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Squashed
2007-Jan-18, 07:52 PM
What is energy?

That is the question.

It has been described as a attribute/property as in a property/attribute of matter.

But what is energy?

Is not energy just motion?

We have kinetic energy (motion) and potential energy (stored) but aren't they both the same thing but on different scales?

Kinetic energy is obviously motion, eh?

But then isn't potential energy just instilled motion?

Take for example chemical energy as released from burning hydrogen with oxygen, the energy contained within the uncombined hydrogen and oxygen atoms is released as photons when the burning takes place.

The photons carry "energy" but isn't the "energy" just momentum which has velocity as a sub-component of its value.

So momentum is motion.

So at the most basic understanding then isn't energy just motion?

So the chemical energy at the atomic level represents the ability of electrons to move faster around their nucleus when the two atoms are uncombined than when the two atoms are combined - so the chemical energy is just motion on a smaller oscillating scale that is converted to tangential linear motion when the burning takes place.

It seems to me that all of energy can be ultimately viewed in its most intrinsic form as just: motion.

There was also in another thread (ATM, shield your eyes) where time was ultimately tied to just motion and not a real dimension of the universe.

If this is true then the common thread to those two topics is: motion.

sirius0
2007-Jan-18, 08:30 PM
A very moving post Squashed! :)







Nevertheless I am trying to think of an intelligent answer.

BISMARCK
2007-Jan-18, 08:59 PM
Maybe this is just semantics, but I would say that energy isn't necessarily motion itself, but the ability to cause motion. Or, as some people say, the ability to do work, since work inevitably entails some kind of motion.

Jeff Root
2007-Jan-18, 09:22 PM
What is energy?
Something a person learns from experience with energy.
Since you use energy in everything you do, you have a lot
of experience with it. You should learn what it is from
that experience.

The more energy a thing has, the more work it can do. So it
is reasonable to say that energy is the ability to do work.



Is not energy just motion?
No. "Motion" is a catch-all term. A change in position over
time (translation) is motion. A change in orientation over time
(rotation) is motion.



We have kinetic energy (motion) and potential energy (stored)
but aren't they both the same thing but on different scales?
You used the word "scales" in a peculiar way, so I can't be
sure what you mean, but for any meaning of the word "scales"
that I'm familiar with, no, kinetic energy and potential
energy are not the same thing on different scales.



Kinetic energy is obviously motion, eh?
It obviously involves and requires motion.



But then isn't potential energy just instilled motion?
Huh?? Do you have any idea what "instilled" means?



Take for example chemical energy as released from burning hydrogen
with oxygen, the energy contained within the uncombined hydrogen and
oxygen atoms is released as photons when the burning takes place.

The photons carry "energy" but isn't the "energy" just momentum
which has velocity as a sub-component of its value.
No. If I haven't confused personalities and maybe even fora,
Ken has repeatedly told you that energy and momentum are
distinctly different quantities, one big difference being that
energy is a scalar and momentum is a vector.



So momentum is motion.
It obviously involves and requires motion.



So at the most basic understanding then isn't energy just motion?
No, but you are right that it involves and requires motion.



So the chemical energy at the atomic level represents the ability of
electrons to move faster around their nucleus when the two atoms are
uncombined than when the two atoms are combined - so the chemical
energy is just motion on a smaller oscillating scale that is
converted to tangential linear motion when the burning takes place.
No. That is a nice, simple Newtonian image, but it doesn't
describe real atoms at all. There is no relation between the
speed of electrons within atoms and the speed, momenta, or
energies of products of combustion, even when you take into
account the energy carried off by photons.



It seems to me that all of energy can be ultimately viewed in its
most intrinsic form as just: motion.
Most of the many different kinds of potential energy do not
require motion.

I have gravitational potential energy relative to the Earth
as a whole. That potential energy can be changed into kinetic
energy which involves motion, or it can remain potential.

Electrostatic potential energy between two electric charges
can be changed into kinetic energy which involves motion, or
it can remain potential.

