Jul 22 2017
Initially, people did not know what a current is. There was just a static charge, but no one understood and did not realize the very nature of electricity.
It took many centuries, until Coulomb developed his theory, and German priest von Klein discovered that the bank could store energy.
By the time Van de Graaff created his first generator, everyone already knew what was the difference between DC and AC. And now it's time for our readers to find this information for personal use.
When the Lord became convinced that it was useless to frighten a flock of sheep with lightning and thunder, he decided to promote the story in a slightly different way.
As a result, human society tried to produce people by:
- Occupation physical culture.
- The development of art.
- The logic that initiated all the sciences.
So gradually, step by step, something more reasonable came out of the beasts. Today, for example, many are shocked that a policeman in the United States can coarsely deal with a Negro woman during her arrest, and some 100-200 years ago Africans were piled up and considered this an example to follow.
It must be said that the moral development of society began precisely in the last decades, when the society openly recognized the fascists as criminals and began to preach and introduce the so-called human rights. Science has evolved much earlier.
Since ancient times, for example, people have seen that tourmaline crystal attracts ash.
Why it happens? It should be said that the properties of piezoelectricity were first described using the example of tourmaline.
At the beginning of the 19th century, it was shown that a crystal, when heated, acquires an electric charge.
Due to the fact that deformation occurred, two poles were formed:
- Southern (analogical).
- Northern (antilogical).
Moreover, if the temperature after heating remains constant, then the electricity disappears. Then the appearance of the poles is observed already during cooling.
In other words, a tourmaline crystal generates electricity as the temperature changes.
Further studies have shown that the size of the potential depends on:
- The cross section of the crystal (cut across the poles).
- Temperature differences.
Other factors have no effect on the amount of charge.
What makes this happen? This phenomenon is called pyroelectricity. Being a dielectric, tourmaline was slowly charging from the current flowing inside. And the charge remained in place (certain surface areas) due to the insulating properties.
Thus, until the conductor pole is closed by the conductor, the crystal will accumulate charge as the temperature changes. The line uniting the poles was called the pyroelectric axis.
Piezoelectricity was discovered by a famous Curie pair based on the same tourmaline in 1880.
It was clear that with a change in the size of the crystal, charges would be produced, it only remained to come up with a technique for conducting the experiment.
Curie used for this static pressure of ordinary mass.
It is clear that the whole experiment is carried out on an insulating surface.
For example, a mass of 1 kg causes an electric charge in the tourmaline crystal of about five hundredths of static units.
How does electric current appear
It is curious that a coherent theory on this issue has not yet been created. For us, it’s important that there are charges in nature, and you can get them by different methods.
During a thunderstorm, this is due to the frictional forces of air masses, moisture molecules and some other phenomena.
The earth is negatively charged, and upward current flows continuously through the atmosphere.
That is, the current is called the movement of charge carriers for any reason. And one of them is the potential difference - the difference in the level of carriers between two points of space.
You can compare it with the pressure of the water. And as soon as the barrier is removed, the flow will rush in the direction where there is less pressure.
Now take the tourmaline crystal analogy
Suppose that charges appeared at its ends, what to do next? It is necessary to cause movement, for example, with a copper streak of a wire.
Combine the poles, and the electric current will flow. The movement of carriers will continue until the potential becomes equal.
At the same time the crystal is discharged. But is the current constant or alternating in this case? In this case, nothing like this can be said about the process.
Alternating and direct current are physical ideals, and are used because of the relative simplicity of obtaining mathematical models and controlling technological equipment with the help of them.
What are the above concepts?
1. By direct current is meant such that the carriers flow in one direction.
This does not mean that their number through the cross section of the medium is the same. Not. In a broader sense, the constant (rectified) current is called the movement of charge carriers in one direction.
But the initial concept in physics requires more stringent conditions.
The current must be formed precisely by a constant number of carriers moving in one direction.
Moreover, these carriers are positive (which contradicts the practice, where electrons are considered as such for the most part).
2. The alternating current is called not just the one where the carriers move in one or the other direction, but do so in time.
That is, half of the wave period runs to the left, and the second to the right.
This is figuratively speaking. Carrier density varies according to the law of a sine wave.
Actually, this is the graph that displays the behavior of the process. At the points of zero crossing, there is no current at all.
And it happens in our network 100 times per second. Consequently, half of the period falls on the movement of carriers in the positive direction, and the second - in the negative.
Total total cycles per second forms 50, which corresponds to a network frequency of 50 Hz.
