12
\$\begingroup\$

I am starting to see a lot of people claiming that convential current is 'wrong' because Franklin made an error when he first started investigating electrostatics, and that later scientists didn't bother correcting the mistake, but preferred to keep the 'convention' (here is a classic example: http://www.allaboutcircuits.com/vol_1/chpt_1/7.html)

I always thought he didn't get it wrong. He said that current is positive in the direction that positive charge flows, and vice-versa. He of course had no way to know which side of two sticks behind rubbed actually gained or lost mass. So he wasn't wrong. What were you taught?

P.S. I can't help but feel we are lucky that he got it 'backwards', because clearly many people are confused about electrostastics (including the author of that text book!) and believe electricity has to involve electrons (an unfortunate name... why couldn't they have been named negatrons...)

\$\endgroup\$
  • 1
    \$\begingroup\$ Electricity does, at a fundamental level, involve electrons. A current, on the other hand, doesn't necessarily have to be just electron transport. \$\endgroup\$ – boardbite Sep 9 '12 at 9:37
  • \$\begingroup\$ It's the same with i, sqrt(-1) -- just because someone had the inspiration of naming it like this doesn't mean it doesn't exist or that its "parent" was right or wrong. \$\endgroup\$ – Vlad Sep 9 '12 at 12:59
  • 5
    \$\begingroup\$ He was wrong, and there's only one way to fix it... xkcd.com/567 \$\endgroup\$ – travisbartley Oct 7 '13 at 7:13
  • 3
    \$\begingroup\$ There are holes in that theory... \$\endgroup\$ – copper.hat Dec 2 '14 at 8:08
  • \$\begingroup\$ Franklin's choice was arbitrary, and does not correspond to the physical reality that most current is carried by electrons. But you should keep in mind that the nature of electrons was not really established until the Millikan oil drop experiment in 1909 (which was part of the reason Millikan won the Nobel Prize in 1923). Being 150 years ahead of the details makes it easy to be "wrong" while getting most of it right. And, for what it's worth, it's perfectly possible to produce a situation where Franklin was correct - a particle accelerator producing a beam of protons. \$\endgroup\$ – WhatRoughBeast Oct 21 '18 at 22:44
21
\$\begingroup\$

Electric current, A.K.A, "conventional current", is an abstract current, the flow of electric charge. From a previous answer I gave here:

Electric current is an abstract current, the flow of electric charge, not a physical current like, say, electron current, the flow of electrons.

But electric charge is a property of things, not a thing, i.e., electric charge is always "carried" by a thing.

So, while an electron current is necessarily an electric current (due the negative electric charge carried by the electron), an electric current is not necessarily an electron current.

For example, in a salt solution, we have two species of electrically charged ions present, the positively charged sodium ion and the negatively charged clorine ion. Imagine that the sodium ions are moving to the right and the chlorine ions are moving to the left.

Obviously, we have two ion currents in opposite directions but there is just one electric current and it must have a direction. The direction of electric current is, by convention, the direction of the flow of positive charge.

So, in this case, both ion currents contribute to an electric current to the right. The first term is due to the positive ions to the right. The second term is due to the negative ions to the left where the negative sign numerically "flips" the contribution to the electric current.

Think about it this way, if I told you that I was travelling at -60mph west, you'd know that I was actually going 60mph east. Similarly, a negative charge current leftward is an electric current rightward.

\$\endgroup\$
  • 1
    \$\begingroup\$ Well-verbalized! \$\endgroup\$ – boardbite Sep 9 '12 at 6:36
5
\$\begingroup\$

I don't think Franklin was "right" or "wrong", as it's just a choice of names.

As far as the particles are concerned, (to put it very roughly) we know one type of particle attracts another type of particle and repels it's own kind. We also know one type doesn't attract or repel itself or the others.
To distinguish between them and their properties, we call them something and say they have a certain type of charge - "Positive", "Negative" or "Neutral".

