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Absolute Boundaries in Space

  1. Sep 9, 2009 #1
    My understanding is that, while at a macroscopic level, my skin may seem like an absolute boundary between my interior and exterior, at a microscopic level, it is full of wholes--we cannot draw a line or arc that represents and absolute boundary between my interior and exterior at a given point in time. I can generalize a skin-boundary for myself, but only in a "fuzzy" way.

    Are fields of electrical charge and magnetic polarity dissimilar to my body in this way? In other words, is the boundary between a positively charged field and a negatively charged field "fuzzy," or do they never interpenetrate or overlap at a given point in spacetime? If your observational capacity is in the center of a negatively charged field and heads out in a given direction towards a positively charged field, what would passing the threshold between the two field charges be like?
  2. jcsd
  3. Sep 9, 2009 #2


    Staff: Mentor

    Fields do overlap. This is called the principle of superposition and is a general characteristic of all systems which are called linear.
  4. Sep 9, 2009 #3
    So if I'm a point at coordinates x,y,z at time t, I can be in a region of positive and negative charge simultaneously?
  5. Sep 9, 2009 #4
    That's right, but you only experience the 'net' field, which is the sum of the two.
  6. Sep 9, 2009 #5
    If I only experience the net field, doesn't that mean I would only experience positive, negative, or neutral charge at any given point in spacetime?
  7. Sep 9, 2009 #6
    Look at an atom from large distances: you experience a very weak net potential U(r) because the fields from the positive and negative charge clouds in atom cancel each other at large distances. Negative and positive ions create long-tailed Coulomb potentials of different signs.
  8. Sep 9, 2009 #7


    Staff: Mentor

    The field is not positive or negative, it is a vector which points in some direction with some magnitude at each point in space.
  9. Sep 10, 2009 #8
    So if you think of labeling the staring end of the field lines positive and the terminal end negative and you have two fields overlapping, considering a point where the fields overlap with the same magnitude but facing the exactly the opposite directions (180 degrees for 2 angle measurements in 3d) the net field you experience is 0. Otherwise add the components of the fields in each direction since they are vectors and overall the magnitude that you experience may be less or more than the magnitude of the either of the fields alone.

    Note: Field lines are just a way to represent a field on paper and are not representing any sort of physical boundary.
    Last edited: Sep 10, 2009
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