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?
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.
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.
