# Explain Energy (is it physical)?

1. Aug 2, 2013

### 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.

2. Aug 2, 2013

### 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.

3. Aug 2, 2013

### Staff: Mentor

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.

4. Aug 2, 2013

### Simon Bridge

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?

5. Aug 3, 2013

### Staff: Mentor

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

I think so.

6. Aug 3, 2013

### A.T.

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

7. Aug 3, 2013

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

8. Aug 3, 2013

### 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?

9. Aug 3, 2013

### 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.

10. Aug 3, 2013

Moment has those units but its not energy.

11. Aug 3, 2013

### ZapperZ

Staff Emeritus
https://www.physicsforums.com/blog.php?b=3203 [Broken]

Zz.

Last edited by a moderator: May 6, 2017
12. Aug 3, 2013

### HallsofIvy

Staff Emeritus
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!)

13. Aug 3, 2013

### Staff: Mentor

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".

14. Aug 3, 2013

### tiny-tim

welcome to pf!

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!

next, have a go at entropy )​

15. Aug 3, 2013

### 256bits

Is not a photon a tiny packet of energy?

16. Aug 3, 2013

### OmCheeto

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......."

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

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

Last edited by a moderator: May 6, 2017
17. Aug 3, 2013

### Naty1

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.

Last edited: Aug 3, 2013
18. Aug 3, 2013

### OmCheeto

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.

Just testing the multi-quote function.

19. Aug 3, 2013

### 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.

20. Aug 4, 2013