As a matter of fact with electric current.
In practice, the form of the current (the dependence of the charge density on time) is not sinusoidal. For various reasons, the form of the graph is distorted.
This, for example, occurs when starting the equipment and stopping, due to induced interference of a different nature.
Thus, the shape of AC and DC is distorted. And it has long been established that it harms the hardware.
Since some methods were needed to combat this scourge, mathematicians invented the so-called spectral analysis.
Many have heard of something similar in the stock market, but in this case it’s rather a different matter: scientists are looking for a mathematical model that would be relatively easy to calculate and predict results.
This method was really found, and his name - spectral analysis. In this case, the oscillation of any shape can be represented as a sum with different specific weights of the simplest sinusoids of different frequencies.
It turns out that many and many components move simultaneously along the chain. And in general, they give a current.
And not necessarily all the components are moving in the same direction as the main mass.
It is possible to imagine this as a group of ants, each of which is dragging in its own direction, and the resulting effect causes the load to move only in one.
We believe our readers will only forget about it.
Therefore, we mention that in addition to the coefficient (amplitude), each component also has a phase (direction), and is called a harmonic.
So, the cascades of technology are arranged so that the useful frequencies (primarily 50 Hz) pass inside the device, and everything else goes to the ground.
This is the sign to solve the problem, which we talked about at the beginning. Any oscillation can be represented as a set of useful and harmful signals and, based on this, the equipment should be designed properly.
For example, all receivers work on this principle: they selectively pass only the current of the desired frequency. Due to this, it is possible to cut off the interference, and the wave is transmitted with minimal distortion over long distances.
We could talk for a long time on this topic, but it is time to give examples of where the types of currents are used.
AC and DC Examples
But, by and large, it happens quite smoothly. And the current flows in one direction and has approximately constant density.
Similarly, work:
- Battery cell phone.
- Battery of any type.
- Laptop Power Battery.
But these are all capacities, but what about the generators?
In nature, there are no direct current sources with the exception of Mother Earth.
It is much more convenient for a person to create rotors, which rotate with a certain frequency, create conditions for the formation of an alternating electric current in the stator coils.
Then the industrial frequency of 50 Hz passes through the wires and is fed through the substation to the consumer.
Be that as it may, adapters can be considered as a source of direct current. These are devices that convert AC to DC.
Suppose cell phones are usually in the order of +5 V, whereas for mobile radios there is a wide variation.
In general, you need to understand that a DC device can function only on the nominal for which it was designed.
Otherwise, either the performance is impaired, or - with large deviations - a complete failure is possible.
This applies to both AC and DC.
Now it is time to say that in the industry the conversion of direct current into alternating current and back is not practiced.
For reasons of economy, all engines operate on three phases. Each of them is an alternating current frequency of 50 Hz.
But we said above that each harmonic has a phase. In our case, it is equal to 120 degrees. A circle is formed by 360 degrees. It turns out that all three phases are equally separated from each other.
Content:
Not the first decade, the debate continues, what kind of current is more dangerous - alternating or direct. Some claim that it is the straightened voltage that carries a greater threat, others sincerely believe that the sinusoid of alternating current, coinciding in amplitude with the beating of the human heart, stops it. But, as always happens in life, how many people - so many opinions. Therefore, it is worth looking at this question purely from a scientific point of view. But it is worth making it in a language that is understandable even for teapots, since not everyone has an electrical education. At the same time, surely anyone would like to know the origin of direct and alternating current.
Where do you start? Yes, probably from the definitions - what is electricity, why is it called variable or constant, which of these types is more dangerous and why.
Most people know that direct current can be obtained from various power supply units or batteries, and the alternating current enters apartments and premises through the power grid and thanks to it household electrical appliances and lighting work. But few people wondered why one voltage allows you to get another and why it is needed.
It makes sense to answer all your questions.
What is electric current?
Electric current is called a constant or variable quantity that arises from a directional or orderly movement created by charged particles — electrons in metals, ions in electrolyte, and both in gas. In other words, they say that the electric current "flows" through the wires.
Some mistakenly believe that every charged electron moves through a conductor from the source to the consumer. This is not true. It only transfers the charge to neighboring electrons, while remaining in place itself. Those. its movement is chaotic, but microscopic. Well, already the charge itself, moving along the conductor, reaches the consumer.
Electric current has such measurement parameters as: voltage, i.e. its value, measured in volts (V) and current, which is measured in amperes (A). What is very important when transforming, i.e. decrease or increase with the help of special devices, one value acts on the other inversely. This means that by reducing the voltage through a conventional transformer, an increase in the current is obtained and vice versa.
