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The waterclock

The waterclock
Materials and mounting
  two half litre plastic bottles, two small straws, two small bolts and relative nuts, coating paint or silicon sealer. Tie together, back to back, the two lids using the two small bolts, open two small holes in the tied lids and pass through them the two straws, setting them at different levels at the opposite sides of the lids. It is important to obtain a tight fit; in case of leakage, stop it with silicon sealer or coating paint.
How to play:   fill one bottle with water, close it with one of the two tied lids, screw to the upper side of the tied lids the other empty bottle and turn the two bottles upside down. Observe the water falling down through the straw at the lower level and the air bubbling up from the other straw. Calibrate the waterclock by marking the level of the water in the upper bottle at regular time intervals. Alternatively, one can fill the bottle with a quantity of water calculated to require a given amount of time to empty.
  • the water passes through the straw which has the lower entrance level in the upper bottle, because there the pressure is larger than at the entrance of the other straw (Stevino’s law),
  • while the water descends, the air rises because the volume available to the air increases in the upper bottle, while it decreases in the lower bottle,
  • since in the lower bottle the volume of air decreases, the pressure increases (Boyle’s law), and the opposite happens in the upper bottle: thus it is effectively the difference of pressure which pushes up the air;
  • time is needed for the water to descend from the upper to the lower bottle; the amount of time depends on the diameter of the straw.
Educational aspects:  
  • discover the different directions of motion of water and air between the two bottles (Pe),
  • discover the importance of the different levels of the two straws and relate it to Stevino’s law (Pd),
  • understand that the pressure of the air changes as the volume changes due to Boyle’s law (Pb),
  • measure the time needed to decrease the level of water (Se),
  • correlate the level with the volumes of air and water (Sc),
  • compare with your expectations.
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Last modified: Thu Nov 16 10:55:54 CET 2000