I’ve noticed how often the word ‘flow’ is electrical flow is often surrounded by inverted commas. So electricity doesn’t really flow. Is the statement below accurate?
Electricity doesn’t actually flow through a wire like water, in reality electrons jumps from atom to atom, this is why atoms with less electrons in the outer orbit are conductors and atoms with their outer orbit full are insulators.
I think “flow” and “current” are used because the analogy between fluid flow phenomena and electrical “flow” phenomena is useful in so many ways. Before electrons were discovered and recognized as the active agency in electricity, there was an arbitrary choice of which direction to describe the flow as occurring (and in bad luck the 50/50 guess turned out wrong, which is why “+” means the absence of electrons). It turns out that a flow of electrons is actually what is going on. They’re not quite a liquid, but you can say sand flows through an hourglass, which might be a bit better model. In some cases, like cathode ray tubes, you’re squirting a stream of them, which is quite similar to spraying with a garden hose. And the description of an “electron gas” in conductors is also quite good.
How precisely do you have to mimic water to have a “flow”? We have a flow of traffic, of money, of all sorts of things.
Your source can be capable of providing a great deal of flow, yet your consumer can take only what it wants. You can drink from a river without taking in all its water.
Yeah, electricity certainly flows, but not like we idealise water to flow. It is doubtful most water flows like we imagine either. Water is just lots of particles jostling around. Electrons jostling about isn’t far removed. But one might want to avoid looking too close, as things will get weird, and those pesky electrons will start to get all smeared.
If you add chemical cells (aka batteries) to the mix we don’t even flow electrons.
The water flow analogue works remarkably well. It doesn’t take much to extend it to include reactive components in a useful manner. So much so that using electrical network analysers to model some fluid systems is a useful trick.
The confusion for me arises when I read that electrons are like jostling marbles. But somehow due to voltage they move these very slow-movin electrons to produce a current. Does this current then flow ? Does it act act the speed of light?
The marble analogue works well too, but perhaps for a different aspect. of understanding. Electrons themselves move quite slowly in a wire. But the effect moves very fast. Imagine a pipe full of marbles. Push a marble in one end, another marble pops out of the other end near instantaneously. The impulse that popped the marble out travelled vastly faster than the movement of the marble. (In fact it travels at the speed of sound in whatever material the marbles are made of.) Electrons, and water in a pipe are the same. The speed of the carrier is significantly less than the speed of the effect. The speed of the impulse in a wire is close to the speed of light. You can walk faster than the electrons move.
In a hydraulic system we push the working fluid around with a pump. The pump creates pressure that causes the fluid to flow. If we want the moving fluid to do work, the fluid acts on the thing doing the work, which results in a resistance to flow. The pressure is applied between the fluid return line and the pump output. The more pressure, the more fluid flows, and the more work we can do.
Same with electricity. You push electrons around with the voltage - which is a measure of the difference in pressure between two nodes in a circuit. Current flows in response to the difference, but only flows as much as is constrained by resistance to flow in the circuit. Pushing current through a resistance does work. The greater the voltage, the more current flows, and the more work is done.
Thank you for this, it is a great explanation.
Derek at Veritasium caught a lot of criticism for the following video, where he claims that electricity doesn’t flow.
He made a follow up where he refuted the critique.
I have no way of telling if his claims are valid or not. I find that his videos are well researched, but even scientists screw up from time to time.
Thank you Francis_Vaughan. That makes perfect sense now.
Thank you for sharing that Charlie Tan. It just goes to show you how unreliable some of these so-called science videos can be!
Did you watch them?
Yes I watched the first one a while ago. I was skeptical. After seeing the criticism I feel more reassured that I was right to be skeptical.
As to the flow velocity and the velocity with which changes propagate, the fluid analogy is helpful here too. If you blow into one end of a long pipe, air comes out the other end pretty quickly. The change propagates at the speed of sound, or very close to it. But it takes a while for the air you blew to make it down the length of the pipe. That’s moving at the flow velocity.
