# What is energy

Ghetalion

Is energy simply kinetic interactions between matter BY matter? My understanding is that chemical and heat are both electromagnetic and are simply photonic flux.

Locrian
It's a scalar quantity associated with the state of a system. That's a pretty vague definition, but since we're in the QM forum, I think it's appropriate.

Ghetalion
Locrian said:
It's a scalar quantity associated with the state of a system. That's a pretty vague definition, but since we're in the QM forum, I think it's appropriate.

A state of a system

Meaning the position of matter within that system, correct?

Antiphon
Ghetalion said:
A state of a system

Meaning the position of matter within that system, correct?

There's more to it than position. But since this is a QM forum,
energy numerically defines the frequency of something.
We don't know know what that something actually is but mathematically
it looks like this:

$$\Psi = e^{i\frac{E}{\hbar}t}$$

Ghetalion
Antiphon said:
There's more to it than position. But since this is a QM forum,
energy numerically defines the frequency of something.
We don't know know what that something actually is but mathematically
it looks like this:

$$\Psi = e^{i\frac{E}{\hbar}t}$$

Frequency suggests a fluxuation of material density... doesn't it?

pmb_phy
Ghetalion said:

Is energy simply kinetic interactions between matter BY matter? My understanding is that chemical and heat are both electromagnetic and are simply photonic flux.
Nobody knows what energy is.

Pete

Homework Helper
"Energy" is a book-keeping device. It was noted pretty early on in physics that in simple collisions, two quantities mass*velocity and 1/2 * mass* velocity2 stayed the same which made them easy to use- one was called "momentum" and the other "kinetic energy". It was observed that the speed of an object (and so it's kinetic energy) increased as it fell, decreased as it went up. "Potential energy" was defined in order to keep "total energy" constant. Of course, if you added friction, you no longer had conservation of those energies but could observe that the temperature increased proportional to the lost energy. Okay, "heat energy" keeps the book keeping straight! As of the advent of relativity, because "e= mc2", we had to include mass itself as a kind of energy!

pmb_phy
HallsofIvy said:
"Energy" is a book-keeping device. ....
Nice post HallsofIvy. You are probably aware of this but that does not define energy. It only defines a property of energy. You'll note that total momentum also satisfies the property you just described.

I took a crack at this topic here

http://www.geocities.com/physics_world/mech/what_is_energy.htm

Pete

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Joethemonkey
first of all i'd like to point out that heat is not electromagnetic, Microwave is electromagnetic.. Heat is the vibration of particles in a substance...

secondly....... its late and i am too tired to finish this post.... *laught out loud* hopefully i'll remember to add another concluding post later... Goodnight

Joethemonkey
oh by the way i mentioned microwave,. because it transfers into heat energy ... *Microwave* ovens use microwaves to heat up food... check out some articles on "howstuffworks.com" that website has some real good information on electromagnetic waves.. light and radio,. energy... a great information source for this kind of thing... Hopefully HowStuffWorks will be able to answer your question,. as i am too lazy and tired right now

DaTario
I would put it like this:

Energy is a class of mathematical combinations of quantitative aspects of one system and its enviroment which forms a way to compute the maximum work that can be done by this system on the parts of its enviroment.

Best Regards

DaTario

Masoud
In my opinion, the best definition of energy would be embedded within M-Theory.

Masoud Zargar

DaTario
Masoud said:
In my opinion, the best definition of energy would be embedded within M-Theory.

Masoud Zargar

Could you please write this best definition of M-theory ?

Best Regards

DaTario

Telos
Maybe we should ask the Department of Energy?

Zelos
im getting sick of this question. ppl ask what energy/matter/charge is. this are proepties matter and things have. its not like its particles that sourond other particles

Telos
Well, Zelos, this is a little different. There isn't a Department of Matter or a Department of Charge.

