More Entropy Tests

Discussion in 'General Slingshot Discussions' started by JoergS, Feb 1, 2012.

  1. JoergS

    JoergS Administrator

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    It is very very cold right now here in Germany - a high pressure system coming in from Siberia.<br><br>I measured minus 8,4 centigrade noon temperature - that is -18 Fahrenheit.<br><br>So I took my band test system outside, stretched the bands and let it sit for 30 minutes or so.<br><br>The bands lost ALL flexibility! No more rubber like behaviour. I saw the "wrinkled" look I noticed a year ago after shooting a sling-x-bow out in the cold.<br><br>About five secs of the heatgun restored the rubber in full.<br><br>This is one more proof: Rubber stores all of its energy thermally. You stretch out a band, it warms up. Release it, it cools down. Cool it down while stretched, and the stretchiness decreases. Cool it down low enough and there is no more stretchiness.<br><br><div style="margin:auto;text-align:center;width:100%">
    <a href="http://3.bp.blogspot.com/-PesYjWiqq5Y/Tykc5OLtimI/AAAAAAAABWg/ABeBf7KwPAw/s1600/Bandtest1.JPG" class="postlink" target="_blank" rel="nofollow"><img src="http://3.bp.blogspot.com/-PesYjWiqq5Y/Tykc5OLtimI/AAAAAAAABWg/ABeBf7KwPAw/s320/Bandtest1.JPG" border="0" alt=""></a><br><br><br><a href="http://4.bp.blogspot.com/-gD0FijYZFmE/Tykc4IAgcDI/AAAAAAAABWY/xrf7Dch89zQ/s1600/Bandtest2.JPG" class="postlink" target="_blank" rel="nofollow"><img src="http://4.bp.blogspot.com/-gD0FijYZFmE/Tykc4IAgcDI/AAAAAAAABWY/xrf7Dch89zQ/s320/Bandtest2.JPG" border="0" alt=""></a><br><br><br><a href="http://1.bp.blogspot.com/-XwxnaALcFJo/Tykc1uw1ZDI/AAAAAAAABWI/9zicG7oxDyg/s1600/bandTest4.JPG" class="postlink" target="_blank" rel="nofollow"><img src="http://1.bp.blogspot.com/-XwxnaALcFJo/Tykc1uw1ZDI/AAAAAAAABWI/9zicG7oxDyg/s320/bandTest4.JPG" border="0" alt=""></a><br><br><br><a href="http://1.bp.blogspot.com/-89WP6bKPj_8/Tykc2mpW_AI/AAAAAAAABWQ/KM6BLlxevqY/s1600/bandtest3.JPG" class="postlink" target="_blank" rel="nofollow"><img src="http://1.bp.blogspot.com/-89WP6bKPj_8/Tykc2mpW_AI/AAAAAAAABWQ/KM6BLlxevqY/s320/bandtest3.JPG" border="0" alt=""></a><br><br><br><a href="http://1.bp.blogspot.com/-OkKK5DeelzQ/Tykc0_Od6bI/AAAAAAAABWA/Swe0sMPnTIQ/s1600/Bandtest5.JPG" class="postlink" target="_blank" rel="nofollow"><img src="http://1.bp.blogspot.com/-OkKK5DeelzQ/Tykc0_Od6bI/AAAAAAAABWA/Swe0sMPnTIQ/s320/Bandtest5.JPG" border="0" alt=""></a>
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  2. Ryan Wigglesworth

    Ryan Wigglesworth Senior Member

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    This is good info, thank you for sharing <img src="http://illiweb.com/fa/i/smiles/icon_smile.gif" alt="Smile" longdesc="2">
     

  3. Flatband

    Flatband New Member

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    Great stuff Joerg! I too noticed the "Wrinkled Effect" when very cold. With all the research done regarding heat and bands ,elongation and power,cut and thickness of various rubber-etc., I always wondered ( I know this may sound totally crazy -but then initially most "out of the box" ideas usually are) if I could get a very thin spring steel wire or some other material,run it along the bands-coiled like a snake the whole band length, and at its base (on the frame somewhere) have a small power source to heat the bands a little and keep them heated. Either that or two thin gauge bands with a thin wire or material in the middle and clamped with adhesive. Granted it's wild,and maybe not possible but some other wild ideas have worked but are cumbersome and not very portable-I.E. heated water,Hot air. I was even entertaining thoughts of " Heat Creme " used for muscle pain and spreading a thin layer of that slop on the bands! I am insane I know, but it's still fun even thinking crazy stuff like this!!! Flatband <img src="http://illiweb.com/fa/i/smiles/icon_eek.gif" alt="Shocked" longdesc="5">
     
