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## What is energy

This is going to sound very bad coming from someone with an A in Physics I, II, and III.

What is energy?

I'm being serious.

Here's what I know.

E=mc2 (there is energy in matter)
E=hf (photons, which are not matter because they have no mass, can have energy)
E=(1/2)mv2 (things that are moving have energy just because they are moving)
E=mgh (there is gravitational potential energy in a mass raised to a certain height)
E=k(q1q2)/r (there is electrostatic potential energy which will cause two like charges to repel)
E=(1/2)CV2 (a parallel-plate capacitor can store energy)

I know there are more examples than this, but this is a good start.

I know energy is a prerequisite to force. In other words, I have to eat food to create energy in my muscles to close a door. The energy is the fuel for the force which causes the acceleration of the door.

Still, I need to know what energy is at a deeper level. I like the tidy idea of conservation of energy, but I'm bothered sometimes that I still don't know the root of energy.
 Recognitions: Gold Member If it moves or if it can make things move its energy.Energy is the ability to do work and work done is force times distance moved in direction of force.

Well, one definition that I know of is of "energy being a measure of an object's capacity to do work". This definition, of course, still doesn't capture the root of what exactly is this entity called "energy".

 I know energy is a prerequisite to force. In other words, I have to eat food to create energy in my muscles to close a door. The energy is the fuel for the force which causes the acceleration of the door.
Actually, the relationship between energy and force is more complex than that. The perception of energy being a "fuel for force" is more of a human thing than a physical phenomenon. A force can convert energy from one form to another eg. an object free-falling in a gravitational field.

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## What is energy

What gives energy the ability to "do stuff"? If energy is the predecessor to force, then is there a predecessor to energy?

Feynman says in his "Lectures on Physics" on page 4-2,
 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. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives "28"—always the same number. It is an abstract thing in that it does not tell us the mechanism reasons for the various formulas.
 Blog Entries: 1 Recognitions: Science Advisor Energy is defined as the source of the gravitational field. Sorry, I know that sounds somewhat indirect, overly sophisticated, and removed from common experience. But ultimately that is in fact what energy is. Just as the answer to "what is charge": charge is the source of the electromagnetic field, so energy is anything that acts as the source of gravity. (More precisely, the source is the stress-energy tensor, and energy is the 00 component of that.) The fact that general relativity is invariant under general coordinate transformations requires that its source must be conserved. And the list of things that are commonly known as forms of energy are just those things that produce a gravitational field, and can be turned into each other.

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 Quote by Dadface Energy is the capacity to do work.
Understood.

 Quote by Bill_K It is the source of the gravitational field.
Should I take a math course on tensors? I have Calc I, II, III, and Diff Eq under my belt so what would the next step be? Also, I've studied special relativity (relativity in non-accelerating inertial reference frames) but not general relativity.

Lastly, according to saim,

Our knowledge of energy is basically limited to being able to measure its effect but not to understand what it is. I've heard that name, Feynman, before. Where can I get his notes/material/books/etc? He sounds pretty stellar.

Wow. Here I was thinking I was Mister Know-It-All. Wrong!

I came across another bothersome thing this semester... My professor told me that electrons "jump" to a new orbit. When I asked him what path they took, he went on about how we don't know. I think it was basically beyond the scope of our Physics III course and has to do with quantum mechanics and the Heisenburg Uncertainty Principal.
 Mentor Energy is not a prerequisite for force.

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 Quote by russ_watters Energy is not a prerequisite for force.
Feel like elaborating?
 @JJBladester: Google "Richard Feynman" and you'll get to learn lots about him. You can also find several of his books online for download. Let me clarify that I agree with Bill_K that ability to produce curvature of spacetime can be considered the defining property of energy. I think what Feynman points toward is a sort of slightly non-scientific understanding of what energy is and that's what I thought you were asking for. @russ_watters: I'm also curious; can you give an example where no energy transfer is involved but a force is present?
 Energy is the value of a measurement. There are different ways to measure things so you end up with different forms of energy. There is also the potential to have energy, which is given it's own form.

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 Quote by LostConjugate Energy is the value of a measurement. There are different ways to measure things so you end up with different forms of energy. There is also the potential to have energy, which is given it's own form.
When you say "engery is the value of a measurement" you leave out a lot of information, which is just the information I am seeking.

Also, isn't potential energy (mgh) just as much energy as kinetic energy (.52mv2) regardless of the fact that it's "the potential to have energy"?

 Quote by JJBladester When you say "engery is the value of a measurement" you leave out a lot of information, which is just the information I am seeking. Also, isn't potential energy (mgh) just as much energy as kinetic energy (.52mv2) regardless of the fact that it's "the potential to have energy"?
Potential energy is defined by the system, it can be tricky treating potential energy as part of the total energy of an object without a clear definition of the system or reference frame.

The process of taking a measurement is to collapse the state into an eigenstate where the uncertainty in your measurement is reduced to zero. The eigenvalue associated with this state is your measured value. So I suppose you could argue that energy is the result of a collapsed wave function. There are many different ways of looking at it, it is after all just a word.

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 Quote by Bill_K Energy is defined as the source of the gravitational field.
Hmmm .. how is energy defined in QM then? Because as I understand it, there is still no unified theory explaining both gravitation and QM.

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