Telos
And there's a State Department. Can't tell if it obeys Pauli Exclusion, though. They all look the same.

mcah5
According to Feymann, energy is just a book-keeping device with the very useful property that energy is conserved. The conservation of energy comes from the fact that experiments are time symmetric (aka "things do not depend on the absolute time.)

Edit: I can't spell Feynman

Telos
That's very interesting, mcah5, because for me, being a human in modern civilization, if I don't have energy, I die.

What is it that I call "energy," then, since it is more than a bookkeeping device? Is it "energy transfer?"

Staff Emeritus
Gold Member
Dearly Missed
Telos said:
That's very interesting, mcah5, because for me, being a human in modern civilization, if I don't have energy, I die.

What is it that I call "energy," then, since it is more than a bookkeeping device? Is it "energy transfer?"

No; if you don't have the right chemical processes, you die. Energy, and its derivatives like free energy, are just abstractions from that. They are important to a description of what is happening, but they are part of the model, not part of the reality.

Zelos
if we can define energy we can denife matter as high concentration of matter and therefor is the matter about matter solved

inha
that makes no sense at all.

Telos
No; if you don't have the right chemical processes, you die. Energy, and its derivatives like free energy, are just abstractions from that. They are important to a description of what is happening, but they are part of the model, not part of the reality.

I understand your last statement, but your first has confused me. You seem to be saying that biochemistry is the basis for our concept of energy. Because that is from whence we abstract the notion?

How are the "right chemical processes" not also an abstraction? Why just yesterday I heard someone accuse another of having the wrong chemical processes.

mcah5
Fine, if you don't like the book-keeping definition then energy is the capacity to do work =)

I think the main problem of trying to define energy is that energy is not quantized, and so we can't imagine a chewy nugget of energy. Personally, I could care less about the "real" definition of energy; problem sets are hard enough without dealing with philosophy.

Locrian
mcah5 said:
I think the main problem of trying to define energy is that energy is not quantized,

Are you sure?

mcah5
I'm pretty sure. You could argue that E=hf means energy is quantized, but f can be any value. And I'm pretty sure there is no lower bound on the frequency of light.

pmb_phy
mcah5 said:
I'm pretty sure. You could argue that E=hf means energy is quantized, but f can be any value. And I'm pretty sure there is no lower bound on the frequency of light.
The assertion that energy is quantized is not meaningful. Before one can state that energy is quantized one must first specify what you're speaking of, i.e. energy of what??

If E = hf refers to the energy of a photon then it can't be assumed that E only comes in certain values. But if E refers to the energy state of an atom then E is quantized. In quantum mechanics there are systems which have a continuos energy spectrum and those which have a descrete energy spectrum. For example, the energy of a free particle is not quantized. The energy of a particle in a box is quantized.

Pete

pmb_phy
mcah5 said:
Fine, if you don't like the book-keeping definition then energy is the capacity to do work =)
That holds true for potential energy and kinetic energy. So that can be taken as a definition of either. But that does not give you a definition of energy, only instances of energy. What am I talking about you ask? Suppose you ask what the definition of life is. If you give examples of life then you've not actually defined life. E.g. a human being is a life form. An ant is a life form. Yet with that knowledge I still don't know what a life form is.

Pete

Telos
Once again we don't seem to have found a satisfactory answer.

Remarkably, we seem to have come to the conclusion that energy isn't real. It only exists in our models and not in reality. How strange that the entirety of physics would depend on something that isn't real? Or am I mistaken?

Enos
It exist and it doesn't. That's the simplest one can put it. It exist because it takes up space. It doesn't exist because it's value is zero.

That's what I think anyway.

pmb_phy
Telos said:
Remarkably, we seem to have come to the conclusion that energy isn't real.
WHAT??? Where did you get that impression???

Is the number 4 real? Is height real? Is life real? Don't hold that something is not real because you've been unable to define it.

Pete

Telos
Enos, that almost sounds like energy "is" space. After all, space tautologically takes up space and has an unwarpped equilibrium value we might as well call zero.