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".