AC and AC current
The first thing to understand is the difference between direct and alternating current. The fact is that the alternating current is not only easier to obtain, although this is also important. Its characteristics allow transmission over any distances along conductors with the least losses, especially at higher voltages and lesser strength. That is why the power lines between the cities are high-voltage. And already in the settlements current is transformed into a lower voltage.
But DC is very easy to get from AC, for which they use multidirectional diodes (the so-called diode bridge). The fact is that alternating current (AC), or rather the frequency of its oscillations, is a sinusoid, which, passing through a rectifier, loses some of the oscillations. This produces a constant voltage (AC) at the output that does not have a frequency.
It makes sense to specify what, after all, they are different.
Current differences
Of course, the main difference between AC and DC is the possibility of transporting DC over a long distance. In this case, if the same way to forward a direct current, it simply will not remain. Due to the potential difference, it is consumed. It is also worth noting that it is very difficult to convert to variable, while in the reverse order such an action is quite easily accomplished.
It is much more economical to convert electricity into mechanical energy with the help of motors powered by speakers, although there are areas in which only direct current mechanisms can be used.
Well, last but not least, alternating current is safer for people. It is for this reason that all appliances used in households and working from DC are low-current. But to completely abandon the use of more dangerous in favor of the other does not work for the reasons mentioned above.
All of the above leads to a generalized answer to the question of how alternating current differs from direct current - these are characteristics that influence the choice of one or another power source in a certain area.
Current transfer over long distances
Some people have a question, to which a superficial answer was given above: why is there a very high voltage along power lines (LEP)? If you do not know all the subtleties of electrical engineering, then we can agree with this question. Indeed, after all, if a voltage of 380 V had come along a transmission line, then it would not be necessary to install expensive transformer substations. Yes, and on their service would not have to spend money, is not it? It turns out that no.
The fact is that the conductor cross section, through which electricity flows, depends only on the strength of the current and on its power consumption and the voltage remains completely out of it. This means that with a current of 2 A and a voltage of 25,000 V, you can use the same wire as for 220 V with the same 2 A. So what follows from this?
Here it is necessary to return to the law of inverse proportionality — in the course of current transformation, i.e. increasing the voltage decreases the current and vice versa. Thus, the high-voltage current is sent to the transformer substation on thinner wires, which ensures less transmission losses.
Transmission features
Just in the loss is the answer to the question of why it is impossible to transfer direct current over long distances. If we consider DC from this angle, then for this very reason there will be no electricity left in the conductor through a small segment of distance. But the main thing here is not energy loss, but their immediate cause, which, again, is one of the characteristics of AC and DC.
The fact is that the frequency of alternating current in public electrical networks in Russia is 50 Hz (hertz). This means the amplitude of the charge oscillation between positive and negative, equal to 50 changes per second. In simple terms, every 1/50 of a second. the charge changes its polarity, this is the difference between direct current - there are practically no oscillations in it. It is for this reason that DC is consumed by itself, flowing through a long conductor. By the way, the frequency of oscillations, for example, in the USA differs from the Russian one and is 60 Hz.
Generation
A very interesting question is how direct and alternating current is generated. Of course, you can produce both one and the other, but here comes the problem of size and cost. The fact is that if, for example, to take an ordinary car, it would be much easier to put a DC generator on it, eliminating the diode bridge from the circuit. But then there is a snag.
If the rectifier is removed from the automotive generator, the volume seems to be reduced, but this will not happen. And the reason for this - the dimensions of the DC generator. Moreover, the cost will increase significantly, and therefore variable generators are used.
So it turns out that to generate DC is much less profitable than the AC, and there is concrete evidence.
Two great inventors in their time began the so-called "war of currents", which ended only in 2007. Opponents in it were Nikola Tesla and George Westinghouse, ardent supporters of alternating voltage, and Thomas Edison, who was behind the application of direct current everywhere. So, in 2007, the city of New York completely switched to the side of Tesla, thereby marking his victory. It is worth a little more detail on this.
Story
The Thomas Edison Company, which was called “Edison Electric Light”, was founded in the late 1970s. Then, at the time of candles, kerosene lamps and gas lighting incandescent lamps, produced by Edison, could work continuously for 12 hours. And although now this may seem ridiculously small - it was a real breakthrough. But already in the 1880s, the company was able not only to patent the production and transmission of direct current through a three-wire system (these were “zero”, “+110 V” and “-110 V”), but also to provide an incandescent lamp with a resource of 1200 hours .
