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Electric dipole moment and Electric field

  1. May 9, 2015 #1
    Hello everyone,

    I would like to ask a couple of questions which are related to electric dipole moment and electric field.

    First one: Let us assume that we have somehow a constant electric field. The obvious thing to say is that any material that contains a Net Charge ( moving one or not moving one) will be affected by this electric field. My question is, is a net charge enough for something, in order to be affected by an external electric field? In a case that something has zero net charge, but DOES have an electric dipole moment, will ''something'' be affected by this electric field inspite of having zero net charge, just because it has a non zero electric dipole moment?

    Second one: In the case that we have a molecule of water which if I am not wrong, DOES have a non zero electric dipole moment, but a zero net charge, which will the result be if we apply an external electric field? A dc current carrying wire since being neutral will not attract or deflect an external charge right? Does it mean that its electric dipole moment is zero?

    Third one: Is electric dipole moment connected with electric field somehow? For example can we have something that has zero net charge and non zero electric dipole moment? Or something that does have a net charge but at the same time zero electric dipole moment? Or is it true to say that something that has electric dipole moment, creates ath the same time its own electric field?

    Fourth one: Does question number four have something similar in magnetism?

    And finally is it true to say that a neutral atom that has only s electrons, due to spherical symmetry, will have zero electric dipole moment since it is not an electric dipole and thus the centers of positive and negative charges are at the same place?

    Thanks a lot for your time. Hope that my questions are clear.
  2. jcsd
  3. May 9, 2015 #2
    1. Both will be affected, but in slightly different ways. An object with net charge will move along the electric field lines, with its direction depending on the sign of its charge. An object with a dipole moment but no net charge will align itself so that the dipole is parallel to the electric field. This happens because the two ends of the dipole are moved in opposite directions by the field until an equilibrium is reached.

    2. You are correct, water molecules do form dipoles. In the absence of an electric field, the positive ends of each dipole are attracted to the negative ends of other dipoles (hydrogen bonding), thereby increasing the energy required to separate the molecules and hence increasing the boiling point. If an electric field is introduced, the dipoles will align as discussed above, but if the electrodes are placed into the water itself, this will break the dipoles apart into separate hydrogen and oxygen atoms (electrolysis). A current-carrying wire does not have an electric dipole moment since there is no overall separation between areas of positive and negative charges.

    3. Yes, an electric dipole moment has an associated electric field. An oscillating dipole (i.e. one where the + and - continually swap places) will produce an oscillating electric field, and this is the principle behind transmission antennae for radio broadcasts etc. Yes to all your other questions.

    4. There are magnetic dipole moments, where + and - are replaced with North and South poles. The oscillating dipole above produces an oscillating magnetic field in addition to the electric field to make an electromagnetic wave. The difference is that it is not possible to have magnetic charge without a magnetic dipole, since as far as we know magnetic monopoles do not exist. Magnetic dipoles will line up in a magnetic field just like electric dipoles in an electric field. This is why magnetic metals in contact with a permanent magnet can themselves become permanent magnets.

    Finally, broadly speaking you are correct. All the positive charge in an atom is concentrated at the centre, so if we assume that the electrons are uniformly spread around the edge of the atom, the dipole moment will average to zero. In practice, however, it is a bit more complicated. Since the movement of electrons around an atom is essentially random (within rules specified by quantum mechanics), there is a probability that at any given time there will be more electrons on one side of an atom than the other, producing a dipole moment. This dipole moment produces a small electric field that is able to induce dipole moments in nearby atoms, such that they become weakly attracted to one another (van der Waals force).
  4. May 10, 2015 #3

    thanks a lot for your immediate and very clear answers. The only ''dark'' point is answer number 4, which as I said is perfectly clear, except that I came up with a couple of other questions after that. Before you may answer, please let me know if you think that it is off topic and I should post a new thread. Here are my new questions:

    1) Based on the fact that you noted, that an oscillating electric dipole produces an oscilatting magnetic field in addition to the electric field, is it correct to assume that when we have an electric dipole, we do have at the same time a magnetic field? Or is it prerequisite to have an oscillating electric dipole in order to have a magnetic field? Because for example a battery is an electric dipole, but not an oscillating one and as far as I know, a battery does not produce any magnetic fields.

    2) A dc current carrying wire is not an electric dipole and thus it does not have any electric dipole moment, or any electric fields produced out of the wire. But in the case that we have an AC current wire we do have a new produced magnetic field, as long as current keeps changing. So: i) Does this magnetic field originate from magnetic dipole moment like an electric dipole moment has an electric field associated with it? ii) if yes is it logical to suppose that when we have AC current, we have somehow forced the creation of Oscillating electric dipoles, which consequently induce the creation of magnetic dipoles, which produce a magnetic field?

    3) Suppose that we have a permanent magnet. If I am not wrong, we call a magnet permanent, if it has all its magnetic dipole moments alligned at one direction and thus produces a total magnetic field. When we place it near a metallic desk for example, it will ''stick'' on it. Why? Because our permanent magnet will force our paramagnetic mettalic material to allign its mangetic dipoles parallel to the magnetic field lines. Is number 3 statement true?

    4) Finally, would it be true to claim that: a material that has a total magnetic dipole moment will be attracted by an external magnetic field, as long as there are some magnetic dipoles which will be alligned to the external magnetic field's lines. But a material that has a constant total electric dipole moment will not be attracted by an external electric field if it has zero net charge( only its dipoles will be alligned with the external electric field ).

    5) Is it true to say: a material with a non constant electric dipole moment will not be attracted by an external electric field if it has no net charge. But it will be attracted by an external magnetic field since: non constant electric dipoles=magnetic dipoles=magnetic dipole moment, which in the case that gets alligned to the external field's lines, will be attracted by it.
  5. May 10, 2015 #4
    1. A magnetic field is set up by moving electric charge, so a static electric dipole will not produce a magnetic field, since the charges are not moving.

    2. As mentioned above, any moving electric charge creates a magnetic field, so both DC and AC wires will have associated magnetic fields around them. The difference with AC wires is that the charges repeatedly change direction, so the direction of the magnetic field lines also changes.

    3. Yes that's essentially true.

    4. Magnetic dipole moments always line up in a strong enough magnetic field (that's how a compass works), but they will be attracted if the 'magnetic flux' is high enough. This depends on the strength of the field and the distance from whatever is generating it. Similarly with electric fields, the dipoles will always line up but attraction can occur if the field is strong enough. In electrolysis, the field is strong enough to rip apart the dipoles and attract their constituent charges to the electrodes.

    5. A non-constant electric dipole will still line up with an electric field, but its direction will switch as the dipole changes. A similar thing happens with the magnetic field.

    While you are correct in saying that an electric dipole has no net charge, this is perhaps misleading. Remember that an electric dipole just means separation between positive and negative charges, so the two ends of a dipole can be thought of as having charge, while the system overall remains neutral.
  6. May 14, 2015 #5
    Thanks you very much. You helped a lot!
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