  4. Antraxx

    Antraxx #7

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    <table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>JoergS wrote:</b></span></td></tr>
    <tr><td class="quote">
    <br>This is one more proof: Rubber stores all of its energy thermally. </td></tr>
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    <span class="postbody"><br><br>Hey Jörg,<br><br>my first time i read through all this heating threads...<br>i hate argueing in a different language, but i will try it anyway...<br>maybe i got it wrong, but the sentence i quoted...well..it´s pretty hard to get that wrong.<br><br>So you think rubber stores the energy that is applied to it as heat?<br><br>Sorry that is not, and can´t be true.<br><br>Rubber is a matrial whose small segments are kind of connected to each other and attract each other...like magnets. the more you strech them out, remove them out of their comfort position, the more they want to get back into their loved position...of course heat is generated during this process, but that can´t be more then waste.<br><br>Just to have some numbers to toss around, lets take a 30cm piece of rubber, 2cm wide.<br>if you lay it flat onto a table or if you want draw it out to 30,001cm, the energy it has stored is <strong>close to zero</strong>. (es hat nur lageenergie bezogen auf die höhe über unserem erdmittlepunkt) it is nearly in his comfort position.<br>Now we draw it out to 1meter. let´s say it heats up to 25°C<br><br>First of all, the energy it takes to heat up those few gramms of rubber for 5 degrees is nowhere near to the energy it can deliver when you release it.<br><br>Second; you say all energy is stored thermally. so if that is true all the energy is stored in those 5°C the band has heated up to...that means (if you are right) if i wait 1hour (and i don´t think it will take that much to cool down to room temperature again...more like 15min or so) and the band has 20° again (but is 1meter long) ALL of the energy you put into it is gone...that would mean it won´t get back to his 30cm relaxed position/can´t fire a shot/the scales should go down to <strong>zero</strong>...but that is wrong. it might have lost a bit of his power, but not (only) because it has cooled down (i will taln about dependance on temp. in a moment) , just because the connections in his inside are getting tired (and i think in those min it takes to cool down to 20° again it will loose very few of its energy) <br><br><br><br>I think you are mistaking it with the effect that the inner power of the rubber is DEPENDANT on heat/temperature. so if the temp. falls below a certain point, the attraction of the elements to each other goes down to zero.<br><br>the same effect with water btw. all those elements are moving and keeping it liquid in a certain "window" of temperature...but if you drop below 0°, the moving stops and the water freezes.<br><br>if you heat up ice again, it will get liquid...same with rubber...the elements are "waking up" and pulling at each other again...but thats not the same as delivering energy to it to fire a shot (because you can´t shoot with a heated but not stretched rubber band).<br><br>Well, thats all shool physics and it´s long ago...just what i think <img src="http://illiweb.com/fa/i/smiles/icon_redface.gif" alt="Embarassed" longdesc="10"></span>
     
  5. JoergS

    JoergS Administrator

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    Antraxx, first of all, these issues have all been sorted out by great physicists, like Werner Kuhn (an Austrian guy) and others. So there is NO dispute in science over these issues. <br><br>Rubber DOES store the energy thermally. <br><br>But you have to create that energy with your muscle power. <br><br>If you pull out a rubber band that is, say, plus degrees cold, it offers, say, 10 kg draw force and will be 20 degrees warm in drawn condition.<br><br>If you do the same to a band that is 20 plus to begin with, then you have to invest, say, 20 kg draw force, and you heat it up to 35 degrees. <br><br>If you let it cool down to 20 degrees, it will have 10 kg draw left. <br><br>If you heat it up to 35 degrees again, it will have the 20 kg back.<br><br>At about -12 or -15, the bands loose all draw weight and are like uncooked pasta. <br><br>Rubber behaves like an ideal gas. Ever shot a co2 air gun? The cartridges cool down when you fire. Same entropy based effect.
     