It was then that the phrase of Thomas Edison was born, which later became known to the whole world, - “We will make electric lighting so cheap that only the rich will burn candles”.
Well, by 1887 in the United States, more than 100 power plants are successfully operating, which produce direct current and where it is used to transmit the three-wire system, which is used in order to at least slightly reduce electric power losses.
But the scientist in the field of physics and mathematics, George Westinghouse, after familiarizing himself with Edison's patent, found one very unpleasant detail - this was a huge loss of energy during transmission. At that time, alternating current generators already existed, which were not popular due to equipment that would work on such energy. At that time, a talented engineer Nikola Tesla was still working for Edison in the company, but once, when he was once again denied a salary increase, Tesla could not stand it and went to work for a competitor who was Westinghouse. At the new location, Nicola (in 1988) creates the first electricity metering device.
It is from this moment that the “war of currents” begins.
findings
Let's try to summarize the information presented. To date, it is impossible to imagine the use (both at home and in the workplace) by any one type of electricity - almost everywhere there is both direct and alternating current. After all, a constant is needed somewhere, but its transmission over long distances is impossible, but somewhere variable.
Of course, it has been proven that the AU is much safer, but what about devices that help to save energy many times while they can only work on DC?
It is for these reasons that the currents now "peacefully coexist" in our lives, ending the "war" that lasted more than 100 years. The only thing that should not be forgotten is that, no matter how much safer than the other (constant, alternating voltage is not important), it can cause tremendous harm to the body, even fatal.
And for this reason, when working with voltage, it is necessary to carefully observe all the norms and rules of safety and not to forget about attentiveness and accuracy. After all, as Nikola Tesla said, you should not be afraid of electricity, you should respect it.
Electric current is called directional, ordered movement of charged particles.
Direct current has stable properties and the direction of motion of charged particles, which do not change with time. It is used by many electrical devices in homes as well as in cars. From DC work modern computers, laptops, televisions and many other devices. To convert AC to DC, special power supplies and voltage transformers are used.
All electrical devices and electrical tools operating on batteries and accumulators are considered to be DC consumers, since a battery is a DC source that can be converted to AC using inverters.
AC Current to DC Difference
Variables are electric current, which can vary in the direction of movement of charged particles and the magnitude over time. The most important parameters of AC are its frequency and voltage. In modern electrical networks, it is AC that has a certain voltage and frequency that is used at various facilities. In Russia, in household power grids, the current has a voltage of 220 V and a frequency of 50 Hz. The frequency of an electrical alternating current is the number of changes in the direction of motion of charged particles per second, that is, at a frequency of 50 Hz, it changes direction 50 times per second. Thus, the difference between alternating current and direct current is that, in alternating current, charged particles can change the direction of motion.
Sources of alternating current on objects of various purposes are sockets. We connect various household appliances that receive the necessary voltage to the sockets. Alternating current is used in electrical networks because the voltage can be converted to the required values with the help of transformer equipment with minimal losses. In other words, it is much easier and cheaper to transport from sources of electricity to end users.
AC transfer to consumers
The path of the alternating current begins with the power plants, on which the most powerful electric generators are installed, from which electrical current flows out with a voltage of 220-330 kV. Through electric cables, the current goes to transformer substations installed in close proximity to electrical consumption objects - houses, apartments, enterprises and other structures.
Substations receive electric current with a voltage of about 10 kV and convert it to a three-phase voltage of 380 V. In some cases, power is supplied to objects with a voltage of 380 V, this is required by powerful household and industrial appliances, but most often at the place where electricity is introduced into a house or apartment , the voltage decreases to the usual 220 V.
AC to DC conversion
We have already figured out that AC is in the outlets of household electrical systems, but many modern consumers of electricity need constant. Conversion of AC to DC is carried out using special rectifiers. The whole conversion process includes three steps:
- Connecting a diode bridge with 4 diodes of necessary power. Such a bridge can “cut off” the upper values of AC sinusoids or make the movement of charged particles unidirectional.
- Connection of a smoothing filter or a special capacitor to the output from the diode bridge. The filter is able to correct the dips between the peaks of the AC sinusoids. Connecting a capacitor seriously reduces the ripple and can bring them to a minimum.
- Connection of voltage stabilizers to reduce ripple.
Current conversion can be carried out in both directions, that is, alternating can also be made from constant. But this process is much more complicated and it is carried out through the use of special inverters, which are distinguished by high cost.