So which direction do electrons really flow?

Question

As I understand it, electrons (which are negatively charged) flow towards positive.

As a convention, we say that current flows from positive to negative. Is this to say that what you connect the positive terminal of a battery to, say, VDD of an IC, and the negative terminal to the IC's GND, what is actually called the "positive terminal" is really ground/negative? Are the electrons flowing from "positive" to VDD, or "negative" to GND, etc?

Are the flowing electrons themselves responsible for the work done, or is it the "holes" moving in the opposite direction?

Answer 1

Conventional current goes from positive to negative, which is the opposite of electron flow which goes from negative to positive.

Neither the electrons nor the holes are responsible for doing the work, it's simply the flow of charge that does the work. The direction of the flow of charge, the electrons, or the conventional current, is not relevant to the amount of work that is being done.

In electronics, when we say "current", we refer to conventional current. If for any reason would we would want to refer to electron flow, we make sure to say so.

The reasons for this are mostly historical.

Answer 2

Positive is positive.

If a pin is "positive" in relation to another pin, then the voltage difference between that pin and the reference pin is positive. In your example VDD is connected to the "positive" terminal, which means the voltage at that pin is higher with respect to GND.

Electrons "flow" from negative to positive, so if a pin is at higher voltage, they will flow towards it. Electron current is therefore from GND to VDD. Conventional current is from VDD to GND.

Answer 3

Voltage is electrical potential energy per charge, i.e. EPE/q. If moving an electron from Point A to Point B takes 1 electron volt of work, then the voltage difference from A to B is 1eV/(-1e) = -1V (note that the notation is a bit confusing, as the 'e' in 'eV' stands for "electron", while the 'e' on the RHS stands for "elementary charge"). Conversely, if an electron travels between two points with a voltage difference of +1V, then it will perform 1eV of work.

If we define current as the signed flow of charge, then a positive charge traveling from the positive terminal to the negative terminal is a positive current from the positive terminal to negative. A negative charge flowing from negative to positive is also a positive current; the negative sign from going the opposite direction and the negative sign from opposite charge combine to give a positive current.

"Positive" and "negative" are a bit misleading, as there's no "absolute zero" of voltage. The "positive" terminal is simply the one with the higher voltage, and the "negative" one is the one with lower voltage.

If one Coulomb of electrons travels from one terminal to another, and the second terminal has a voltage that is 5V higher than the first, then we have -1C of going across -5V of voltage, giving (-1C)(-5v), or 5J, of work. The electrons do work by being negative charge going across a negative voltage difference.

As something that's somewhat analogous, if you release a helium balloon (negative weight), it will float upwards, and the work done will be the weight of the balloon times the signed change in height. Both the weight and the displacement will be negative, giving a positive amount of work done by the balloon.

Answer 4

As I understand it, electrons (which are negatively charged) flow towards positive.

TL;DR -- Commonly true, but in some cases, application of this principle leads to incorrect results.

Long version:

I wish to make a subtle point, but it is one that is worth grasping.

Electrons are accelerated toward positive a more positive field, but they may move in the opposite direction. In fact, they very often move in the opposite direction, and knowing this fact is essential to understanding the operation of bipolar junction transistors.

All particles have thermal energy, and this causes them to move randomly. This random motion results in particles diffusing from areas of higher concentration to areas of lower concentration. This is called diffusion current. Charged particles are also accelerated by electric fields (or if they are moving, by magnetic fields as well). Although the predominant motion of electrons is often random, an electric field will cause a net drift in the motion of electrons from negative to positive. This is called drift current. The situation is analogous to the random motion of air molecules, which have random velocities near the speed of sound, and wind, which is a net average movement of air molecules. Just as the wind moves much slower than the molecules in the air, the drift velocity of electrons is much slower than their random thermal motion. Much, much, much slower.

Diffusion current and drift current are often in opposite directions.

The electrons in both kinds of current have momentum, and will continue to travel in the same direction even when they leave an electric field, until they collide with some other particle (which may be "composite" such as an atom). The average distance electrons travel before colliding is their mean free path. If the electric field is reversed within the mean free path, the electron will decelerate, but in electric circuits, not by much.

So why is this important? In an (NPN) bipolar junction transistor operating in its saturation region, the emitter is more negative than the base. And, as the model which says that electrons move from negative to positive predicts, electrons move from the emitter to the base. However, in a saturated NPN transistor, the collector is more negative than the base. If the model which says that electrons flow from negative to positive were entirely accurate, electrons would not flow from the base to the collector. And yet they do! Electrons are accelerated in the positive direction of an electric field, but may move in the opposite direction.