The direct current (CD) is the flow of charged particles in the same direction, usually inside a conductor or other medium. These charged particles can be electrons, in the case of a material such as copper, or ions of various kinds..
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:
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..
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).
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..
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 decreased intensity, is still direct current..
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.
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 operate with direct current, it always comes equipped with a device or charger with which to perform the conversion..
A direct current circuit comprises a series of elements such as resistors, capacitors and others, connected by conductive wires and fed 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..
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.
Another common element in direct current circuits is the endurance. All conductors present some opposition or resistance to the passage of current and their symbol is a jagged line.
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 in the bulb to light up and give off heat..
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 amperes (A), and resistance in ohms (Ω).
If the resistance of the filament in the circuit above 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
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 lights system and the starter motor work.
The vast majority of portable electronic equipment for domestic use is powered by direct current and comes equipped with a battery and a charger..
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..
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.
It is the one whose graph current vs time is a horizontal line, parallel to the axis of time.
The following graph shows two direct currents of variable intensity, obtained from an alternating signal through an appropriate circuit:
Differences between alternating and direct current.
Alternating current circuits.
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