What kind of current is called constant. Electric current is direct and alternating. The difference between direct current and alternating current. How does direct current differ from alternating current and how is it converted?
Despite the fact that electric current is an indispensable part of modern life, many users do not even know basic information about it. In this article, skipping the basic physics course, let’s look at how it differs D.C. from variable, as well as how it is used in modern domestic and industrial conditions.
Difference in current types
We will not consider what current is here, but will immediately move on to the main topic of the article. Alternating current differs from direct current in that it continuously changes in the direction of movement and its size.
These changes are carried out in periods of equal time intervals. To create such a current, special sources or generators are used that produce an alternating EMF (electromotive force), which changes regularly.
The basic circuit of the mentioned device for generating alternating current is quite simple. This is a rectangular frame made of copper wires, which is attached to an axis and then rotates in the field of a magnet using a belt drive. The tips of this frame are soldered to copper contact rings that slide directly over the contact plates, rotating synchronously with the frame.
Under the condition of a uniform rhythm of rotation, an EMF begins to be induced, which changes periodically. It is possible to measure the EMF generated in the frame with a special device. Thanks to the appearance, it is possible to determine the EMF variable and with it AC.
In graphical execution, these quantities are typically depicted in the form of a wave-like sinusoid. The concept of sinusoidal current often refers to alternating current, since this type of current change is the most common.
Alternating current is an algebraic quantity, and its value at a specific time instant is called the instantaneous value. The sign of the alternating current itself is determined by the direction in which the current flows at a given time. Therefore, the sign can be positive or negative.
Current characteristics
For a comparative assessment of all possible alternating currents, criteria called AC parameters, among which:
- period;
- amplitude;
- frequency;
- circular frequency.
A period is a period of time during which a complete cycle of current change occurs. The amplitude is the maximum value. The frequency of alternating current was the number of completed periods in 1 second.
The parameters listed above make it possible to distinguish various types alternating currents, voltages and EMF.
When calculating the resistance of different circuits to alternating current, it is permissible to connect another characteristic parameter called angular or circular frequency. This parameter is determined by the speed of rotation of the above-mentioned frame at a certain angle in one second.
Important! You should understand the difference between current and voltage. The fundamental difference is known: current is an amount of energy, and voltage is called a measure.
Alternating current gets its name because the direction of movement of electrons changes continuously, as does the charge. It has different frequencies and electrical voltages.
This is the distinguishing feature from direct current, where the direction of electron movement is unchanged. If the resistance, voltage and current are constant, and the current flows only in one direction, then such a current is constant.
For the passage of direct current in metals, it is necessary that the source of constant voltage be closed on itself using a conductor, which is the metal. In some situations, a chemical energy source called a galvanic cell is used to generate direct current.
Current transmission
AC power sources are regular outlets. They are located at facilities for various purposes and in residential premises. Various electrical devices are connected to them, which receive the voltage necessary for their operation.
The use of alternating current in electrical networks is economically justified because the magnitude of its voltage can be transformed to the level of required values. This is accomplished using transformer equipment with minor losses allowed. Transportation from power sources to end consumers is cheaper and easier.
The transfer of current to consumers begins directly at the power plant, where a variety of extremely powerful electrical generators are used. Electric current is obtained from them, which is sent through cables to transformer substations. Often, substations are located near industrial or residential electrical consumption facilities. The current received by the substations is converted into three-phase alternating voltage.
Batteries and accumulators contain direct current, which is characterized by stable properties, i.e. they do not change over time. It is used in any modern electrical products, as well as in cars.
Current conversion
Let us consider separately the process of converting alternating current into direct current. This process is carried out using specialized rectifiers and includes three steps:
- The first step is to connect a four-diode bridge of a given power. This, in turn, makes it possible to specify unidirectional motion for charged particles. In addition, it lowers the upper values of sinusoids characteristic of alternating current.
- Next, a smoothing filter or a specialized capacitor is connected. This is done with diode bridge on the way out. The filter itself helps correct the valleys between the peak values of the sinusoids. And connecting a capacitor significantly reduces ripple and brings it to minimum values.
- Then voltage stabilizing devices are connected to reduce ripple.
