# Conservation of energy

This topic was created by LeeE .

1. #### Conservation of energy

Here's a little thought experiment that occurred to me a couple of days ago, which I can't figure out...

Take a spring, made of something that can be dissolved in a suitable solvent, and a clamp, made of something that's non-soluble in the aforementioned solvent, that can be used to hold the spring in a compressed state.

Compress the spring, apply the clamp and immerse the lot in the solvent.

The spring, whilst held in its compressed state by the clamp, is storing the energy used to compress it.

So what happens to the energy stored in the compressed spring once the spring has been dissolved by the solvent?

It would seem most likely that the energy is transferred to the solvent, but in what form and by what mechanism?

1. #### Re: Conservation of energy

The form would be heat energy. The mechanism is the mechanical energy of the stirring by the spring when/as it breaks(down).

Note that I'm not going into the energy involved in the chemical break-down of the spring, which while probably exothermic, might be endothermic.

I hope I didn't just help a 14 year old with simple Physics homework ...

1. #### Re: Conservation of energy

Nope, you're not helping a 14 yr old with homework and, anyway, I suspect it's a bit too subtle for a 14 yr old physics student.

Assume the spring is perfectly manufactured, so that it dissolves perfectly uniformly; at no point does the spring break, as it were; the only relaxation in the tension of the spring would come from the erosion of the material of the spring between the bounds of the clamp; I can't see this producing a degree of stirring comparable to the energy stored in the spring.

Also assume that the compression of the spring remains within the elastic region of deformation of the spring material.

Not being a chemist, I'm thinking that the reaction must be exothermic because we're not adding energy, and if a reaction takes place when we're not adding energy then it must be exothermic, with the exothermic energy that's released from the chemical process coming from the breaking of the chemical bonds in the solvent.

So how does the elastic deformation of the spring increase the energy released by the chemical reaction? That's what I can't figure out.

1. #### Re: Conservation of energy

Remember, the by-product of the chemical reaction of the materials in question retain the mechanical energy, until released. This also works at the atomic level. Thus the heat.

2. #### Re: Conservation of energy

Compression and extension are electromagnetic phenomena at the atomic level. If the material is stretched, the atoms are further apart than the rest position, so the "spring energy" is electromagnetic potential energy. When the atomic bonds are released (because of the chemical reaction), this will be transferred into kinetic energy at the atomic scale (i.e. the atom will depart the surface of the material with significant additional velocity).

But recall: At the atomic scale, kinetic energy and heat are the same thing. Temperature is proportional to the mean square velocity of the atoms in the material.

Bulk material: Spring Energy => Atomic level: Electromagnetic potential energy.

Bulk material: Heat Energy => Atomic level: Kinetic energy.

and:

Bulk material: Chemical reaction => Atomic level: Electrons changing orbit

Bonus: Hooke's law.

Springs are complicated - when a spring is under compression one side of the wire is compressed but the other is extended. Neither of these are linear forces, but all smooth curves are almost straight lines if you zoom in enough. The point of a spring is that very small deflections are added up over the length of a very long wire. This gives an almost linear effect.

Springs closely obey Hooke's law not because it is a physical law, but because they are carefully manufactured to closely obey Hooke's law.

1. #### Re: Conservation of energy

Ta for that explanation - I hadn't thought down deeply enough to get to the electromagnetic level of the spring material.

3. #### Re: Conservation of energy

"Not being a chemist, I'm thinking that the reaction must be exothermic because we're not adding energy..."

We don't operate at 0K so there's plenty of energy available. An (unlikely) endothermic reaction would be indicated by the reaction vessel falling below ambient temp and thus absorbing energy from its surroundings (ignoring potential cooling from evaporation etc...)

2. #### Re: Conservation of energy

Since I do not want to embark on a complex study of springs to understand exactly how they store energy I have to admit my answer is somewhat speculative. Taking this experiment from "thought" mode into a real physical experiment would be challenging because the ability of the solvent to dissolve the spring implies some significant energy exchanges as the solvent will react with the spring metal to release energy independently of the metal being under tension. Taking the explanation down to the molecular/atomic level I would imagine that the tension of the spring comes about due to some deformation of the crystal lattice or perhaps some minuscule deformation of orbiting electrons. I wonder if quantum mechanics could be brought into the discussion here? But I won't go there. So as the solvent attacks the crystal lattice and pulls off the metal atom by atom the tiny amount of energy held by each particular atom being under some kind of deformation will likely impart some that energy into movement (kinetic energy) and pass that energy into the solvent around it thereby increasing the temperature. It might be possible to do two experiments placing a large number of springs into a large vat of acid once under tension and once not under tension and see if there if an observable difference in temperature behavior.

1. #### Re: Conservation of energy

This very subject was discussed on something on Radio 4 a couple of weeks ago (sadly I don't recall the programme). Their conclusion was exactly as discussed above - save that the amount of energy involved was miniscule and significantly below the level at which it would be possible to measure any temperature changes.

That energy is stored in a spring is easy to demonstrate with an elastic band: stretch it and it will get hot - energy has been added to it. Let it cool to ambient and then relax it; it will cool further.

1. #### Re: elastic band

That's not "the energy stored" that's energy being lost due to failure of the elasticity: hysteresis loss

2. #### What about the clamp?

What happens to the clamp, or rather the energy in the clamp, when the spring is dissolved? The clamp compresses the spring, so there is an interaction of forces between them.

(I've just speed read the discussion, must read it again carefully and think about it for a bit. But the clamp question just popped up in my mind.)

1. #### Re: What about the clamp?

The clamp slowly releases its energy into deforming the spring which is weakening as it dissolves.

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