Video Transcript
In this video, we’re talking about
the somewhat shocking concept of electric current. Electric current is something we’re
able to witness in the natural world, for example, strikes of lightning. And it’s also something we
experience up close. We probably all know what it’s like
to get a mild electric shock when we put our hand near a metal object on a very dry
day. That shock comes from the discharge
of electric current.
And of course, in addition to these
examples, there are many more examples of controlled applications of electric
current. We use it to turn on lights,
operate appliances, and heat and cool our homes. Though electric current may seem to
be a very mysterious phenomenon, understanding it really comes down to understanding
how electric charges, both positive and negative charges, interact with one
another. But before we talk about electric
charges, let’s talk for a bit about a different type of current.
We’re all familiar with currents of
water. This is what flows in a river or in
a stream. The way that water flows can help
us understand how electricity flows, what electric current is. Think about the current in this
river. We might describe it as fast moving
or slow moving or somewhere in between. When we talk this way, we’re really
referring to the speed of the water as it moves along, whether that’s fast or
slow. But really when it comes to
current, that’s not quite the whole story.
To see why, let’s consider a cross
section of this stream. Let’s say that our cross section
looked like this. It had this particular width to the
stream and this particular depth. And we’ll say that all the water in
the stream is moving along at the same speed in the same direction. Okay, that’s one scenario.
Now let’s imagine a second one with
a different cross section to our stream. Now imagine that our stream cross
section looked like this, same width as before, and all the water in the stream is
moving at the same speed as before. But now the stream is much
deeper. We can see that, in this case,
there’s a much larger overall volume of water that passes by our observation point
standing by the stream. Here’s the point.
Current of any type, whether a
current of water or a current of electricity, is a description of the amount of
flowing material that passes by a point over some time span. And we can write that down over
here. This is our definition of what
current in general is.
So now that we understand that,
let’s get back to those electric charges we saw earlier, the positive and the
negative charges. Here we have those electric
charges, and we know that they have opposite signs. And what does this mean? Well, it means that they attract
one another. That is, there’s an attractive
force that draws the positive charge towards the negative and the negative charge
towards the positive. These charges want to attract one
another. And we know that the opposite is
true. That is, we know that if we had two
positive charges, then they would push one another away. The force on them will be like
this. And likewise, if we had two
negative charges, again the charges are the same and so they repel one another. There’s a force to push them away
from each other.
This simple fact that unlike
electric charges attract one another and like electric charges repel one another is
the reason for electric current. In other words, it’s the thing that
makes electric charges move through a circuit or through a loop. To see why that’s so, let’s
consider an object called a conductor, which is made up of many many positive as
well as negative charges.
Now in this little sketch here, we
show our conductor with just a few represented charges but understand there are many
many more than we’ve shown here. In any case, if we counted up all
the negative charges that we see in the conductor and then we counted up all the
positive charges, we will get the same number in each case. Overall, this conductor has the
same number of positive as negative charges.
But here’s why it’s called a
conductor. We call this a conductor because
the negative charges shown here on our sketch, the charges in gold, are fairly
mobile charges. It’s easy to pull them off of the
atom they’re attached to and have them move around the conductor. In other words, if we bring other
electric charges near this material, then this conductor is able to respond to
that. Charge actually flows within
it.
That said, if we just leave the
conductor there, say sitting on a tabletop, then no charge will flow across it
because we’re not providing any sort of electrical push. We’re not giving the charges in it
any reason to move. But we can do that. We can provide that push by
connecting our conductor up to a battery.
A simple way to think of what a
battery does is it sends positive charges out in one direction and negative charges
out in the other direction. Here our battery sends positive
charge in a clockwise direction and negative charge in a counterclockwise
direction. When the positive charge reaches
the left side of our conductor, we can see what will happen by recalling our
attraction and repulsion of electric charges that we learned over here. The positive charge in our circuit
will exert an attractive force on these negative charges on the left side of our
conductor. As a result, these negative charges
will start to move to the left.
Remember, they’re able to because
this is a conductor. As these negative charges leave the
conductor, that will open up a region in the conductor which is depleted of negative
charges. The other negative charges in the
conductor will be drawn to that region. And they’ll all move to the left
themselves. This process carries on through the
whole length of the conductor, minus charges, negative charges, moving along to the
left.
If this was the only part of the
process, then very quickly our conductor would run out of mobile electric charges to
contribute. But that’s where the negative
charges that are sent out by the battery in the other direction come into play. These charges are able to replenish
the supply of mobile and negative charges in the conductor by moving in from the
right side.
All of these interactions between
the positive and negative charges we have here are governed by these simple
rules. Opposite charges attract, and like
charges repel. The overall effect of all this is
that negative charges, the minus signs, are able to move counterclockwise
continuously through this circuit. We have a flow of electric charge,
in other words, an electric current.
Once we have an electric current
set up in this circuit, our next step is we wanna quantify it. We wanna know just how much current
is flowing. That brings us back to our
definition of current. It’s the amount of flowing
material, in this case electric charge, that passes by a particular point in some
amount of time.
