Explain Energy (is it physical)?

[xmenfan]

Hi,

I'm going to be a senior in high school this coming year. I want to ask what exactly energy is and if it is physical?

I know that we cannot see with our naked eye what energy is, but is it something physical and microscopic? What is it like?

Thank you very much.

 

[Simon Bridge]

You can see kinetic energy in much the same way you can see speed or (closer) momentum.
Energy is a description of the forcefullness of an action or potential action - simplistically put, it is a property of movement and of a potential for movement.

 

[Drakkith]

The usual definition is that energy is the ability of one system to perform work on another system. I like to say it's the ability for one object to cause a change in another object. These changes include acceleration, heating, etc.

 

[Simon Bridge]

The usual definition is that energy is the ability of one system to perform work on another system.

I've never been happy teaching to that one - the same sources usually also say that work is the change in energy or energy comes from work. (i.e. the "OmCheeto on energy" quote in your sig :) )

We usually teach physics starting with forces - which are easier to visualize: there's a kinesthetic sense to the concept.

Then we move the student more and more to the energy description since it is more useful and the math is easier.

Admittedly, students usually meet $\small W=Fd$ before they meet $\small W=\Delta E$ and thermodynamics likes to distinguish work energy from heat energy or something doesn't it?

 

[Drakkith]

I've never been happy teaching to that one - the same sources usually also say that work is the change in energy or energy comes from work. (i.e. the "OmCheeto on energy" quote in your sig :) )

Yes, unfortunately the definition of work is generally poor in my opinion.

Admittedly, students usually meet $\small W=Fd$ before they meet $\small W=\Delta E$ and thermodynamics likes to distinguish work energy from heat energy or something doesn't it?

I think so.

 

[A.T.]

There is a fact, or if you wish, a law governing all natural phenomena that are known to date. There is no known exception to this law – it is exact so far as we know. The law is called the conservation of energy.

It states that there is a certain quantity, which we call “energy,” that does not change in the manifold changes that nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says there is a numerical quantity which does not change when something happens.

It is not a description of a mechanism, or anything concrete; it is a strange fact that when we calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.

It is important to realize that in physics today, we have no knowledge of what energy “is.” We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. It is an abstract thing in that it does not tell us the mechanism or the reason for the various formulas.

From : http://www.phy.davidson.edu/fachome/swp/courses/PHY110/Feynman.html

 

[Dadface]

Anything that moves or anything that can make things move has energy.

 

[Simon Bridge]

I thought of the Feynman quote too, but then I realized it didn't answer the question - even though it does describe why it is useful. Feynman goes on to talk about children's building blocks.

Similarly - pointing to all the things that have energy does not explain the concept, does it?

It's actually a tricky question that tends not to be handled well - most sources rely on intuitive ideas or getting the student to do lots of examples to get a feel for what is meant.
http://ocw.mit.edu/courses/physics/8-21-the-physics-of-energy-fall-2009/lecture-notes/

Wikipedia says: "It is impossible to give a comprehensive definition of energy because of the many forms it may take, but the most common definition is that it is the capacity of a system to perform work. " .. and then goes on to talk about what work is.

@xmanfan: has any of this been useful?

 

[ModusPwnd]

My thought is that energy is that which has the units of kg m²/s². If it has those units, its energy. If it doesnt, its not. Whether you want to consider it physical or not is a matter of taste, it makes no difference to any theory or observation.

 

[Dadface]

My thought is that energy is that which has the units of kg m²/s². If it has those units, its energy. If it doesnt, its not. Whether you want to consider it physical or not is a matter of taste, it makes no difference to any theory or observation.

Moment has those units but its not energy.

 

[ZapperZ]

Hi,

I'm going to be a senior in high school this coming year. I want to ask what exactly energy is and if it is physical?

I know that we cannot see with our naked eye what energy is, but is it something physical and microscopic? What is it like?

Thank you very much.

https://www.physicsforums.com/blog.php?b=3203

Zz.

