Springs and acid

[bonj2]

Bonj

Allow a bit of poetic license-

Take a perspex ruler. Rest one end on a table, have it vertical and push down on it with your hand until it is bowed.

Now, take a knife and shave away at it, all the time keeping the pressure on. What's going to happen?

The amount by which I will have to push down with my hand to keep it in the same position will gradually decrease. If I keep the same pressure, then I would guess it would gradually start to bend more as I shaved some off.

The potential energy isn't in the 'shape', the potential energy is in the actual molecules of matter that are being compressed together at a higher compression level than equilibrium.

 

[Dave5N]

The potential kinetic energy is dissipated as kinetic energy through the dispersion of the particles of the spring as it corrodes. This in turn leads to an increase in heat energy in the system.

 

[marinyork]

I'm not sure the clip will exert a constant pressure though by Newton's 3rd Law. Taking a crude view of Hooke's law User's constant force would appear to be true but I'm not sure this is the case once you're messing around with the properties of the spring by the action of the acid.

 

[Dave5N]

And you were doing so well, until

Also, possibly more boringly, it takes a lot more to create potential energy than it does to create the same amount of joules of heat energy - in other words, it would take an awful lot of compression of a very big spring to produce a significant temperature rise in a significant volume of acid if this was carried out. And even then some of the heat energy generated might be consumed by the chemical reaction so this would have to be taken into account if you were trying to predict the temperature rise.

A lot more what? Are you unfamiliar with the first law of thermodynamics?

 

[cisamcgu]

Boringly, it will be dissipated as a little tiny bit of heat energy .... it would take an awful lot of compression of a very big spring to produce a significant temperature rise in a significant volume of acid if this was carried out. And even then some of the heat energy generated might be consumed by the chemical reaction so this would have to be taken into account if you were trying to predict the temperature rise.

Hmmm... OK, so it would be converted into heat, increasing the temperature of the acid, and therefore increasing the rate of the reaction, so leading to the spring being dissolved faster ?

Maybe ...

 

[Dave5N]

Hmmm... OK, so it would be converted into heat, increasing the temperature of the acid, and therefore increasing the rate of the reaction, so leading to the spring being dissolved faster ?

Maybe ...

Marginally.

 

[biking_fox]

NO it's not converted into heat.

Heat is created as the spring is compressed - this is the inefficency of the compression process. It takes say 1J to compress it but the spring only stores 0.8J the 0.2J difference is lost as heat.

However the compressed spring still has 0.8J of stored potential energy and assuming the spring cannot at any stage change shape this has to go somewhere.

Probably as heat, but none of the mechanisms proposed so far allow for it. Dissolving a metal in acid is an exothermic reaction, so maybe the compressed spring would be very slightly more exothermic - This could be tested in a very sensitive calorimeter, but would be very hard to do.

gbb - "infers that at some point, the original amount of acid wasnt, or wouldnt,be enough....but of course, it would be enough..."

Why of course?

 

[domtyler]

After the new orleans hurricane, there was enough potential energy left in the rubble to actually rebuild the city

When?

 

[magnatom]

What if a duck was sitting on the spring?

It'll get a nasty acid burn on its bottom.

 

[Dave5N]

NO it's not converted into heat.

Heat is created as the spring is compressed - this is the inefficency of the compression process. It takes say 1J to compress it but the spring only stores 0.8J the 0.2J difference is lost as heat.

However the compressed spring still has 0.8J of stored potential energy and assuming the spring cannot at any stage change shape this has to go somewhere.

Probably as heat, but none of the mechanisms proposed so far allow for it. Dissolving a metal in acid is an exothermic reaction, so maybe the compressed spring would be very slightly more exothermic - This could be tested in a very sensitive calorimeter, but would be very hard to do.

gbb - "infers that at some point, the original amount of acid wasnt, or wouldnt,be enough....but of course, it would be enough..."

Why of course?

1. Yes it is. As I explained, the potential kinetic energy is released as the spring dissolves/oxidises and fragments. The kinetic energy in the fragments is converted to heat as they deaccelerate through the system. (the acid).

2. The enthalpy change of the chemical reaction is not affected by the potential kinetic energy in the spring. It is a change (an exothermic reaction which releases heat energy) in chemical potential energy. The compressed spring cannot be 'slightly more exothermic'. This 'explanation' is a thermodynamic non-sequiter.

3 There is also an increase in entropy in the system as the spring metal reacts with the acid.

 

[Jaded]

OK, OK, it was only a potential joke.

 

[Dave5N]

And when/how do you convert a potential joke into a working one?

 

[biking_fox]

RE: the compressed spring would be very slightly more exothermic - I meant the exothermic energy of the acid reaction with the compressed spring could be greater than that of the uncompressed spring - I should learn to write more clearly

Why can the enthalpy of the bonds not be increased when the spring is compressed?

I've asked some clever bods because I like these sort of puzzles, They like the conversion to kinetic fragments idea, which I hadn't fully understood from your first post Dave,

BUT

Consider when the compressed spring is only partially dissolved - before it snaps.

A thinner spring produces less energy when expanding than a thicker one, hence it stores less potential energy when compressed. Our partially dissolved (now thinner) spring has therefore lost some energy somewhere?

 

[Jaded]

And when/how do you convert a potential joke into a working one?

With a lot of acid...

 

[Dave5N]

With a lot of acid...

Very good indeed!