum mechanics to say that energy is mass is frequency. But it'd probably be simpler to argue me on the E=mc2 front.

Don't you wish this forum has a thumbs up/thumbs down thing like you tube so we could see what everyone agree'd with?

 

[Dale]

And the mass diminished in this quote is equal to the relativistic mass of the photons. The conservation of relativistic mass is the conservation of evergy ...

Everytime you increase an objects energy (possibly giving it kinetic energy), you increase its mass. Everytime you increase its mass, you increase its energy. To measure its mass, you are measuring its energy. They are equivilent, they are the same thing: Energy is mass.

In the first paragraph, you correctly used the term "relativistic mass". The second paragraph would be correct if you also continued to use the term "relativistic mass".

Usually in modern physics, the unqualified term "mass" refers to the invariant mass rather than the relativistic mass. The reason is what you have identified here, that the relativistic mass is equivalent to total energy, so there isn't a need for a separate name.

So, if we use the usual approach that "mass" refers to invariant mass then it is true that anything with mass has energy, but it is not true that anything with energy has mass. Therefore, energy is not mass, at least not in modern usage.

 

[superg33k]

In the first paragraph, you correctly used the term "relativistic mass". The second paragraph would be correct if you also continued to use the term "relativistic mass".

Usually in modern physics, the unqualified term "mass" refers to the invariant mass rather than the relativistic mass. The reason is what you have identified here, that the relativistic mass is equivalent to total energy, so there isn't a need for a separate name.

So, if we use the usual approach that "mass" refers to invariant mass then it is true that anything with mass has energy, but it is not true that anything with energy has mass. Therefore, energy is not mass, at least not in modern usage.

Thanks Dale. In lectures we always use mass as rest mass but after reading that conservation of mass is equivalent to conservation of energy I figured everyone else must use mass as relativistic mass. Anyway, I will use mass as rest mass from now on. Therefore (rest) mass is not always conserved.

From my prevo Einstein quote rest mass can be converted into photons.

Would you agree that (rest) mass is a form of potential energy?

 

[Dale]

Would you agree that (rest) mass is a form of potential energy?

Yes.

 

[SpectraCat]

"IMO, if you want an answer that cannot be directly expressed mathematically you are asking for something that cannot be given by physics."

Energy is expressed mathematically as fxd.

No it is not. Energy is a path independent state function of a system. Work is a path-dependent quantity, and in fact is only defined in terms of *changes* in energy between two systems. That's why we define energy as a measure of the capacity of a system to do work.

What's more, the work done moving a mass from point a to point b is NOT a fixed quantity. The difference in energy between that mass when it is at point a and when it is at point b *is* a fixed quantity .. that is why we call it a state function.

 

[JJBladester]

@DaleSpam, @SpectraCat, @Drakkith, et al.

I haven't chimed in much since originally posting the question "What is energy?" I have read all posts (all 10 pages thus far) and am developing a much closer idea of it than before.

Energy describes the state of a system or the state of two or more interacting systems. Energy is a quantity that tells us what is happening (in terms of movement) or what could happen if things change. When I say "things change" I am referring to the position of certain masses with respect to other masses or a change in the velocity of one or more masses.

You can touch mass but you cannot touch energy. You can feel the effects of energy but not energy itself.

Back to my original post... You eat food which gives your body and muscles a capacity to do work (*energy). The work you perform is defined by how you push the door (which has rest mass). You may press straight-on or press the door at an angle; you may press the door near the top or bottom, too. You may also vary the magnitude of your pressing of the door.

*(The door has energy just by the fact that it has rest mass but I'm not talking about that here.)

I like to think of rest mass as something you can touch. If it's not rest mass, it cannot be touched. Relativistic mass is energy that acts as mass but is not mass and cannot be physically touched.

When I say "acts as mass," I am referring to the inertial property of mass (that mass resists a change in acceleration due to an outside force).

With a statement like "this system has energy," one would be stating that the system can "do stuff," namely, the system, or some of its parts, can speed up, slow down, or change directions.

 

[Dale]

You can touch matter, and matter has mass, but I am not sure you can touch mass.

There are many other properties of matter, such as volume, color, charge, etc., would you also say that you can touch volume, color, charge, etc.? If so, then why would you single out energy as being the one untouchable property, and if not, then why would you single out mass as being the one touchable property?

 

[superg33k]

Personally I am still having difficulty with the whole circular thing:

Energy is ability to work
Work if forces times distance
Force is mass times acceleration
Mass is potential energy
And I guess potential energy can become energy

It makes me thing that all these terms cannot be independant.

As well as I am having difficulties making crystal clear the differences between energy and potential energy.

If anyone thinks they can give a nice clear explenation to me that would be awesome. Otherwise if I'll have to think how to formulate a clear question.

 

[JJBladester]

You can touch matter, and matter has mass, but I am not sure you can touch mass.

There are many other properties of matter, such as volume, color, charge, etc., would you also say that you can touch volume, color, charge, etc.? If so, then why would you single out energy as being the one untouchable property, and if not, then why would you single out mass as being the one touchable property?

Matter is anything that has mass and takes up space. Mass is "how much matter something has". Mass is simply a quantity telling us how much matter is present.

I made a bunch of really good points and you jumped to a very minor detail which I hope to have made clear above. Anyway, what about the rest of my post?

With the statement I've made above, you could just use the word "matter" where I had used "rest mass" in my previous post and I think what I had said would be correct from there...

 

[Dale]

I made a bunch of really good points and you jumped to a very minor detail which I hope to have made clear above. Anyway, what about the rest of my post?

I thought the rest was fine. The part about energy describing the state of a system is good. And you specifically mentioned the definition of energy in the eating food bit. And replacing mass by matter when referring to touching makes it all good IMO.

