General > General Technical Chat

Why conventional current sucks!

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kc1980:
In the beginner's sub-forum, we recently had a brief discussion on conventional current and how it has no impact on real-world calculations.  Well, I recently had a confusing situation that I blame squarely on "conventional current" (conventional current is defined as positive charges flowing in the opposite direction as electrons).

http://en.wikipedia.org/wiki/File:Hall_effect.png

Consider the first illustration on the Hall Effect wiki page: http://en.wikipedia.org/wiki/Hall_effect

Case 1, Real Current (flowing electrons):
The illustration shows electrons flowing through a uniform magnetic field and deflecting as one would guess using the left-hand rule for electrons; accordingly, a voltage is induced in the metal plate with the top being more negative than the bottom.

Case 2, Conventional Current (flowing positive charges):
If you convert the illustration to conventional current and pretend that there are positive charge carriers flowing in the opposite direction as the electrons, you will find -- using the right-hand rule for positive particles -- that the top of the metal plate should be more positive than the bottom.

How can that be possible?

In reality, the majority charge carriers are the negative electrons and the positive charges are bound in the metal lattice.  We also have to keep in mind that "holes" play a significant role only in semiconductors and not metals.  Case number 2 is somewhat correct in that it predicts the direction in which the current is deflected; however, the notion of conventional current incorrectly predicts the charge distribution.  :P

XynxNet:
Left hand rule appliece to both situations. Conventional current with positive charges is mathematical equivalent to current with negativ charges in the other direction. So the same left hand rule appliece.

But shafri is right. Don't forget that the hand rule is just a matematical artifact of the convention how we set up or coordinate systems. Understanding the underlying principle is the important thing. Otherwise the Hall effect can become quite confusing, for example in some semiconductors electrons can behave like positiv charges and this results in an inverted hall voltage.