 

[HallsofIvy]

The real problem is that xmenfan hasn't said what he (or she) means by "physical". He appears ("is it something physical and microscopic") to mean an actual material "object" which, of course, it isn't. I suspect that the best response was Simon Bridges's initial "You can see kinetic energy in much the same way you can see speed or (closer) momentum." Now, xmenfan can ponder whether "speed" is a material "object".

(Personally, I have always thought of "energy" as a "bookkeeping" device. Any time we have a situation in which energy does not appear to be conserved, we define a new "form" of energy so it is!)

 

[Dale]

The real problem is that xmenfan hasn't said what he (or she) means by "physical". He appears ("is it something physical and microscopic") to mean an actual material "object" which, of course, it isn't.

I agree. I think the OP should clarify.

In my usage energy is clearly "physical", but it is not "material". I think the OP means "material" rather than "physical".

 

[tiny-tim]

welcome to pf!

hi xmenfan! welcome to pf!

… what exactly energy is and if it is physical?

I know that we cannot see with our naked eye what energy is, but is it something physical and microscopic? What is it like?

is temperature physical? is pressure physical?

you can't create or destroy energy …

there's only so much energy in the universe, and all you can do is move it around!

(i see you've asked the same question about space …

next, have a go at entropy )​

 

[256bits]

Is not a photon a tiny packet of energy?

 

[OmCheeto]

https://www.physicsforums.com/blog.php?b=3203

Zz.

After seeing this question so many times, I decided that the best thing to do was ask the OP several* similar questions:

What is color? Is color a thing? If so, then how much does a "blue" cost?
What is length? Is length a thing? On which shopping aisle do I find an "inch"?
etc, etc.

Energy is not a thing, just as color and length are not "things".
They are measurements, or attributes, or, as wiki defines energy, a property of a system.

"a system" is probably the only unique quality of energy. You can have a blue marble. You can have a basketball that is 1 foot in diameter. But if you replaced everything in the universe with a single electron, there would be no energy.

"Energy is like the Tango dance..."

...
Personally, I have always thought of "energy" as a "bookkeeping" device. ...

I'll take that as confirmation that Drak's signature is correct.

----------------------
* Actually, I think I always asked them just one question; "What is Red?"

 

[Naty1]

I'm going to be a senior in high school this coming year. I want to ask what exactly energy...

and we all wish we could provide a straight forward all inclusive answer.

While energy is 'abstract', along the lines of the Feynman quote, it remains because it is a very useful concept in physics. And while energy as the ability to do work is a decent introductory explanation it's many facets takes a while to understand. Like,maybe,years.

As a simple example, work [as noted] is often discussed as a force times a distance. So If I push as hard as I can against a wall and nothing moves, no work is done but I am sure burning up energy. In fact I am using food energy, say calories as a unit of measure, just sitting here thinking about what to type next. I burn energy whether I am tapping on the keyboard or not...although the typing uses a bit more energy.

Are you in an air conditioned room right now? A type of work is being done by that air conditioner, as it expels heat outside and likely some power company is charging you for that via their measure of watt hours [or 'kilowatt hours' meaning thousands of watt hours] consumed...that is their measure of the energy being consumed...they keep track of that energy use via an electric meter.

If you take chemistry, you'll find the chemical reactions involve electromagnetic energy...the energy of orbital electrons get rearranged in different chemical bonds. And radioactivity involves changes in nuclear energy...the energy of the nucleus itself in an atom...as does fission and fusion as in our sun. So the forms of energy are indeed numerous.

 

[OmCheeto]

Is not a photon a tiny packet of energy?

Not according to wiki.

But according to "e=mc²", everything is energy!

And "What is a photon?" is a question I've been asking for decades. I'm pretty sure I'll die without knowing the answer. But that's ok. I can totally relate with Feynman's onion. When in high school, I knew exactly how a diode worked. In college, I became confused as to how they worked. When I arrived at PF, and tried to understand them at a quantum level, I decided that I had not a clue how they worked.

hi xmenfan! welcome to pf!


is temperature physical? is pressure physical?

you can't create or destroy energy …

there's only so much energy in the universe, and all you can do is move it around!

