# What is Energy?

I've been throwing this term energy around for a while now, and thinking about it I have absolutely no idea what it is. Is it something that actually exists in the universe, or just a construct that we use to simplify problems?

Terms like kinetic energy, and even gravitational potential energy (from a Newtonian standpoint) are a little bit easier to understand because they are exist in everyday life, but when you get E&M (I've only studied classical E&M) you have this idea of a field, the field has energy and you need to calculate it? How does a field have energy?

Consider a conducting bar, moving on rails with a large resistor at one end bathed in a strong, uniform magnetic field. As the bar moves, the magnetic flux increases, inducing a current in the circuit and thus energy is lost in the resistor as heat, so the bar must be slowing down (it never actually stops even though it moves a finite distance, as i've calculated). I can ascertain what happens in this situation because I know about E&M, but it bothers me that I don't feel like I understand what is actually happening. If you showed me that and I didn't know physics, I would say the bar moves an infinite distance. How can some invisible field (with energy) stop a real, moving object? This is the whole idea of transfer of this energy which I don't know what it is?

Is there a good model of a physical interpretation of this phenomenon? Is it just a mathematical construct?

Here is a real physical example of a conducting bar moving in a non-uniform magnetic field. I was working in a group using Enrico Fermi's cyclotron magnet for bending a very high energy beam of muons. The pole tip diameter was about 13 feet. We had opened up the gap so it was roughly 40 inches. The field in the center was about 1.5 Tesla, but we needed to measure it exactly. I was chosen to crouch/walk into the gap and place a NMR (nuclear magnetic resonance) probe in the center with the magnet ON. After placing the probe in the center, we needed to put the probe on a non-magnetic support. The only one we could find locally was a piece of 12 inch high. 6 inch wide, 24 inch long aluminum I beam. I started to pull the I beam into the magnet, and was met with resistance-lots of it. It was like pulling the I beam through molasses (honey). This was a problem only in the edge field, not in the center of the magnet (uniform field), as long as I rotated the I-beam only about a vertical axis (parallel to magnetic field). Rotating about any horizontal axis met with resistance. See eddy current explanation at:
http://en.wikipedia.org/wiki/Eddy_current

Andy Resnick
Energy is something I need more of.

wow that story sounds pretty hardcore.

I was thinking about it, and you could define energy simply as the ability to do work, but then your stuck with the question again, what is work

Energy is anything that can be converted into mass , or that is already mass.
this may not be 100% accurate but i am trying to relate it to E=mc^2
like fore example a photon which is energy can decay into an electron positron pair.

If you could find out what energy is as a force, then i think you may find the answer to life itself.

Perhaps.

Energy is anything that can be converted into mass , or that is already mass.
this may not be 100% accurate but i am trying to relate it to E=mc^2
like fore example a photon which is energy can decay into an electron positron pair.
Photons above roughly 5-10 MeV produce e-p pairs, which then produce heat (ionization). Photons under this energy produce heat via photoelectric effect or Compton scattering.
The end result of nearly all energy sources (including fissionable and fusionable materials)produce heat as an end product. If you want to watch the last throes of energy wasted as heat, put an ice cube in a glass of water.

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Dale
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2020 Award
you could define energy simply as the ability to do work, but then your stuck with the question again, what is work
Work is force times distance (W=f.d)

Is it something that actually exists in the universe, or just a construct that we use to simplify problems?
Is there a difference?

Work is force times distance (W=f.d)
How would you explain light using this.

If it is moving or if it can make things move it has energy.We can calculate energy from the work formula or the variations of it such as the K.E. formulae. Energy is conserved(mass itself being a form of energy) and everything has energy.But what is energy and why is it as we observe it to be?At the most fundamental level we don't know.One of the books I read on energy had the following as the opening sentence;
Energy does not make sense.

russ_watters
Mentor
How would you explain light using this.
You wouldn't.

Perhaps you could rephrase the question...?

russ_watters
Mentor
But what is energy and why is it as we observe it to be?At the most fundamental level we don't know.
Yeah, we do - "the most fundamental level" is e=w=f*d. People who are looking for some deeper meaning to the word beyond the physics definition are attempting to attach a significance to it that simply doesn't exist and is completely unnecessary. IMO, this results from confusion about a possible link between the scientific and mystical definitions for the word "energy". In physics, "energy" is simply a name given to a useful mathematical relationship. That's it.

Why don't people look for similar "most fundamental level"/deeper meanings for the word "speed"? If there should be more to energy, there should be more to speed too.

E=hf

Why don't people look for similar "most fundamental level"/deeper meanings for the word "speed"? If there should be more to energy, there should be more to speed too.
people do look for deeper meanings in speed , relativity , time dialtion .
the faster you go time slows down and your mass becomes bigger , as i'm sure you are aware.

.But what is energy and why is it as we observe it to be?At the most fundamental level we don't know.
Yeah, we do - "the most fundamental level" is e=w=f*d. People who are looking for some deeper meaning to the word beyond the physics definition are attempting to attach a significance to it that simply doesn't exist and is completely unnecessary. IMO, this results from confusion about a possible link between the scientific and mystical definitions for the word "energy". In physics, "energy" is simply a name given to a useful mathematical relationship. That's it.

Why don't people look for similar "most fundamental level"/deeper meanings for the word "speed"? If there should be more to energy, there should be more to speed too.
Just one of the things we observe energy to be is a conserved quantity.At the most fundamental level we don't know why this is the case,it is just that our definitions and observation show that it is so.Are you saying that we do know why energy is conserved?

Dale
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How would you explain light using this.
Light can exert a force on an object. When that object moves the light does work according to W=f.d. Therefore light has the ability to do work (energy).

are you talking about radiation pressure , so would W=(F)(d) come up with the same

Does light have kinetic energy?

Dale
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Yes.
so would W=(F)(d) come up with the same answer as E=hf
Yes.
Does light have kinetic energy?
I would say that a photon's energy is purely kinetic energy.

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ok thanks for clarifying
Can we define energy as anything that affects time . for example time dilation and gravitational time dilation
like if our speed increased time would slow down and our kinetic energy would increase
and if we were in a strong gravitational field we would be around a lot of mass
which we could relate to E=mc^2 and we would experience gravitational time dilation
And light creates a gravitational field which would also have gravitational time dilation

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Just one of the things we observe energy to be is a conserved quantity.At the most fundamental level we don't know why this is the case,it is just that our definitions and observation show that it is so.Are you saying that we do know why energy is conserved?
Energy is conserved in those systems where the equations of motion are independent of time. This is a consequence of Noether's theorem. So, yes, we do know why energy is conserved: forces in the universe, say from Coulomb's law for example, do not depend on time.

Dale
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Can we define energy as anything that affects time
Why would we want to do that?

Stick with the standard definition (capacity to do work). If you redefine existing terms you just get confusion, miscommunication, and pointless semantic arguments.

Now, if you want to use a new word then I have nothing against the rest of what you said.

russ_watters
Mentor
E=hf
That too. Perhaps the key discoveries of the Rennaisance scientists are deriving the relationships between different kinds of energy.
people do look for deeper meanings in speed , relativity , time dialtion .
the faster you go time slows down and your mass becomes bigger , as i'm sure you are aware.
That's not equivalent - those are implications of speed, not "meanings" (definitions) of speed. People don't accept the definition of energy because they think there should be more to the definition than there really is. No one has ever asked on this forum why speed isn't s=d^2/t, but people ask all the time why you have c^2 in e=mc^2