Well, we've spent many videos talking about electrostatic
fields and the potential on a charge or the potential energy
of a charge when it's in one place.
But let's see what happens where, given a potential, what
happens when we actually allow the charge to move?
And this will probably be a lot more interesting to you,
because you'll learn how much of the modern world works.
So let's say that I have a source of voltage.
Let me see how I want to draw that.
I'm going to draw that like that.
I'll draw it in yellow.
So this is my source of voltage,
often known as a battery.
This is the positive terminal.
This is the negative terminal.
It's a whole other subject, a whole other video, and I'll
make one eventually, of how a battery works.
But let's just say that no matter how much current--
well, actually, let me explain in a second, but no matter how
much charge flows out of one side of a battery to the other
side of the battery, that somehow the
voltages remain constant.
So that's kind of a non-intuitive thing, because
we learned about capacitors, and we will learn more about
capacitors in the context of circuits, but what we learned
about a capacitor is that if we got rid of some charge on
one end, the total voltage across the
capacitor will decrease.
But a battery is this magical thing.
I think it was invented by Volta, and that's why we call
everything volts and voltage and all of that.
But it's this magical thing that, even as one side loses
charge to the other side, that the actual voltage, or the
potential between the two sides,
actually remains constant.
That's the magic of a battery.
So let's just assume that we have one of these magic
instruments.
You probably have one in your calculator or your cellphone.
And let's see what happens when we allow the charge to
actually travel from one side to the other.
So let's say that I have an ultra-good conductor.
Let's say it's a perfect conductor.
It's normally drawn straighter than what
I'm capable of doing.
And no, I haven't had anything to drink
before making this video.
So what did I do here?
So in the process of kind of connecting this positive
terminal to the negative terminal of the battery, I'm
also exposing you to common schematic notation for
electrical engineers and electricians,
et cetera, et cetera.
So what this is, these lines here essentially are wires.
There's no reason why I drew it at a right angle here.
I just did that to be neat, those right angles.
And it's assumed that this wire is an ideal conductor,
that charge can flow freely without being impeded.
This thing right here, this scratchy line, this is a
resistor, and this is something that will actually
impede the charge.
It'll keep the charge from going as fast as possible.
And then, of course, out here, this is a
perfect conductor again.
Now, which way will the charge flow?
Well, I think I've mentioned this before, but in electric
circuits, it's actually the electrons that are flowing.
The electrons are those small particles that are going
really, really fast around the nucleus of an atom.
And it's actually the electrons that have this
fluidity that allow it to flow through a conductor.
So the actual movement of objects, if you call an
electron an object, some would argue that they're almost just
notional objects, but the actual flow is the electrons
from the negative terminal to the positive terminal.
But the people and all who originally created circuit
schematics and were the pioneers of electrical
engineering and electricians and whoever, I don't know who
came up with it, they decided to say-- and I think the point
here was to confuse people-- that the current flows from
the positive to the negative.
So the direction of the current is normally given in
this direction, and current is specified by I.
And what is current?
Well, current-- so wait.
Actually, before I tell you what is current, just
remember, even though people say that the current-- and
most textbooks do this, and if you become an electrical
engineer, people will often say that the current is
flowing from the positive terminal to the negative
terminal, the actual flowing of things actually occurs from
the negative terminal to the positive terminal.
It's not like somehow these big heavy protons and nuclei
are somehow traveling this way.
Once you compare the size of an electron to a proton, you
would realize how crazy that is.
It's the electrons, these little super-fast particles
that are moving through the conductor from the negative
terminal this way.
So you could almost view this current as, the lack of
electrons are flowing this way.
I don't want to confuse you.
But anyway, just remember that this is the convention, but
the reality is to some degree the opposite of the
convention.
So what is this resistor?
Well, as the current is flowing-- and I want to stay
as close as possible to reality so you have a good
visualization of what's going on.
As the electrons are flowing, you have these little
electrons, and they're flowing in this wire.
And we assume for some reason this wire is just so amazing
that they don't in any way bump into any of the atoms of
the wire or anything.
But when they get to this resistor, that's when these
electrons start bumping into things.
They start bumping into the other
electrons in this material.
So this is the resistor right here.
They start bumping into the other
electrons in this material.
They bump into the atoms and molecules in this material.
And in the process, the electrons essentially slow
down, right?