* * * *

Afterburner asked exactly these same questions a few months ago:

http://www.bautforum.com/showthread.php?t=40813

Energy is involved in everything that happens. Motion probably
is also involved in everything that happens. They are not the
same thing.

-- Jeff, in Minneapolis

Dr Nigel
2007-Jan-18, 09:57 PM
To try to add a bit more clarification about different forms of energy:

Kinetic energy - motion

Electrostatic potential energy - stored energy by means of an electrostatic field. Note the absence of motion.

Magnetic potential energy - as above, but a magnetic field instead. No motion of the entity that has the magnetic potential energy (although a magnetic field is only generated by movement of charges).

Gravitational potential energy - picture a ball at the top of a hill. It is not moving, but it has the potential to move by converting gravitational potential energy into kinetic energy.

Thermal energy (heat) - this is incoherent vibration of molecules (in a liquid or gas) or by bending / stretching and relaxing of chemical bonds (in solids and liquids). There is movement, but nothing makes any net motion in any given direction.

Sound - this is also a vibration, but travelling as waves of compression / relaxation through a substance.

Light (or electromagnetic radiation) - a photon has energy that relates to its wavelength (remember, they all travel at the same speed). This energy is often represented as an oscillation in both an electric and a magnetic field. It is associated with electrons in particular (electrons can interact with photons in ways that transfer energy from one to the other - usually, this means the absorption or emission of a photon by an electron), but nuclear forces can generate very high-energy photons (gamma radiation).

Chemical potential energy - this has two descriptions. One is as the potential of a substance to participate in chemical reactions (often called its redox potential), which is a kind of macroscopic way of thinking. The other is in terms of the energy of individual electrons that are part of molecules or atoms, but in locations that allow the energy to cause chemical reactions to occur (reading this back, it sounds a bit obscure, but the truth would require a lot of explanation).

Nuclear potential energy - this is the energy "stored" in atomic nuclei that can be released in nuclear reactions (usually as either gamma photons or as high-velocity particles). If you look at a table of the precise atomic weights of all the elements, you can see that the average mass of a nucleon (i.e. a proton or neutron) changes from one element to the next. This change in mass is the energy of the strong nuclear force (remember that mass and energy are equivalent, E=mc^2).

So, no, energy is not motion, but several forms of energy involve motion of some kind, and the transfer of energy often involves motion.

I hope this helps.

hhEb09'1
2007-Jan-18, 10:09 PM
We have kinetic energy (motion) and potential energy (stored) but aren't they both the same thing but on different scales?

Kinetic energy is obviously motion, eh?

But then isn't potential energy just instilled motion?I too am curious about this remark.

An object that is said to have potential energy is sometimes relatively motionless. Like a rock perched on top of a cliff. How do you involve motion in that instance?

Aerik
2007-Jan-18, 10:36 PM
Energy is a measurement of how much work can be done. Contrary to what cartoons and new-age ignoramuses tell you, energy is not something you can measure - it *is* the measurement.

punkrockbong151
2007-Jan-19, 12:27 AM
energy is anything that needs a same or stronger resistence to stop or slow it, that easy

Aerik
2007-Jan-21, 08:13 AM
You seem very confused, punkrockbon151. "resistance" there would seem to be a force, and the "it" would be a mass, and no, energy cannot be "anything" no matter what vague criteria you attach to it. Energy isn't a thing - it's a measurement.

Another thing about energy is that it's a measurement that can only be made when it is traversing a gradient, that is moving from a high to a low. Take heat, for example. People often confuse heat and temperature. Temperature is a measure of the kinetic motion of particles in a volume of matter, and can be measured instantaneously at any time. Heat is a measurement of energy, and can only be measured when it is moving from a 'hot' object to a 'cool' object, using factors such as the masses of the objects, time, the before/after temperatures of the objects, and the 'specific heat' of the objects, which is quite another can of worms.

Energy is also modal. No type of energy is the same. Thermal energy, hydro-electric energy, kinetic energy, potential energy, mechanical energy... The medium matters.

hhEb09'1
2007-Jan-21, 01:09 PM
Another thing about energy is that it's a measurement that can only be made when it is traversing a gradient, that is moving from a high to a low. How do you measure potential energy? :)
Energy is also modal. No type of energy is the same. Seems tautological--if they're different types, they're different, not the same.

