Video Transcript
In this lesson, we will learn what
an electric current is and how to determine the direction of an electric current in
a circuit.
Electric current is something we’re
all able to witness in the world around us, for example, getting a mild electric
shock when we touch metal on a dry day. This is caused by a discharge of
electric current. Electrical devices such as
lightbulbs, phones, and laptops also require an electric current to make them
work. Even big natural events such as
strikes of lightning are caused by electric currents. But before we can talk about
electric current in detail, we first need to remember what electric charge is.
This is a property that all matter
has, and it tells us how an object will behave in an electric field or when it is
close to other charged objects. Any object or particle can either
have a positive electric charge, a negative electric charge, or have no charge at
all. Particles with no electric charge
are called neutral particles. In this lesson, we’ll always draw
positively charged things in red, often with a plus sign with them to remind us that
they’re positive. And we’ll always draw negatively
charged things in blue, often with a minus sign to remind us that they’re
negative. The charge of a particle tells us
how it will behave when it’s near anything else with a charge. For example, two positively charged
particles will always move away from each other, and the same happens for two
negatively charged particles. They always move away from each
other.
However, when a positively charged
particle is near a negatively charged particle, the two are attracted to each other
and will move towards one another. Just like with the poles of a
magnet, opposite electric charges attract each other and the same electric charges
repel each other. Neutral particles are neither
attracted nor repelled to charged particles or spaces. They don’t move at all in response
to an electric charge. Let’s now recall that everything we
see around us is made up of atoms. This includes things like the
materials that make up tables and chairs, the screen we’re watching this video on,
the air we breathe, and even our own bodies. Everything is made of atoms.
Atoms have a nucleus at their
center, which we’re drawing in red because the nucleus is positively charged. The plural of nucleus is nuclei,
and nuclei are actually made up of even smaller particles called protons and
neutrons. But we don’t need to go into that
for this lesson. Around the nuclei of an atom, there
are smaller particles called electrons, which we’re drawing in blue because they’re
always negatively charged. Now, the atoms that make up
different materials can have different numbers of electrons and different sized
nuclei. And this is what gives different
materials different properties.
Let’s now take a look at how these
particles can behave inside a material. Let’s think about a large but flat
piece of material. So this could be wood, plastic,
metal, or anything really. Since every material is made of
atoms, let’s pretend that on this face here, we can see all the individual atoms
that make up this material. We would see the nuclei of each of
the atoms, which we draw in red because they have a positive charge. And we’d also see the electrons
that surround the nuclei, which we draw in blue because they have a negative
charge. In reality, there’d be many, many
more atoms than we’ve drawn here. But we’re just drawing a few so we
can see what’s going on in this material.
Sometimes the electrons and the
nuclei of an atom can separate from each other. And the electron can move around
within the material. Sometimes these moving free
electrons can pair up with another nucleus in the material. And sometimes they just keep moving
freely through the material. The type of material that the atoms
are in determines how easy it is for the electrons to move within that material. For example, if the material here
is plastic, then it’s very difficult for the electrons to move away from a
nucleus. And they usually don’t move very
much at all and stay very still.
However, if the material is a
metal, the electrons and the nuclei are bonded together very weakly. And as a result, the electrons are
constantly moving through the material. And this is exactly why metals are
such good conductors of electricity. Often, each of the electrons is
moving in a random direction through the metal. But if every electron is moving in
the same direction like this, then we call this a flow of electrons. Since every electron has a negative
electric charge, we could also say that this flow of electrons is the flow of
electric charge through the material. In fact, when the electrons are all
moving in the same direction, when there is a flow of electric charge in a material,
this is what we call an electric current. An electric current is the flow of
electric charge through a material.
So now we know what electric
current is, let’s take a look at how this works in a simple electric circuit. Let’s start with a very simple
circuit, which is just a light bulb and some wire. So in this diagram, we have a bulb
at the top of our circuit. And then we have wire, which
connects one side of the bulb to the other side of the bulb. Usually when we draw wire, we would
just use one line to represent the wire. But here, we want to look at the
electrons which are moving around inside the wire. So we’ve drawn it with some
thickness like a pipe. But it’s important to remember that
this only represents one wire connecting the bulb to itself. This wire is usually made of a
metal such as copper or iron so that we have free electrons constantly moving
throughout the wire.
However, before we start thinking
about the electric current in this circuit, let’s remember that we don’t usually
draw pictures of the components of our circuit like this bulb here as they would
look in real life. But rather, we use symbols to
represent them. And these symbols are usually
easier and quicker to draw. The symbol we use for a bulb is a
circle with a cross in the middle. So this symbol here represents a
bulb. And we can draw a wire in the same
way as we have already. So the two sketches we have here on
either side of the screen are thought of as equivalent, with the one on the right
using symbols to represent the one on the left. And we call the one on the right,
which uses symbols, a circuit diagram.
