Understanding the concept of energy

[fog37]

TL;DR

Concept of energy

Hello Forum,
Energy, like space and time, is a common concept but little understood. I think Feynman himself stated energy is hard to describe...
I have a few observations in regards to energy and I would like your feedback on them:

• While force, which represents an interaction between two different systems, causes the change in one of the system's configuration and/or state of motion, energy measures the change in the system's configuration and/or state of motion. What does it mean that a system has a certain energy? I think it means that it has the potentiality of changing its own configuration or the configuration/state of motion of another system. Somehow, the textbook definition of energy as the ability of doing mechanical work is deemed incorrect and flawed. Why so?
• All forms of energy seem to boil down, macroscopically and microscopically, to two fundamental energy forms: kinetic energy $KE$ and potential energy ##PE# (I guess radiation would be the 3rd energy type). For example, besides the bulk, organized, macroscopic kinetic energy of a moving body, thermal energy represents the internal and disorganized kinetic energy of the body's microscopic constituents. Chemical energy is the potential energy between molecules and atoms. Nuclear energy is the energy deriving from the particles inside the nuclei.
• As far as potential energy $PE$ goes, the most common $PE$ forms are gravitational potential energy $PE_g$ and elastic potential energy $PE_{elastic}$. Both $PE$ forms involve two or more entities interacting among each other (PE only makes sense for a system of object).
• After relativity, we learned that mass and energy are intimately related. A point particle has rest mass+$KE$ while a composite system has rest mass+$KE+PE$.

I always thought that only $PE$ differences mattered and that the configuration having $PE=0$ was arbitrary in the sense that we could assign $PE=0$ to whatever system configuration. But I recently learned that PE is absolute in some sense. However, I read a recent article by Sherwood which consider a system of two objects with the same mass $m$ (an electron and a positron), both at rest and so far apart that their interaction is negligible. Their kinetic energy is zero and the total system energy of this system must be $2mc^2$ . The total energy of the system is the sum of the rest energies, kinetic energies, and the potential energy associated with the interaction. The energy and momentum four-vectors will not transform under a reference frame change unless the potential energy value is $PE=0$, which means that the $PE$ value cannot be arbitrary...

Energy is measured indirectly. Can we measure the energy of a system or can we really measure only energy differences? There seems to be no problem measuring the $KE$ of a system. What about $PE$? Can we measure $PE$ or just its changes?

Thanks!

 

[Dale]

Summary:: Concept of energy

Energy, like space and time, is a common concept but little understood.

I disagree. People just seem to like a little mystery so they create it even where it is not necessary. Plus, I guess it sells more pop-sci books that way.

 

[robphy]

These references mentioned above might help the discussion

A unified, contemporary approach to teaching energy in introductory physics
American Journal of Physics 87, 504 (2019); https://doi.org/10.1119/1.5109519
Ruth Chabay and Bruce Sherwood and Aaron Titus

Pseudowork and real work
https://brucesherwood.net/?p=134

https://www.feynmanlectures.caltech.edu/I_04.html
Conservation of Energy (start at &t=23m03s )

 

[fog37]

I disagree. People just seem to like a little mystery so they create it even where it is not necessary. Plus, I guess it sells more pop-sci books that way.

Do we really deeply understand time and space? I know we have the relativistic spacetime model...but it seems that we still don't have a full theory that connects with QM. Even gravity, such a common force, is still mysterious (gravitons have not been found, etc.)

 

[Dale]

Do we really deeply understand time and space? I know we have the relativistic spacetime model...but it seems that we still don't have a full theory that connects with QM. Even gravity, such a common force, is still mysterious (gravitons have not been found, etc.)

So before we can claim to understand anything we must understand everything? Sorry, that is an argument I don’t accept.

There are many genuine mysteries, and personally I think it cheapens those to pretend that non-mysterious things are mysterious

 

[A.T.]

I think Feynman himself stated energy is hard to describe...

It's difficult to describe in layman terms, because it's an abstract quantity, not some substance or intrinsic object property. But this applies more or less to all physics.

 

[danielhaish]

Do we really deeply understand time and space? I know we have the relativistic spacetime model...but it seems that we still don't have a full theory that connects with QM. Even gravity, such a common force, is still mysterious (gravitons have not been found, etc.)

but griavital wave have been measured before and it hard to find its quant because we don't have enough measures but I believe that the same as with light the gravity wave is not Continuous . and I don't think physics is close book i think the all concept might change all the time but we still can describe the nature with it

 

[haushofer]

Energy is a very abstract idea in physics, like money in economics. But in general the idea is well-understood, I'd say. Mainly in general relativistic context there are some difficulties (what's geometry, what's energy, what's the nature of Lambda).