(i see you've asked the same question about space …

next, have a go at entropy )​

Just testing the multi-quote function.

 

[Simon Bridge]

As we advance in our learning we come to realize how little we understand ... we understand less and less and console ourselves, finally, that our uncertainty now is more useful than the understanding we used to have. (And then we get annoyed when someone who is still quite certain insists that our uncertainty means they are right.)

And that's my philosophy quota for today.

 

[A.T.]

Semi related part where Feynman criticizes using energy as an explanation of an observed phenomenon:

http://www.youtube.com/watch?v=fkIymBSCM4Q&t=22963

 

[technician]

The usual definition is that energy is the ability of one system to perform work on another system. I like to say it's the ability for one object to cause a change in another object. These changes include acceleration, heating, etc.

The Earth causes the Moon to move in an orbit.
Acceleration is involved here. How do you bring 'energy' into what the Earth causes to happen to the moon.
We are all familiar with energy 'running out' or being transferred. How does that fit with the Earth Moon set up?

 

[tiny-tim]

As we advance in our learning we come to realize how little we understand ... we understand less and less and console ourselves, finally, that our uncertainty now is more useful than the understanding we used to have.

yes, and if your teacher asks if you understand entropy, just reply "whenever you try to make me understand something, the total knowledge in the universe decreases"

 

[Drakkith]

The Earth causes the Moon to move in an orbit.
Acceleration is involved here. How do you bring 'energy' into what the Earth causes to happen to the moon.
We are all familiar with energy 'running out' or being transferred. How does that fit with the Earth Moon set up?

Other than the transformation of potential energy into kinetic energy and back again, it doesn't.

 

[voko]

Semi related part where Feynman criticizes using energy as an explanation of an observed phenomenon:

http://www.youtube.com/watch?v=fkIymBSCM4Q&t=22963

I do not think he criticizes using energy. I think he criticizes using the word energy without explaining what it really means. Later on, when he uses the "Sun is shining" explanation, he talks about accumulated solar energy.

I should also say that I dislike that he says it makes more sense to say that something has energy because it moves. That would mean something stationary does not have any energy, which contradicts that "accumulated solar energy" thing he states later.

 

[technician]

Other than the transformation of potential energy into kinetic energy and back again, it doesn't.

So you would agree that 'energy' is not the explanation for centripetal force.
It seemed to me that you were confusing Force and energy and their effects.

 

[Drakkith]

So you would agree that 'energy' is not the explanation for centripetal force.

I would agree. Energy is not the explanation for any force. Forces give rise to energy, not the other way around.

It seemed to me that you were confusing Force and energy and their effects.

Nope. Sorry if I confused anyone.

 

[technician]

I would agree. Energy is not the explanation for any force. Forces give rise to energy, not the other way around.



Nope. Sorry if I confused anyone.

Great...cheers

 

[voko]

We need to 'explain' forces when we use them. We can, however, do physics entirely without forces (but with energy).

 

[technician]

We need to 'explain' forces when we use them. We can, however, do physics entirely without forces (but with energy).

Fully agree with this.
Energy, in principle, is easy to deal with, it is a scalar quantity and therefore a simple addition problem
Forces...more difficult...evidence is in many posts here.

 

[OmCheeto]

We need to 'explain' forces when we use them. We can, however, do physics entirely without forces (but with energy).

Entirely? (My motto is "Never say never, and never say always", as there is always some smartypants who will know an exception to the rule)

Now, I do admit to using energy balance almost entirely in my calculations, but I can think of one experiment where force had to be measured. It was to weigh something on a balance scale. I have to go to work soon, so I don't have time to scratch my head and figure out how to describe a balance scale in terms of "energy".

