6
\$\begingroup\$

Is conventional flow a real, physical thing or is it something we still use for historical reasons?

• Electron flow is the actual current flow and the textbooks are the other way because of Benjamin Franklin.

• Conventional current is real and electrons just go the other way.

I know nothing about this and searching on the internet just made me even more confused.

\$\endgroup\$
7
  • 1
    \$\begingroup\$ We made it up basically. \$\endgroup\$
    – Andy aka
    Feb 2, 2021 at 14:57
  • \$\begingroup\$ You can think about electric current like water. Do you consider a river to be a bunch of water molecules flowing individually or is it a current/river? Its the same thing for electrons. There are so many electrons flowing that it can be assumed to be a uniform flow. \$\endgroup\$
    – Parker
    Feb 2, 2021 at 15:09
  • 2
    \$\begingroup\$ It simply took a long time after Benjamin Franklin before anyone could actually measure the charge on an electron ... and find out it was -ve. \$\endgroup\$ Feb 2, 2021 at 15:24
  • 3
    \$\begingroup\$ We made up pretty much everything of what we consider "classical physics". No one really knows what is a "real physical thing". We only have equations which are more or less approximating what we observe. \$\endgroup\$
    – Eugene Sh.
    Feb 2, 2021 at 15:45
  • \$\begingroup\$ Electron current was invented to help explain the operation of vacuum tubes. \$\endgroup\$ Feb 2, 2021 at 15:59

7 Answers 7

22
\$\begingroup\$

Is conventional flow a real, physical thing or is it something we still use for historical reasons?

It's a model we use to describe flow of charge. "All models are flawed but some are useful" - and this one is very useful.

Electron flow is the actual current flow and the textbooks are the other way because of benjamin franklin ...

It is true that the mobile charges in metals are electrons but it is not always the case. In other situations the mobile charges are positive ions.

Don't get hung up on this. You don't need to think about electrons for most electronic engineering. Just volts, amperes, watts, ohms, henries and farads will get you a long way.

Give Benjamin some capital letters - even if you think he got it backwards.

\$\endgroup\$
8
  • 7
    \$\begingroup\$ @KH I think in a battery the cations literally migrate in the opposite direction from the electrons. I don't think free electrons flow inside aqueous solution batteries. \$\endgroup\$
    – mkeith
    Feb 3, 2021 at 5:37
  • 3
    \$\begingroup\$ Molten salts and solutions of salts in water are both conductive, and in both cases whole ions move. \$\endgroup\$ Feb 3, 2021 at 7:59
  • 4
    \$\begingroup\$ @mkeith -- proton beams are a thing in particle accelerators :) \$\endgroup\$ Feb 3, 2021 at 12:38
  • 2
    \$\begingroup\$ Aside from positive ions, you are going to get in a horrible tangle if you try to understand how a bipolar junction transistor works without talking about "holes moving". And unlike positive ions, the holes don't move at all, they simply get created and destroyed as the electrons move. \$\endgroup\$
    – alephzero
    Feb 3, 2021 at 13:54
  • 2
    \$\begingroup\$ @alephzero It is more complicated than that, otherwise you wouldnt have the semiconductor hall effect. Holes in semiconductors are as real as electrons in semiconductors. \$\endgroup\$
    – Matt
    Feb 3, 2021 at 19:34
8
\$\begingroup\$

The important concept is that current is the net flow of charges across some boundary. This is the basic physics of current. This is what you should remember.

Current flow is inherently directional. Early scientists could have defined the positive direction to be the other way, so that electron flow is the positive current flow direction. But they didn't. It is not a big deal. All the mathematics works just fine the way it is. It is not some tragic mistake. It does not mean that current is imaginary or made up.

Current also has a magnitude. The number of charges per second passing through the boundary. One coulomb of charge per second is one ampere (aka, 1 amp).

To sum up, current flow is very real. The magnitude is measured in coulombs per second (amperes) and by convention, the positive direction of current flow is opposite to the electron flow.

\$\endgroup\$
6
\$\begingroup\$

"Current" may be the flow of physical positive charge (protons or positive ions, for example) in one direction or physical negative charge in the other direction. It doesn't matter whether how we define "conventional current" as long as we are consistent.

