# What is energy

1. Jul 27, 2008

### jefswat

I have always been curious as to what energy is. you can't see it or touch it so it can't be an actual material. I finally came to the conclusion that energy is just a way of describing a position or change in position. would i be correct by looking at energy in this way? I couldn't think of any exceptions but my experience is limited.

2. Jul 27, 2008

### poopcaboose

Well technically speaking energy is measured in joules, which is really a measurement of force times distance, and force is mass times acceleration, so it can be seen as the amount of work required to accelerate a mass over a measured distance.

3. Jul 27, 2008

### Staff: Mentor

Energy is defined as the capacity to do work. Work is force times distance. Change is position is just distance.

4. Jul 27, 2008

### jefswat

Yes, so couldn't you say that because of the position of a given object it has energy? What I am saying is any kind of energy is related to the position of the object.

5. Jul 28, 2008

### Staff: Mentor

It is related to the position of the object (or rather, the change in position), yes. That's not quite what you said in the first post, though.

6. Jul 28, 2008

### casanova2528

If you can confidently answer that question in your own way, you've done what many physicists have tried to do and a few have accomplished.

Taking the fundamentals of thermodynamics with a brief mentioning of particle physics, Energy describes the relationship between movement, capacity to move, and to give off light. More specifically, the 2nd law tells us that heat transfer is necessary for work. The 1st law tells you E = k + u. You can make your deductions from there.

Of course...particle physics and quantum mechanics opens another door.

7. Jul 28, 2008

### Staff: Mentor

You are exploring different types of energy and laws associated it, not defining energy itself.

8. Jul 28, 2008

### Andy Resnick

Energy is a scalar quantity (a number) that characterizes a system. In a way, it's a lot like mass- a scalar that characterizes a material object.

9. Jul 29, 2008

### quantumcore

this question is difficult as we use energy to define certain things and work , but on its own energy is a fundamental thing that cannot be physically defined exactly as you have to use few undefined quantities to define other things ( like Axioms ) , same as it is very difficult to define charge , but it is a fundamental quantity that is there but we can not exactly define it. If there is no matter than it does not mean that there is no energy as few of us regarding it with movement of matter.

10. Jul 30, 2008

### the_awesome

well if its just a change in position then what about potential energy? or am i just reading wrong?

11. Jul 30, 2008

### Staff: Mentor

It isn't just change in position. That's what I said. Potential energy is a change of position of a certain amount of mass up or down in a gravitational or magnetic field. Again, you need the change in position and the mass to know the energy (and, of course, the field strength).

12. Jul 30, 2008

### HallsofIvy

I've always thought of energy as a "book keeping device"! As physics progressed, more and more types of energy- kinetic, potential, heat,...- were added to keep energy conserved; culminating in "E= mc2".

13. Jul 30, 2008

### Andy Resnick

Energy is clearly a property of a physical system, like charge, spin, mass, length, etc, so I think it's more than just a 'book keeping device".

I also agree that the different *expressions* of energy- gravitational potential, entropic, kinetic, electromagnetic, electrostatic, strain, etc. can complicate the issue.

And even worse, many physical processes are associated with energy *changes* (free energy, for example) rather than a definite energy state.

Even so, the energy can be written down and there are clear relationships between processes and changes to the energy.

I guess this puts me in the 'shut up an calculate' school of thought....

14. Jul 31, 2008

### Domenicaccio

But then, what is "work"?

Energy is the capacity to do work, and work is the effect of changing someone else's energy...

It's not so easy to define, just as it ain't easy to define "mass" or "charge". You can try to define mass in terms of how it resists being accelerated by a force, but then how do you define "force" without using mass?

15. Jul 31, 2008

### Andy Resnick

Energy is not a capacity to do work. Entropy, for example, is a form of energy but can not, by definition, perform work.

On the face of it, F = ma is a definition of mass. It's really a defintion of force-a kinematic relationship of a dynamic quantity to something measurable (the acceleration).

Similarly, dU = Q - W is simply a conservation law- it is not a definition of energy in terms of heat or work, nor does it restrict forms of energy to dissipationless work and workless dissipation.