This process, if necessary, can be carried out in two directions, converting direct and alternating current.
Another distinctive feature is the propagation of electromagnetic waves in relation to space. It has been proven that direct current does not allow electromagnetic waves to propagate in space, while alternating current can cause them to propagate. In addition, when transporting alternating current through wires, induction losses are much less than when transmitting direct current.
Rationale for current selection
The variety of currents and the lack of a single standard is due not only to the need for various characteristics in each individual situation. In solving most issues, the advantage is in favor of alternating current. This difference between the types of currents is determined by the following aspects:
- Possibility of transmitting alternating current over long distances. Possibility of conversion in heterogeneous electrical circuits with ambiguous consumption levels.
- Maintaining a constant voltage for alternating current is two times cheaper than for direct current.
- Conversion process electrical energy directly into mechanical force is carried out at significantly lower costs in AC mechanisms and motors.
Current is the movement of electrons in a certain direction. It is necessary for electrons to move in our devices too. Where does the current in the outlet come from?
A power plant converts the kinetic energy of electrons into electrical energy. That is, a hydroelectric power plant uses running water to rotate a turbine. The turbine propeller rotates a ball of copper between two magnets. Magnets cause the electrons in the copper to move, which causes the electrons in the wires that are connected to the ball of copper to move, resulting in a current.
The generator is like a water pump, and the wire is like a hose. The generator-pump pumps electrons-water through wires-hoses.
Alternating current is the current that we have in the outlet. It is called variable because the direction of electron motion is constantly changing. AC power from outlets has different frequencies and electrical voltages. What does it mean? In Russian sockets the frequency is 50 hertz and the voltage is 220 volts. It turns out that in a second the flow of electrons changes the direction of electron movement and the charge from positive to negative 50 times. You can notice a change in direction in fluorescent lamps when you turn them on. While the electrons are accelerating, it blinks several times - this is a change in direction of movement. And 220 volts is the maximum possible “pressure” with which electrons move in this network.
In alternating current, the charge is constantly changing. This means that the voltage is either 100%, then 0%, then 100% again. If the voltage were 100% constant, then a huge diameter wire would be needed, and with a changing charge the wires could be thinner. It's convenient. A power plant can send millions of volts through a small wire, then a transformer for an individual house takes, for example, 10,000 volts, and delivers 220 to each outlet.
Direct current is the current you have in your phone battery or batteries. It is called constant because the direction in which the electrons move does not change. Chargers They transform alternating current from the network into direct current, and in this form it ends up in batteries.
In the 21st century, electronics have become very popular. Many people want to learn more about radio engineering and start reading special books, although much in the books is not clear. And so they begin to get confused and ask a lot of questions. They can’t find suitable and understandable websites about electronics, where they can briefly and simply understand what’s what. But we’ve gone a long way, okay, let’s get down to business. The task is to tell more and more clearly about direct and alternating current.
D.C
Before the time when there were no radio receivers and radio communications, there was a current that flowed in one direction - it was called constant, on the graph it is depicted as a straight line, as shown in the figure below.
Let's figure out what the operating principle of this current is, and it is very simple. Because direct current flows only in one direction. Powerful power plants produce alternating current; it must be converted to direct current. Direct current can only be created by a galvanic cell. A galvanic cell is an element that produces direct current, that is, a regular battery. We will not analyze the principle of operation of the battery; the main thing for us now is that only direct and alternating current fit into your memory. Let's say we have generated a direct current, it will begin to move from plus to minus, be sure to remember this.
AC
Now we move on to alternating current, all radio communication has appeared, alternating current has become a highlight. Consider the graph of alternating current. You immediately noticed these strange letters, we don’t need them, except for one - T. Alternating current has a peculiarity, it can change its direction, for example: it moves first in one direction, then in the other. This process is called oscillation or period. In the figure, the period is indicated by this very letter T. It can be seen that there is a wave above the t axis, and below it there is also a wave. This means that above the axis this is a movement towards plus, and below, a movement towards minus, in other words, this is a positive half-period, why a half-period, because two half-periods are equal to T, that is, equal to a period, which means they are still half-periods. Period is the same as fluctuation. Several oscillations completed in 1 second are called frequency. So, we figured out what direct and alternating current are, I think we got it figured out.