They're bumping into things.
So essentially, the more things that there are to bump
into, or the less space there are for the electrons to flow
through, the more that this material is going to slow down
the electrons.
And as we'll see later, the longer it is, that only
increases the chance that electrons bump into things.
And this is called a resistor and it provides resistance,
and it dictates how fast the current flows.
So current, even though the convention is it flows from
positive to negative, current is actually just the flow of
charge per second.
So we could write that down.
I know I'm saying this in kind of a disjointed way, but I
think you get what I say.
Current is flow of charge, so change in charge per second,
or per change in time, right?
So the way you could think about it is, what is voltage?
Voltage is how badly does current want to flow?
So if there's a high voltage difference between these two
terminals, then the electrons that are sitting here, these
electrons want to really badly get here, right?
And if the voltage is even higher, these electrons want
to get there even more badly.
So before people understood that voltage was just a
potential difference, they would actually call this
desire of the electrons to get from here to here the
electromotive force.
But what we've learned now, it's not actually a force.
It's just this potential difference that makes the-- we
could almost view it as an electrical pressure, and
that's what people used to actually call voltage,
electrical pressure.
How badly do the electrons want to get from here to here?
As soon as we give the electrons a path through this
circuit, the electrons will start traveling.
They'll start traveling, and we assume that this wire
provides no resistance, that they can travel as
fast as they want.
But when they get to this resistor, they start bumping
into things, and this limits how fast the
electrons can travel.
So you can imagine that if this object right here is
somehow the rate-determining factor in how fast the
electrons travel, no matter how fast the electrons can
travel after that, this was the bottleneck.
So even though electrons can travel really fast here, they
have to slow down here, then they could travel really fast
here, the electrons here can't travel any faster than the
electrons through this.
Well, why is that?
Because if these electrons are traveling slower, so the
current here is lower-- current is really just the
rate at which the charge is traveling, right?
So if the current is lower here and the current was
higher here, we would essentially end up having a
buildup of charge someplace here while all of the current
were waiting to travel through this.
And we know that that's not the case, that all of the
electrons actually travel at the exact same rate through
the entire circuit.
I'm going in the opposite of the convention right now,
because the convention is that somehow we have the positive
things traveling this way.
But I want to give you a really intuitive sense of
what's going on in a circuit, because I think once you
understand that, once problems get a lot more complicated,
they won't be so daunting.
So what we know, and this is called Ohm's law, we know that
the current is actually proportional to the voltage
across the circuit.
So we know that voltage-- or we could view it the other
way, that the voltage is proportional to the current
through a circuit.
So the voltage is equal to the current times the resistance,
or you could say that the voltage divided by the
resistance is equal to the current.
This is called Ohm's law, and this is true whenever we're at
a constant temperature.
We'll go into more depth later, and we'll learn that if
a resistor actually has temperature increases, then
its particles and its molecules are moving around
more, they have higher kinetic energy.
And then it's even more likely that electrons will bump into
them, so actually, the resistance increases with
temperature.
But if we assume at a constant temperature for a given
material-- and we'll also learn later that different
materials have different resistivities.
But for a given material at a constant temperature in a
given configuration, the voltage across a resistor
divided by the resistor is equal to the current that
flows through it.
An object's resistance is actually measured as ohms, and
it's given by the Greek letter Omega.
So let's do a simple example.
Let's say that this is a 16-volt battery, so the
potential difference here is 16 volts between the positive
and the negative terminal.
So it's a 16-volt battery.
Let's say that this resistor is 8 ohms. What is the current
flowing through-- and I keep doing it in the opposite of
the convention, but let's go back to the convention.
What is the current flowing through this circuit?
Well, it's fairly straightforward.
It's just Ohm's law, V equals IR.
The voltage is 16 volts, and it equals the current times
the resistance, times 8 ohms. So the current is equal to 16
volts divided by 8 ohms, which is equal to 2,
and this is 2 amperes.
Or sometimes they're called amps, and that's
the units for current.
But as we know, all current is, is the amount of charge
per amount of time, so an ampere is just 2 coulombs per
second, right?
Oh, I'm already at 11 1/2 minutes.
So I will leave you there.
You now know the basics of Ohm's law and maybe a little
bit of intuition on actually what's going on in a circuit,
and I will see you in the next video.