  6. Antraxx

    Antraxx #7

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    <table width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td><span class="genmed"><b>JoergS wrote:</b></span></td></tr>
    <tr><td class="quote">
    <br>If you pull out a rubber band that is, say, <font color="red"><strong>?</strong></font> plus degrees cold, it offers, say, 10 kg draw force and will be 20 degrees warm in drawn condition.<br><br>If you do the same to a band that is 20 plus to begin with, then you have to invest, say, 20 kg draw force, and you heat it up to 35 degrees. <br><br>If you let it cool down to 20 degrees, it will have 10 kg draw left. <br><br>If you heat it up to 35 degrees again, it will have the 20 kg back.<br>
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    <span class="postbody"><br><br>Well, i´m not quite sure if it´s just a communication problem.<br>i don´t doubt the above written lines (your missing a number tho)...they fit what i said about rising entropy due to more temp. (=more energy, those inner chains have more power when the are warmer)<br><br>the problem here is, as i said, the sentence, storing all energy thermally. you are only comparing different draws at different starting temp.<br><br>compare it to a ball that i lift up on a table. his energy has increased, because i lifted it...it can deliver energy when it drops....and it can stay there for 100 years and still has that energy stored.<br><br>so if i glue the end of a rubberband to the floor and attach a toy car to it, now i draw out the car 25cm and fix it. bandtemp rises. so if that temp is back to normal and i unlock the car, it will still be pulled back, eventho the temp, thus the stored energy you say, is gone.<br>i don´t doubt that under a certain temp. the car will stay where it was or that heating the bands up will result in a faster pull!<br><br>if you say it stores as temp. it automatically means the energy is gone after the temp is gone, and that is not true. more heat just powes up the entropy effect of those polymeres.<br><br>Es handelt sich dabei um einen <a href="http://de.wikipedia.org/wiki/Entropie_%28Thermodynamik%29" class="postlink" target="_blank" rel="nofollow">entropischen</a> Effekt, diese Form der Elastizität wird als <i>Entropie-Elastizität</i><br> bezeichnet. Sie stellt in der Regel den größten Anteil des elastischen <br>Effekts eines Elastomers. Da die Drehbewegung umso schneller und <br>effizienter erfolgt, je mehr Energie für diese vorhanden ist, steigt <br>dieser Effekt mit der Temperatur. Der Anstieg der Elastizität mit der <br>Temperatur ist ein sehr typisches Merkmal von Elastomeren. Die <br>Temperatur, unterhalb deren thermische Energie nicht mehr ausreicht, die<br> Drehbewegungen zu vollführen, heißt <i><a href="http://de.wikipedia.org/wiki/Glas%C3%BCbergangstemperatur" class="postlink" target="_blank" rel="nofollow">Glasübergangstemperatur</a></i>, unterhalb der Glasübergangstemperatur verlieren Elastomere ihre typische Eigenschaft.<br><br><table class="spoiler" width="90%" cellspacing="1" cellpadding="0" border="0" align="center">
    <tr><td style="cursor: pointer;"><span class="genmed"><b>Spoiler:</b></span></td></tr>
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    <td class="spoiler_closed">*</td>
    <td class="spoiler_content hidden">Ums nochmal auf Deutsch zu sagen: Unter "store" verstehe ich "Speichern". So wie eine Batterie eben Energie speichert...die ist eben erst leer, wenn wirklich keinen Elektronen mehr fließen. EIn Gummiband behält aber seine elastischen Eigenschaften und die Elastomere "ziehen" trotzdem weiterhin an einem Gewicht, auch wenn sich die 5° um die es sich erwärmt hat wieder verabschiedet haben. Mehr Temperatur begünstigt lediglich die elastischen Eigenschaften. Mit mehr Hitze haben die Coils auch mehr Energie (die ziehen sie nunmal aus der Wärme) um sich wieder zusammenzuziehen. Darum passen die Werte wie Du sie oben genannt hast ja auch alle. Aber unter Speichern versteh ich halt die 10kg die man reinsteckt werden in der Steigenden Temperatur gespeichert, also 10kg=DeltaX Temp. Was bedeuten würde wenn Delta X wieder Null ist ist auch die Energie weg...und dem ist nicht so.<br>Sicher ne Definitionssache, mir ist der Satz im englischen Wiki auch bewusst...Internet juckt mich aber nicht so...zumal es wie oben zu sehen im deuschen so steht wie ich es auch interpretiere. Würd mich die Aussage eines Physikprofessors hier an der RWTH zu mal interessieren.</td>
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    <span class="postbody"></span></span>
     
  7. konni

    konni New Member

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    You are absolutely right here. As can be found here at Wikipedia:

    "the ability of an elastomer to do work depends (as with an ideal gas) only on entropy-change considerations, and not on any stored (i.e., potential) energy within the polymer bonds. Instead, the energy to do work comes entirely from thermal energy..."

    However the arguments of user Antraxx seem correct as well:
    if you let a cocked rubber cool down to the surrounding temperature, it can contract and propel a projectile nevertheless.

    A contradiction?

    No. Both is true because when contracting, the temperature decreases, resulting in even cooler bands - below the surrounding temperature. So all available propulsion was based on thermal energy.

    But this comes with a surprise, as conventionally, it is not possible to gain energy out of a system with temperature already homogeneously distributed.

    There very trick behind is: Temperature and thermal energy is different and their relation depends on the number of free microscopic states (lesser entropy).

    The stored energy of a rubber band lies in the reduced number of molecular states as the rubber tension reduces the freedom of polymers to spin/oscillate along C-bonds. Thus, these give their kinetic (i.e., thermal) energy to the remaining options of free movement, that is why temperature increases. If you release the rubber, the temperature energy spreads again equally along these options of free movement, it cools down, and the rubber contracts as the spinning/oscillating C-bonds reduce the polymer length.