Gary Novosielski, professeur de physique à la retraite
Mise à jour il y a 4w · L'auteur dispose de réponses 2.7k et de vues de réponses 1.2m
First of all, everyone who says that current is the flow of electrons should go back to school. It’s not; it’s just NOT. Reach up and slap yourselves, right now. I’ll wait…. Oh, and I don’t care if your teacher(s) told you it was the flow of electrons. It is not, and they should go back to school too.
Okay. Pay attention:
Current is the flow of des charges. Charged particles if you like. Are electrons the only charged particles? No. Okay, you in the back sit down and stop calling out. Electrons are ne pas the only particles that can move. That’s wrong. Yes, I know protons don’t move around, they’re locked in the nucleus. But the entire nucleus can move; the entire atom can move, or rather the whole ion, because remember, the particles have to be charged.
And that’s exactly what happens in a glass of salt water if you put a current through it. Positively charged sodium cations move toward the cathode (negative terminal) while negatively charged chloride anions move toward the anode (positive terminal). That’s where we get the names cation and anion
The unit of electric charge is the coulomb (symbol=C). And charge can be positive or negative. So a coulomb (or any amount of charge) can be positive or negative.
The unit of electric current is the ampere (symbol=A). An ampere is a current flow of one coulomb per second ( 1 A = 1 C/s). Obviously, if the number of coulombs is positive, the current in amperes is positive, and if the number of coulombs is negative, the current in amperes is negative.
In particular, a positive current flow is a flow of +1 C/s, and a negative current flow is a flow of -1 C/s. But a current of +1 A can be a flow (each second) of a coulomb of positive particles flowing one way, a coulomb of negative particles flowing the other way, or a half coulomb of each, flowing in two different directions (which is what’s going on in the salt water.)
So why do soooo many people get this wrong, and say that electric current is the flow of electrons? Because it’s true in metals. In metals (and also in vacuum tubes, for those of us older than dirt). Most people’s experience with electricity is confined to currents in wires. And wires are made of what? Metal.
Can it be that simple? I’m afraid so. When people picture an electric current, they picture a current in a wire. Wires are metal, and metals are held together with (surprise!) metallic bonds, in which the nuclei of the metal atoms are surrounded by a “soup” of easily mobile valence electrons. That’s why metals are shiny, and that’s why they conduct electricity. Freely mobile electrons.
So, in wires, the charged particles that are moving are electrons, and electrons have a negative charge. A coulomb of electrons has a value of -1 C. And -1 C moving one way is exactly the same as +1 C moving the other way.
But don’t most currents flow in wires? Isn’t that the right way to think about it?
No. Most current does not flow in wires. A lot does, mainly on Earth. But in lightning, which dwarfs the output of power plants, the current flows through a plasma. Ionized air. What are the charge carriers? Free electrons in one direction and positive nuclei in the other direction, simultaneously although, in terms of charge, not equally, due to the mass disparity between electrons and ions.
So in metal wires, it’s electrons, but in the remaining approximately 100% of the observable universe, nearly all the current flows through plasma.
But I digress. The basic question is: Did Franklin get it “wrong”? Not really; he made an arbitrary choice about something that is completely arbitrary. He could have called the charges east and west, or hither and yon, or yin and yang, and it wouldn’t have mattered. To use them in a math formula we’d still have to arbitrarily assign positive to one charge and negative to the other, just as we arbitrarily assign positive to upward motion and negative to downward motion. (And not all textbooks agree on that!)
This is no more difficult to understand than the idea that a ball moving eastward with a negative velocity is actually moving westward. If you want the math to work, you have no choice. Electric charges are identified as + and -. Get over it.
William Beaty, Ingénieur électricien 35yrs, amateur d'électrostatique, site Amasci
Mise à jour il y a 14h · L'auteur dispose de réponses 348 et de vues de réponses 2.3m
Courant électrique is not a flow of particles. Instead, “Electric current” is an abstract concept: currents are all the flows of all charged particles inside a conductor, added together.
So, with a metal wire, whenever you grab the wire and move it along, the protons flow. But the electrons flow too. Two exactly opposite charges are both flowing. Since “Electric Current” is the proton flow moins the electron flow, moving the copper will create zero current. The two flows perfectly cancel each other. (If they didn’t, if current was really just electron-flow, then we’d create enormous amperes whenever we jerked a piece of copper around!)
Next, suppose a wire in a circuit contains an electron-flow, say half an ampere. If you now move the wire along, you’ve added extra motion to the electrons. Also you’ve added exactly the same motion to the copper’s protons. The two have opposite charge, so the forward proton-current adds to the forward electron-current, and this adds up to zero amperes of “electric current.” We’re still left with the original half-ampere, even though the protons and electrons are now both moving. Because of the wire’s motion, the electron-flow might even be gigantic. But the Courant électrique didn’t change, because any difference in the electron-motion was cancelled out by a gigantic proton-motion.