Remember: The outlet always has 220 V AC - it is very dangerous. One hit can even kill a person, so be careful!
You should remember: the movement of direct and alternating current; graphs of direct and alternating current; What is frequency, half-cycle, period.
By the way, I forgot to say what frequency is measured in. Remember: frequency is measured in Hertz. Let's say there are 50 vibrations per second, this means that the frequency is 50 hertz. This way you can define any other values. Bye everyone, Dmitry Tsyvtsyn was with you.
A long time ago, scientists invented electric current. The first invention was the permanent one. But later, while conducting experiments in his laboratory, Nikola Tesla invented alternating current. There were and are many differences between them, according to which one of them is used in low-current equipment, and the other has the ability to overcome different distances with minor losses. But a lot depends on the magnitude of the currents.
AC and DC current: difference and features
The difference between alternating current and direct current can be understood based on the definitions. In order to better understand the operating principle and features, you need to know the following factors.
Main differences:
- Movement of charged particles;
- Mode of production.
Variable current is a current in which charged particles are capable of changing the direction of movement and magnitude in certain time. The main parameters of alternating current include its voltage and frequency.
Currently, public electrical networks and various facilities use alternating current, with a certain voltage and frequency. These parameters are determined by the equipment and devices.
Pay attention! In household electrical networks, a current of 220 Volts and a clock frequency of 50 Hz is used.
The direction of movement and frequency of charged particles in direct current are unchanged. This current is used to power various household devices, such as televisions and computers.
Due to the fact that alternating current is simpler and more economical in its production method and transmission over various distances, it has become the basis for the electrification of objects. Alternating current is produced at various power plants, from which it is supplied to the consumer through conductors.
Direct current is obtained by converting alternating current or through chemical reactions (for example, an alkaline battery). For conversion, current transformers are used.
What voltage level is acceptable for a person: features
In order to know what values of electric current are permissible for a person, appropriate tables have been compiled that indicate the values of alternating and direct current and time.
Electric current exposure parameters:
- Strength;
- Frequency;
- Time;
- Relative humidity.
The permissible touch voltage and current that flow through the human body in various modes of electrical installations do not exceed the following values.
Alternating current 50 Hz, should be no more than 2.0 Volts and a current of 0.3 mA. Current with a frequency of 400 Hz with a voltage of 3.0 Volts and a current strength of 0.4 mA. Direct current with a voltage of 8 and a current of 1 mA. Safe exposure to current with such indicators is up to 10 minutes.
Pay attention! If electrical installation work is carried out at elevated temperatures and high relative humidity, these values are reduced by three times.
In electrical installations with voltages up to 100 Volts, which are solidly grounded or the neutral is insulated, the safe touch currents are as follows.
50 Hz alternating current with a voltage range from 550 to 20 Volts and a current strength from 650 to 6 mA, 400 Hz alternating current with a voltage from 650 to 36 Volts, and direct current from 650 to 40 Volts, should not affect the human body within the range of 0.01 to 1 second.
Dangerous alternating current for humans
It is believed that alternating electric current is the most dangerous for human life. But this is provided, if you do not go into details. Much depends on various quantities and factors.
Factors influencing hazardous exposure:
- Duration of contact;
- The path of electric current;
- Current and voltage;
- What is the resistance of the body?
According to the rules of the PUE, the most dangerous current for humans is alternating current with a frequency that varies from 50 to 500 Hz.
It is worth noting that, provided that the current does not exceed 9 mA, then anyone can free themselves from the current-carrying part of the electrical installation.
If given value is exceeded, then in order to free yourself from the effects of electric current, a person needs strong help. This is due to the fact that alternating current is much more capable of stimulating nerve endings and causing involuntary muscle spasms.
For example, when you touch the live part of the device with the inside of your palm, the muscle spasm will cause the fist to clench more strongly over time.
Why is alternating current more dangerous? At identical values current, alternating current has a several times stronger effect on the body.