 

[Dale]

Personally I am still having difficulty with the whole circular thing:

Energy is ability to work
Work if forces times distance
Force is mass times acceleration
Mass is potential energy
And I guess potential energy can become energy

It makes me thing that all these terms cannot be independant.

There is an enormous difference between independent and circular. These terms are clearly not independent as you can write mathematical expressions relating each of them to the others. However, just because they are all related (not independent) does not in any way imply that their definitions are circular.

Mass and energy are equivalent as discussed above, but they are defined differently. I won't repeat the definition of energy yet again, but mass is defined operationally. I.e. there is a specific physical experiment (balance scale) which can be performed which will yield a number, and that number is called mass.

When you go through all of the chain of definitions for any physical quantity you find that they are all eventually defined in terms of a series of one or more experiments which can be performed to determine the quantity. In the end this is what prevents physics definitions from being circular.

 

[James A. Putnam]

"No it isn't. Provide a credible reference or stop saying something that you know is wrong.

You have been provided the definition of energy many times with several credible references. It is becoming obvious that you have a hidden agenda and are not interested in anything else."

Weidner, Sells, Elementary Modern physics, 1960, Allyn and Bacon, pages 69-70; "We next ask, 'What is the relativistic kinetic energy K?' To find this, we define, as in classical physics, the kinetic K to be the total work done in bringing a particle from rest to the final speed v under a constant force F."

They go on to derive Einstein's energy equation beginning with the integral of force times distance.

Robert Resnick, Introduction to Special Realtivity, 1968, Wiley, Pages 120-121 "In Newtonian physics we defined kinetic energy, K of a particle to be equal to the work done by an external force in increasing the speed of the particle from zero to some value u. That is, (He goes on to derive Einsteins energy equation beginning with the integral of force times distance) were F*dl is the work done by the force F in displacing the particle through dL."

Energy is force times distance. After it is calculated its sume total can be equated to other quantities and qualities such as 1/2mv^2.

I have no hidden agenda. The question of "What is energy" is not answered by saying it is the capacity to do work. It is answered by saying that it is the sum total of force times distance. The answer ends there because there is no answer for what force is. There is nothing wrong with running out of answers. There is something wrong with not pursuing meanings that exchange words for other words. Real meaning should be pursued until scientific explanations run out. Then we know where to search for extending our knowledge of the fundamentals.

James

 

[superg33k]

There is an enormous difference between independent and circular. These terms are clearly not independent as you can write mathematical expressions relating each of them to the others. However, just because they are all related (not independent) does not in any way imply that their definitions are circular.

Mass and energy are equivalent as discussed above, but they are defined differently. I won't repeat the definition of energy yet again, but mass is defined operationally. I.e. there is a specific physical experiment (balance scale) which can be performed which will yield a number, and that number is called mass.

When you go through all of the chain of definitions for any physical quantity you find that they are all eventually defined in terms of a series of one or more experiments which can be performed to determine the quantity. In the end this is what prevents physics definitions from being circular.

Thanks Dale. I don't know if it was your explination or my baked beans for dinner but I think I get it...

There is a bunch of stuff we measure directly, like mass, position, time. Theres a bunch of stuff we measure indirectly like momentum (by measuring its mass and speed) and energy (mass and speed again). Some of these are always conserved, like energy and momentum. Obviously not all of these are independant.

When I think about the world, I like to think there is some 'real' indepandant quantities, and some other made up useless ones (or for simplifying maths). But which are the 'real' ones, the ones we measure directly, or the ones which are conserved? A question left for the philosphizing.

Annoyingly this is the way I've always thought about it before wondering why. Now I have wondered why and nothing has changed.

I still can't decide if rest mass is energy or potential energy. I think its energy. All energy internal to the object in discussion I think is energy, all energy from its position in space (i.e. external to the object) is potential energy.

Why am I so stupid?

 

[Dale]

Weidner, Sells, Elementary Modern physics, 1960, Allyn and Bacon, pages 69-70; "We next ask, 'What is the relativistic kinetic energy K?' To find this, we define, as in classical physics, the kinetic K to be the total work done in bringing a particle from rest to the final speed v under a constant force F."
...
Robert Resnick, Introduction to Special Realtivity, 1968, Wiley, Pages 120-121 "In Newtonian physics we defined kinetic energy, K of a particle to be equal to the work done by an external force in increasing the speed of the particle from zero to some value u."

These are different from the usual "energy is the capacity to do work" definition, but they are clearly not "energy is force times distance" either. These definitions are in fact the time-reverse of the usual "capacity to do work" definition, and since Newtons laws and SR are time-reverse symmetric they are equivalent in those contexts. Furthermore, they only apply to kinetic energy, and not to any other form of energy.

This certainly doesn't support your case at all, try yet again.

 

[Drakkith]

I have no hidden agenda. The question of "What is energy" is not answered by saying it is the capacity to do work. It is answered by saying that it is the sum total of force times distance. The answer ends there because there is no answer for what force is. There is nothing wrong with running out of answers. There is something wrong with not pursuing meanings that exchange words for other words. Real meaning should be pursued until scientific explanations run out. Then we know where to search for extending our knowledge of the fundamentals.

I've already given you the definition of force. Again, this comes back to you not accepting the definition we have given it. You can argue that it doesn't explain it all you want, in the end science has defined it as such. If you want to continue to disagree, then you should re-read the rules to the forums you agreed to when signing up.

 

[James A. Putnam]

These are different from the usual "energy is the capacity to do work" definition, but they are not "energy is force times distance" either. These definitions are the time-reverse of the usual "capacity to do work" definition, and since Newtons laws and SR are time-reverse symmetric they are equivalent in those contexts.

