# What drives rubber bands

usljoo
ive heard somewhere that it is heat that does that, but the effect is supossed to be so small so that one cant even notice.
and on the other hand the effect is so large that it resists the motion of your hand for example so something is clearly wrong here it cant be only heat can it it must also be the process that is on work with springs

Homework Helper
The heat is related to the loss of energy by a rubber band, not the properties of the molecular bonds that allow the rubber band to act as a spring when stretched or compressed. During deformation, the rubber band generates heat and loses energy. If the force is reduced so that the rubber band returns to it's original shape, the loss of energy results in the force on the "return path" being less than on the "deformation path". This is called elastic hysteresis.

http://en.wikipedia.org/wiki/Hysteresis#Elastic_hysteresis

usljoo
hehe didnt know that histeresys exists also for spring forces i thought its only for magnets

The heat is related to the loss of energy by a rubber band, not the properties of the molecular bonds that allow the rubber band to act as a spring when stretched or compressed.
No, heat has everything to do with the spring-effect of the rubber band. When you are stretching a rubber band, the bonds are not stretching. Where have you seen bonds that can allow a material to stretch 100%+ and not break? 10% is the limit for these things.

What's really going on is that the polymer chains in the structure of rubber band are jumbled up. When you stretch the band, because these chains are linked at some places, you are straightening these chains out. When you do that, the random motion of the links in the chain cause a net force trying to contract the chain back into a jumbled up form.

If you want to look at it from perspective of thermodynamics, when you straighten the chains, you reduce the entropy, and therefore, increase free energy. This energy is directly proportional to the temperature of the band. A warm rubber band has higher spring constant than a cold band.

look at this then pls, i mean i cant believe to hear this from a nobelist but maybe he just slipped a bit :D wasnt his day

That's exactly the same thing as I described. That's exactly what happens. I've ran two demos that actually demonstrate this. First, you can suspend a weight on a rubber band and then heat the band. It will contract, pulling the weight higher. Second, you can demonstrate how this works with real chains on a vibrating surface.

usljoo
No, heat has everything to do with the spring-effect of the rubber band.

thats what you said k2

Homework Helper
The issue here is one of terminology, temperature (the energy of the molecules) versus heat (the transfer of thermal energy). As mentioned in the post above, what allows the rubber band to stretch is the jumbled up polymer chains that get straightened out. As mentioned in the video the energy of the other molecules between the polymer chains are trying to keep the chains jumbled up, and stretching the rubber band decreases the amount of space available for those in between molecules, which increases the pressure and energy of those molecules between the chains. This creates an increase in temperature, which in turn radiates heat, which causes the total energy of the rubber band to decrease, resulting in hysteresis. When the rubber band is allowed to return to it's original length, temperature decreases, and the rubber band will absorb heat (assuming the rubber band is not in a vacuum).

A similar process happens if rubber is compressed, temperature is increased during compression, heat is radiated, energy is decreased, and again hysteresis occurs.

I assume that if a rubber band was stretched in air that was externally heated and cooled to prevent any heat transfer from or to the rubber band, then hysteresis would not occur.

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usljoo
Which parts did you find incorrect?

as you can see on the video he clearly says that heat is responsible for the rubber band and not like with the spring.

but im just wondering because it would take an enormous amount of heat to resit the your hand and the rubber band should be glowing red.

and another thing, when you lite it up it glows like wuush

usljoo
The issue here is one of terminology, temperature (the energy of the molecules) versus heat (the transfer of thermal energy). As mentioned in the post above, what allows the rubber band to stretch is the jumbled up polymer chains that get straightened out. As mentioned in the video the energy of the other molecules between the polymer chains are trying to keep the chains jumbled up, and stretching the rubber band decreases the amount of space available for those in between molecules, which increases the pressure and energy of those molecules between the chains. This creates an increase in temperature, which in turn radiates heat, which causes the total energy of the rubber band to decrease, resulting in hysteresis. When the rubber band is allowed to return to it's original length, temperature decreases, and the rubber band will absorb heat (assuming the rubber band is not in a vacuum).

A similar process happens if rubber is compressed, temperature is increased during compression, heat is radiated, energy is decreased, and again hysteresis occurs.

look, do you really find it logical that energy of those molecules bouncing perpendicular can resist your hand. they clearly generate heat and your histeresis is correct but if it wouldnt be for other molecular bounds then it could never resist so much or when then the rubber should light up hot red or violet or whatever.

and in the video he says that its not the same as with springs and that here heat does the work

Homework Helper
energy of those molecules ... they clearly generate heat
What's clear is that their temperature increases. The increase in temperature then causes heat to be radiated, except in the special case I mentioned where the gas (or liquid) that surrounds the rubber band is externally heated (or cooled) to prevent the transfer of of any thermal energy between the rubber band and the surrounding gas (or liquid).