 

[voko]

Entirely? (My motto is "Never say never, and never say always", as there is always some smartypants who will know an exception to the rule)

Now, I do admit to using energy balance almost entirely in my calculations, but I can think of one experiment where force had to be measured. It was to weigh something on a balance scale. I have to go to work soon, so I don't have time to scratch my head and figure out how to describe a balance scale in terms of "energy".

The balance scale does not measure any force. It measures a distance or simply a co-incidence. We infer from some principles that there is a particular measure of force related to the observed distance.

However, we can equally say we measure energy. In a balance scale, that would be the difference of energies stored in the deformed arms of the scale; at any given deformation the scale then seeks the configuration with the minimal potential gravitational energy.

My remark was of a more general nature. It is possible to formulate all of physics without using the concept of force. The Hamiltonian and Lagrangian formulations would exemplify that.

 

[OmCheeto]

The balance scale does not measure any force. It measures a distance or simply a co-incidence. We infer from some principles that there is a particular measure of force related to the observed distance.

However, we can equally say we measure energy. In a balance scale, that would be the difference of energies stored in the deformed arms of the scale; at any given deformation the scale then seeks the configuration with the minimal potential gravitational energy.

My remark was of a more general nature. It is possible to formulate all of physics without using the concept of force. The Hamiltonian and Lagrangian formulations would exemplify that.

Remind me in my next lifetime, not to be your lab partner.

Om; Ok, step one is we have to measure this on the scale.
Vo; No! We are going to measure its energy.
Om; What?
Vo: Scales are lame. They are interpolations of force.
Om; We only have an hour. What are you proposing?
Vo; We are going to solve the Hamiltonian and Lagrangian formulations.
Om; This is General Physics, and we don't know how to do that here.
Vo; Silence Mortal!
Om; Isn't the balance scale solution just a limit of the frictional coefficient of the fulcrum bearing?
Vo; Silence!

 

[Drakkith]

Remind me in my next lifetime, not to be your lab partner.

You can be my lab partner Om. We'll grab Phinds as well and several cases of our favorite beverages and measure the energy transfer rate of body heat into cold liquid after consumption. Or if we're too drunk for that there's always drinking songs. I hear Phinds does a wonderful impression of Freddy Mercury singing in a scottish accent after a few drinks...

 

[OmCheeto]

You can be my lab partner Om. We'll grab Phinds as well and several cases of our favorite beverages and measure the energy transfer rate of body heat into cold liquid after consumption. Or if we're too drunk for that their's always drinking songs. I hear Phinds does a wonderful impression of Freddy Mercury singing in a scottish accent after a few drinks...

Go home Drak! You obviously recognize that I'm drunk!

[edit] ps. This morning, I started out with a bathroom scale, but decided, after about 10 seconds of thought, that voko was correct, as the change in my height could be directly correlated to the change in spring energy. Hence, my balance scale.

 

[voko]

This morning, I started out with a bathroom scale, but decided, after about 10 seconds of thought, that voko was correct, as the change in my height could be directly correlated to the change in spring energy. Hence, my balance scale.

Indeed. With a spring scale, the energy explanation is obvious. With a balance scale, it is a bit trickier, but it is trickier because a force-based explanation would also be trickier.

 

[OmCheeto]

Indeed. With a spring scale, the energy explanation is obvious. With a balance scale, it is a bit trickier, but it is trickier because a force-based explanation would also be trickier.

I don't see how a force based explanation would be tricky at all.

If I put a penny on each tray of the scale, the force exerted by each penny will be the same, and given my scale has equal length arms, the torque about the fulcrum will be zero, and the scale will be in balance.

And the arms of my scale don't appear to be deforming all that much.

The balance scale actually reminds me of the normal force. N=mg. I suppose I could manipulate that equation and get energy somehow.

hmmm...

I'm just not getting this energy based scale, and I'm late for work again.

ps. 8 old pennies weigh more than 9 new pennies, but less than 10. The fulcrum on my homemade penny scale is a piece of thread, and makes it very sensitive.

