Can you imagine building a motor from latex gloves? Physics teachers Ludwig Eidenberger and Harald Gollner, and their students Florian Altendorfer and Christoph Eidenberger, show how, exploiting the reversible thermodynamic processes of thin layers of latex.
In 2006, we took part in the Experimentale 2007w1 in Wels, Austria, a biennial regional science exhibition for schools. The inspiration for our project came from a simple effect described in the chapter on thermodynamics of the popular physics book Feynman Lecturesw2: a stretched rubber band contracts when heated. This is unusual, since most materials expand when heated, rather than contract.
We concentrated on how best to visualise this effect, and developed an elevator and a motor, as well as two types of refrigerator – all driven by the expansion and contraction of latex, a type of rubber. The advantages of latex over other types of rubber are its high quality, lack of additives, the thin layers in which it is available – plus the added fun factor of using coloured condoms.
This interdisciplinary project was a valuable experience for everyone involved: we collaborated with art and technology teachers on the experimental design; the students translated texts, designed posters and a websitew3, and practised their presentation and communication skills, not only in German, but also in English and Spanish.
They carried on working on the project for over a year after leaving school in 2007, and the whole team are still running teacher workshops on the project, such as in Technorama, The Swiss Science Centerw4 in Zürich (2009), and in Berlin, Germany (2010)w5.
Latex is a polymer consisting of long, chain-like molecules of repeating isoprene units (C5H8). In its relaxed state, the chains are interlinked at a few points. Between a pair of links, each monomer can rotate freely about its neighbour, and at room temperature, latex stores enough kinetic energy for them to do so. When latex is stretched, though, the monomers are no longer able to oscillate, and their kinetic energy is given off as excess heat. When the expanded latex is heated, the process is reversed: the latex absorbs the heat, the molecular motion increases, and the latex contracts.
This is a simple experiment, suitable for students aged 11 and above, to introduce the thermodynamics of latex, as it shows the conversion of energy. A latex glove emits heat when expanded and absorbs heat when it contracts again.
Kinetic energy -> thermal energy
As this effect is reversible, a stretched latex glove contracts when heated by a spotlight. We can use this to build an elevator.
Thermal energy -> kinetic energy
The difference between the effect that stretched latex contracts when heated and heat expansion should be mentioned.
The elevator can be built by students with little effort and basic materials (such as a Lego® construction set). In the teacher workshop we ran in Zürich, participants built very diverse elevators with great success entirely without instructions. Instructions that are too detailed will constrain the students’ creativity – try formulating the aim instead: the effect that the stretched latex band will contract when heated should be visualised. Mechanisms to enhance the visibility of this effect could be a lever, a pulley, etc. Below are some guidelines you might like to use.
This experiment can be used
This experiment takes advantage of the thermodynamics of rubber to generate power, creating a thermal engine.
The latex spokes contract on the side that is heated, so the centre of mass shifts. Thus the wheel starts turning, and due to the cooling-down of the spokes on the other side, a continuous energy conversion is possible.
Thermal energy -> rotational energy
This experiment can be used
As an introduction, expand a latex glove and wait for it to emit its heat to the surrounding air. If you now let the glove contract, it will be cold.
Kinetic energy -> heat transmission
The following experiment illustrates that the latex motor is a reversible process. If the hula hoop runs in guide rolls (powered by an electric motor) and the axis is not in the centre, the condom spokes are warm on the expanded side and cool on the other side. The resulting temperature difference can only be visualised using an infrared camera (see images).
Motor: temperature difference -> rotation
Refrigerator: rotation -> temperature difference
The following is a variation of the experiment above, and can be used to explain the concept of a refrigerator (a closed circle which absorbs heat on one side and emits heat on the other side).
Thus, the latex loop is permanently expanded (warm) on one side and permanently relaxed (cold) on the other side. This machine produces a temperature difference of about 10 °C.
This is a simplified version of Experiment 3 which can easily be performed in class without a lot of preparation. It is best done in groups of three students.
For higher heat emission, stretch the latex close to its elastic limit.
Very thin layers of latex give the best results. Use thin latex gloves (disposable gloves) or condoms. Replace the materials after some time to ensure good results.
Note: some students may be allergic to latex, so be sure to check.
Let the students invent new machines based on those effects!