This certainly doesn't support your case at all, try yet again.

I don't have to try again to get it right. I just need to try again to communicate that the word work is a stand in for change of energy. If the force is a constant, it can be taken out of the integral and K=fxd. Here it can be plainly seen that kinetic energy is force times distance. The word work can be substituted, but it adds no clarity to explaining "What is energy?" I will grant you that the use of the word work is wide spread in texts. Energy is presented as, what I also have referred to, the sum total of force times distance. That sum can be evaluated for a variable force at any instant of time and it will be recognized as energy. The energy is changing. The word work refers to the change in energy. If this is unacceptable to you, then, from my point of view, nothing is lost by rephrasing the original question to: What is work? The change in words still leaves us with explaining what force is or admitting that we do not know how to explain the existence of either work, presumably an abstract way of referring to the fact that change of position is ocurring, or energy, another abstract way of referring to the fact that change of position has occurred. Since energy can be calculated at any point along the way, energy is another abstract way of referring to the fact that change of position is ocurring.

With regard to your 'hidden agenda' remark: My point has to do with the unknown nature of force or cause. All empirical evidence involves patterns in changes of velocity resulting from the action of force or cause. We know about the patterns. We know about effects. We do not know about cause. Presumably there is a cause for the existence of energy. That cause is unknown. Whatever it is that is convertible between energy and mass, it is not a sum total. It is a transferrence of that unknown cause. If you see this as a hidden agenda, so be it. I leave it in your hands. My answers exist elsewhere. I just thought this was an important question to clarify that we do not know what energy is any more than we know what work is or that we know what force is. We only know the effects of force. The resulting effects can have whatever names we wish to give them, but, the introduction of those names do not answer our questions. Best wishes to you.

James

 

[James A. Putnam]

I've already given you the definition of force. Again, this comes back to you not accepting the definition we have given it. You can argue that it doesn't explain it all you want, in the end science has defined it as such. If you want to continue to disagree, then you should re-read the rules to the forums you agreed to when signing up.

What is the definition of force? You posted this: "In physics, a force is any influence that causes a free body to undergo a change in speed, a change in direction, or a change in shape." What is an influence? These questions have to do with physics, not language. Trading words for words explains nothing. When I ask: What is force? I am asking what property of the universe is responsible for change of velocity? It is ok to not yet know that property. It is not ok to pretend that changing the word force for influence is a physics answer.

James

 

[Drakkith]

What is the definition of force? You posted this: "In physics, a force is any influence that causes a free body to undergo a change in speed, a change in direction, or a change in shape." What is an influence? These questions have to do with physics, not language. Trading words for words explains nothing. When I ask: What is force? I am asking what property of the universe is responsible for change of velocity? It is ok to not yet know that property. It is not ok to pretend that changing the word force for influence is a physics answer.

James

It is a physics answer. It is THE physics answer. It is NOT ok to pretend that science doesn't know what it's talking about just because you disagree with how we define something. What property of the universe is responsible for the change in velocity? That IS force.

 

[James A. Putnam]

It is a physics answer. It is THE physics answer. It is NOT ok to pretend that science doesn't know what it's talking about just because you disagree with how we define something. What property of the universe is responsible for the change in velocity? That IS force.

Exchanging words for words is not doing physics. No one knows what force is including you. You only know about effects. Yes we know that force exists. In other words, we know that cause exists because effects exist. We do not know what cause is. All of our knowledge consists of observing patterns in changes of velocity. We do not know what those patterns exist.

James

 

[Drakkith]

Exchanging words for words is not doing physics. No one knows what force is including you. You only know about effects. Yes we know that force exists. In other words, we know that cause exists because effects exist. We do not know what cause is. All of our knowledge consists of observing patterns in changes of velocity. We do not know what those patterns exist.

James

I know exactly what force is as it is defined by physics. That is what you don't understand. I'm not arguing that some mystical unknown "thing" is or isn't force, which is what you seem to be implying. THAT is not doing science, that is something else entirely.

 

[James A. Putnam]

I know exactly what force is as it is defined by physics. That is what you don't understand. I'm not arguing that some mystical unknown "thing" is or isn't force, which is what you seem to be implying. THAT is not doing science, that is something else entirely.

You do not know what force is. What you know is a wordy definition that passes the real answer further down the line. Cause is not mystical. Cause is real. If that which remains unknown but is clearly proven to exist is to be referred to as mystical, then what is physics? Is physics restricted to only that which is currently taught. Certainly that is not true. Physics is about learning the nature of the universe. The nature of the universe is still out there waiting for us to discover it.

 

[Drakkith]

You do not know what force is. What you know is a wordy definition that passes the real answer further down the line. Cause is not mystical. Cause is real. If that which remains unknown but is clearly proven to exist is to be referred to as mystical, then what is physics? Is physics restricted to only that which is currently taught. Certainly that is not true. Physics is about learning the nature of the universe. The nature of the universe is still out there waiting for us to discover it.

This has devolved into a pointless argument and I will take no further part in it. If you want to continue to believe that we don't know what force and energy is, then go ahead. You have simply missed the point entirely. Good day to you sir.

 

[KOSS]

In reply to Putnam's post #85, and most of the following discussion since,
I likened energy to money as an abstraction. JamesA.Putnam did not like it...

JamesA.Putnam said:

"Energy is like money? I read the earlier messages. Money is real. It can be held. It can be contained and tested for its physical effects. It is physical effects that concerns physics. Do we know what energy is or do we not know what it is? Likenesses, meaning analogies, do not count."