The electron is a lepton (type of fundamental particle) with a charge of -1e. e here is the unit of elementary charge. The proton has a charge of +1e, which is comprise of three quarks (two "up" and one "down") having a charge of +2/3, +2/3, -1/3 adding up to a total of +1.

Then everything else goes from here. As the link you give in your question says, we usually associate positive with "surplus" so it makes more sense for whatever has more of something to be the positive side. However, what Franklin had been calling "Positive" was the side with less electrons. Rather than swap the definitions round, they simply assigned the electrons a negative charge instead.

It's a bit like pipe with water flowing downwards through it - we say the current is in the direction the water is flowing. It would be confusing for may to say the current was flowing in the opposite direction, but this is how it is in electronics (i.e. we call the "water" negative) If we imagine the air bubbles flowing in the opposite direction, this is what we term "holes" (i.e. lack of an electron) and provide a mental image of the positive charge flow.
Of course, in a substances other than metal wires the current can be comprised of "real" positive particles or ions, as well as negative ones, so we can't always assume the current is an electron flow as Alfred mentions.

\$\endgroup\$
  • \$\begingroup\$ Ben Franklin was wrong about the direction of flow of actual charge carriers in metal. It's a matter of context. From the context of circuit design, current flow conventions tend to be arbitrary. Only when the mechanism is involved does it really make a difference. So, as a Physicist/Scientist, Mr. Franklin was in error. As an Engineer, direction of flow is more likely to be artigrary. Only if the engineer is designing, say, a Cathode Ray Tube, or new Battery, or anything where the physical elements of charge are involved, would an Engineer have any concern for the actual Physics of flow. \$\endgroup\$ – ReverseEMF Apr 4 '18 at 22:39
1
\$\begingroup\$

Several people have pointed out the choice is arbitrary and there are scenarios where positive charges are mobile. But to get at the real intent of the question, instead of saying, "right" or "wrong" let's phrase the question as, "Now that we have access to knowledge that Ben Franklin and his peers did not, if we were in the position of creating the naming convention, would we make the same choice? Or would it come down to the flip of a coin?" The answer is absolutely not! Everyone would agree the best answer is to name electrons positive and protons negative (and we would call the side of a battery that the electrons flow out of the positive terminal). Everyone would agree this is the preferred convention because the most ubiquitous form of current is electron flow, and the other examples, such as positive hydrogen ion flow in a fuel cell would be the rare exception where the particle flow was the reverse of the conventional current.

\$\endgroup\$
  • \$\begingroup\$ Can you imagine the confusion that would cause! Or use Conventional Current AND Electron Flow as required. \$\endgroup\$ – StainlessSteelRat Mar 1 '17 at 16:13
  • 1
    \$\begingroup\$ It's fortunate that Ben Franklin made the 'wrong' choice, or today we would have to work mostly with 'negative' voltages. Rather than being a 'rare exception' almost all modern electronic devices work with positive voltages (which would be 'negative' if current flow was the same as electron flow) because silicon transistors and ICs prefer it. \$\endgroup\$ – Bruce Abbott Mar 1 '17 at 20:50
  • \$\begingroup\$ I believe @Bruce Abbot is incorrect. Take ΔU=qΔV. Under the current conventions, if we want electric energy to decrease as current flows through a light bulb (ΔU is negative), and charge is negative, and voltage drops across the bulb, then the equation must actually be ΔU=-qΔV to get all the desired signs. If electrons were considered positive, then the equation would be written ΔU=+qΔV, resulting in voltage drops and energy loss in a resistor, as well as voltage gain and energy gain as an electron goes through a battery. Equations have conventions built into them too. \$\endgroup\$ – Paul B Mar 10 '17 at 19:52
  • \$\begingroup\$ @ Paul B Electrical energy is current x voltage x time between two points. Doesn't matter whether the voltage drop is positive or negative, the only difference is which way around you measure it (one way needs a '-' sign, the other doesn't). My point is:- the only advantage of Ben Franklin's choice is not having to write so many '-' signs in the circuits we commonly use today (though not all - I worked with negative voltages for 15 years because telephone exchanges use positive earth). \$\endgroup\$ – Bruce Abbott Mar 10 '17 at 23:32
0
\$\begingroup\$

