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When discussing sources and sinks for the property of divergence of electric fields, why is the positive polarity uniquely selected as a source and the other a sink? Aren't they both sources and sinks of opposite polarity? The negative is a source of negative field and a sink for positive field and the positive a source for positive field and a sink for negative field. This conditions students (me) to think in a confused way.

Please don't address nomenclature. The question of charge naming convention has an answer here: Why is the charge naming convention wrong?. This question is about the physical properties of the fields.

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    $\begingroup$ "The negative is a source of negative field" - ??? Not sure what a negative field is. The electric field at a point has a magnitude (which is never negative) and a direction. $\endgroup$
    – Alfred Centauri
    Commented Dec 23, 2017 at 1:49
  • $\begingroup$ @Alfred Centauri I am speaking of course of electrically negative and electrically positive. Not in the numerical sense of being less than zero. A field magnitude if it exists is of course always positive in the numerical sense. Doesn't the underlying field have flux in both directions. The negative field vector points in the positive direction and the positive field vector points in the negative direction. $\endgroup$
    – DMac
    Commented Dec 23, 2017 at 2:32
  • $\begingroup$ "The negative field vector points in the positive direction and the positive field vector points in the negative direction." - I have several electromagnetic textbooks (from both an EE and a physics perspective) and I don't recall anything like that. It's true that the E field vectors, due to a point charge, are directed radially outward for a positive charge and directed radially inward for a negative charge. If you have a positive and a negative point charge (an electric dipole), the electric field of the dipole can be written as the sum of the fields due to each charge (superposition). $\endgroup$
    – Alfred Centauri
    Commented Dec 23, 2017 at 3:47
  • $\begingroup$ Is this what you mean by positive and negative fields? $\endgroup$
    – Alfred Centauri
    Commented Dec 23, 2017 at 3:48
  • $\begingroup$ What I mean is that there seems to be two different fields because they produce opposite deflections on moving test charges. There is no way to convert the field of an electron into the field of a proton & vice versa. It seems that they must be intrinsically different. $\endgroup$
    – DMac
    Commented Dec 23, 2017 at 16:43

3 Answers 3

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What I mean is that there seems to be two different fields because they produce opposite deflections on moving test charges. There is no way to convert the field of an electron into the field of a proton & vice versa. It seems that they must be intrinsically different

If there is an intrinsic difference, then it must be detectable in principle.

Suppose that there is an electric field in some region of space containing no charge and that, in this region, the electric field is uniform in magnitude and parallel to the $z$ axis:

$$\mathbf{E} = E_0\mathbf{\hat{z}}$$

Can you think of a way, using only a test charge in this region of space, to determine if this field is due to positive or negative charge, i.e., to detect this intrinsic difference you believe exists?

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  • $\begingroup$ You are right about detectable in principle, but I really spoke too strongly. I should have said "should be intrinsically different". I'm thinking a lot about this, but everything I can think of has major gotchas. I would like to point out though that conventional current (positive current even +ions etc.) move according to the [right hand rule]:(en.wikipedia.org/wiki/…) and negative current (electrons) move according to a left hand rule. I wonder if that makes them intrinsically different. Topologically would that make them the same?? $\endgroup$
    – DMac
    Commented Dec 23, 2017 at 22:47
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There is no "positive field" and "negative field", there is only the electric field (and the magnetic one, but that's another discussion). The reason the positive charge centers are considered sources and not sinks is because of an arbitrary convention, the math would work out just as well if it were the other way around, although there might be a few more sign problems if the "negative source" convention were used.

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  • $\begingroup$ Thank you for your answer, but other than a reference to the nomenclature issue I'd like to avoid discussions of nomenclature because it obscures the discussion of the actual field properties. I wonder if you could edit your posting to remove the part of the discussion refering to just the nomenclature. $\endgroup$
    – DMac
    Commented Dec 23, 2017 at 16:51
  • $\begingroup$ I didn't discuss nomenclature. The way I see it, there are two separate conventions at play in electrical phenomena: 1) electrons are negative and protons are positive and 2) electric field lines flow from positive charges to negative charges. I did my best to leave the first convention alone, as it seemed that your question did as well, and focus on the second. $\endgroup$
    – No Name
    Commented Dec 24, 2017 at 3:19
  • $\begingroup$ However, considering your discussion with Alfred Centauri, I will add something taking that into account. $\endgroup$
    – No Name
    Commented Dec 24, 2017 at 3:41
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Urgent Mission
(XKCD #567)

As far as the electric fields themselves are concerned, which side is positive (source) and which side is negative (sink) is just a convention. However, when it comes to the flow of electricity, our convention is backwards. In the majority of cases, the charge carrier is an electron which starts from the negative side (sourced from the sink) which travels "backwards" through the circuit to the positive side (sinked into the source)! All because Benjamin Franklin had to pick a side to be positive, and he picked the less convenient one.

In semiconductors sources and sinks become more important. Each region of semiconductor is doped to be either P-type or N-type. This creates either a glut of holes (atoms which are short 1 electron) or a glut of mobile electrons. In these situations, the direction becomes important because one charge carrier can be replenished faster than the other. Of course even here, the difference is contextual. PNP transistors use the exact opposite charge carrier of NPN transistors.

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  • $\begingroup$ "In semiconductors sources and sinks become more important" The point of my interest in the question is that it seems that sources and sinks are important in some unknown or underappreciated way. Aspects of the phenomena seem to be ignored including whether the flux travels and whether it travels in both directions, do the fields themselves have rotation, would the field from a positive particle. Do the fields from static charges have actual propagation. If they have actual propagation shouldn't they have magnetic fields (they don't) etc. Very confusing. $\endgroup$
    – DMac
    Commented Dec 23, 2017 at 17:20
  • $\begingroup$ From above: If the fields themselves have rotations, would the fields from both positive and negative particles have opposite rotations? I wonder @Cort Ammon if except for the carton and the reference to nomenclature, if you could remove the discussion of nomenclature. There is a good discussion of that topic referenced in the edited question now. $\endgroup$
    – DMac
    Commented Dec 23, 2017 at 17:31
  • $\begingroup$ The charge-sign convention isn't wrong. $\endgroup$
    – Emilio Pisanty
    Commented Dec 29, 2017 at 19:35

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