In this explainer, we will learn what the main types of electric current are and what sources produce them.

Electric current is the flow of electric charge. The unit of electric current is the ampere, which we give the symbol A.

A current of 1 ampere, or 1 A, is equivalent to 1 coulomb of charge passing a point in one second, or 1 C/s.

There are two different types of electric current that are used in everyday electrical appliances.

One of these is called direct current, which is often abbreviated to dc. It is used by devices such as digital watches and calculators.

Direct current is current of constant value. This means that the amount of charge flowing past a point in the circuit will remain the same as time passes.

Importantly, direct current is only ever in one direction.

The graph below shows current against time for a direct current.

Looking at the graph, it is clear that the current remains constant. It stays at its starting value.

So, a direct current is represented on a graph of current against time by a straight, horizontal line.

### Example 1: Understanding Direct Current

Which of the following sentences correctly describes direct current?

- Direct current has a variable magnitude and always has the same direction.
- Direct current has a constant magnitude and always has the same direction.
- Direct current has a variable magnitude and its direction can reverse.
- Direct current has a constant magnitude and its direction can reverse.

### Answer

This question tests knowledge on the direction and magnitude of a direct current.

Remember that a direct current is a constant current. It has a constant magnitude.

It does not reverse direction.

This means that sentence B is the correct answer.

Another type of current that is used a lot in electrical appliances is alternating current, or ac.

Mains electricity is the electricity used in homes. It is alternating current, so any device that is plugged into the mains uses alternating current.

An alternating current does not have a constant value. Its value is always changing. This means the amount of charge flowing past a point in the circuit changes over time.

More specifically, alternating current periodically reverses direction.

This is shown in the graph below. This shows current against time for an alternating current.

Notice how between 0 seconds and 1 second, the value of the current is positive. This means that the charges are flowing in one direction, let’s say clockwise, around the circuit.

Then, between 1 second and 2 seconds, the value of the current is negative.

A negative current means that the charges are now flowing in the reverse direction, so the charges are now flowing counterclockwise.

This repeats regularly, as the value of the current is again positive between 2 and 3 seconds. It is negative again between 3 and 4 seconds.

First, the charges flow clockwise, then counterclockwise then clockwise again, and so on.

### Example 2: Understanding Alternating Current

Which of the following sentences correctly describes alternating current?

- Alternating current has a constant magnitude and always has the same direction.
- Alternating current is a current that never reverses direction.
- Alternating current is a current that periodically reverses direction.
- Alternating current is a current that can sometimes reverse direction.

### Answer

This question is testing knowledge on the direction and magnitude of an alternating current.

The word “alternating” in “alternating current” tells us that it must be changing. Alternating current does not have a constant magnitude.

Remember that this change is periodic where the current periodically reverses direction. This tells us the correct sentence is C.

The graph below shows current against time for both an alternating current and a direct current.

Line 1 is direct current. This is a straight horizontal line as it has a constant value and direction.

This means that the charges will always be flowing in the same direction around the circuit.

Line 2 is an alternating current. It periodically reverses direction.

This means that at regular time intervals, the value of the current changes between positive and negative, and the charges will go from flowing in one direction around the circuit to flowing in the opposite direction.

### Example 3: Distinguishing between Direct and Alternating Current Graphs

The graph below shows the electric current over time for two different circuits. Which of the following sentences is correct?

- Both line 1 and line 2 represent an electric current that is constant.
- Line 1 represents an electric current that periodically reverses direction.
- Line 2 represents an electric current that periodically reverses direction.
- Line 1 represents an electric current that is constant.
- Both line 1 and line 2 represent an electric current that periodically reverses direction.

### Answer

This is a question about understanding information shown in a graph.

Line 1 varies over time and goes below zero. This tells us it reverses direction. It does this periodically. This means it is an alternating current.

Line 2 is a constant current and does not go below zero. This tells us it does not reverse direction.

Line 2 is a direct current. This tells us that the correct sentence is B.

If we need a direct current in a circuit, we can use a chemical cell or battery, like the one shown in the photo below.

If instead, we need an alternating current in a circuit, we could use a dynamo to generate it, like the one shown in the image below.

### Example 4: Understanding That a Battery Produces a Direct Current

A company is making circuits. They need their circuits to have a constant electric current that does not change direction. What is their best choice for a source of electric current?

- A dynamo
- A chemical battery
- Either a dynamo or a chemical battery

### Answer

Let’s start by recalling what each of these objects is.

A dynamo is a device that can be used to produce an alternating current. An alternating current is one that periodically reverses direction.

This means that at regular time intervals, the value of the current changes between positive and negative, and the charges will go from flowing in one direction around the circuit to flowing in the opposite direction.

A chemical battery, like the one shown in the image below, is an object that can be used to produce a direct current.

A direct current has a constant value over time, and if a circuit has a direct current in it, the charges will always be flowing in the same direction in that circuit.

The graph below shows current against time for a direct current.

The company requires a source of electric current that provides a constant current that does not change direction.

A chemical battery produces this, but a dynamo does not, so the answer is B.

### Example 5: Understanding That a Dynamo Produces an Alternating Current

A company is making circuits. They need their circuits to have an electric current that periodically reverses direction. What is their best choice for a source of electric current?

- A dynamo
- A chemical battery
- Either a dynamo or a chemical battery

### Answer

Let’s start by recalling what each of these objects is.

A chemical battery is an object that can be used to produce a direct current.

A direct current has a constant value over time, and if a circuit has a direct current in it, the charges will always be flowing in the same direction in that circuit.

A dynamo, like the one shown in the image below, is a device that can be used to produce an alternating current.

An alternating current is one that periodically reverses direction.

This means that at regular time intervals, the value of the current changes between positive and negative, and the charge will go from flowing in one direction around the circuit to flowing in the opposite direction.

The graph below shows current against time for an alternating current.

The company requires a source of electric current that periodically reverses direction.

A dynamo produces this, but a chemical battery does not, so the answer is A.

### Key Points

- Direct current is a type of current that has a constant value.
- Direct current is in one direction.
- Alternating current is a type of current with a varying value.
- Alternating current periodically reverses direction.
- A chemical cell or battery is a source of direct current.
- A dynamo is a source of alternating current.