Franklin is not wrong, it's just a convention. Charge carriers can be positive (like in p type semiconductor materials or positive ions in an electrolyte) or negative (like in copper conductors). Defining the flow of current in the same direction as the flow of positive charge simplifies the equation of electromagnetics and eliminates the need to established what type of carrier is there (positive or negative). It just assumes that the carrier is positive and applies the electromagnetics equation or electrical theorems (i.e KVL or KCL, etc) without worrying about the actual carrier and gets the correct result independent of the charge carrier. Just remember that the actual flow will depend on the type of carrier after all the computation.

We could have defined the conventional flow the same as the flow of electrons but it would slightly complicate the electromagnetics equation. However this flow is still not correct for a p-type material or in a positive ion carrier, so the same argument ensues (but we have a more complicated electromagnetics formula). The conventional flow of current we have today was not chosen due to Franklin's theory, but it is the most convenient notation.

As a side note: We could have chosen (during the discovery of electrons and protons) that the charge of electron is positive and the charge of proton negative. What is stopping us to view it this way? It's just a convention.

\$\endgroup\$
0
\$\begingroup\$

Franklin was wrong but not for the reasons people usually think. He was a proponent of the single fluid theory of electricity which reasoned that all electrical effects were due to an excess or absence of a single kind of electric fluid. He decided that in electrostatic experiments common for the day, that what was in reality the negatively charged body, was the body that had a lack of electric fluid. If he had decided that the negatively charged body was positive (excess of fluid) then conventional current would match the net direction of electron flow (keep in mind that in reality there is a random motion with a slow net motion in one direction), but he would STILL be wrong because we have two kinds of charge, not one and a positive charge flow is also current.

Anyone that says Franklin was wrong for the normal reasons is giving higher status to free electrons (over e.g. positive ions) simply because they're more familiar with them.

\$\endgroup\$
  • \$\begingroup\$ I think you protest too much. In the kinds of experiments Franklin did, and the kinds of circuit situations that are most familiar to electrical engineers, electrons are the predominant charge carriers by far. We see very little movement of copper or silicon ions. Even the "holes" in silicon are an abstract interpretation of electron vacancies. The only real exception is primary batteries. \$\endgroup\$ – Elliot Alderson Oct 22 '18 at 1:23
  • \$\begingroup\$ No argument other than your last (incorrect) statement. Even it wasn't, calling this this an exception implies that somehow the ions moving in those primary batteries don't constitute a "real" current. You help prove my point. If the signs we're flipped would you consider current from ion flow to be "wrong"? Real understanding of current has to allow for charge carriers of both polarities. The designation of a conventional direction means the choice of polarity doesn't matter as long as we're consistent. Reversing the convention just moves the goalpost for those that don't truly understand. \$\endgroup\$ – denki Nov 14 '18 at 0:46
  • \$\begingroup\$ Can you suggest other examples of positive ion flow that should be "most familiar to electrical engineers"? I didn't say, or imply, that ion current was not real current...of course it is, but most engineers don't need to think about it explicitly in most situations they encounter. \$\endgroup\$ – Elliot Alderson Nov 14 '18 at 19:00
  • \$\begingroup\$ I think I'm not fully understanding your position, what is it (other than that I protest too much)? The next most familiar example of current due to positive charge flow (of course if we're discounting holes in semiconductors) to electrical engineers would maybe be neon signs and associated phenomena, though that's just a guess. \$\endgroup\$ – denki Nov 16 '18 at 1:47
  • \$\begingroup\$ ...also I feel like I should add, since you're emphasizing engineering, the definition of current is the domain of physics, not electrical engineering. We (EEs) build on top of that. a practicing engineer should understand that positive current is an abstraction that could arise from positive charge in one direction or negative charge in the other and not concern themselves with which one it is, unless they are operating in a domain in which it actually matters (e.g. devices where mobility comes into play). \$\endgroup\$ – denki Nov 16 '18 at 2:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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