Why is plus on top?

[Martin Czech]

Why do we write from left to right?  Why do people walk with their feet
on "ground" , even if the virtual image in your eye is upside down.
And even worse, why is there up and down in space flight movies where
we know that there shouldn't be any up or down?

By convention. People need convention. Not so long ago the earth was
assumed to be the center of the universe...  People just can't stand a
relative view, they need something absolute...

For me it makes no difference. Using these nice arrows in the schematic
one can define an assumed current or potential direction.  It can turn out
later on that the actual current or potential direction is the opposite,
indicated by a minus sign. It doesn't matter, as long as one stays consistent
with his own arrow system. So what?

It is true that the "technical" direction of current differs from
the actual physical electron drift direction. Errr, in most cases. In
semiconductors we have the nice construction: "hole", i.e. a missing
valence electron.

This has a positive charge and moves in opposite to the electron current.
So the technical direction is right for the holes, well and also for
the positive ions in batteries.

Remember the symbol for battery? A long plate and a short plate.  The
short plate is eaten by some acid (it is getting shorter and shorter),
whereas this material solves as positive ions thus releasing electrons
into the wire. The electrons come from the short plate. And they move to
the long plate, where they combine with some positive charged material
in the "soup". I.e. there is a positive charge drift in the opposite
direction in this soup as well (otherwise charge would come out of
nothing, this is still one of the most important laws of physics: charge
and energy totals stay constant).

So, for the positive charge carrieres the usual arrows are correct.

m.c.