What Electric Current Continually Flows in One Direction

The flow of electrons, protons, and ions is called electric current. In simple words, the flow of charge is called electric current.

In conductors or wire, electric charge is carried by moving electrons. In semiconductors, electric charge is carried by electrons and holes. In an electrolyte, electric charge is carried by ions.

Like the electrons, protons also have an electric charge. But they cannot move (carry charge) from one place to another place. They always reside at the center of the atom. So the protons do not conduct electric current.

Electric current is of two types based on the direction of charge carriers. One is Alternating Current and another one is Direct Current.

In this tutorial, Direct Current is explained.

Direct Current definition

Direct Current is an electric current that flows in only one direction (backward ← or → forward).

                                                  Or

Direct Current is an electric current generated by the charge carriers (electrons) that are flowing in only one direction (backward ← or → forward).

Direct Current is also sometimes simply referred to as DC.

What is Direct Current (DC)?

Direct Current flows only in one direction. The electric current flowing through a semiconductor diode is an example of Direct Current. A diode is an electronic device that allows electric current in only one direction. However, diode does not produce a pure Direct Current (DC). Because a small current flows in the opposite direction. This current was generated by the minority carriers in the diode. However, the electric current generated by the minority carriers is negligible. Thus, the diode acts as a DC source.

Direct Current is produced by the sources such as batteries, thermocouples, fuel cells, and solar cells.

Direct Current can flow through a conductor such as wire and semiconductors such as diodes. However, Direct Current does not flow easily through an insulator.  Because insulator is a poor conductor of electricity.

In Direct Current, the electrons always flow from the negative end of the battery to the positive end of the battery.

Direct Current (DC) example

The best example for Direct Current (DC) is a battery. We use batteries in TV remotes, AC remotes, mobile phones, motorbikes, cars and in many other types of equipments. If you look close to these batteries, you will find plus (+) and minus (-) signs printed on it.

The plus (+) sign printed on the battery represents positive terminal and the negative (-) sign printed on the battery represents negative terminal. When the battery is connected to a circuit, the electrons (negative charges) starts flowing from the negative terminal of the battery to the positive terminal of the battery. The charge carriers in the battery always move in the same direction.

Symbol of DC voltage source

The symbol of DC voltage source is shown in the below figure. In the below figure, the longer vertical line or plus sign (+) represents the positive terminal of the battery while the shorter vertical line or minus sign (-) represents the negative terminal of the battery.

The conventional current always flows from the positive terminal of the battery to the negative terminal of the battery. However, the actual DC current always flows from the negative terminal of the battery to the positive terminal of the battery.

Direct Current (DC) electrical circuit

Consider an electrical circuit consisting of a battery, on/off switch and a bulb.

Let us assume that initially, the switch is turned off. Turn off means a breakdown in the electrical current path. This broken electrical path circuit is called open circuit.

We know that air is a poor conductor of electricity.

When the switch is in off state, no current (electrons) flows from this open path. So the bulb will not turn on. Current always flow in a circuit where there is no broken path.

When the switch is turn on, the circuit is completed without any broken path. So the electric current (electrons) starts flowing from the negative terminal of the battery.

When these electrons flow through the bulb, they supply energy to the bulb. As a result, the bulb will turn on. The remaining electrons will move towards the positive end of the battery. Thus, electric current flows through the closed circuit. The battery will continuously supply energy to the bulb unless the switch is turn off.

From the above circuit, we observe that electric current (electrons) is flowing in only one direction. So this current is known as the Direct Current (DC).

In electronic devices such as computers, mobile phones, and TV remotes we use  Direct Current (DC) or batteries as the energy source.

Current direction

Even though the electrons flow from the negative (-) end of the battery to the positive (+) end of the battery, the current direction is indicated as flowing from positive (+) to negative (-). This is due to the convention.

Before the discovery of electrons and protons, Benjamin Franklin observed that something is moving through the electrical wires. He does not know what is moving through the wires. Because at that time, electrons and protons are not yet discovered. So he called these moving things as the charge.

He assumed that charge always flows from a higher concentration region (excess charges region) to a lower concentration region (fewer charges region). He called the higher concentration region as the "positive" region and the lower concentration region as the "negative" region. That means according to his assumption, charge always flow from positive (+) to negative (-).

We know that electric current means flow of charge. According to Benjamin Franklin assumption, the electric current flows from positive to negative.

However, after the discovery of electrons, scientists realized that the actual electric current was carried by electrons.

So in reality, the electric current direction was from negative (-) to positive (+). But we are still following the conventional current direction. That is from positive (+) to negative (-).

Alternating Current (AC) to Direct Current (DC)

We can convert the electric current from an AC source into Direct Current by using an electronic device called rectifier. A rectifier converts Alternating Current (AC) into Direct Current (DC).

In an oscilloscope, the Alternating Current (AC) is often represented by a sinusoidal waveform whereas a Direct Current (DC) is represented by a straight line.

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Source: https://www.physics-and-radio-electronics.com/blog/direct-current-dc/