Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Il Palazzo]

I feel like we're talking past each other. I've never said PE somehow "vanishes". Quite to a contrary - I keep saying PE is as real as any other energy, as can be seen by weighing the system with added PE.

You say PE is released in many ways. I object to this not because it can't happen(of course it can!), but it doesn't have to happen and the system still does have more energy than before, in a measurable way.

The main beef I have with your explanation, is that it reinforces the misconception that PE is somehow 'not real', and if you look close enough you'll see other kinds of energy making it up.

Let's take a rubber band that you mention. If you look close enough at the interactions between the molecules it's made of, you end up not with some other kind of energy, but with PE of the Coulomb force between the charges. There's nothing more or less in there than just the change in position in the force field - exactly like with gravity.
That there is some part of the work you do on the band that gets converted to heat is not very much relevant, and a distraction imo.

[Putnam]

I keep saying PE is as real as any other energy, as can be seen by weighing the system with added PE.

You keep making this claim, but... has it actually been measured? I feel like the change in weight would be less than the reduction in weight based on distance from the center of gravity, much less measurable by any apparatus.

[Il Palazzo]

I keep saying PE is as real as any other energy, as can be seen by weighing the system with added PE.

You keep making this claim, but... has it actually been measured? I feel like the change in weight would be less than the reduction in weight based on distance from the center of gravity, much less measurable by any apparatus.

First of all, I mean the mass of the system(i.e., if you could weigh an imaginary box containing it, or "weigh" its mass by observing how some other body orbits the system). Of course the local gravitational effects dominate over any mass changes when weighing an object within the system.

For most objects gravitational PE contribution to the total energy(including the rest mass of its components) is negligible. It is a major contribution to the mass of neutron stars, though, as the difference between the baryonic mass(how massive it should be based on just to adding the mass of all the particles that make it up) and the gravitational mass(what curves the spacetime) can be as high as 20%.
You can find it discussed briefly in the begining of this article:
http://iopscience.iop.org/0004-637X/614/2/914/fulltext/57649.text.html#rf8
or mentioned in questions #4 and #7 here:
http://www.astro.umd.edu/~miller/teaching/questions/neutron.html

Whether it has or hasn't been actually observed with neutron stars I don't know.

But it's of little importance, because this is in no way contentious, or at least shouldn't be. It's the result of combining the conservation of energy and relativity. As you add PE or any other kind of energy by performing work, you increase the total energy of the system. And the total energy of the system is what curves space. Which is the same as saying its gravitational mass causes gravity and adding energy increases the gravitational mass.

But as far as it's hard to observe with gravity, due to its feelbleness compared to other fundamental forces, it can be better seen with e.g.,electromagnetic or strong forces.
The concept of binding energy, for example, is just taking the difference between potential energy of the system's components at infinity and when brought together.
It's what's behind the mass deficit in nuclear reactions(and we know these work, don't we?), and energy release in chemical reactions(fire!). The binding energy of whatever attractive force field is negative, and gets released each time the components get more tightly bound. The bound system is lighter due to the lost energy.
Conversely, it takes energy to pull the components apart, whether you do it all the way to infinity, or just a little bit, you increase the total energy of the system(its mass) as you do it.