# Magnetic field due to a current carrying conductor is___?

## Main Question or Discussion Point

In an infinitely long current carrying wire (assuming zero resistance) say there is a definite current.
"Having current in it" means there is a flow of charge in it, which means if positive charge moves in one direction then negative charge moves in opposite direction. As we know that a charge in motion will have to produce a magnetic field which is precisely given by the famous "Biot-Savart 's Law" for a current carrying wire. But is it not this for a normal current carrying wire, that the magnetic field produced by positive charges is equal to that produced by negative charges.
So, when we derive the expression for magnetic field due to an infinitely long current carrying wire the magnitude is given by |B|=Uoi/2 (pi)d where d--->perpendicular distance of the point from the wire and i--->current in the wire.
And that's the expression derived without considering the fact that "in a current carrying wire both positive and negative charges flow" but "only positive charges flow"..there are many textbooks which say exactly the same.
So is this not a violation of obtaining result for something which is not the proclaimed one??
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warning..
the probability of this poster to contain irony is finite and maybe nearly one..(or even not)

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Matterwave
Gold Member
I don't really get your question, could you rephrase it a little?

One can get the magnetic field due to an infinitely long wire by applying Ampere's law, or Biot-Savart's law. The charges which flow in current are negative charges of the electrons. The positive nuclei of the atoms themselves don't move. The current is defined to move the opposite way as the electrons, but that is just a convention we use. We could redefine convention and change the current to the other direction, but we'd have to redraw every schematic, and redo all the positive/negative signs on all those equations. This turns out to be too big of a hassle.

Born2bwire
Gold Member
It doesn't matter what the charge makeup of the current actually is. We can model the current as a flow of ions, electrons, or a mix of the two but the end result is still the same.

without considering the fact that "in a current carrying wire both positive and negative charges flow" but "only positive charges flow"..there are many textbooks which say exactly the same.
So is this not a violation of obtaining result for something which is not the proclaimed one??
I don't think that I fully understand what you are asking.

But, as has been posted above, I don't think that positive charges flow in a wire.

In semiconductors, positively-charged "holes" (vacancy in the valence band) can flow in electric fields. See

http://en.wikipedia.org/wiki/Semiconductor

The total sum current (electrons plus holes plus ions etc.) is continuous everywhere. It doesn't matter what the makeup of the current is.

Bob S

So, the flaw is there in my question..actually i thought that both positive charges(atomic nuclie) as well as electrons move in a conductor while there is a current in it..there i was wrong..