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A speck of dust that carries charge in a electric field?

  1. Jan 18, 2015 #1
    A speck of dust has a mass of 1 x 10-18 kg and carries a charge equal to that of one electron. Near to the Earth's surface it experiences a uniform downward electric field od strength 100NC-1 and a uniform gravitational field of strength 9.8Nkg-1.

    Draw a diagram which shows the electric field pattern and the sign of the charge on the surface.

    I find my college's homeworks very confusing in wording and just generally to understand. I can do more stuff with exam Q's most of the time, but these homework bring me out of the safe zone because I can never actually understand what they are trying to say.

    First of all, It says the speck of dust itself has a charge, does that mean that I can treat it as a point charge?

    Secondly, if it's equal to one electron's charge, does it automatically make the charge negative? The hw sheet says "Electronic Charge = 1.6 x10-19", while the formula booklet says there is a minus before all that so I don't know whether it is a positive charge or a negative charged dust speck.

    Thirdly, if it is a point charge, does it mean that it will have its own radial field that, depending on whether it is positive or negative, goes outwards or towards the charge? And that there is going to be a background electric field that is going downwards from most positive to least positive at the same time?

    Fourthly, what does it mean by ''the sign of the charge on the surface''? Does it mean that I label the particle negative or positively?

    Since this forum requires an attempt at answer:
    20h63nm.png
     
    Last edited: Jan 18, 2015
  2. jcsd
  3. Jan 18, 2015 #2

    mfb

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    That depends on what you want to calculate. For the sketch, it should not matter (a sphere is easier to draw clearly).
    Probably, as an electron has a negative charge.
    It will have its own electric field, independent of the size of the speck of dust. You probably don't need this field.
    Probably, but I don't see the point in this part of the homework question.
     
  4. Jan 19, 2015 #3

    rude man

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    You should show the E field pointing from the earth to the speck (quiz: why not from the speck to the earth?). The surface they refer to must be the earth, since it's obvious what the charge on the speck is. The earth's surface in the immediate vicinity of the speck will have charge induced due to the presence of the speck.

    The wording on this problem does indeed leave much to be desired.
     
  5. Jan 19, 2015 #4

    mfb

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    Ah, good idea.
    I doubt that's relevant, considering the tiny charge of the speck.

    Yes, the wording of the problem is problematic.
     
  6. Jan 19, 2015 #5

    rude man

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    But, it's a tiny problem! The flux lines have to end somewhere, and that somewhere is the earth.
     
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