# What is energy?

1. Apr 14, 2004

### Math Is Hard

Staff Emeritus
I have trouble understanding and visualizing this concept because I want to think of something tangible. I am not sure if I should be thinking of something tangible, though.
Are photons energy, or are they a just a by-product of electrons jumping between energy levels?
Sorry for such a rudimentary question. I just started my first physics class this week and wanted to make sure I don't start off with any wrong assumptions.

2. Apr 14, 2004

### repugno

Measure of the ability of a system to do work.

Last edited: Apr 14, 2004
3. Apr 14, 2004

### Math Is Hard

Staff Emeritus
So it's a measurement only?
This is what got me curious:
The other night I watched that program "The Elegant Universe" and they talked about string theory. It was stated that matter at it's basest level was composed of vibrating strands of energy.
Doesn't make sense. Will I understand this later on?

4. Apr 14, 2004

### Les Sleeth

I think I know why it doesn't make sense to you. When one hears energy is the capacity to do work, and then one hears matter is energy . . . somehow it is hard to connect the two. I am not a physicist, but I have thought about how to explain what energy is.

As a term energy was probably first applied in the seventeenth century (borrowed from Aristotle’s energeia) to help explain the quality of motion, or “vis viva” in things. Today people who are spiritually inclined may speak of energy as well when referring to properties of consciousness, life, God, soul and ethereal peculiarities. Unfortunately the popularization of the energy concept has led to considerable misconceptions about it. Science writer Paul Davies writing in his book Superforce explains, “What made it appealing was that energy is always conserved, never created or destroyed.” Davies goes on to say, “When an abstract concept becomes so successful that it permeates through to the general public, the distinction between real and imaginary becomes blurred. . . . This is what happened in the case of energy. . . . Energy is . . . an imaginary, abstract concept which nevertheless has become so much a part of our everyday vocabulary that we imbue it with concrete existence.”

If we are to be accurate with our terms, then it must be understood that science claims first rights to the word energy and assigns it a very specific meaning, which is the capacity to do work. In science, energy is more of a mathematical and measuring tool of movement than anything actual. The thermodynamic law that states energy is “never created or destroyed” is really meant to support calculations in physics that gauge and record the path of movement power.

For example, the energy concept can help describe what happens in a mechanical system, say when electricity moves an electric motor. Fuel is burned and produces energy, which is transferred to a generator, which is transferred to electrical energy, and then back to kinetic energy as it turns the motor. If one adds up all the movement power used, plus that lost to heat and friction, it will total the amount of movement power, or energy, started with. So there was no energy created in the process and none was destroyed by the process; the power of movement was simply transferred from one thing to another.

And then there is me, who is afflicted with a disturbing need to understand things , and so has to wonder:

When the fuel is burned up and the generator stops, when electricity no longer flows and the motor comes to a halt, where is all that energy just used? Maybe it wasn’t destroyed, but it is gone from the system and it is gone for good. If it did survive,where did it go, and what was that movement power in the first place?

5. Apr 14, 2004

### turin

The term measure is does not mean measurement in this context. A different way to say it would be, "energy is the ability/potential to do work." Here, two synonyms are suggested. But, as pointed out by LW Sleeth, the physics concept of energy was never intended to be defined in words.

This is inaccurate (i.e. ambiguous). If one believes that E = mc2, then I've never heard of a macroscopic process in which this statement could be considered true. Certainly, this is not true in the stated example.

6. Apr 14, 2004

### Les Sleeth

To be honest, I made my post to test it for accuracy. So I am very open to being corrected if someone finds a problem with what I said. I am going to debate you now to see if I can make my case, or if I need to adjust how I think of energy.

I can't say I fully understand why you find a contradiction between E = mc2 and what I said. Was it me describing energy in terms of "movement power," or was it substracting heat and friction, or was it neither, both or . . . If energy is defined as the capacity to do work, how do we know work is done? Isn't it only when we observe movement? Matter may contain a lot of energy, but the only way we know that is when we release it and observe how it moves things.

It seems to me there are two ways of looking at energy. One is for everyday measurement; that is where we say energy is the capacity to do work. Then there is the theoretical aspect which recognizes a lot of energy is packed into an atom. However, it also seems more than energy is there because "c2" tells us light is present too. Hmmmmmmm . . .