Dr Nigel
2007-Jan-21, 03:05 PM
How do you measure potential energy?

Calculate from the field potential, which can be measured.


Seems tautological--if they're different types, they're different, not the same.

Er ... yeah, that's what Aerik said. No two types are the same.

hhEb09'1
2007-Jan-21, 03:16 PM
Calculate from the field potential, which can be measured.How do you measure gravity field potential?
[quote]Er ... yeah, that's what Aerik said. No two types are the same.Yeah, if they are two different types, then they are not the same type :)

peteshimmon
2007-Jan-21, 04:38 PM
I have asked this before! Is matter in a
gravitating body doing work? Either attracting
other matter or holding matter up? :):):)

hhEb09'1
2007-Jan-21, 04:45 PM
I have asked this before! Is matter in a
gravitating body doing work? Either attracting
other matter or holding matter up? Depends. Can you give a specific example? Obviously, the Earth is doing no work on my computer, which is just sitting here on my desk (it doesn't have the supersecret password).

Ken G
2007-Jan-21, 05:53 PM
I would say that energy is a useful construct primarily because of the importance of forces, and also because forces tend to come as a function of position (called "conservative" forces, mediated by fields), rather than a function of time. If you have known forces as a function of position, then the concept of energy emerges naturally, because it is natural to ask what happens when an object is allowed to be acted on by that force, and it also natural to note that an object being acted on by a force will undergo the more significant change the farther it is allowed to move under the action of that force (you could also phrase it the longer the amount of time the force acts, but since forces are more often functions of position rather than functions of time, it is more useful to look at the distance traversed). All this gives you the concept of work, as has been mentioned above.

So what is the connection between work and energy? It is called the work-energy theorem, which basically asks, what dynamical property of the object is only a function of the force and the distance over which it acts? Given that the force will produce an acceleration, when you multiply (i.e., integrate) that by the distance, what you have is mass times acceleration times distance. If you think of this integral as being over time instead of over distance, then you replace each little distance segment in the integral by the speed times the time interval, giving you mass times acceleration times velocity times time interval. That sounds complicated, until you recognize that acceleration times velocity is the same thing as half the rate of change of the square of the velocity! So when you do a work integral for a force accelerating an object, you are really doing a time integral of the rate of change of 1/2*m*v^2. Look familiar? You end up with the change in kinetic energy, and that's where the connection between work and motion comes in most simply.

So we have the question, which came first, the concept of kinetic energy or the concept of work (which is potential energy, in the conservative-force point of view)? I think we see from the above that the answer is work, and potential energy, are actually concepts that then give rise to the concept of kinetic energy. But of course kinetic energy then becomes the crucially important concept in making things "happen", while potential energy is really just a kind of bookkeeping measure to make sure energy is conserved, like an accountant making the bottom line come out right. Still, if we weren't interested in forces that depend on location being in effect over known distances, we would never have conjured up the concept of kinetic energy.

Squashed
2007-Jan-21, 11:50 PM
A lot of interesting answers and here is a further elaboration on my part.

The formula for momentum is mass times velocity: p = mv

So there is definitely velocity (motion) present in momentum.

But then energy is momentum times velocity as we can see in these two energy formulas:

KE = 1/2mv2

E = mc2

Again I see only two components: mass and velocity.

When I think of "dark energy" I wonder: "What component is dark?"

The velocity portion can't be "dark" because we "see" the velocities that cause us to surmise that a "dark energy" must be afoot.

If the mass portion is "dark" then doesn't that go against the "dark matter" dictate because dark matter is only supposed to interact with baryonic (normal) matter through gravity - and the dark energy is dispersing the universe (causing expansion) instead of pulling it together?

Tim Thompson
2007-Jan-22, 01:26 AM
Well, the standard undergraduate definition of energy is entirely operational in nature. Energy is the ability to do work. Work is defined as a force applied through a distance (force x distance). If you push on the wall, but it does not move, then you have done no work. You might not feel like that, but since the distance is zero, and force x distance is zero, you are officially workless. Of course, this tells you absolutely nothing about what energy really is, but rather what it does. And that's the real bottom line. There is no known answer to the original question. What is energy? Nobody knows. But we do know what it does, and that's really all we need to know, to do any physics we need to do.