You missed the point James! Firstly, the analogy with money is only to aid understanding, most analogies are never intended to be exact correspondences, if so we wouldn't call them analogies. Besides that, JaredJames picked up the essence of the analogy well when he noted that "money doesn't exist"...you cannot say coins and dollars define money because while it's true they are forms of money, that doesn't cover many, many other forms of money, as JaredJames pointed out. This is again analogous with the confusion so many people have, when they think they have a definition of energy as a concrete reality, someone comes along and easily disabuses them of the notion because it cannot cover all forms of what we refer to as energy in physics. The work=energy definition is an example. It is not a sufficient definition. Just like coins are not a sufficient definition of money.

So, in short, the money analogy was never intended to "count" as a definition. The definitions of energy pertaining to relative states of systems and time translation symmetry are probably the best you are ever going to get, since, I submit, they cover everything that is considered "energy" in modern physics. It agrees with the use of the Hamiltonian in QM as the generator of time evolution. Furthermore, the relational definition is precisely what you are looking for when you say you are concerned only about the physical effects. It is indeed the relationships that count, and pretty much only relationships that ever count, all else is unnecessary add-ons (I claim, I'm not saying I'm an oracle who knows all ;-)

Aside: there are even recent attempts to place the foundations of quantum mechanics and quantum gravity on a purely relational basis (eg., http://arxiv.org/abs/quant-ph/9609002). That's not addressing the "what is energy?" question directly, but if the ideas have merit and become mainstream it would do a lot to ease future physics student's qualms about the question 'what is energy?'.I'll reiterate a bit more:
It's dopey to say energy is force times distance or capacity to do work, since that doesn't cover many forms of energy like heat energy with zero free energy and so forth. My plea is to realize how we use energy in physics. You should soon convince yourself that it is a relationship between states of a system, so it's purely an abstract concept, which BTW explains why there is all the confusion when folks think about trying to grasp and touch the stuff, or try to define it in concrete terms, you can't! Or if you do, you will find that you've only got a partial definition that doesn't cover other forms of energy.

 

[James A. Putnam]

In reply to Putnam's post #85, and most of the following discussion since,
I likened energy to money as an abstraction. JamesA.Putnam did not like it... ...(Putnam's dots to show that I deleted much of the quote.)...

I'll reiterate a bit more:
It's dopey to say energy is force times distance or capacity to do work, since that doesn't cover many forms of energy like heat energy with zero free energy and so forth. My plea is to realize how we use energy in physics. You should soon convince yourself that it is a relationship between states of a system, so it's purely an abstract concept, which BTW explains why there is all the confusion when folks think about trying to grasp and touch the stuff, or try to define it in concrete terms, you can't! Or if you do, you will find that you've only got a partial definition that doesn't cover other forms of energy.

I don't have strong feelings about the money analogy. I do prefer more direct answers, but, analogies can certainly be useful, so, for me, yours stands for what you explained it to be. I could do without the 'dopey' description. I would have stated it as "...energy is force times distance or capacity to do work... is a mathematical description. We don't have a physical description outside of patterns of effects. Heat is energy intransit, so I am not clear on your point of separating types of energy. I could use a little friendly further explanation of your point with regard to the zero free energy reference. I am willing to learn because I am driven to learn. It is just that I don't need to learn new words for expressing the same thing. I am not saying that you do this. I am only requesting that explanations delve into the actions of physical properties at least to the extent that they are presently known.

 

[SpectraCat]

I don't have strong feelings about the money analogy. I do prefer more direct answers, but, analogies can certainly be useful, so, for me, yours stands for what you explained it to be. I could do without the 'dopey' description. I would have stated it as "...energy is force times distance or capacity to do work... is a mathematical description. We don't have a physical description outside of patterns of effects. Heat is energy intransit, so I am not clear on your point of separating types of energy. I could use a little friendly further explanation of your point with regard to the zero free energy reference. I am willing to learn because I am driven to learn. It is just that I don't need to learn new words for expressing the same thing. I am not saying that you do this. I am only requesting that explanations delve into the actions of physical properties at least to the extent that they are presently known.

James,

You never responded to my post #158 ... would you care to?

 

[James A. Putnam]

No it is not. Energy is a path independent state function of a system. Work is a path-dependent quantity, and in fact is only defined in terms of *changes* in energy between two systems. That's why we define energy as a measure of the capacity of a system to do work.

What's more, the work done moving a mass from point a to point b is NOT a fixed quantity. The difference in energy between that mass when it is at point a and when it is at point b *is* a fixed quantity .. that is why we call it a state function.

SpectraCat, I missed this one: Energy is not path independent. It is true that the final calculation of the sum total of force times distance is path independent. However, all along the path followed by force times distance, the calculation can be made instantaneously and that sum total will be energy. So the change in energy follows the path of force times distance. It is only the use of two different words that makes energy as the final sum total path independent while its word susbstitute work is acknowledged to be path dependent. In any ncase, you used the word capacity. What is capacity that it should clarify the cause of change of velocity?

Your second point, I think, is clarified by my same answer. So long as vector type changes of direction are taken into account, work can vary wildly and so can energy. It does remain the case that final results are independent of the path taken on a generalized work diagram. If, I make instantaneous calculations of sum totals continuously during the path traveled, I might even say, a generalized energy diagram. However, formally speaking, keeping to separate terminologies, your point is well taken. I am not so interested in the debate about whether the word work or the word energy is more textbook appropriate for a given circumstance as I am interested in the cause of changes of either. That cause must exist but no one knows what cause is. Until cause can be explained, we are only speaking about effects. If energy is only another word for specific effects, then, I find the position that effects can be convertible into matter to be incomplete. Perhaps this point is viewed here as a separate subject. Thank you for your message. Any corrections you have to offer to my opinion are welcome.