Since alternating current affects nerve endings and muscles, it is worth understanding that this also affects the functioning of the heart muscle. From which it follows that when contacting alternating current, the risk of death increases.
An important indicator is the resistance of the human body. But when struck by alternating current with high frequencies, the body resistance is significantly reduced.
What magnitude is direct current dangerous for humans?
Direct current can also be dangerous for humans. Of course, variable, ten times more dangerous. But if we consider currents in different quantities, then constant can be much more dangerous than alternating.
The effects of direct current on humans are divided into:
- 1 threshold;
- 2 threshold;
- 3 threshold.
When exposed to direct current at the feather threshold (the current is noticeable), your hands begin to tremble a little and a slight tingling sensation appears.
The second threshold (not releasing current), ranging from 5 to 7 mA, is the lowest value at which a person cannot free himself from the conductor on his own.
This current is considered not dangerous, since the resistance of the human body is higher than its value.
The third threshold (fibrillation), with values of 100 mA and above, the current has a strong effect on the body and on internal organs. In this case, the current at these values can cause chaotic contraction of the heart muscle and lead to its stop.
The strength of the impact is also influenced by other factors. For example, dry human skin has a resistance of 10 to 100 kOhm. But if the contact occurs with a wet skin surface, then the resistance is significantly reduced.
Only a few are able to really understand that alternating and direct current are somehow different. Not to mention naming specific differences. The purpose of this article is to explain the main characteristics of these physical quantities in terms that people without technical knowledge can understand, and also to provide some basic concepts concerning this issue.
Visualization challenges
Most people have no difficulty understanding concepts such as “pressure,” “quantity,” and “flow,” because in their everyday life they are constantly faced with them. For example, it is easy to understand that increasing the flow when watering flowers will increase the amount of water coming out of the watering hose, while increasing the water pressure will cause it to move faster and with more force.
Electrical terms such as "voltage" and "current" are usually difficult to understand because you cannot see or feel the electricity moving through cables and electrical circuits. It is extremely difficult for even a novice electrician to visualize what is happening at the molecular level or even clearly understand what, for example, an electron is. This particle is beyond human sensory capabilities and cannot be seen or touched unless a certain amount of it passes through the human body. Only then will the victim definitely feel them and experience what is commonly called an electric shock.
However, exposed cables and wires appear completely harmless to most people simply because they cannot see the electrons just waiting to take the path of least resistance, which is usually the ground.
Analogy
It's understandable why most people can't visualize what's going on inside ordinary conductors and cables. Trying to explain that something is moving through metal defies common sense. At the most basic level, electricity is not that different from water, so its basic concepts are quite easy to grasp when compared electrical circuit with plumbing system. The main difference between water and electricity is that the former fills something if it manages to escape from the pipe, while the latter requires a conductor to move electrons. By visualizing the pipe system, the technical terminology is easier for most to understand.
Voltage as pressure
Voltage is very similar to electron pressure and indicates how fast and with what force they move through a conductor. These physical quantities are equivalent in many respects, including their relationship to the strength of the pipeline-cable. Just as too much pressure ruptures a pipe, too much voltage destroys or pierces a conductor's shielding.
Current as flow
Current is the flow rate of electrons, indicating how many electrons are moving through the cable. The higher it is, the more electrons pass through the conductor. Just as large amounts of water require thicker pipes, large currents require thicker cables.
Using the water circuit model allows you to explain many other terms. For example, power generators can be thought of as water pumps, and electrical loads can be thought of as a water mill that requires water flow and pressure to rotate. Even electronic diodes can be thought of as water valves that only allow water to flow in one direction.
D.C
The difference between direct and alternating current becomes clear from the name. The first represents the movement of electrons in one direction. It is very easy to visualize it using a water loop model. It is enough to imagine that water flows through a pipe in one direction. Common devices that produce direct current are solar cells, batteries, and dynamos. Almost any device can be designed to be powered by such a source. This is almost the exclusive domain of low-voltage and portable electronics.
Direct current is quite simple, and obeys Ohm's law: U = I × R. It is measured in watts and is equal to: P = U × I.