To think about any electric current
in our circuit, let’s draw some of the electrons that are in the wires of this
circuit. In both of these diagrams, we’re
using blue circles to represent the electrons and to remind us that they have a
negative electric charge. Since the wire in our circuit is
made of metal, the electrons will be constantly moving throughout the wires. However, each electron will be
moving in a random direction, so there will be no flow of electric charge in one
particular direction around our circuit. This means there’s no electric
current in our circuit.
In order to get a flow of electric
charge in one direction in our circuit, we need to add a cell to our circuit. Let’s start by doing this in the
left-hand diagram, where we can just draw a cell as it would look in real life. And we can then add the cell to our
circuit diagram by remembering the symbol for a cell. The symbol we use to represent a
cell is two vertical lines separated by a small gap. The longer vertical line represents
the positive terminal of the cell, while the shorter line, which is usually drawn as
thicker, represents the negative terminal of the cell.
From now on, let’s just work with
our circuit diagram. So let’s redraw this bigger so we
have more room to see what’s going on inside the circuit. Remember that this symbol
represents the bulb in our circuit. The blue circles represent the
electrons in the wires. And this symbol represents the cell
in our circuit. We don’t have to keep drawing the
little minus and the little plus sign on the symbol for a cell. But let’s keep them there for this
lesson to remind us which terminal is which. So now that there’s a cell in our
circuit, how do we know that we have a flow of electric charge in one particular
direction?
Remember that every single electron
has a negative electric charge. And we’ve seen that two objects
with a negative charge are repelled from each other. This means that every electron in
the wire of the circuit is repelled from the negative terminal of the cell. However, we’ve also seen that
negative charges and positive charges attract one another. So all of the electrons in the wire
are attracted towards the positive terminal of the cell. Since the electrons have to stay in
the wire, this means they start to move like this, away from the negative terminal
of the cell in an anticlockwise direction, through the bulb in our circuit, and then
back down towards the positive terminal of the cell.
Since every electron is now moving
in an anticlockwise direction, this means we have a flow of electric charge in our
circuit. And hence, there’s an electric
current. This electric current powers the
bulb in our circuit. And hence, the bulb will turn on
now that there’s a cell in the circuit too. It’s important to remember that the
electrons were always present in the circuit even before we added the cell. But there’s only an electric
current when all of the electrons are moving in the same direction, when there’s a
flow of electric charge. The presence of electrons doesn’t
give us an electric current. We only have an electric current
when there’s a flow of electric charge in one particular direction.
It’s also important to remember
that while the electrons flow around the circuit, the nuclei stay still and do not
move around the circuit. We now need to cover one slightly
confusing naming convention used when we talk about electric currents. The term conventional current is
often used when we’re talking about electric circuits. Conventional current is exactly the
same as the electric currents we’ve talked about already, except it’s defined to be
in the opposite direction to the electron flow. So whatever direction the electrons
are flowing in, conventional current is in the opposite direction. So in our circuit here, the
conventional current is in the opposite direction to the orange arrows because these
represent the electron flow around the circuit.
Finally, now that we know what
causes an electric current, we also need to know the units of electric current. This unit is called the ampere,
which is usually abbreviated to amp and written as a capital A. That is, we would usually write one
amp is equal to one capital A. The more electrons that flow
through a point in the circuit every second, the more current is in the circuit. So the current is more amps. Okay, let’s now take a look at an
example question to do with electric current.
Complete the following sentence:
The conventional current in a wire is in the blank the flow of electrons in the
wire. (A) Opposite direction to, (B) same
direction as.
Okay, so in this question, we’re
being asked about the current in a wire. So let’s start by drawing a sketch
of a wire. So in this sketch, the red circles
represent the nuclei of the atoms in the wire, and the blue circles represent the
electrons in the wire. The question tells us that in the
wire, there is a flow of electrons. So let’s assume that in our wire,
the electrons are flowing to the right, so in the direction of this arrow. So all of our electrons are moving
in the same direction, which in this case is to the right.
We can recall that electric current
is the flow of electric charge in a material. In our wire, this electric charge
is in the direction of the electron flow, so it’s to the right. But let’s also recall that
conventional current, which is what this question is asking about, is defined in the
same way as electric current. But the direction of the
conventional current is in the opposite direction to the flow of electrons in the
material. So for our wire, if the electrons
are flowing to the right, this means the conventional current in the wire must be to
the left. And therefore, we have our answer
to this question. And that answer is (A) opposite
direction to. And so our full completed sentence
reads: The conventional current in a wire is in the opposite direction to the flow
of the electrons in the wire.
Let’s now end by summarizing the
key points we’ve learned in this lesson. We’ve seen that electric current is
the flow of electric charge, either through a material or around a circuit. Conventional current is in the
opposite direction to the electron flow. The units of electric current are
amps, which is denoted with a capital A. So electric current is the flow of
electric charge, and it’s measured in units of amps.
