Do you know the difference between alternating and direct current?

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Electricity is a fascinating phenomenon that makes our lives much easier and therefore the terms alternating current and direct current are well known, but do we really know the difference between alternating and direct current?

There are two types of electric current: alternating current and direct current.

Electricity is a very common concept in our lives and one that we use on a daily basis. It occurs naturally in nature, for example in lightning or in electric fish capable of giving a good shock. And although the effect of static electricity has been known for centuries, it took a long time to study this phenomenon. Some of the pioneers were Galvani, Volta, Coulomb, Ampère, Faraday and Ohm. Many of these names are well known because they ended up naming some of the units that are used today to measure the different magnitudes of this phenomenon. 

For the use of electricity, as we use it today, it was essential to know the concept of electric current - the flow of electric charge through a material, normally a conductor - whose charge is measured in coulombs per second, from which we derive its intensity, whose unit is called the ampere. However, in order to understand the concept of electric current correctly, it is necessary to clarify that there are two types: alternating current and direct current. Before talking about the differences between alternating current and direct current, let's clarify what each of them consists of. 


As its name indicates, alternating current is that in which the voltage regularly changes its polarity (positive or negative) 50 times per second, in the case of European grids, therefore, if we look at the waveform most commonly used in alternating current it shows a sinusoidal shape, which is also the one most efficiently generated by rotating machines such as alternators or generators. 

This system was devised by Nikola Tesla and its ease of transformation makes it much more suitable for bringing electricity to industries and homes. The alternating current can be transformed, increasing its voltage by means of transformers to travel much more cheaply and efficiently and then, on approaching the point of consumption, its voltage is reduced again, making it completely safe for industrial or domestic use.

The symbol for alternating current is known worldwide as AC, an acronym for alternating current. As a curiosity, the name of the famous band AC/DC stands for Alternating Current and Direct Current.


In contrast to the concept of alternating current, there is the concept of direct current in which the voltage does not change polarity. This is the type of current usually found in batteries, in which one terminal is always positive and the other negative, and when shown in a diagram it is illustrated by a straight line without variations. There are also generators that directly produce direct current, such as photovoltaic generators.


In addition to those already described, the main differences between alternating current and direct current can be found in the following aspects: 

Polarity: As already mentioned, direct current has a single polarity while alternating current constantly switches between positive and negative.

Transported energy: Both direct current and alternating current can be transported over power lines. However, the equipment necessary to adapt the voltage to the different uses is much simpler and cheaper in alternating current. On the other hand, the transport of direct current electricity allows for better control of current flows. On the other hand, energy can be transported inside chemical elements, i.e., batteries, cells, etc., which is not possible in the case of alternating current.

Connection: If we remember how batteries work, we will remember that, in direct current, for there to be current, it is essential to connect each wire with the correct polarity. This is also not the case with alternating current, where it does not matter how the devices are connected, for example, if the plugs in the house are turned around, they work without any problems. 

Voltage used: Both AC and DC voltages can vary, usually depending on the location. For transport it is more efficient to use higher voltages, while lower voltages are used for receivers, both for safety and cost reasons. 

Storage: Energy transported by direct current can be stored, as in the case of batteries, independently or in association with generation, whereas this cannot be done with alternating current. 


Because there are these two types of alternating and direct current, of which we have already seen their differences, there is also equipment to modify the type of current, when necessary, usually based on power electronics. In general, they are called converters, although they are often given specific names, such as rectifiers, which only switch from AC to DC, or inverters, which only switch from DC to AC.

To adjust the AC voltage up or down, transformers are used, which are electromechanical elements that are more robust and cheaper than power electronics. The same function is possible between different DC voltage levels, but in this case, it is essential to use converters.

The clearest example is when a power plant produces electricity at a very high voltage in order to allow the current to travel faster over long distances. However, this same current must be down-regulated, using transformers that are close to the final destination (be it homes or industry), to ensure that the voltage that reaches them is adequate for their consumption, which is not the same.

The opposite is the case, for example, when a laptop or smartphone battery charger is connected to the household mains. The mains use alternating current and it is transformed into direct current by the rectifier before it reaches the device.

Photovoltaic generators follow the reverse process, as the panels provide direct current which, to connect it to our home, we transform into alternating current. The equipment that performs this function is a photovoltaic inverter. Finally, if we add battery storage to this installation, we will go from AC to DC and from DC to AC, by means of a converter.