 

[SpectraCat]

SpectraCat, I missed this one: Energy is not path independent.

Yes, it is. It is a state function. The amount of kinetic (or potential) energy a body has is determined only by its state at the time of measurement ... the past history of the body is irrelevant to its energy. As you can see, the integral of "force times distance" is most assuredly NOT independent of past history .. it is *defined* by past history. The quantity represented by the integral of "force times distance" is called work in physics. Work is a *change* in energy of a system. In order for a system to do work on its surroundings, it must end up with less energy than it started with. This is why we say that energy is a measure of the capacity to do work.

It is true that the final calculation of the sum total of force times distance is path independent. However, all along the path followed by force times distance, the calculation can be made instantaneously and that sum total will be energy. So the change in energy follows the path of force times distance.

Now you are perhaps getting the idea .. that last statement is close to correct ... work is the *PATH-DEPENDENT CHANGE* in energy as a force is applied to a system over a particular path. If you have two well defined states of a system, the energy difference between those two states will also be a well-defined quantity. However the work required to move the system between those two points is NOT well-defined until you have specified a path (i.e. a trajectory where the position and force vectors are specified at every point).

It is only the use of two different words that makes energy as the final sum total path independent while its word susbstitute work is acknowledged to be path dependent. In any ncase, you used the word capacity. What is capacity that it should clarify the cause of change of velocity?

You seem to think this is all just semantics ... it is not. Energy and work have the same units, but they are not interchangable concepts for the reasons I gave above.

 

[KOSS]

Hi again JamesP, SpectraCat et al., "once more unto the breach dear friend,..." and maybe a few more times after this.

Replying to JP's point:

"Heat is energy intransit, so I am not clear on your point of separating types of energy."

My point is that energy clearly exists in different forms, kinetic and potential and work, and then as an eigenvalue of an operator in QM formalisms, etc., etc.. None of these forms of energy suffices in itself as a definition or explanation of "what energy is". So, my point is that to seek an explanation for "what energy is" in any particular form, such as the Work theorem (which is just a definition of Work in terms of other observables, and hence is a relational concept, it is not a definition of energy) is futile. You'll never pin down energy that way. To be fair, I haven't defined or attempted to definitively answer the original question posted by JJBladester. All I've attempted is an argument for why a rational physicist might realize or accept that our current concept of energy is a pure abstraction, an abstraction nonetheless that has immediate and motive physical consequences---it provides a framework for doing physics, viz. the Hamiltonian formalism of classical mechanics and the operator formalism of QM. And an abstraction nonetheless that can be crisply and unambiguously defined in various forms, unified, I think, by the notion of time translation invariance in CM and time evolution in QM.

If you look at his JJ's post, he provides a few examples of types of energy. So I rest my case on this point---that there are multiple forms of energy, none of which can or should be taken as a universal definition, though each is fine and clear as a definition of one form of energy in the given context & scope intended (which isn't always spelt out, which may add to some of the confusion about this topic).

Even the "energy as a measure of the capacity of a system to do work" definition is incomplete. As you picked up on JP, there is also "free energy". And a system with zero free energy does not have a capacity to do net work! So once again, we see an attempt to wrap things up with an all-encompassing definition of energy based upon one form or use of the concept of energy which fails.

Note, free energy = 'the energy in a physical system that can be converted to do work'.
So when a system has zero free energy, such as a gas that has reached thermal equilibrium, or a dead battery, then there is no capacity to convert energy to work, and yet the system (pick one) is clearly not at absolute zero temperature, so it still has heat energy. But talk to the experts on thermodynamics for further elaboration, since thermodynamics is not my particular specialty.

Next, JP wrote:

"I am not so interested in the debate about whether the word work or the word energy is more textbook appropriate for a given circumstance as I am interested in the cause of changes of either. That cause must exist but no one knows what cause is. Until cause can be explained, we are only speaking about effects. If energy is only another word for specific effects, then, I find the position that effects can be convertible into matter to be incomplete. Perhaps this point is viewed here as a separate subject."

This is an interesting turn in the discussion, well worth exploring.

I may be out of my depth now, but I would suspect that physics does not seek to inquire into ultimate causes, even though naive physicists and even professionals slip into imagining they are delving into causation from time to time---I think that's a conceit and a delusion, but that's just MHO. "We" (the collective Borg ;-) gave this enterprise (seeking ultimate causes) up once "we" accepted the least iota of quantum indeterminism. Modern physics seeks only to inquire into fundamental laws, and seeks to uncover the most parsimonious laws needed to describe physical phenomena. How can we ever hope to deduce ultimate causes? I for one don't see how it is possible, but maybe I'm philosophically too unsophisticated to see something obvious here?

In other words, IMHO, although I think your (JP's) desire to want to know the causes of the changes in state of a system that require energy principles to explain them, is admirable and worthy, I do not think it is a physics problem. Feel free to argue otherwise, I'd be interested...

The physics problem is to find mathematical laws that predict the changes in state. The only physics answer I suspect is circular, which is not a bad thing. We use energy to predict changes in state or relational states, and energy is defined essentially as a predictor of changes in state. (More or less that's my overly crude summary of what physicists can say about causes.) The physicist is merely a tinkerer who tries to nudge the mathematical relationships to get the predictions conforming closer to observed reality. Seems like kind of a let down huh? But it needn't be viewed that way, just look at the triumphs of technology and blisteringly beautiful mathematics that such endeavours have spawned. To seek anything more from physics is probably akin to a god-delusion or god-complex, where physics is god. I for one don't want to go there.

