On Fri, Sep 7, 2012 at 1:00 AM, Abd ul-Rahman Lomax <a...@lomaxdesign.com> wrote: > At 08:16 PM 9/6/2012, Harry Veeder wrote: >> >> If a spring is compressed by a force at room temperature, the spring >> will return to its original length once the force is removed. >> In the language of CoE the compressed spring is said to "store" the >> energy of the work done by the force. >> >> Now compress the spring again and then place it in a bath of liquid >> nitrogen. The spring will not return to its original length once the >> force is removed. >> At this stage I would say some of the "stored energy" has vanished and >> CoE has been violated. > > > Seriously? Why would you say that? > > You could create a much simpler "violation." You slip a rectangular box of > the right size over the compressed spring, so it can't return to its > original size. Where did the potential energy go?
It is still present because the box is performing the same compression task as the weight. > First of all, I'm not so sure about the liquid nitrogen doing what is > claimed about the spring. But if it does, it might do so in several > different ways. For starters, energy is conserved in a closed system. In the > situation described, the liquid nitrogen is heated by the spring, i.e., > energy is removed from the spring and is transferred to the liquid nitrogen. > Placing an uncompressed spring in the liquid nitrogen will also result in a transfer heat. If the uncompressed and compressed spring both start at room temperature, then it seems to me the loss of stored energy is not commensurate with the transfer of heat. > I could imagine that the spring, with serious pressure on it, might just > break in the liquid nitrogen. > > If a compressed spring is dissolved in acid, again, the energy of > compression is -- theoretically by CoE -- transferred to the acid, it would > be heated a little. As the spring dissolved, it would become weaker, it > would be likely to break before complete dissolution. All the energy, > ultimately, would end up as heat in the acid, but the dissolution is also > exothermic. Not easy to measure the contribution from the spring energy. > > The "length" of the spring is a red herring. The energy stored in the spring > is the force exerted integrated over the distance travelled. At the same > temperature, we expect with a perfect spring that it will return to the same > length if released, and that does work equivalent to the return distance. > However, an imperfect material may lose its "spring," and will not return to > the same length. Where has the energy gone? > > It's the same problem, really. > > From my observation of bending metal, the energy goes into heating the > spring, i.e, as the material is inelastically deformed, it heats. > Bend metal back and forth, it heats, to the extent that the bending is not > converted into spring tension. I've felt metal bent repeatedly back and > forth get quite hot. Upon first compression the spring warms, but it will cool so this warming is not commensurate with the store of energy represented by the load on the spring. Harry
