Study Material

Direct current

The direct current (DC) is the flow of charged particles in one direction, usually within a conductor or other means. These charged particles can be electrons, in the case of a material such as copper, or ions of various kinds. direct current examples

This is the type of current that is generated when a cell or battery is connected to a light bulb to turn it on, as shown in the following image: direct current examples

Simple direct current circuit.
Figure 1.- Simple direct current circuit.

It happens that the atoms of metals such as copper, silver and others, have in their outermost shell at least one electron that moves easily within the material. Under normal conditions this movement is random, but as soon as a potential difference is established between the ends of the conductor, the electrons can move in the same direction, thus creating a direct electric current.

Since metals have these free electrons, they are good current conductors, however all materials can conduct electrical current to a greater or lesser extent. direct current examples

Since the current I is a flow of charges in time, it is mathematically defined as the amount of electric charge Δq, which circulates through the cross section of a conductor during a time interval Δt:

I = Δq / Δt

The unit for electric current in the SI International System is the ampere, abbreviated A and equivalent to the coulomb / second (C / s).

How is direct current generated?

Direct current needs a closed circuit or path to flow and can be generated in various ways. One of the most common is through cells (batteries), fuel cells and accumulators, which have two fixed terminals and opposite signs. direct current examples

Inside the batteries, chemical reactions take place that separate the electrical charges and establish the potential difference necessary for the current to be produced.

Separated charges, the electrons leave the negative terminal, where they are at a higher potential and are attracted to the positive terminal of lower potential, moving throughout the circuit in the same direction.

Over time the chemical reactions slow down and the battery wears out. Despite this, the current it produces, even though it is of diminished intensity, is still direct current.

Converting alternating current to direct current direct current examples

Direct current and alternating current diagram
Figure 2. Direct current and alternating current diagram

Another method of obtaining direct current is by converting alternating current. The current that reaches homes is alternating, therefore its direction changes cyclically, but it can be converted into direct current by having certain circuit elements such as diodes, capacitors and transformers.

These elements are responsible for keeping the current flowing in only one direction. It sounds a bit complicated, but the necessary circuits, known as rectifier circuits, are not difficult to implement. The reverse process, that is, changing direct current to alternating current is a bit more complex, although it can also be done. direct current examples

Alternating current is preferred for mass use because it is more easily transported and distributed, and with fewer losses, from generating stations. For this reason, if a device is designed to work with direct current, it always comes equipped with a device or charger with which to carry out the conversion.

Direct current circuits

A direct current circuit comprises a series of elements such as resistors, capacitors and others, connected by conductive wires and supplied with a direct voltage source.

There are specific symbols for each of these elements, for example direct voltage sources, which supply dc current are symbolized by two parallel lines of different lengths or a circle with the symbol of equality inside. direct current examples

In the following image V is the voltage and I is the current. Note that the current is drawn leaving the positive pole of the battery, although it is known that electrons, negatively charged, carry the current. This is done by convention.

Symbols for voltage used in direct current circuits.
Figure 3. Symbols for voltage used in direct current circuits.

Another common element in direct current circuits is resistance . All conductors present some opposition or resistance to the passage of current and their symbol is a jagged line. direct current examples

The filament of the bulb has resistance, in this way, the circuit of figure 1 is schematically represented as shown in the figure below.

Once switch S is closed, current can flow through resistor R, causing the filament of the bulb to light up and also give off heat.

Schematic representation of a direct current circuit, on the left with the switch open there is no current, and on the right, with the switch closed a direct current is established.
Figure 4. Schematic representation of a direct current circuit, on the left with the switch open there is no current, and on the right, with the switch closed a direct current is established.

Ohm’s law

Ohm’s law is used to solve simple direct current circuits like the one in the previous section, being valid for a large number of conductive materials:

V = I ∙ R

Where V is the voltage, I is the intensity of the current and R is the electrical resistance. In International System units, voltage is measured in volts (V), current in amps (A), and resistance in ohms (Ω).

Numerical example

If the resistance of the filament in the above circuit is R = 150 Ω and the voltage is 12 V, the current flowing through the circuit is:

I = V / R = 12 V / 150 Ω = 0.08 A

Direct current applications

Batteries and batteries direct current examples

The cells and batteries that power watches and toys supply direct current. They come in different sizes and voltages depending on the needs of the design.

The car battery is another example of the use of direct current, thanks to which the lighting system and the starter motor work.

Chargers for laptops and cell phones

The vast majority of portable electronic equipment for home use is powered by direct current and comes equipped with a battery and charger. direct current examples

Photovoltaic cells

They consist of electronic circuits to capture and convert sunlight into electrical energy through the photoelectric effect. These devices always provide a direct current, which can be converted to alternating current using an inverter.

Examples of direct current

As previously explained, direct current is one that flows in the same direction, however, its magnitude can be variable. For example, when a common battery is discharged, the current it produces is less than when it was new.

The following images show different examples of direct current, starting with a constant current over time. The others are variable, however, they all flow in the same direction, therefore they are direct currents. direct current examples

Constant current direct current examples

It is the one whose current vs. time graph is a horizontal line, parallel to the axis of time.

Constant direct current. 
Figure 5. Constant direct current.

Variable intensity direct current

The following graph shows two direct currents of variable intensity, obtained from an alternating signal through an appropriate circuit: direct current examples

Two examples of direct current of variable intensity.
Figure 6.- Two examples of direct current of variable intensity.

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