Lesson Video: Electric Current Science

In this video, we will learn what an electric current is and how to determine the direction of an electric current in a circuit.

14:11

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.

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