\$\endgroup\$
1
  • \$\begingroup\$ This is a key point -- it's the relative motion of the positive and negative charges that matters. \$\endgroup\$
    – Adam Haun
    Feb 3, 2021 at 15:52
6
\$\begingroup\$

Conventional flow == hole flow

While it's not wrong to consider conventional current as merely a bookkeeping trick, a more illuminating way of looking at conventional current flow comes from semiconductor theory. In solid-state devices, conventional current has a definite charge carrier, namely the hole. (A hole is a spot where an electron has gone AWOL from the semiconductor lattice, and can be created by the presence of a P-type dopant, or simply by random ionization.) This is quite important when working with semiconductors, especially when trying to understand minority-carrier phenomena, but can usefully be extended to metals as well.

\$\endgroup\$
2
  • \$\begingroup\$ Holes don't "flow" at all. They get created and destroyed, but they don't actually move. \$\endgroup\$
    – alephzero
    Feb 3, 2021 at 13:58
  • \$\begingroup\$ @alephzero They move. In a big P-channel CCD, we can push them around centimeters. \$\endgroup\$
    – John Doty
    Feb 3, 2021 at 15:28
5
\$\begingroup\$

Kind of made up and still being used for historical reasons. But the math doesn't care because in most cases it is symmetrical enough to work out. Like keeping track of how shadows move instead of how the light moves. Think of it as a book keeping trick...like negative dollars in accounting.

\$\endgroup\$
4
\$\begingroup\$

Paraphrasing my own answer to a similar question on Physics.SE:

Have you ever played the sliding 15 puzzle? There are fifteen sliding pieces, numbered 1 to 15, arranged in a 4x4 grid. It has one "hole" where there is no piece. You can move any piece adjacent to the hole into that space.

Sliding-block 15 puzzle

As you play the puzzle, which do your fingers actually move: the numbers, or the hole? Of course, your fingers only move the numbers. You don't actually touch the hole.

Does the previous question really matter? Would you get the same outcome if you actually moved the hole? Indeed, the game would exhibit the same behavior, even if it was the hole that actually moved.

Is it sometimes useful to think of the hole moving? Yes, it's an effective part of solving the puzzle. A skilled player can move the hole anywhere he wants, and can even move the hole around in a closed circuit. Even more important, a skilled player knows that there are times to think of the puzzle as moving the numbers, and other times when it is better to think of it as moving the hole.

So is the hole a real, physical thing? In some contexts, yes; in other contexts, no. Most of the time, it does not matter. Use the convention appropriate to the task.


It's also worth noting that electrons are not the only charge carriers:

  • Positive ions flow in the electrolyte of batteries and electrolytic capacitors.

  • In electrolysis, hydrogen fuel cells, and mitochondrial membranes, positive H+ ions move.

  • Positive charge can flow in semiconductors in the form of holes, which are more than just an absence of electrons, due to their different dispersion relation.


The ultimate answer to your question is that using the convention which is most useful to a given problem is more important than worrying about what physically represents the charge. Because like the 15-puzzle, the outcome will be the same.

\$\endgroup\$
2
  • \$\begingroup\$ The hole is a perfectly good fermion, with all the quantum behavior that implies. \$\endgroup\$
    – John Doty
    Feb 4, 2021 at 0:45
  • 1
    \$\begingroup\$ I want to award an "insightful metaphor" badge to this answer :) \$\endgroup\$
    – orithena
    Feb 4, 2021 at 9:46
1
\$\begingroup\$

Conventional current assumes that charges are positive and flow from from positive terminal of a voltage source to the negative terminal. Ammeters and semiconductors are setup to indicate conventional current. Put a positive voltage on a ammeter and it will indicate a forward current even thought the true physical current is in the opposite direction. Put a positive voltage on the arrow of a diode and it will conduct, but the true physical current direction of charge will flow away from the arrow. Conventional works well if one does not care about the physical direction of the current. It does away with spurious negative signs in calculations. If current direction is needed, a good method to use is to do your calculations assuming positive charges. If working with semiconductor holes or positive ions in electro-chemistry, do nothing because they are positive charges. If working with electrons, flip the physical direction.

\$\endgroup\$

Not the answer you're looking for? Browse other questions tagged or ask your own question.