With electricity, if you shove extra electrons into one end of a long wire, electrons are available at the other end to do something (like illuminate a lamp connected there) very quickly, at the speed of light or a largish fraction of it like 2/3 c. But the particular electrons you put in there will take a long time to get to the far end. They move at the “drift velocity”, which is an obscure enough thing that I’ve never heard of it except in curiosity conversations like this one despite a degree and career in physics.
There are places the electrons go very fast. The CRT is such a place. Other electron tubes, and X-ray tubes, are others.
I’m sorry but if you really, really want a more correct answer, you need to watch the second video which clarifies what the first video was trying to say and clarifies the critiques. Or you can stick with what you got and that satisfies most engineers. But be warned, if you really try to understand what’s going on, it’s fields all the way down. (That’s quantum field theory, not the green pastures of dreamscapes.)
The second video correctly refutes the criticisms of the initial video. The point is that the simplified models that most people get from physics class (I know this first hand from having used multiple physics books both in learning and teaching) of electrons moving through the wires and transferring energy through that movement break down when asking any complex questions about circuits.
The movement of charges is necessary for conduct in a material, and you are correct that full outer orbits an insulator makes, but it’s more complicated than that which Derek, if you consider those two videos together, explains almost as simply as is possible. Though I think he could have benefited from bringing up a short discussion on cause and effect.
The water flow analogy is a simple model, which is why it’s often used. But there are lots of problems with it, obviously. Unlike water in a pipe, the energy that’s being transferred isn’t directly due to the “push” or flow of the electrons in the wire. The energy is really in the fields that surround the wires. Furthermore, a battery or power supply is not an “electron pump.” Heck, electrons don’t even flow through a battery.
An accurate description of what’s going on is much more complex, and requires things like Maxwell’s Equations, Poynting vectors, etc. And when you do this, you discover the electric and magnetic fields in a circuit are the primary things that make it work, and the conductors and free electrons are sorta secondary, and just there to guide the fields in certain directions.
Analyzing a circuit using Maxwell’s Equations is a PITA, though. Fortunately we can do a lot of analysis using results from Maxwell’s Equations (and other things) that were formulated a long time ago that simply involve voltage, current, etc. Unlike electric and magnetic fields, these are easy to measure, calculate, and understand. But it’s always helpful to keep in mind that it’s a bit of a façade, and that the fields are what really make it work.
A better way to visualize the free (mobile) electrons in a conductor is to imagine them as a gas moving around the stationary atoms.
Good point! I agree – generally.
But I propose two counterexamples.
One is resistance heating. The electrons bump into obstacles and transfer momentum to them, giving them vibrations that are the essence of thermal energy. At least, this is how I understand it.
The other is in a solid state heat pump, wherein the electrons interact with phonons and drag them along with the stream against the thermal gradient. Phonons are quanta of vibration; in thermal physics the vibration is the molecular motion of heat or thermal energy. This counterexample is, admittedly, kind of obscure.
What do you think, Crafter_Man, are these valid counterexamples to the general tendency that the energy really acts in the fields?
Actually, isn’t it also true that, if you’re talking specifically about the magnetic fields, those are fueled by relativistic effects of the moving electrons, right? It’s not clear to me how the magnetic fields being dissipated drags on the electrons, but I think it must, right?
Thanks GWF_Hafel. I will take a look at the second one.
Thank you naita. I will watch the second video.
Yes, that’s my understanding, too: in a resistor, the electrons are accelerated by the electric field that traveled from the source to the load. The same electric field can also cause insulators to heat up via dielectric heating.
In other words, the energy in the fields has to go somewhere. Some of it goes off into space as an EM wave, too.
Interesting tidbit: when you splice a small gauge wire to a large gauge wire, the electrons will speed up when they enter the small gauge wire. This means there will be more collisions, and the wire will be hotter than the small gauge wire.