In line with this definition for
current, let’s go ahead and pick a particular point in this circuit. It could be anywhere. But it’s the point where we’ll
watch for the flow of electric charge. Let’s say that point is right here
in the top left corner of our circuit. So say we stand here and we watch
that particular point. And as we watch, we count every
single negative charge that passes by that point. And more than that, say we have a
stopwatch with us so that we’re able to count the number of charges that pass by
that green point in our circuit over some particular time interval.
Let’s say that, using our
stopwatch, we count off one second of time passed and that, in that one second, we
count 27 negative charges flowing past that particular point in our circuit. So we had 27 negative charges pass
by a point in our circuit in one second. And what our definition for current
says is if we divide that amount of flowing material by the time interval of one
second, then that is a measure of the current in our circuit.
What we found then is that the
current in our circuit, which we symbolize using the letter capital 𝐼, is equal to
27 negative charges passing by a point in the circuit every one second. This brings up the question though,
how much charge is in 27 negative charges? We’d like to quantify that
somehow.
Well, it turns out that all of the
negative charges we’ve been talking about that are in our conductor have a
particular electric charge to them. This amount of charge, which we
often abbreviate using letter 𝑄, is equal to negative 1.6 times 10 to the negative
19th coulombs, where coulomb is the unit of electric charge. Just like meters is the unit of
distance or seconds is the unit of time, coulombs is the unit of electric
charge. Knowing this number is great
because we can substitute it in for the symbol we’ve had here of negative electric
charge.
Now we actually know how much
charge that is. With that substitution in, now look
at our expression for the current 𝐼. We have some amount of flowing
material, charge, divided by some amount of time. If we went ahead and calculated
this fraction, that would give us the current running through this circuit. But what would the units of that
current be?
Well, we can see that they would be
coulombs per second. But there’s another name for a
coulomb per second. The official unit of electric
current is called the ampere. And we symbolize it using a capital
A. One ampere is defined in terms of
coulombs and seconds. In fact, one ampere is equal to one
coulomb of charge passing by a point in our circuit every one second.
If we take a look back at the
amount of charge that each one of the negative charges in our circuit holds, we can
see that we would need billions and billions of those single negative electric
charges to make up one coulomb of total charge, far more charges than we could
count. Nonetheless, that’s the definition
of an ampere of current. It corresponds to one coulomb of
charge passing by every second.
Let’s get some practice now with
electric current through an example.
Which of the following is the
correct formula for the amount of charge flowing through a point in a circuit in a
given time? 𝑄 represents the amount of charge,
𝐼 represents the current, and 𝑡 represents time.
In this example, we’re looking for
the correct formula out of the four choices we’re given, A, B, C, and D, that
represents the amount of charge, 𝑄, flowing through a point in a circuit over a
given time. In other words, we want to know
mathematically how it is that charge 𝑄, current 𝐼, and time 𝑡 are related.
It will be helpful to us to recall
the general definition of just what a current is. Current is a measure of the amount
of flowing material passing by a point in some amount of time. When it comes to electric current,
that flowing material is charge. We represent that using the letter
𝑄. And we can represent some amount of
time using the letter 𝑡.
Since current has to do with the
amount of charge passing a point per unit time, we can divide 𝑄 by 𝑡, and that
will give us the current, which we symbolize with 𝐼. We now have a mathematical equation
for current. Let’s see if we find this equation
anywhere among our answer options.
Looking first at answer option A,
we see that that claims that current is equal to charge times time. But our equation shows that current
is equal to charge divided by time. This means that option A is not the
correct formula. All the remaining options after
this one have 𝑄 isolated by itself on one side of the equation. In order to see how our equation
compares, let’s algebraically rearrange it so that 𝑄 is on one side by itself.
To do this, we can take our
equation and multiply both sides of it by the time passed 𝑡. When we do this, 𝑡 on the
right-hand side of the equation cancels out since it’s in both numerator and
denominator. Our equation now reads 𝑄, the
charge, is equal to 𝐼 times 𝑡. And we see that, of the three
choices B, C, and D, it’s D that matches up with this expression. So the correct formula for the
amount of charge flowing through a point in a circuit over a given time is 𝑄 is
equal to 𝐼 times 𝑡.
Let’s summarize what we’ve learned
so far about electric current. We saw in this lesson that electric
current is the amount of charge flowing past a point in some amount of time. If we write that as an equation, we
can express it as 𝐼, electric current, is equal to charge, 𝑄, divided by time,
𝑡. We also saw that the flow of charge
happens thanks to interactions between positive and negative electric charges.
We saw that these opposite charges
attract one another, while on the other hand if we have a pair of like or similar
electric charges, they repel one another. And we also saw that the unit of
electric charge is the coulomb, while the unit of current is the ampere. And we saw they’re related to one
another this way, that one ampere is defined as one coulomb of charge passing a
point in one second. So then electric current is the
flow of electric charge over time.