Because of its simple equations and behavior, direct current is relatively easy to conceptualize. The first power transmission systems, developed by Thomas Edison back in the 19th century, used only this. However, the difference between alternating current and direct current soon became apparent. Transmission of the latter over considerable distances was accompanied by large losses, so after a few decades it was replaced by a more profitable (at that time) system developed by Nikola Tesla.
Although commercial power grids around the planet now use alternating current, the irony is that technological advances have made direct current transmission high voltage more efficient over very long distances and under extreme loads. What, for example, is used when connecting individual systems, such as entire countries or even continents. This is another difference between AC and DC. However, the former is still used in low-voltage commercial networks.
Direct and alternating current: differences in production and use
While alternating current is much easier to produce using a generator using kinetic energy, batteries can only create direct current. Therefore, the latter dominates the power supply circuits of low-voltage devices and electronics. Batteries can only be charged by DC current, so the AC mains current is rectified when the battery is the main part of the system.
A common example would be any vehicle—motorcycle, car, and truck. The generator installed on them creates alternating current, which is instantly converted to direct current using a rectifier, since there is a battery in the power supply system, and most electronics require constant voltage. Solar cells and fuel cells also produce only direct current, which can then be converted to alternating current if necessary using a device called an inverter.
Direction of movement
This is another example of the difference between DC and AC. As the name suggests, the latter is a stream of electrons that constantly changes its direction. Since the late 19th century, almost all domestic and industrial electrical power throughout the world has used sinusoidal alternating current because it is easier to obtain and much cheaper to distribute, except in a very few cases of long distance transmission where power losses force the use of the latest high-voltage direct current systems.
AC has another big advantage: it allows energy to be returned from the point of consumption back to the grid. This is very beneficial in buildings and structures that produce more energy than they consume, which is quite possible using alternative sources such as solar panels and The fact that alternating current allows for bidirectional energy flow is a major reason for the popularity and availability of alternative power sources.
Frequency
When it comes to technical level Unfortunately, it becomes difficult to explain how alternating current works, since the water circuit model does not quite fit it. However, it is possible to visualize a system in which water quickly changes direction of flow, although it is not clear how it would do anything useful. Alternating current and voltage constantly change their direction. The rate of change depends on the frequency (measured in hertz) and for household electrical networks usually 50 Hz. This means that voltage and current change direction 50 times per second. Calculating the active component in sinusoidal systems is quite simple. It is enough to divide their peak value by √2.
When alternating current changes direction 50 times per second, that means incandescent light bulbs turn on and off 50 times per second. The human eye cannot notice this, and the brain simply believes that the lighting is on all the time. This is another difference between AC and DC.
Vector mathematics
Current and voltage not only constantly change, but their phases do not match (they are not synchronized). The vast majority of AC power loads cause phase differences. This means that even the simplest calculations need to use vector mathematics. When working with vectors, you can't simply add, subtract, or do any other scalar math operations. With constant current, if one cable carries 5A to a certain point, and the other carries 2A, then the result is 7A. In the case of a variable, this is not the case, because the result will depend on the direction of the vectors.
Power factor
The active power of an AC powered load can be calculated using the simple formula P = U × I × cos (φ) where φ is the angle between voltage and current, cos (φ) is also called power factor. This is how direct and alternating current differ: in the first, cos (φ) is always equal to 1. Active power is needed (and paid for) by household and industrial consumers, but it is not equal to the complex power passing through the conductors (cables) to the load, which can be calculated using the formula S = U × I and measured in volt-amperes (VA).
The difference between direct and alternating current in the calculations is obvious - they become more complex. Even the simplest calculations require at least a mediocre knowledge of vector mathematics.
Welding machines
The difference between direct and alternating current also appears when welding. Arc polarity has great influence on its quality. Electrode-positive welding penetrates deeper than electrode-negative, but the latter accelerates the deposition of metal. With direct current, the polarity is always constant. With variable it changes 100 times per second (at 50 Hz). Constant welding is preferable, as it is produced more smoothly. The difference between AC and DC welding is that in the first case, the movement of electrons is interrupted for a split second, which leads to pulsation, instability and loss of the arc. This type of welding is rarely used, for example, to eliminate arc wander in the case of large diameter electrodes.