Rising, rising. Here at Cottesloe Beach, Perth, Australia. © CC by SA 2.0 go_greener_oz
In the scientific press, we call them the MacGyvers. With fishing line, a glass marble and an electronic scale, the researchers Stephen Hughes and Darren Pearce reproduced in vitro the effects of climate change on the oceans. Ready for the experience?
Do you know about Archimedes’ principle? The principles that indicates that “the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces”? It is this force that two physics professors of an Australian university, Stephen Hughes and Darren Pearce, used to demonstrate that when the temperature of the oceans rises, these expand, before even considering the melting of the ice caps.
This phenomenon called thermal expansion (or thermal dilatation) is “responsible for the essential part of the rise in the sea level of the last century”, explain the researchers in the European Journal of Physics, before clarifying that “in the future a more and more important quantity of the rise in sea level will be due to the melting of the ice caps in Greenland and Antarctica, as well as the melting of the glaciers everywhere in the world and particularly in the Himalayas”.
The melting of the ice caps in the Arctic from 1987 to 2014 (NOAA, American agency for atmospheric and oceanic observation):
As temperature increases, the water molecules move faster and collide more often, which has the effect of spacing them out: the density of the water decreases and its volume increases. The thermal expansion being difficult to demonstrate by directly measuring the volume of water (less than 0.2 millimeters per degree Celsius for 1 liter of water), the professors decided to rely on Archimedes’ principle to prove it.
“My first year physics students find this experience quite simple since the volume of the marble can be measured precisely by the professor before the experience and the students can read all the measures they need directly on the digital instruments, without having to pay too much attention to the measure of the volume”, explains professor Stephen Hughes.
Beware: this is precision work. Here are the simplified instructions for the experience (for the complete process, with the equation to measure the mass of the marble, see the study published in the European Journal of Physics).
Equipment
You will need a wooden box, an electronic scale, a glass beaker, a glass marble, fishing line and an electronic thermometer with an immersion sensor.
The rising of water in vitro step by step
1. Place the electronic scale on the top of the wooden box. Affix the fishing line (the electronic scale will need to have a hook under its pan);
2. Fix the glass marble to the fishing line using Araldite glue. The marble must remain one centimeter under the water surface;
3. Fill a beaker with 500 ml of water and add ice so that when it melts, the water temperature is below 1 degree;
4. Regularly measure the temperature of the water with the immersion sensor. After each measurement, take out 20 ml of water to replace them with 20 ml at room temperature. Mix the water well before each measurement;
5. Repeat the measurements with water heated to 40 degrees to increase the water temperature to approximately 35 degrees. As the temperature rises, its density decreases and the weight of the marble increases: this is the thermal expansion measured by Archimedes’ principle. You can also use this online density calculator.
Stephen Hughes and Darren Pearce’s study in European Journal of Physics (subject to a fee)