You might as well just say that the machine masters that run our universe are the cause of these relationships and laws that we uncover. I think physics can only uncover the relationships, it cannot say much about ultimate causes. Whether you've agreed that energy is an abstraction or not, this I think is nearly the end of the road for a physics forum on the topic. While I admire the desire to want to know the ultimate causes of things, as a physicist I think one has to ultimately settle for a mathematical description. That needn't forbid philosophers from wondering about why the description must be such as the physicists discover. But it's a different game. Not a bad one, but different.

Here's another spanner in the works: who said causes must exist? If we live in a Multiverse of the type outline by Tegmark and others, then it is dumb blind luck that the laws and rules physicists uncover continue to be stable and seem to be consistent. There is no cause of things in a Multiverse, only patterns that can be treated by sentient beings observing them as causal. In a true Multiverse this is a delusion. I myself do not subscribe to such metaphysics, but I would find it hard, maybe impossible, to deny Tegmark his suppositions. Maybe he is right. I can't disprove his grand theory. My only pointer being that physics is on a very tenuous ground when it attempts to pronounce that there are known causes of phenomena. The sagacious physicists would probably prefer to just say, "this here <insert formalisms> is how I define energy, these are it's implications <insert consequences> and the causes of these relationships are metaphysical, not scientifically accessible."

I know that sounds like giving up, but it is not. It is merely demarcating the boundaries of what we scope out as "physics", separate from metaphysics and philosophy & ontology.

Could I just add: I think a lot of people get this confusion about physics. They look to physics for ultimate answers, which is a sort of conceit or prejudice of our modern enlightened scientific age, which has achieved so much for us technologically, but in real terms has done nothing to explain the "why" of the universe in anything other than purey abstract mathematical-relational terms.

Looking to physics for knowledge about ultimate causes is I think a fine pursuit and one that humanity should pursue. Indeed, this is one of the greatest things about physics---it reveals to us knowledge about causal structures, or at least the apparent causal patterns. But thinking that physics will provide the actual answer to the ontological questions of "what the causes are" is naive and I think a fore-doomed quest. And dare I use another analogy (only for helping colour my argument you understand!): imagine the inside of the proverbial elephant is an eternal black hole, so we cannot ever observe it's internal structure, but we can see it's outer form, which looks just like an elephant. Physics (the human scientific enterprise) stands kinda' in relation to our universe like an external observer trying to understand what makes the elephant tick, where the elephant is the universe, its' skin and air and tusks are what we can observe, and we are some sort of insects or bacteria living in it's skin desperately trying to understand it's behaviour and innermost laws.

Sorry if this poor poetic analogy doesn't satisfy ya. I'm doin' my best. you'll have to interpret further for yourself and maybe ask for more clarification about what I'm trying to say...unless you think I'm nuts---in which case ignore my ramblings.

 

[Dale]

I don't have to try again to get it right. I just need to try again to communicate that the word work is a stand in for change of energy.

No, you need to find a credible reference which actually agrees with your position. So far you have not been able to find any credible reference which defines energy as force times distance. Here is another source which contradicts your position: http://en.wikipedia.org/wiki/Work_(physics) it says W = f.d and that W = ΔKE. Therefore, by substitution f.d = ΔKE ≠ E.

Consider an object of mass 1 kg initially moving at 1 m/s in the absence of any external force. According to your definition, E = F.d = 0.d = 0, which is in contradiction to the actual kinetic energy of 1/2 J. If the mass collides with another mass then it can exert a force of 1 N for a duration of 1 s, during which it will have traveled a distance of 1/2 m. Thus the mass has the capacity to do F.d = 1 N 1/2 m = 1/2 J of work, in agreement with the standard definition of energy as the capacity to do work and in agreement with the actual kinetic energy of 1/2 J.

Now, consider other forms of energy. For example, consider an electron at rest in the absence of any external force, which has a mass energy of .5 MeV. Again, according to you E = F.d = 0.d = 0, which is in contradiction to the actual mass energy of .5 MeV. The electron may be anhilated producing a photon which can do .5 MeV work on any charged particle, in agreement with the standard defintion of energy as the capacity to do work and in agreement with the actual mass energy of .5 MeV.

Your definition is not just a different way to say the same thing, it is wrong. It is not supported by the two references you cited, it is contradicted by all of the references I have cited, and it demonstrably fails in the two examples shown above.

 

[JJBladester]

My point is that energy clearly exists in different forms, kinetic and potential and work, and then as an eigenvalue of an operator in QM formalisms, etc., etc.. None of these forms of energy suffices in itself as a definition or explanation of "what energy is". So, my point is that to seek an explanation for "what energy is" in any particular form, such as the Work theorem (which is just a definition of Work in terms of other observables, and hence is a relational concept, it is not a definition of energy) is futile. You'll never pin down energy that way. To be fair, I haven't defined or attempted to definitively answer the original question posted by JJBladester

First of all, KOSS, thank you very much for the time and effort you put into your thoughtful post. I read it twice through.

Ultimately, as I've stated before, energy describes the state of a system or the state of two or more interacting systems. Energy is a quantity that tells us what is happening (in terms of movement) or what could happen if things change. When I say "things change" I am referring to the position of certain matter with respect to other matter or a change in the velocity of one or more pieces of matter.

When you say "God-delusion," I am saddened that you must think that way. You put faith in physics. You don't know that in another few years a new theory will shatter our existing "knowledge". You don't know that the sun will rise tomorrow. "Now faith is the substance of things hoped for, the evidence of things not seen." This is from Hebrews 11:1. In other words, God is not explained by physics. Yes, physics is evidence that we, as humans, are making imminse progress. However, and I think you and everyone else will agree, we don't know the root cause for everything, particularly energy.

At any rate, regardless of your origin or religion, we do physics because curiosity demands it. That is the bottom line for me. I need to know, thus I find it valuable to spend my time studying and pondering the inner workings of this amazing and beautiful world.

