This episode was made possible by generous supporters on Patreon.
Hey Crazies.
Welcome to part three of our series on electrodynamics!
In part two, I made the following comment:
We usually have a bunch of charges moving together in something we call a current.
Hmm, I think that deserves a little more attention.
If you've been watching this channel for a while, you know language is tricky business
especially in science.
You have to be careful with your word choices.
The word "current" is usually used to describe a fluid like water moving continuously
in a certain direction.
But does charge actually move like water?
We certainly thought it did when we gave it that name.
But to answer this question, we need to delve a bit deeper than usual into materials.
First, materials are made of atoms.
A bunch of protons and neutrons surrounded by a cloud of electrons.
Those electrons are the lighter looser particles, so they do the moving in materials.
Second, we like to divide materials into two categories: conductors and insulators.
A conductor is a material that freely allows the flow of charge.
An insulator is a material that does not allow the flow of charge.
Either type of material can become charged.
What matters is whether or not the charge can move once it's there.
Category Alert!
OK, OK! I’ll mention it!
Stop!
Quick disclaimer: always be cautious of categories.
They give us little bite-sized pieces our limited brains are capable of understanding,
but reality consistently resists that kind of simplicity.
Like most things, materials fall on a spectrum: Conductors on one end
and insulators on the other.
No material is a perfect example of either type,
but the three best conductors are silver, copper, and gold, in that order.
It's no coincidence that those all fall in the same chemical group.
They each have one very loose electron on their outer edges.
That alone doesn't make them a good conductor though.
A material isn't made of one atom.
It's made of a bunch of them.
We need to see how those atoms work together.
Atoms are always bonded by their electrons.
In metals like copper, the electrons tend to form one big cloud for the entire material.
So this really comes down to the quantum states of the electrons in the material.
Specifically, the energy levels.
In a single atom, electrons can be in a variety of energy levels,
but when atoms are together in materials, those levels are separated into bands:
A conduction band and a valence band.
The lower energy levels don't matter because they’re buried.
What separates conductors from insulators is how much energy it takes for an electron
to jump from the valence band to the conduction band.
For insulators like glass, the gap is really big.
In semiconductors like silicon, the gap is small.
In conductors like copper, there is no gap.
Valence electrons are conduction electrons.
That's what makes them so conductive.
There are already electrons in the conduction band ready to go.
If they get a reason to move through the material, they’ll do it.
But is that how water moves?
Well, kind of.
We want to be careful with our comparison here.
The analog for charge is the water.
Those are normal-scale things.
The analog for a single electron is a water molecule.
They're microscopic things.
We need to keep those two things separate.
Let's consider water first.
Say we've got a long pipe with some water at rest inside.
The water molecules look like this, moving around randomly.
If the water begins to move through the pipe the molecules move too
because they're what make up the water.
They still have the random motion, but they now also have forward motion.
A similar thing happens with the charge in a copper wire.
That wire is full of conducting electrons just like the pipe was full water.
Individual electrons are conducting around the copper randomly.
If we connect a power source though, it'll act like pump.
The electrons will still have the random motion, but also some forward motion.
Except electrons are quantum particles, so we don’t know where they are exactly.
Grrr! You never let me get away with anything.
Nope.
OK, so according to quantum mechanics, if we know the energy of the electrons,
we don’t really know their position.
Those measurements are mutually exclusive.
We don't really know where individual electrons are in this wire,
but we do know the whole collection of them is drifting forward.
In fact, there's something called drift velocity that tells us how fast that drift is
and it's a lot slower than you might think.
In this copper wire, it's only about an inch and half per hour.
That’s 4 centimeters per hour.
Wait a minute, then how come this flashlight turns on instantly?
Because changes in the electric field transfer at the speed of light.
A flashlight is pretty simple: a couple batteries, a light bulb, and switch.
It could easily take an hour for the charge to get from the battery to the light bulb.
But, once the switch is flipped,
the battery's electric field travels across the circuit at the speed of light
and all the charge in all the conductors moves at nearly the same time.
How cool is that?
Anyway, charge moves a lot slower in a wire than water does through a pipe,
but that's OK.
Electric current isn’t about speed.
It's measured as the amount of charge over time, not distance over time.
There's a lot of charge in a wire, so a high current doesn't actually need to move that fast.
But does charge move through the entire volume of the wire?
Sure.
Well, at least when the current is direct.
Alright, one more thing.
There are two different ways charge can flow.
Direct Current or DC, which is a steady flow in one direction,
and Alternating Current or AC, which is a continuously changing flow that goes back and forth.
Can’t you do either of those with water too?
Well, yeah, but there's something hiding in the details.
The inside edge of a pipe can put drag on the water,
so it moves more freely near the center.
For electric current in a wire, it's the other way around.
The behavior of the atoms in the center restricts the number of electrons that can flow,
so they actually move more freely near the edge.
In DC, electrons flow through the entire volume in spite of this,
but, for AC, the flow is mostly limited to the outer edges.
It's called the skin effect.
What was the original question again?
Does charge flow like water?
Right. Uh, yeah, kind of.
If you're not concerned with where the moving charged particles actually are in the material,
then water flow is a good analogy for electric current.
So good in fact, it has a name: the hydraulic analogy.
There's a hydraulic equivalent for every circuit component you could imagine.
It's kind of fascinating.
So, got any more questions about electric current?
Please ask in the comments.
Thanks for liking and sharing this video.
A special thanks goes out to Patreon patrons like Kenny Holmes
who help keep this show going with their generous support.
Don’t forget to subscribe if you’d like to keep up with us.
And until next time, remember, it's OK to be a little crazy.
By far, the most common comment from the magnet video was:
What about Neodymium magnets? Neodymium is an element!
Yes, it is, but the Neodymium magnets are not an element.
They're Neodymium-2 Iron-14 Boron.
They’re actually a molecule, not an element.
Anyway, thanks for watching!