In other words, Electric Current is a “differential flow” between the protons and electrons of a conductor. It never was a flow of electrons. (Ha, your textbooks lied to you. Go look up Lies to Children,those incorrect oversimplifications which must be un-learned in later grades. Electron-flow is one such lie.)
OK, now suppose we take a simple flashlight circuit, then rotate the whole thing slowly backwards, so the copper is rotating, but the forward electron-current is halted. This means that the total current has become a circular flow of protons! The ring of movable electrons has stopped, but the positive-charged copper nuclei are still making the amperes.
See what’s going on? We can easily create a genuine proton-current by first using a battery to create a usual current in the wire, then next moving the conductor backwards in relation to the electron motion. Do it right, and the electrons stop totally. (It’s even possible to detect this happening in a moving conductor, by performing a Hall-effect physics experiment, proving that the moving wire contains a flow of positives.)
Finally, let’s ignore the wires for a moment. We can really make our brains hurt by examining electric currents inside human bodies. (Or, just use a hose full of blood plasma, or even salt-water, with wires shoved into either end.) Send some amps through the liquid. There are no free electrons inside of saltwater. There are no mobile electrons inside of people! (Electron flow is a thing about metals only.) In the salty water, we’ll find a flow of positive-charged sodium atoms, the +Na ions. At the same time, -Cl chloride ions are flowing backwards. Also, there are +K potassium positive ions flowing forwards, but moving slower than the sodiums. Aaaaaand also there are bare protons moving fast (the +H hydrogen ions,) and lots of -OH ions going backwards.
So, in our salt-hose, what is the true direction of electric current? Which of the five flowing particles do we chose? We can’t make a choice. All the particles are creating genuine electric current.
The idea called “Electric current” was created to solve this entire problem. We just ignore all the separate particle flows. We ignore their speed, and ignore the population-density. Ignore the percentages of different particles (many with different charge.) Ignore their actual direction of motion. Instead, we add it all up into a single number called “Amperes of conventional current.”
Oh yeah, did you know that Conventional Current is also called “Electric Current?” And also called “Amperes?” Your ammeter measures conventional current (it cannot detect electron flows or proton flows.) Ammeters add up all the flows together, then give a number which isn’t the electron flow. Instead, the amperes are the value of conventional current, which is the sum of all charged-particle motions your conductor. Proton-flow becomes part of it too, whenever the metal wires move or vibrate.
Speaking of lies to children, beware of people who discuss Ben Franklin. They’ll try to convince you that Ben Franklin named the positive charges wrong. Actually, they themselves are wrong. They think that electric currents are flows of electrons only. So, yes, if that were true, it would make sense if electrons had been named positive at the beginning. Unfortunately, protons can flow just fine (in acids, salt water, in the ground, in fuel-cells, inside people, etc.) If we change the standards, so north is now south and left is now right and positive is now negative, it might help us understand vacuum tubes during Radar Technician training-camp during world war two. But it makes our understanding of batteries even worse, since the current through the battery isn’t electrons. And if we tried to make the leap from vacuum tubes to transistors, or even to diodes, we’d hit a solid wall …since we’d been taught that all currents are electron-flows. The “n-type minority currents” and the “hole motion” become unacceptable heresies. They, they-they-they don’t exist, and YOU CAN’T MAKE ME BELIEVE IN THEM, and these stupid “transistors” cannot ever be useful we all better just stick to nice simple vacuum tubes like our 1943 textbooks said!!!
Finally …why does electron-flow give a negative ampere reading? It’s because electrons have negative charge, and whenever they go backwards, they create un numéro, an abstract concept called “Electric Current,” which has a positive value. Electrons are like electricity-antimatter! But Courant électrique isn’t really a flow in the first place. Instead it’s a flow-difference, a relative motion between opposites, and the true flows are being ignored when we talk about currents.
Just remember that saltwater-hose one last time, with equal and opposite flows in opposite directions, the clouds of +Na and -Cl particles passing through each other, and no single “true” particle direction. The average flow in our hose could even be zero exactly, with opposite particles moving backwards at exactly the same speed, yet the amperes can still be enormous.
Steven Wilson, BS/MSEE, 50+yrs NW R&D EE, Power/Control/Comm Systems
Réponse d'origine: Why do electrons move in the opposite direction as the flow of an electric current in a material conductor?