Perhaps in doing so, we can make our neighbor's life easier. Perhaps we find a certain thrill that we can't quite explain. Physics is just cool to us and thus we are bound to it.

... And you're right, KOSS, physics is not God. God is God. If we were God, we would know the cause to everything as JP is hoping for.

 

[Drakkith]

At any rate, regardless of your origin or religion, we do physics because curiosity demands it. That is the bottom line for me. I need to know, thus I find it valuable to spend my time studying and pondering the inner workings of this amazing and beautiful world.

There is no problem in seeking answers. However, there is a problem in not listening when an answer is given to you.

When you say "God-delusion," I am saddened that you must think that way. You put faith in physics. You don't know that in another few years a new theory will shatter our existing "knowledge". You don't know that the sun will rise tomorrow. "Now faith is the substance of things hoped for, the evidence of things not seen." This is from Hebrews 11:1. In other words, God is not explained by physics. Yes, physics is evidence that we, as humans, are making imminse progress. However, and I think you and everyone else will agree, we don't know the root cause for everything, particularly energy.

No, we don't know what theories will appear in the future, however I will gladly accept the new theory provided there is sufficient evidence.

The search for a "root cause" of everything is futile. One can always ask "why". And what does god have to do with this? Please, leave God out of this.

 

[JJBladester]

There is no problem in seeking answers. However, there is a problem in not listening when an answer is given to you.

I have listened and I have thought about every post to my original question, 'What is energy?" There is no problem in my interpretation of what energy is according to the responses to the original question and my own seeking. Understand that it is okay to be both a physicist and a Christian simultaneously.

You seem very black and white.

 

[JaredJames]

This is the vacuum thread all over again.

We have energy, a clearly defined term in physics and we have people arguing the definition is wrong.

In the vacuum thread we had a clear definition and people arguing it was wrong because it didn't include EM radiation. Which is nonsense. What they were discussing was a different issue.

This is identical to what is happening here. Just because you arent happy with the physics definition, doesn't mean anything.

 

[Ryan_m_b]

When you say "God-delusion," I am saddened that you must think that way. You put faith in physics. You don't know that in another few years a new theory will shatter our existing "knowledge". You don't know that the sun will rise tomorrow. "Now faith is the substance of things hoped for, the evidence of things not seen." This is from Hebrews 11:1. In other words, God is not explained by physics. Yes, physics is evidence that we, as humans, are making imminse progress. However, and I think you and everyone else will agree, we don't know the root cause for everything, particularly energy.

No scientist has "faith" in science, we accept the conclusion that the current evidence shows. It's easy for people to mistake what scientists say as absolutes but in reality anything said is with the caveat "to the best of our knowledge...". You might want to look up the relativity of wrong (from which my signature comes from).

The fact that we do not have a thorough understanding of the formation of the universe does not mean we cannot define energy and certainly does not mean it's logical to slot god into the unknown

 

[aether seraph]

first let me say thank you for asking the question you did in the first place. most would not. i too am not content with the current understanding of "energy"

currently i believe it to be a result of the relationship between space and matter. the flip side to that model is that time is also a result of that relationship. I am not sure if my own belief will help u but it does help me hold the universe in my head.

interestingly the post which stated that gravity is the root of energy is directly in line with this model. if you would like me to elaborate i will. but i don't want to be out of line and de-rail your thread.

 

[Dale]

I would like to remind everyone of the rules that we each agreed to when we signed up. There are specific sections on overly speculative posts and on religious discussions. Please click on the link labeled "rules" at the top of the page for details.

This site is for discussing and learning mainstream science, not for promoting personal theories. The mainstream definition of energy is the capacity to do work.

 

[jollyPHYSICS]

Energy is simply the readiness for a body to assume an assignment over a distance in an time determined by the will called the force and manipulated/controlled/overseen by gravity/friction.

 

[James A. Putnam]

Koss,

A very well written and helpful answer.

"While I admire the desire to want to know the ultimate causes of things, as a physicist I think one has to ultimately settle for a mathematical description. That needn't forbid philosophers from wondering about why the description must be such as the physicists discover. But it's a different game. Not a bad one, but different."

Actually my point was that we do not know what cause is. It was not a question where I answer it or anyone else needs to answer it. I brought that point up to make clear, my viewpoint, that definitions such as 'force is influence' are unhelpful. The answer, as I see it is that 'we do not know what force is'. I see no problem with recognizing that which is known versus that which is not known. Effects are known. Cause is unknown. My own response to 'What is energy?' is that the answer depends upon explaining force, but, we cannot explain force. Therefore, I added to the discussion, in effect, that we do not know what energy is. I still see no value in debating that the acceleration of a particle who's energy is constantly in existence and changing according to the sum of differential values of force times distance, is really not a measurement of energy. If the book answer is different, let the book answer rule. As for me, I will continue to look beyond answers that are formed from mere exchanges of words.

Thank you for taking the time to write a detailed informative message.

James

 

[James A. Putnam]

Yes, it is. It is a state function. The amount of kinetic (or potential) energy a body has is determined only by its state at the time of measurement ... the past history of the body is irrelevant to its energy. As you can see, the integral of "force times distance" is most assuredly NOT independent of past history .. it is *defined* by past history. The quantity represented by the integral of "force times distance" is called work in physics. Work is a *change* in energy of a system. In order for a system to do work on its surroundings, it must end up with less energy than it started with. This is why we say that energy is a measure of the capacity to do work.