 

[Naty1]

I suppose I could manipulate that equation and get energy somehow.

why not E = mc²

 

[voko]

I don't see how a force based explanation would be tricky at all.

If I put a penny on each tray of the scale, the force exerted by each penny will be the same, and given my scale has equal length arms, the torque about the fulcrum will be zero, and the scale will be in balance.

And the arms of my scale don't appear to be deforming all that much.

It gets tricky when the masses are unequal. Explain then why the scale finds some tilted equilibrium rather than rotating all the way till the arms are vertical (or blocked by something).

The simple case of equal masses is just as simple from the energy standpoint: they have equal potential gravitational energies.

 

[Dale]

It gets tricky when the masses are unequal. Explain then why the scale finds some tilted equilibrium rather than rotating all the way till the arms are vertical (or blocked by something).

The simple case of equal masses is just as simple from the energy standpoint: they have equal potential gravitational energies.

This doesn't work as an explanation. An equilibrium position is a position of minimum energy for the whole system, not a position of equal energy for two parts of a system. The minimum energy configuration is with the heavy mass at the bottom, just like with forces.

 

[voko]

This doesn't work as an explanation. An equilibrium position is a position of minimum energy for the whole system, not a position of equal energy for two parts of a system.

The equality of masses, the "untilted" equilibrium, and the equality of the potential energies of the masses are all equivalent. If potential energies are not equal, then the equilibrium is tilted, or the system is not in equilibrium at all.

 

[Dale]

The equality of masses, the "untilted" equilibrium, and the equality of the potential energies of the masses are all equivalent. If potential energies are not equal, then the equilibrium is tilted, or the system is not in equilibrium at all.

You are using energy concepts completely wrong here. To find the equilibrium position never has anything to do with the equality of GPE of the two masses separately. You look at the system as a whole and find the minimum total energy. That is the equilibrium position. The separate GPE of each side has nothing to do with it.

 

[voko]

You are using energy concepts completely wrong here.

If you think that my statements in #40 are incorrect, I would like to see some justification, not just something which I find highly subjective. I you so wish, I could provide formal proofs of the statements.

To find the equilibrium position never has anything to do with the equality of GPE of the two masses separately. You look at the system as a whole and find the minimum total energy. That is the equilibrium position. The separate GPE of each side has nothing to do with it.

I believe I said quite precisely what the equality of potential energies of the masses on a balance scale has to do with the untilted equilibrium. If you believe that is incorrect, I would like to know why.

 

[AlephZero]

I don't see how a force based explanation would be tricky at all.

It gets tricky when the masses are unequal. Explain then why the scale finds some tilted equilibrium rather than rotating all the way till the arms are vertical (or blocked by something).

It's not tricky, if you consider the geometry of the scale.

The central pivot is higher than the points where the pans hang from the beam.

When the beam tilts, one pan moves further away from the pivot (horizontally) and the other moves closer to the pivot.

The tilted equilibrium position is when the moments of the two weights about the pivot are equal (smaller weight x larger distance = larger weight x smaller distance).

 

[voko]

It's not tricky, if you consider the geometry of the scale.

The central pivot is higher than the points where the pans hang from the beam.

This is not the only possible design of the balance scale.

 

[Dale]

If you think that my statements in #40 are incorrect, I would like to see some justification

Consider a balance with raised fulcrum, equal arms, equal chains, and equal pans holding equal masses. The equilibrium position is the one with the minimum total energy, and coincidentally the GPE of the masses on each side is the same.

Now consider a balance with raised fulcrum, equal arms, unequal length (but equal mass) chains, and equal pans holding equal masses. The equilibrium position is again the one with the minimum total energy, and the GPE of each side is unequal since the one on the longer chain is at a lower potential.

Now consider a balance with raised fulcrum, one arm longer than the other, equal chains, and equal pans holding unequal masses that balance. The equilibrium position is once again the one with the minimum total energy, and the GPE of each side is unequal since the larger mass has more GPE.