Again I see only two components: mass and velocity. When I think of "dark energy" I wonder: "What component is dark?"
Look at the equation for the potenial energy of an electromagnetic field (http://scienceworld.wolfram.com/physics/ElectromagneticField.html). Where are the mass & motion? At best, you will find the definition of energy, as a force applied over a distance, in the integral for the general definition of potential energy (http://scienceworld.wolfram.com/physics/PotentialEnergy.html). The field has no obvious mass or motion, and static fields are perfectly capable of doing work.

Energy is simply the ability to do work. Dark energy is not simply mass & motion. Like the electromagnetic field, it might be some other kind of field, at work in the universe pushing things around. Or, maybe dark energy is a manifestation of a changing geometry of the universe, an expansion of space itself, in which case it would not be "energy" at all.

sirius0
2007-Jan-22, 01:27 AM
When I think of "dark energy" I wonder: "What component is dark?"

The velocity portion can't be "dark" because we "see" the velocities that cause us to surmise that a "dark energy" must be afoot.

If the mass portion is "dark" then doesn't that go against the "dark matter" dictate because dark matter is only supposed to interact with baryonic (normal) matter through gravity - and the dark energy is dispersing the universe (causing expansion) instead of pulling it together?


This thread I started is a little similiar.
http://www.bautforum.com/showthread.php?t=48493

Dr Nigel
2007-Jan-22, 10:03 AM
How do you measure gravity field potential?

Measure the attractive force between two objects, such as a bowling ball and the Earth. This measurement can also be called the weight of the object in that specific gravitational field.

Take a 1 kg mass. Its weight is 9.8 Newtons at Earth's surface. The Earth's gravitational potential field strength is therefore 9.8 N/kg (at Earth's surface). Because of the way the SI units work out, the gravitational acceleration in the same field is 9.8 m/s^2 (i.e. N/kg and m/s^2 are equivalent, although one measurement is made at rest and the other is derived from motion).

'kay?

Dr Nigel
2007-Jan-22, 10:17 AM
...
But then energy is momentum times velocity as we can see in these two energy formulas:

KE = 1/2mv2

E = mc2

Again I see only two components: mass and velocity.

When I think of "dark energy" I wonder: "What component is dark?"

...

In addition to Tim Thompson's answer, you are considering these two equations equivalent without explaining why. Although the two equations have an equivalent formulation, they are describing different phenomena which are not directly equivalent.

In the formula for kinetic energy, v represents the velocity of a macroscopic object, and m represents the mass of that object. The object's velocity will be relative to our own reference frame.

In the formula for mass-energy equivalence, m is the observed change in mass of a particle (such as an atom taking part in a nuclear reaction), and c is not a macroscopic velocity, but the speed of light, which is a constant property of electromagnetic interactions. As such, it is independent of our reference frame.

So, although both equations relate to energy, they relate to energy of different types, which should not necessarily be considered equivalent.

Wayne McCoy
2007-Jan-22, 05:09 PM
So, a photon's energy, which can be extremely high, is based upon its wavelength, and it has momentum without mass. Is its energy potential or kinetic? What work does this energy do when a photon is absorbed by an electron? Sometimes, when an electron absorbs a photon, the electron "moves" to a higher orbital, but in a jump rather than continuous motion. What is the work done, since the higher orbital does not necessarily represent distance? What happens to an electron's momentum when a photon is emitted?

Just a few questions for thought.

Dr Nigel
2007-Jan-22, 07:31 PM
So, a photon's energy, which can be extremely high, is based upon its wavelength, and it has momentum without mass. Is its energy potential or kinetic?

Neither - its energy is electromagnetic radiation. A photon can be considered to be a "packet" of waves travelling through electric and magnetic fields.


