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About the Author: livescience


  1. Oh god, it’s the simple electrostatics experiement that has people in different disciplines arguing with each other about how it works, and using it to “prove” multiple different things because they’re clearly the only thing that could cause this.

    (“Water is polar”, “tap water has ions in it”, “polarization affects electrically neutral molecules”)

  2. Okay bois,

    The plastic cup, much like a balloon rubbed on your head, is charged. Even though water is neutral overall, it is polarized into a dipole. The oxygen atom has a much stronger affinity for the shared electrons than the hydrogen atoms, so the electrons spend more time around the oxygen atom, making it slightly more negative on that side. Let’s say the cup is positively charged and you bring it close. The oxygen atom side of H2O would be attracted and the hydrogen side repelled. Because the oxygen atom is closer, even by just a few angstroms or so, it feels a stronger pull than the positive hydrogens are repelled. This net attraction pulls all the water molecules, bending the stream. “Does this work with a negatively charged rod?” Hell yeah it does. Just this time, the hydrogens feel the pull and the oxygen is repelled, so it is spun around 180 degrees. This proves the molecular dipole is the leading cause for this reaction and not ions in the water.

    TL;DR – water molecules are polar, line up, and are attracted to a charged rod/cup.

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