7. Apr 14, 2004

### Tom Mattson

Staff Emeritus
That's quite a nitpick there, Turin. If the energy is low enough, we don't have to take relativity into account. The change in mass of the fuel during burning could not be more than a tiny fraction of an eV per bond broken.

It would be true enough for any mechanical engineer.

8. Apr 14, 2004

### Tom Mattson

Staff Emeritus
The flow of power will not be simultaneous with the emptying of the fuel tank. That last bit of electricity will continue to power the motor until it is used up. And as will all electromechanical loads, some will be converted to motion and some to heat. The temperature of the whole system will be (for a time) higher than it was before operation.

Don't forget that a "system" is an abstract concept that only exists in the minds of people. If we can draw an imaginary boundary around what we want to call a "system", and no matter or energy is transfered across that boundary, then we can apply the same conservation laws to that "system" that we believe hold for the universe in total. So, the energy is only gone from "the system" if we define "the system" not to include the surrounding air, or anything else to which the heat may escape.

9. Apr 14, 2004

### Les Sleeth

Thanks, I hadn't thought of that.

I see that, however . . .

(. . . and I am still "testing" my understanding) when I say it is gone from the system and gone for good, I mean once energy has passed through everything we understand as a system. If I am to take conservation as absolute, which it seems you are saying I should, and we consider the limits of the universe the extent of all systems, then I don't see how energy is necessarily conserved except as it passes through the relative subsystems that compose the overall universe. I say that because the universe is expanding, and the current thinking seems to be it is energy (of one sort or another) which is causing that. To me that means expansion must expend energy, and then that energy becomes unavailable not only to relative situations, but to the only absolute situation we know of: the universe.

So in the end, isn't energy (if we rely on current thinking) "disappearing"?

10. Apr 14, 2004

### Tom Mattson

Staff Emeritus
We saw in our motor example that the end product (after the motion has stopped) is heat, right? So, where does the heat go? If our system boundary is finite, the heat will eventually cross it and be transferred (by conduction, convection, or radiation) to a larger system, one which includes the air in the room.

But as the heat spreads, it will approach the boundary of that system and eventually cross it, and so on. But we will at some point reach an end to all this, such that the heat cannot cross any more boundaries, simply because there are no more boundaries for it to cross. Each subsystem will come to thermal equilibrium with every other subsystem, and their common temperature will be just a little higher than it was before I ran my motor.

The energy isn't being lost, the universe is getting hotter.

I'm a bit out of my depth here with general relativity, but what I do know is that the energy-momentum tensor, which is the source term for the tensor that describes the structure of spacetime, is written in terms of energy and momentum densities, not energy and momenta themselves. If I've got that right, then it's not that the energy-momentum is disappearing, it's that it is becoming more sparse. But when integrated over the whole of space, the total is a constant.

11. Apr 14, 2004

### Les Sleeth

This confuses me a bit because what I've read tells me the universe is getting cooler. The worry over the universe's heat death, for instance . . . see

I think I see what you mean however -- that the energy which seems to be disappearing is merely participating in "equilibrium." Yet I still have a logical concern . . .

Energy is commonly thought of as a distinct property. Yet even if it sacrifices itself to equilibrium, it really does in the end disappear as a distinct property. To me that suggests that either energy has left the system we call our universe, or energy was never really and truly distinct from the universe to begin with. In other words, whatever it is that created the universe might be some monistic property which some set of unrecognized conditions cause to appear as distinct and separate properties.

I know this is the classic physics area, and I shouldn't be talking so speculatively. But I can't resist pointing out that one cannot have it both ways. If energy is totally conserved, doesn't it imply unity at the root of everything?

12. Apr 14, 2004

### Tom Mattson

Staff Emeritus
Right, what I meant was that the universe is just a little hotter than it would be if the motor had not run. It does dump heat into the surroundings, but the expansion of the universe (which is a competing effect) is winning, and so the universe is cooling overall.

I don't know what you mean by "distinct", but maybe this will answer your question:

Heat is a form of energy.