It is a simple fact which is true of many systems of many different kinds: equilibrium is determined by extremizing the total energy. A stable equilibrium is a minimum, and an unstable equilibrium is a maximum. The equality of the GPE of different parts of a system is a red herring, it is an irrelevant and occasional coincidence rather than the feature which defines equilibrium.

 

[voko]

Consider a balance with raised fulcrum, equal arms, equal chains, and equal pans holding equal masses.

Equal arms, equal chains and equal pans were implicit requirements. Sorry if that was not clear.

It is a simple fact which is true of many systems of many different kinds: equilibrium is determined by extremizing the total energy. A stable equilibrium is a minimum, and an unstable equilibrium is a maximum.

I never debated this.

The equality of the GPE of different parts of a system is a red herring, it is an irrelevant and occasional coincidence rather than the feature which defines equilibrium.

"The equality of the masses of different parts of a system is a red herring, it is an irrelevant and occasional coincidence rather than the feature which defines equilibrium."

True, but how useful in the particular case we are talking about?

 

[Dale]

Equal arms, equal chains and equal pans were implicit requirements. Sorry if that was not clear.

This is not the only possible design of the balance scale.

Interesting.

True, but how useful in the particular case we are talking about?

An irrelevant conincidence is never useful, just coincidental. Particularly on a forum where we are trying to provide correct and factual information so that people can learn about physics. Physics is about a small set of underlying principles which you can apply to understand many systems, not a long laundry list of random coincidences that can only be applied to one specific system each.

 

[voko]

Interesting.

It is entirely possible to build scales that violate this:

"The central pivot is higher than the points where the pans hang from the beam."

yet meet the requirements I listed above.

An irrelevant conincidence is never useful, just coincidental.

The purpose of a scale is to measure the equality of mass. Being able to establish the equality of masses, however irrelevant and co-incidental it may be in the grand scheme of things, is all important in this particular domain. I do not understand why you are trying to say it is irrelevant. The equality of associated GPEs is very obviously equivalent.

A few messages ago I stated very explicitly that the minimum of potential energy is what defines the equilibrium. So I am not debating this, and I am not trying to invent some other principle. I am merely stating that as easily as someone can say a scale measures force, one can say it measures energy. Which was, if you look a few further messages back, in the context of my statement that it is possible to do physics without force as a concept.

 

[Dale]

The purpose of a scale is to measure the equality of mass. Being able to establish the equality of masses, however irrelevant and co-incidental it may be in the grand scheme of things, is all important in this particular domain. I do not understand why you are trying to say it is irrelevant. The equality of associated GPEs is very obviously equivalent.

Yes, the purpose of any balance scale is to measure the mass (not necessarily equality of mass). The equality of GPE of each side is irrelevant and coincidental for only for a specific design, as I showed. The important thing for all balance scales of all designs is that the total energy be minimum at the equilibrium position when the unknown mass is correctly measured.

A few messages ago I stated very explicitly that the minimum of potential energy is what defines the equilibrium. So I am not debating this, and I am not trying to invent some other principle.

Then you should have stopped at that point, before you started inventing some other principle. And once it was pointed out that you were inventing some other principle you should have just said "oops" and moved on. This needn't have turned out as the big production that it has.

I am merely stating that as easily as someone can say a scale measures force, one can say it measures energy. Which was, if you look a few further messages back, in the context of my statement that it is possible to do physics without force as a concept.

I agree with that. It is possible to do physics without forces and it is possible to analyze a balance scale in terms of energy. I didn't object to your overall point, only to your description of the equilibrium condition as being due to equality of GPE, which you should have recognized as being a valid objection given your previous comments.

 

[AlephZero]

It is entirely possible to build scales that violate this:

"The central pivot is higher than the points where the pans hang from the beam."

Of course it's possible to build something that way.

Do you have a reference (preferably with a picture or drawing) of a pracitcal balance scale built that way? Unless it has some other clever design features, I think it would be useless because the equilibrium position would be unstable.