What work does this energy do when a photon is absorbed by an electron? Sometimes, when an electron absorbs a photon, the electron "moves" to a higher orbital, but in a jump rather than continuous motion. What is the work done, since the higher orbital does not necessarily represent distance? What happens to an electron's momentum when a photon is emitted?


Well, if the electron is part of an atom or molecule when it absorbs a photon, the energy becomes chemical potential energy. The electron becomes "excited" and the atom or molecule gains the potential to participate in more types of chemical reactions than would otherwise be the case (this is how plants turn sunlight into sugar). If the electron is "free", then its momentum changes as a result of absorbing a photon.

When an "excited" electron emits a photon (which need not be the same wavelength as the photon it absorbed in the first place, which is how fluorescence works), its chemical potential energy is reduced. When a "free" electron emits a photon, its kinetic energy and momentum are reduced (this was first observed in the strong magnetic fields in synchrotrons, so is called synchrotron radiation).

Does this answer your questions?

Ken G
2007-Jan-22, 07:38 PM
So, a photon's energy, which can be extremely high, is based upon its wavelength, and it has momentum without mass. Is its energy potential or kinetic? Interesting question. I would expect most physicists would view it as kinetic energy, but as energy is what energy does, the label isn't so important.


What work does this energy do when a photon is absorbed by an electron? Sometimes, when an electron absorbs a photon, the electron "moves" to a higher orbital, but in a jump rather than continuous motion. What is the work done, since the higher orbital does not necessarily represent distance?What you are pointing out is that the entire concept of "work" as a force times a distance is a classical notion (and indeed you don't even need to have forces explicitly appear in quantum mechanics, you can work directly from energy). Welcome to reality in physics-- the words that answer any question will require certain assumptions being made about what the question is asking. In quantum mechanics, energy is an eigenstate of the energy operator, but saying that it's what you get when work gets done sounds a lot easier! There is also a way to view energy as the "canonical conjugate" of time, which lends insight when you realize that the systems that conserve energy are precisely the systems that have no explicit dependence on time (i.e., no external influences that could distinguish one moment from another). So I suppose the most general answer to "what is energy" is that it is a mental construct that takes on a rather different flavor in various different applications.


What happens to an electron's momentum when a photon is emitted?

A photon is a quantum mechanical entity, so you have to use the language of quantum mechanics to answer this. That also means that you have to include measurement. So if you measure the momentum of a photon, and the momentum of the electron that emitted it, and you haven't measured anything else that could mess with these measurements, then the total momentum will be conserved in that process. They are "entangled."

Wayne McCoy
2007-Jan-22, 08:39 PM
There is also the principle that energy and time are complementary in the quantum mechanical sense. This means, for example, that energy can be gained from the vacuum for very short periods of time, permitting particles to tunnel through the walls of potential wells, and given back to the vacuum on the other side. Similarly for electron-positron pair creation. Precise measurements of energy yield very imprecise measurements of time (Heisenberg). Again, classical mechanics gives no insight into "work" at this level of examination. Work becomes an interpretation of what is going on.

Maybe for an ur-definition of energy we have to go to the vacuum.

sirius0
2007-Jan-24, 01:59 AM
Perhaps energy at the quantum level is simply ( :) ) statistical bias. I.e. it ensures (almost) that a given particle will in the next instant be imediately next to where it was last. Perhaps it is this bias that allows things to move and/or exist in the first place (the bias allows a particle to be non-virtual/real)

Ken G
2007-Jan-24, 09:15 AM
The statistical bias sounds reminiscent of the Heisenberg uncertainty principle, which is in turn related to Huygens' principle, which is deeply related to constructive wave interference. I think the way energy comes in at the quantum level is that it is connected to frequency, and the frequency controls where you will get constructive interference-- for nonrelativistic particles, the higher frequency is associated with some shortening of the wavelength, but the wavelength doesn't shorten as fast as the frequency increases, so the region where the interference is constructive advances at a faster and faster speed the higher the energy. Put differently, the nature of the motion is not dependent on energy alone, it is dependent on the combination of the energy and the momentum (i.e., frequency and inverse-wavelength), or the kinetic energy vs. rest energy (which is the same issue).