So, when the temperature of a system rises or falls, its thermal energy content has changed. When (edit: if?) the so-called "heat death" of the universe occurs, all the energy that has ever existed will be unusable for work, and in theory that amount of energy will be equal to the total amount of energy (in all its forms) that has ever existed. It hasn't gone anywhere, it has just changed form.

Is that what you were getting at?

edit: re-wrote a part I didn't like too much

Last edited: Apr 14, 2004
13. Apr 15, 2004

### Les Sleeth

I wrote an answer and then deleted it after deciding this probably isn't the best place to be theorizing about energy losing its distinctness.

But I am still wondering about turin's criticism when I said, "if one adds up all the movement power used, plus that lost to heat and friction, it will total the amount of movement power, or energy, started with." Turin responded that "If one believes that E = mc2, then I've never heard of a macroscopic process in which this statement could be considered true."

Your reply seemed to be that if I were to be precisely correct, I'd need to take into account relativity. Would you (or anyone) explain why?

14. Apr 15, 2004

### zoobyshoe

15. Apr 15, 2004

### Les Sleeth

Thanks Zooby. Actually I understand that much already. What I am trying to understand now is turin's criticism. Is it because when I spoke of all the energy adding up in my example, I didn't include the energy still remaining in the burned fuel mass? If so, it is because I was only referring to the energy released for work as fuel is burned. So I am trying to understand if there is a problem with that latter idea or not. As I understand it, the energy released when fuel is burned moves through the system and does work, and the amount of work done plus what heat is lost equals the amount of energy originally released by the burned fuel.

16. Apr 15, 2004

### Tom Mattson

Staff Emeritus
In chemical reactions (such as burning fuel), the main source of energy comes from the breaking fo chemical bonds. It turns out that there is also a very slight mass difference after the reaction, and the lost mass is converted to energy a la Einstein (E=mc2). But the contribution of this mass to the total energy is so minute, that there is not a chemist or engineer in the world who uses E=mc2 in any theoretical model of chemical reactions. It is such a low-order effect that it is just not necessary, and the correction is below the threshold of measurement error anyway.

17. Apr 16, 2004

### Les Sleeth

I see, thanks. I am happy to know that.

I was looking at Math Is Hard's posts and realized no one had answered what had confused her. She pointed out the fact that in string theory matter is believed to be little vibrating strings of pure energy seems to conflict with the idea that the term "energy" is mostly an abstraction used for the purpose of measurement.

I have the same problem with how energy (as a word) is used, such as when someone says a photon is pure energy, yet light seems to maintain its integrity, at least as vibration, whether it gains or loses energy. In the way I described it to her, calling it "movement power," it is hard to imagine a little string of movement power, or a little string of capacity to do work.

18. Apr 16, 2004

### Tom Mattson

Staff Emeritus
Actually, it's an abstraction that is used for the purpose of theorizing. No one actually measures energy. Rather, experimentalists measure state variables that are used in mathematical functions which we call the different forms of energy.

Velocity is measured, from which kinetic energy is calculated.

Temperature is measured, from which thermal energy is calculated.

Position relative to a mass is measured, from which gravitational potential energy is calculated.

Frequency of a photon is measured, from which photonic energy is calculated.

Energy isn't measured, it's calculated.

That's a misuse of the term "energy". A photon is not to be identified with energy, a photon has energy, which as I noted above is calculated from its frequency.

Just as with photons, the same can be said of strings: They aren't to be identified with energy, they have an energy.

edit: fixed some tags

19. Apr 16, 2004

### Tom Mattson

Staff Emeritus
First thing to note is that energy is not a tangible substance. In fact, energy doesn't exist anywhere in the universe!

Energy has no reality apart from its (mathematical) functional dependence on state variables that do have reality (see my post to Les above).

As I just remarked above, photons are not to be identified with energy. Energy is a mathematical invention, but photons are real. Having said that, deexcitation of atoms is not the only way to produce them. They can be produced from any deexciting system (semiconductor, molecule, nucleus, nucleon, etc.) or by accelerating charges.

20. Apr 17, 2004

### Math Is Hard

Staff Emeritus
Thank you. I appreciate all the responses. This was the clarification I was looking for.