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usljoo
What's clear is that their temperature increases. The increase in temperarture then causes heat to be radiated, except in the special case I mentioned where the gas (or liquid) that surrounds the rubber band is externally heated (or cooled) to prevent the transfer of of any thermal energy between the rubber band and the surrounding gas (or liquid).

i have no problem with that but thats not my question

Homework Helper
Apparently the focus on temperature or heat is confusing things here. Molecules have an energy related to their mass and average speed2. This is their kinetic energy and temperature is proportional to this kinetic energy (Kelvin scale). If the molecules are constrained within some amount of space, then the density and kinetic energy of the molecules results in some amount of pressure.

Imagine that those in between molecules are like millions of little pockets of air trapped inside the rubber band. When the rubber band is stretched the volume of those little pockets is decreased, resulting in an increase in pressure trying to restore the rubber band back to it's original shape, resulting in an increase in tension of the rubber band as it is stretched, but the shape of the chains and those pockets allows the rubber band to be stretched quite a bit (latex rubber can be stretched up to 600% or so).

The decrease in volume also increases the temperature, which can further increase the pressure, until the increased temperature is reduced through radiation of thermal energy (heat). However it's the "pressure" of those molecules that is creating the tension in a stretched rubber band, and the increase in temperature is a "side effect" also related to the decrease in volume of those "pockets" of molecules between the polymer chains.

What bothers me about the video explanation is that the tension is related to mechanical energy of the affected molecules, but thermal energy can be also used to describe the kinetic energy of the affected molecules. This is similar to the pressure versus mechanical energy density issue when thermal effects are also included.

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usljoo, you seem to be confused on the difference between energy and force.

usljoo
usljoo, you seem to be confused on the difference between energy and force.

how did you come up with that

usljoo
Apparently the focus on temperature or heat is confusing things here. Molecules have an energy related to their mass and average speed2. This is their kinetic energy and temperature is proportional to this kinetic energy (Kelvin scale). If the molecules are constrained within some amount of space, then the density and kinetic energy of the molecules results in some amount of pressure.

Imagine that those in between molecules are like millions of little pockets of air trapped inside the rubber band. When the rubber band is stretched the volume of those little pockets is decreased, resulting in an increase in pressure trying to restore the rubber band back to it's original shape, resulting in an increase in tension of the rubber band as it is stretched, but the shape of the chains and those pockets allows the rubber band to be stretched quite a bit (latex rubber can be stretched up to 600% or so).

The decrease in volume also increases the temperature, which can further increase the pressure, until the increased temperature is reduced through radiation of thermal energy (heat). However it's the "pressure" of those molecules that is creating the tension in a stretched rubber band, and the increase in temperature is a "side effect" also related to the decrease in volume of those "pockets" of molecules between the polymer chains.

What bothers me about the video explanation is that the tension is related to mechanical energy of the affected molecules, but thermal energy can be also used to describe the kinetic energy of the affected molecules. This is similar to the pressure versus mechanical energy density issue when thermal effects are also included.

sure there is pressure from heat and everything you said is right yes and im not quite getting what bothers you exactly but i can smell it its the same that bothers me ;)

Homework Helper
Short version - the tension in the rubber band is related to the mechanical energy within the rubber band, not to the thermal energy. The thermal energy is also affected by stretching the rubber band, and the thermal energy also affects the mechanical energy, but the tension is directly related to the mechanical energy, and the thermal energy is a "side effect".

As an obvious example, a rubber band kept streched will eventually lose all of it's increase in thermal energy via radiation, returning to it's original temperature, but it will continue to generate tension, although slightly less tension as it cools to ambient temperature.

usljoo
so i hope it is clear what bothers me and that is that heat is supposed to be responsible for the motion of the rubber and not other bounds of molecules.

and yes of course is there heat generated just like the video says that, exerts some pressure on those chains, but it could never be enough to get even close to the pressure that my hand would put on the rubber OK? and the only possibility witch i could think of this being possible would be if the heat in the rubber would be enormous so that it would probably start to radiate in ultraviolet or something like that but even then it would probably not be possible because rubber has a small volume and thereby a relatively small number of particles.

and of course i im not confused by energy and force those concepts are quite familiar to me and even if i would be confused my them my question still makes perfectly sense

usljoo
Short version - the tension in the rubber band is related to the mechanical energy within the rubber band, not to the thermal energy. The thermal energy is also affected by stretching the rubber band, and the thermal energy also affects the mechanical energy, but the tension is directly related to the mechanical energy, and the thermal energy is a "side effect".

As an obvious example, a rubber band kept streched will eventually lose all of it's increase in thermal energy via radiation, returning to it's original temperature, but it will continue to generate tension, although slightly less tension as it cools to ambient temperature.

that is exactly what i mean and the video says it otherwise, so he did make a mistake right?

Homework Helper
that is exactly what i mean and the video says it otherwise, so he did make a mistake right?
It's an issue of terminology. Apparently he's using the term "heat" to include both the mechanical and thermal energy of the rubber band. I don't like using the term heat in that fashion.

Wiki definition of "heat"

http://en.wikipedia.org/wiki/Heat#Definitions

usljoo
he could never be doing that i mean who would no i dont think so but ok

and he also says that its different than with springs so nope he was definitely being stupid that day :D