Something interesting I noticed some years ago about the properties of elastic bands.
First, the experiment:
Take a rubber band (a fairly chunky one, say a quarter inch) and hold it between the thumb and first finger of each hand. Hold it taut (But NOT stretched) and touch it briefly against your upper lip just under your nose (Because this area is particularly sensitive to temperature). Now, quickly and firmly stretch the rubber band (Taking such precautions that you might deem necessary bearing in mind that this is being done just in front of your face!
) and quickly touch it against your upper lip again. You will notice that it is warmer!
Now for the interesting bit, allow the band to relax and touch it against your upper lip once more. You will notice that it is cooler! Indeed, if you allow it to cool down before allowing it to relax it will become quite cold!
This is interesting! Warming up when stretched can be dismissed as simply internal friction heating. Cooling down when relaxed suggests that something rather more intriguing is happening!
My own interpretation is that the thermal capacity of rubber can change depending on its degree of stretch. The model given for the behavior of rubber always envisages tangled up molecules that are straightened out when stretched and become tangled up again when relaxed, I can see a situation where the stretching of the molecules reduces their degrees of freedom so in order to contain the same amount of thermal energy they need to vibrate more vigorously in the degrees of freedom they still have. (IE The specific heat capacity has been reduced so it has to become hotter in order to store the same amount of thermal energy) When the band is relaxed, the process is reversed.
Using this property, it should be entirely possible to build a rubber band heat pump! The COP will be pretty dismal but I think it would certainly be possible to generate a measurable effect. In principle (since heat engines rely on reversible processes) it should also be theoretically possible to build a heat engine, In practice the internal friction would be wayyy to high to make it work (Though I would imagine that with suitable measuring equipment the “Heat engine” effect could be detected on a “Driven” system)
So, The question is…
Is my analysis correct?
And,
Might there be other “Variable heat capacity” materials out there that might be exploited to make a practical machine based on this principle?
First, the experiment:
Take a rubber band (a fairly chunky one, say a quarter inch) and hold it between the thumb and first finger of each hand. Hold it taut (But NOT stretched) and touch it briefly against your upper lip just under your nose (Because this area is particularly sensitive to temperature). Now, quickly and firmly stretch the rubber band (Taking such precautions that you might deem necessary bearing in mind that this is being done just in front of your face!
) and quickly touch it against your upper lip again. You will notice that it is warmer!Now for the interesting bit, allow the band to relax and touch it against your upper lip once more. You will notice that it is cooler! Indeed, if you allow it to cool down before allowing it to relax it will become quite cold!
This is interesting! Warming up when stretched can be dismissed as simply internal friction heating. Cooling down when relaxed suggests that something rather more intriguing is happening!
My own interpretation is that the thermal capacity of rubber can change depending on its degree of stretch. The model given for the behavior of rubber always envisages tangled up molecules that are straightened out when stretched and become tangled up again when relaxed, I can see a situation where the stretching of the molecules reduces their degrees of freedom so in order to contain the same amount of thermal energy they need to vibrate more vigorously in the degrees of freedom they still have. (IE The specific heat capacity has been reduced so it has to become hotter in order to store the same amount of thermal energy) When the band is relaxed, the process is reversed.
Using this property, it should be entirely possible to build a rubber band heat pump! The COP will be pretty dismal but I think it would certainly be possible to generate a measurable effect. In principle (since heat engines rely on reversible processes) it should also be theoretically possible to build a heat engine, In practice the internal friction would be wayyy to high to make it work (Though I would imagine that with suitable measuring equipment the “Heat engine” effect could be detected on a “Driven” system)
So, The question is…
Is my analysis correct?
And,
Might there be other “Variable heat capacity” materials out there that might be exploited to make a practical machine based on this principle?