Addressing this in particular: "As you can see, the integral of "force times distance" is most assuredly NOT independent of past history .. it is *defined* by past history. The quantity represented by the integral of "force times distance" is called work in physics. Work is a *change* in energy of a system. "

I would appreciate your view on this: The integral of force times distance is independent of past history. That is why a constant force can be substituted to represent all examples of force times distance. That is why a constant force can be used to derive Einstein's energy equation.

Now you are perhaps getting the idea .. that last statement is close to correct ... work is the *PATH-DEPENDENT CHANGE* in energy as a force is applied to a system over a particular path. If you have two well defined states of a system, the energy difference between those two states will also be a well-defined quantity. However the work required to move the system between those two points is NOT well-defined until you have specified a path (i.e. a trajectory where the position and force vectors are specified at every point).

"Now you are perhaps getting the idea .. that last statement is close to correct ... work is the *PATH-DEPENDENT CHANGE* in energy as a force is applied to a system over a particular path. " I think I have the idea. Still, I would be interested in your explanation about why a change in energy is not a recognized as simply a change in energy, but rather, work?

You seem to think this is all just semantics ... it is not. Energy and work have the same units, but they are not interchangable concepts for the reasons I gave above.

I put forward the view that energy and work have the same units because they are the same thing. Any differences are semantical. The introduction of the word 'capacity' for purposes of mediating between the two is, from my viewpoint, semantical. Still, if the book answer must be that they are different things, then let the book answer rule. However, they both result from the integral of force times distance and they both suffer from the unknown nature of force. Is there an instance where work occurs that I cannot equate every step, even infintesimal steps, with the existence of energy?

I gave my contribution to this thread in a few earlier messages. If my explanations for my views are unacceptable, I don't need to push them, presenting them is enough. I do welcome input from others and I thank you for your explanations.

 

[Dale]

I put forward the view that energy and work have the same units because they are the same thing. Any differences are semantical.

This is not correct. The fact that in general E\ne \Delta E = W is not merely semantics. They are different quantities, not the same quantity by different names.

Is there an instance where work occurs that I cannot equate every step, even infintesimal steps, with the existence of energy?

No, but there are instances where energy exists that you cannot equate with the occurence of work. I gave some specific examples above.

 

[James A. Putnam]

This is not correct. The fact that in general x\ne \Delta x is not merely semantics. They are different quantities, not the same quantity by different names.

Thesum total is energy and the change is a change in energy.

No, but there are instances where energy exists that you cannot equate with the occurence of work. I gave some specific examples above.

I don't think that I am required to equate all examples of energy with work. Work is a name for a specific event. That event is a change in energy in specific circumstances. Those circumstances do not cover all examples of energy. In other words, the word energy will always apply while the word work is a limited substitute applied to a limited set of circumstances.

I did respond to your earlier message, but, I must have missed pushing the right button or perhaps it just got lost in cyberspace. Anyway the jist of my message was that I am retracting my shorthand description of energy is force times distance for my clearer statement that energy is the sum total of force times distance.

 

[Dale]

Work is a name for a specific event. That event is a change in energy in specific circumstances. Those circumstances do not cover all examples of energy.

Then it cannot be the definition of energy. The definition must cover all examples or it is not the definition.

Anyway the jist of my message was that I am retracting my shorthand description of energy is force times distance for my clearer statement that energy is the sum total of force times distance.

I understood that to be your meaning and the counter examples in post 182 disprove your definition.

 

[James A. Putnam]

Then it cannot be the definition of energy. The definition must cover all examples or it is not the definition.

No it doesn't.

I understood that to be your meaning and the counter examples in post 182 disprove your definition.

For me, your examples cited showed that you did not apply my intended meaning. The examples were clearly erroneous. Furthermore, a sum total exists between any two points. Even if those points are infinitesimally close, the sum total of the difference between those two points is still a sum total. Differential quantities of energy are not inherently different from accumulations of differential quantities of energy. The examples you cited do not represent disproval of my definition.

 

[Dale]

The examples were clearly erroneous.

Then show clearly the error. Apply your definition of energy to an electron at rest in the absence of any external force and show how you get the correct mass energy of .5 MeV and where I made my mistake in using your definition to get 0.

 

[James A. Putnam]

Then show clearly the error.

The error was that you repeatedly included a zero value for force on the basis that my position is simply that energy is the simple product fxd. You substituted zeros into that simple equation as if it was a valid disproval of my position. Zero force in those examples mean zero energy. I never intended that and I am not being that silly. I say that the calculation of energy exists for any calculation that can be called work. I say that, in general, energy is the sum total of force times distance between any two points. I say that units of measurement belong always to the same property. I would appreciate your view on that last sentence.

 

[Dale]

The error was that you repeatedly included a zero value for force on the basis that my position is simply that energy is the simple product fxd. You substituted zeros into that simple equation as if it was a valid disproval of my position. Zero force in those examples mean zero energy. I never intended that

What other value besides 0 could I possibly put in for force in a situation with no force? I am only using your definition as you have stated it.

If you feel that I worked them out incorrectly then kindly work out the two examples I posted in the way that you intended your definition of energy to be used.

 

[James A. Putnam]

Then show clearly the error. Apply your definition of energy to an electron at rest in the absence of any external force and show how you get the correct mass energy of .5 MeV and where I made my mistake in using your definition to get 0.

Did you change your message? I quoted you but the quote missed including the electron at rest question. Just wondering if I am doing something wrong in creating these posts?

An electron can exert force for an unknown reason. If there is nothing to exert that force upon, the nature of the electron is not changed. So far, the discussion involves differentiating between work and kinetic energy. I see no work performed by an electron that has nothing to exert its force upon. However, the electron retains its ability to exert force upon another charge particle should that circumstance occur. The page I am on now doesn't show your original message. I will end this message and again look up your previous message.