What is energy

DaleSpam

I don't know why this seems so complicated. Energy is the capacity to do work, and work is force times distance (dot product). Not very confusing at all. I think several of you are going out of your way to make this seem mysterious and complicated when it isn't.

vinniewryan

True, energy as a figure of math is just that. I was referring more to the physical reason for why energy works.

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.

vinniewryan

When I was thinking of 'energy' I was thinking alive energy. My bad.

Drakkith

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.

russ_watters

A book sitting on a table exerts a force, but expends/absorbs no energy.

Drakkith

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.

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.


@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." :)

Drakkith

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.

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.

DaleSpam

Yes.

If you like to be unnecessarily confused that is certainly your perrogative, but that is the definition of energy, nothing more.

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.

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.

Drakkith

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.

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.


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".

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%.

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!

pergradus

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.