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So far, we've been saying that currents are moving charges.
But the natural question is "what charges?" Positive charges or negative ones, or both?
Here is the thing, in the beginning, scientists thought that moving charges in their experimental circuits were positive charges.
That is, that negative charges were stationary. That's what they thought.
It follows, that when they found a current flowing, let's say, to the right; they thought it was due to positive charges moving to the right.
It was a long time before experimenters found out that, for the most part, it's negative charges, electrons what move in an electric circuit.
Bummer! You'd think that they would have changed the definition of currents as the flow of negative charges, right? Wrong! They didn't!
They did not! How come? Well, from a macroscopic point of view, it is rather hard to tell whether, in a conductor with a current,
it's positive charges moving to the right, or negative charges moving to the left. The external signs are the same!
So, they kept the convention of direction for currents.
In this "conductor", if we measure 5A flowing to the right, in reality, it may be five positive coulombs moving to the right every second.
or five negative coulombs moving to the left every second.
Or even, it could be two positive coulombs moving to the right and three negative coulombs moving to the left every second.
For the most part, it does not make a difference to us at the macroscopic level.
We still say that there is a five amp current flowing to the right.
And that is the current convention that physicists and engineers use for directions of currents: It's ...
Along with that convention, there is another one.
In an electric field, when we say that a point is higher, we mean "it's higher for a positive charge".
How is that? An electric field drags positive free charges in one direction, and negative free charges in the opposite direction.
That means that, for a negative charge, "downward" is the other way!
Then, in an electric field, when we say up, or we say down, we really mean "up" or "down" for a positive charge.
And that is the other convention.
That way we know that if no source of energy is in a section of a circuit path, Current flows" downhill".