Simply put, they do or don’t flow in the opposite direction, depending on how you define “current”. Current is defined as the movement or transfer of electric charge per unit time. If you look at a conductive material on the atomic level, the applied voltage causes electrons in the outer orbits of the material’s atoms to be dislodged, so they are then free to flow through the material to an adjacent atom, which has previously lost one of its own outer shell (known as valence) electrons, leaving the latter’s atom with a net positive charge. The net positive charge of an atom that has previously lost an electron, serves as the attractive force that pulls on a free electron originating from an adjacent atom. This electron transfer happens very rapidly throughout the material, with a general propagation in a particular direction, based on the voltage or potential difference that is applied to the material, which is the overall driving force that is “pushing” the electrons loose from their respective atom’s valence shells on one end, and “pulling” on the free electrons on the other. The rate that this electron transfer takes place is known as the current flow, measured in amperes (or coloumbs/sec)
You can visualize this on a physical level by imagining Chinese Checkers board whose holes are almost all filled with marbles, with a handful of empty holes. If you then tilt the board (this gravitational force which acts on the marbles is analogous to the applied voltage), the marbles are going to start to be pulled/fall out of their respective holes, and roll downward on the board. The marbles in adjacent holes are going to do the same thing, and as each marble vacates a hole, a space is opened up for another marble (having left an adjacent hole) to fall into, as all the marbles collectively move downward on the board, pulled by the force of gravity toward the ground. The more you tilt the board (i.e., the higher the applied voltage, the higher the rate that the marbles move (i.e., current flow) from the higher end of the board to the other.
Using this analogy, in basic electrical theory, the marbles correspond to electrons or negative charge carriers, while the holes in the board also correspond to “holes”, only in electrical theory taught at the engineering level, these are positif charge carriers, moving in the opposite direction of the opposite direction of the electrons.
In most introductory courses on electricity and electronics, current flow is described in terms of movement of electrons, or negative charge carriers, because it’s easier to conceptualize movement of electrons rather than holes. In engineering studies, particularly in semiconductor theory, current flow is often described in terms of positif charge carriers, and is known as the conventionnel direction of current flow. This is used because conventional current flow is consistent with the direction of energy transfer (from a higher energy to a lower energy state).
As an aside, if you ever encountered transistors, such as the symbol for the NPN bipolar junction transistor in the circuit below, and wondered why the arrow on the base-emitter junction is drawn the way it is, it’s because the arrow points in the direction of trou flow, and opposite to the direction of électron flow. Regardless of which way you characterize it, you’re referring to the même electric current, or movement of electric charge..
Alan Wu, Lapsed physics major, EECS graduate, retired engineer
Réponse d'origine: Free electrons move opposite to current flow why?
Alfe Berlin, worked at Technical University of Berlin
Répondu il y a 25w · L'auteur dispose de réponses 99 et de vues de réponses 75.4k
Réponse d'origine: Why current is in the opposite direction of flow of electrons when a conductor is charged by a battery?
When the orientation of currents and voltages was fixed by physicists, they weren’t aware yet of the particle-based reasoning behind that. They assumed some kind of movement from one electrode (they called plus) to the other (they called minus). So they assumed—without fixating this—positive charges.
Only later (when more experiments had been conducted) they found out that, actually, the particles moving were negatively charged, namely the electrons. By that time, alas, it was too late to change the plus/minus orientation that was already common.
By the way, electrons aren’t really moving that fast through the conductors. The effect of the electromagnetic fields (which could be called »the current«) travels with the speed of light (in the conductor medium), but the electrons are way slower. (The page »How fast do electrons travel when moving as an electrical current through copper wire?« gives the number of .25mm/s for electrons in 3mm² copper wire at 10A.)
So, in a way, the movement of the electrons is not »the current« but just a side effect anyway.
Tamara Ivy Iverson, Retired/Disabled, electricity, electronics, communication & interest in physics.
Répondu il y a 68w · L'auteur dispose de réponses 528 et de vues de réponses 291.8k
Réponse d'origine: Why do electrons flow in the opposite direction of an applied electric field?
You are asking about electron flow from - to + and hole flow, (conventional current from + to -)”. An electron current consists of electrons “hopping” from one atom to the next. The holes are open spaces in the electrons in each of the atoms, so when an electron goes from atom “A” to atom “B”, there is a hole in electron A which doesn’t actually move - it is filled by another electron from a neighboring atom. So while electrons are moving from atom “A” to “B” to “C” etc, the holes APPARENTLY move from atom “C” to atom “B” to atom “A” So the “hole flow” is in the opposite direction of electron flow, even though the atoms involved don’t move, they just act like a fire brigade, handing atoms from right to left while the empty buckets appear to move left to the right.
I hope this clarifies this instead of muddying it up.