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Mass of water molecules in an electric field

  1. Feb 2, 2015 #1
    I'm watching a Leonard Susskind video about the Higgs Boson where he gives an example of how an electric field can change the mass of a water molecule. If you put a water molecule between two capacitor plates (so the electric field is uniform and the field lines are parallel), it will tend to align with the field because it's a dipole. The tendency to align represents a potential energy, so a 'misaligned' water molecule in the field picks up extra mass from this potential energy.

    In the video, he shows a diagram of two water molecules in the field, one aligned with the field, the other upside down (so the first is lower mass than the second). Then he says that the field "increases one mass, decreases the other mass". I understand the mass increase, but I'm confused about why the molecule that is already lined up with the electric field will have a mass decrease. Wouldn't this molecule keep the same mass it had before the introduction of the electric field? It seems that if it doesn't need to rotate (already aligned), then there's no extra potential energy, so it's in the same energy state as it would be without the field.

    at about 35:25 minutes in.
  2. jcsd
  3. Feb 3, 2015 #2


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    As it is not really a mass (it is just the energy), the scale is arbitrary - he chooses "between the two extremes" as zero because it is convenient.
  4. Feb 3, 2015 #3
    Thanks for the reply. At first I was thinking that the molecule which was already in the low energy alignment would not be affected in any way by the field, but I'm realizing that something actually does change. It might not be affected immediately, but the molecule in the field would require more force to rotate in most directions than without the field. It was just chance it happened to be aligned that way, but the average molecule will probably be at the middle of the two extremes (assuming a group of randomly oriented molecules), so it seems natural to label this state as zero to describe the effect in general.
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