# What is energy

by JJBladester
Tags: energy, energy conservation
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 P: 32 True, energy as a figure of math is just that. I was referring more to the physical reason for why energy works.
 Mentor P: 17,534 That isn't so complicated either. Energy works because the fundamental laws of nature are invariant under time translations. Per Noether's theorem, this leads to a conserved quantity: energy. I just don't understand why energy gets singled out as something particularly mysterious sometimes. It is no more nor less mysterious than any other physical quantity.
 P: 32 When I was thinking of 'energy' I was thinking alive energy. My bad.
Mentor
P: 12,005
 Quote by DaleSpam That isn't so complicated either. Energy works because the fundamental laws of nature are invariant under time translations. Per Noether's theorem, this leads to a conserved quantity: energy. I just don't understand why energy gets singled out as something particularly mysterious sometimes. It is no more nor less mysterious than any other physical quantity.
 Quote by vinniewryan When I was thinking of 'energy' I was thinking alive energy. My bad.
In response to DaleSpam, and using vinniewyran's post as an example, I believe that the concept that energy is "something" is due to the widespread portrayal of energy in media. (TV, Movies, Books, ETC)

As has been said already, energy is defined as the ability for a physical system to do work on another physical system.

Let's look at an example of a misunderstanding:

"Where does the energy come from to provide the force for gravity?"

Answer: Gravity doesn't require energy to provide it's force. Neither do any of the other funamental forces. There is NO expenditure of energy required for a photon to attract an electron or repel another proton. It simply happens. The conditions of the system, how far one particle is from another, and etc, are what we use to difine Energy. IE a 10kg weight dropped from a height of 10 m from the earths surface will aquire a certain amount of kinetic energy. This is measured by what happens when that weight impacts the ground. The weight will cause a certain amount of deformation in the dirt, throw up an amount of dirt into the air, ETC.

As vinnie's post states, many people have this concept of "energy" that all living things posess. As far as I know, physics doesn't agree on this. Everything a living organism does is a result of burning fuel to provide the energy necessary for it's cells to do their jobs. Hence how you will cease to live if you do not eat.
Mentor
P: 22,313
 Quote by JJBladester Feel like elaborating?
A book sitting on a table exerts a force, but expends/absorbs no energy.
 Mentor P: 12,005 Recently, scientists from Texas discovered something amazing: A 5th fundamental force. The believe that this force only manifests in very high energy situations and usually results in the destruction of all matter around the area. They named this force Chuck Norris after the only known example to date. Sorry. Just bought a new Chuck Norris fact book.
 PF Gold P: 287 I see that the subject of energy is touchy to some and deep to others. I don't disagree that seeing energy in terms of its manifestation (ability to do work) is bad... But I would also say that you have tunnel vision. There's a lot more going on and it's awesome! Physics is all about breaking physical systems down and understanding the causes and effects. It seems like we don't have a "cause" for energy at this point in physics. That's okay with me. Perhaps we will someday. Then we'll be asking what the cause of that is... and so on, until we meet our Maker. @DaleSpam Are you an engineer? I'm actually studying to become one, so your answers to "what is energy" seem like answers an engineer would give. However, I like to think of it as more than "the ability to do work." @Drakkith You crack me up. I was thinking "Really, a 5th fundamental force, that's great..." Then I saw "Chuck Norris." :)
Mentor
P: 12,005
 Quote by JJBladester I see that the subject of energy is touchy to some and deep to others. I don't disagree that seeing energy in terms of its manifestation (ability to do work) is bad... But I would also say that you have tunnel vision. There's a lot more going on and it's awesome! Physics is all about breaking physical systems down and understanding the causes and effects. It seems like we don't have a "cause" for energy at this point in physics. That's okay with me. Perhaps we will someday. Then we'll be asking what the cause of that is... and so on, until we meet our Maker.

We already have a defined cause of energy. Various particles and forces acting upon other particles and forces. The location and strength of these particles and fields determines the energy of everything.
PF Gold
P: 287
 Quote by Drakkith There is NO expenditure of energy required for a photon to attract an electron or repel another proton. It simply happens.
Drakkith, I appreciate your explanations, but the words "It simly happens" just don't sit well with me. Perhaps that is becuase I need to continue my study of physics.

Thanks to you and all for the replies to my question.
Mentor
P: 17,534
 Quote by JJBladester @DaleSpam Are you an engineer? I'm actually studying to become one, so your answers to "what is energy" seem like answers an engineer would give.
Yes.

 Quote by rede96 However, I like to think of it as more than "the ability to do work."
If you like to be unnecessarily confused that is certainly your perrogative, but that is the definition of energy, nothing more.
Sci Advisor
P: 1,395
 Quote by russ_watters A book sitting on a table exerts a force, but expends/absorbs no energy.
That definition would seem to indicate that energy is exclusively a transient quantity in physics, like work or heat. I don't think that is the case. For example, in statistical thermodynamics, the internal energy, total energy and enthalpy of a system are all state functions, meaning that they are path independent parameters of the system, like mass or temperature. So it seems that thermodynamically, that book has a non-zero energy even when it is sitting on the table. Even from a macroscopic classical physics definition of energy being "the ability to do work", that book has gravitational potential energy (due to the force on it), thus it has the ability to do work.
P: 156
 Quote by DaleSpam That isn't so complicated either. Energy works because the fundamental laws of nature are invariant under time translations. Per Noether's theorem, this leads to a conserved quantity: energy. I just don't understand why energy gets singled out as something particularly mysterious sometimes. It is no more nor less mysterious than any other physical quantity.
The problem is the assumption that these ideas we use like "energy", "force" and "field" describe real physical things.

Personally I just believe that they are convenient, but abstract concepts from which we can build up a set of equations that work. I really don't think anyone understands the actual physical mechanism by which two charges attract or on mass attracts another, but people have come up with ideas that for all purposes seem to be consistent with observation - however that doesn't make them real.
Mentor
P: 12,005
 Quote by JJBladester Drakkith, I appreciate your explanations, but the words "It simly happens" just don't sit well with me. Perhaps that is becuase I need to continue my study of physics. Thanks to you and all for the replies to my question.
I'm not sure why it doesn't sit well. Why does an electron attract a proton? Because negative charges attract each other. Well why is that? Because that's how it works. But why? Because it's how the universe works. See where I'm going? It doesn't matter how far we go down or how much we explain, someone can always ask why. At a certain point you simply have to accept the fact that that's just how it works.

 Quote by SpectraCat That definition would seem to indicate that energy is exclusively a transient quantity in physics, like work or heat. I don't think that is the case. For example, in statistical thermodynamics, the internal energy, total energy and enthalpy of a system are all state functions, meaning that they are path independent parameters of the system, like mass or temperature. So it seems that thermodynamically, that book has a non-zero energy even when it is sitting on the table. Even from a macroscopic classical physics definition of energy being "the ability to do work", that book has gravitational potential energy (due to the force on it), thus it has the ability to do work.
Don't confuse the different types of energy. The thermal energy contained in a book on a table is most definately not zero. The book has zero gravitational potential energy UNTIL it is either pushed off the table or the table is removed or whatever.

 Quote by pergradus The problem is the assumption that these ideas we use like "energy", "force" and "field" describe real physical things. Personally I just believe that they are convenient, but abstract concepts from which we can build up a set of equations that work. I really don't think anyone understands the actual physical mechanism by which two charges attract or on mass attracts another, but people have come up with ideas that for all purposes seem to be consistent with observation - however that doesn't make them real.

These "ideas" ARE what we observe and measure. The equations we used are 100% based upon observation of how the universe works. There is absolutely nothing abstract about them. The only thing abstract here is the idea that Energy is "something".
PF Gold
P: 287
 Quote by Drakkith These "ideas" ARE what we observe and measure. The equations we used are 100% based upon observation of how the universe works. There is absolutely nothing abstract about them.
And Newtonian Mechanics and Galilean Transformations work and are 100% based on observation... Oh wait... Until 1905 when Einsten crushed our idea of space and time.

We cannot take for granted what we "know". 100% is never 100%.
Mentor
P: 12,005
 Quote by JJBladester And Newtonian Mechanics and Galilean Transformations work and are 100% based on observation... Oh wait... Until 1905 when Einsten crushed our idea of space and time. We cannot take for granted what we "know". 100% is never 100%.
And? You aren't understanding what I'm saying. Energy will NEVER be something physical, somthing tangible. Why? Because it isn't! This isn't something that we just don't know about yet. Our definition of energy describes how different objects in a system interact with each other. When a weight falls to the ground we can describe precisely what the effects will be at impact because we know how fast it will hit, the properties of the weight and the earth, the acceleration due to gravity, the resistance of air, ETC. Simply saying that the weight has X amount of potential energy is a short way of describing all of that before it happens using known laws.

A similar example is temperature. What is temperature? It is NOT something physical or tangible either. It is a measure of the average kinetic motion of the particles that make up an object. Will we suddenly discover that temperature is "something" in the future? No!
P: 156
 Quote by Drakkith And? You aren't understanding what I'm saying. Energy will NEVER be something physical, somthing tangible. Why? Because it isn't! This isn't something that we just don't know about yet. Our definition of energy describes how different objects in a system interact with each other. When a weight falls to the ground we can describe precisely what the effects will be at impact because we know how fast it will hit, the properties of the weight and the earth, the acceleration due to gravity, the resistance of air, ETC. Simply saying that the weight has X amount of potential energy is a short way of describing all of that before it happens using known laws. A similar example is temperature. What is temperature? It is NOT something physical or tangible either. It is a measure of the average kinetic motion of the particles that make up an object. Will we suddenly discover that temperature is "something" in the future? No!
Uh, you just described what temperature is - the random motion of molecules. That is a physical description of the quantity we call "temperature".

There is no same physical description for why two charges attract, and how one charge "senses" another - they just do. We use this make-believe thing called an electric field to describe the interaction, but this idea of a "field" tells us absolutely nothing about the physical mechanism of the attraction.

And seriously, if you want to continue this stance that you are the guy who understands all of this, then you should know you're basically claiming to have a better understanding of physics than Richard Feynman. Youtube his lecture "the character of physical law" and humble thyself.
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P: 12,005
 Quote by pergradus Uh, you just described what temperature is - the random motion of molecules. That is a physical description of the quantity we call "temperature"
That was my intention.

 There is no same physical description for why two charges attract, and how one charge "senses" another - they just do. We use this make-believe thing called an electric field to describe the interaction, but this idea of a "field" tells us absolutely nothing about the physical mechanism of the attraction.
My description was not about how electric charges attract and such. It was only about what our definition of energy is. What do you mean by saying that an electric fiield is make believe? It is a way of describing how charged particles interact. How is that make believe?

 And seriously, if you want to continue this stance that you are the guy who understands all of this, then you should know you're basically claiming to have a better understanding of physics than Richard Feynman. Youtube his lecture "the character of physical law" and humble thyself.
I'll be sure to take a look at it when I get home. Until then, I'd be willing to bet he would agree with me. (But if he doesn't, then so be it.)
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P: 17,534
 Quote by pergradus The problem is the assumption that these ideas we use like "energy", "force" and "field" describe real physical things.
The word "energy" is a well-defined term with a clear and unambiguous meaning which has been experimentally measured. Unlike the word "real".

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