Determine whether the current is direct or alternating. Alternating current and direct current: the difference
Electric current is the transfer of charge or the movement of charged particles between points with different electrical potentials. Electrical charge can be carried by ions, protons and/or electrons. IN Everyday life Almost everywhere, the movement of electrons through conductors is used. There are usually two types of electricity - alternating and direct. It's important to know what D.C. different from variable.
Direct and alternating current
Any phenomenon that cannot be seen or “touched” directly is easier to understand using analogies. In the case of electricity, we can consider water in a pipe as the closest example. Water and electricity flow through their conductors - wires and pipes.
- The volume of flowing water is the current strength.
- The pressure in the pipe is tension.
- Pipe diameter is conductivity, the reciprocal of resistance.
- Volume per pressure - power.
The pressure in the pipe is created by the pump - the pump pumps harder, the pressure is higher, more water flows. The diameter of the pipe is larger - the resistance is less, more water flows. The source produces more voltage - more electricity flows. Wires are thicker - less resistance, higher current.
For example, you can take any chemical source power - battery or accumulator. Its terminals have pole designations: plus or minus. If you connect a corresponding light bulb to the battery, through the wires and the switch, it will light up. What happens? The negative terminal of the source emits electrons - elementary particles carrying a negative charge. Along the wires, through the switch connectors and the lamp spiral, they move towards the positive terminal, trying to equalize the potential of the terminals. As long as the circuit is closed across the switch connectors and the battery is not dead, electrons flow in a spiral and the light bulb is on.
The direction of movement of charges remains unchanged all the time - from minus to plus. This is direct current, it can be pulsating - weaken or increase.
For many reasons application only DC voltage inappropriate: Take, for example, the inability to use transformers. Therefore, by now a system of supply and consumption has developed AC voltage nutrition, for which they are created Appliances.
There is a simple answer to what is the difference between direct and alternating current. In this light bulb example, the voltage on one terminal of the power supply will always be zero. This is the neutral wire, but on the other - the phase wire - the voltage changes. And not only in size, but also in direction - from plus to minus. Electrons do not flow in orderly rows in one direction, on the contrary, they rush back and forth, the same particles run back and forth along the incandescent spiral and do all the work. Changing the direction of electricity and gives the very concept of “variable”.
Advanced network settings
In addition to voltage, force, power and resistance/conductivity, two new features appear that describe processes. These parameters are required, just like the first four. When any of them changes, the properties of the entire chain change.
- Form.
- Frequency.
The type of voltage change graph plays a big role. Ideally, it has the form of a sinusoid with smooth transitions from value to value. Deviations from the sinusoidal shape can lead to poor energy quality.
Frequency is the number of transitions from one extreme state to another per certain time. The European standard of 50 Hz (hertz) means that the voltage changes plus and minus 50 times per second, and the electrons change direction a hundred times. For reference: doubling the frequency leads to a fourfold reduction in device dimensions.
If the socket has an alternating current of 50 Hz and 220 V (volts), then this means that the maximum supply voltage in the network reaches 380 V. Where does this come from? IN permanent network The voltage value is constant, but when there is a change, it either falls or rises. These 220 V are the value of the effective voltage of a sinusoidal current with an amplitude of 380 V. That is why the form of change in values is so important, because if it differs greatly from a sinusoid, the effective voltage.
Practical significance of the differences
This is what it is, alternating and direct current. It's not that difficult to figure out what the difference is. There is a difference and a very big one. A DC source will not allow you to connect a welding, or any other, transformer. When calculating insulation or capacitors, the maximum voltage value, rather than the effective voltage, is taken for breakdown. After all, the thought may certainly arise: “why do you need 400-volt capacitors in a 220-volt network?” Here is the answer, in a 220 V network the voltage reaches 380 V at normal operation, and in the event of a minor failure, 400 V is not the limit.
Another "paradox". A capacitor has infinite resistance in a DC network, and conductivity in an AC network; the higher the frequency, the lower the resistance of the capacitor. With coils it’s different - an increase in frequency causes an increase inductive reactance. This property is used in oscillatory circuit- the basis of all communication.
In this article we will tell you what alternating electric current and three-phase alternating current are.
Concept of variable electric current given in the general education physics textbook educational institution- schools. - a current in the form of a harmonic sinusoidal signal, the main characteristics of which are the effective voltage and frequency, changes in direction and magnitude over time.
Frequency– this is the quantity complete changes polarity of alternating electric current in one second. This means that the current in a regular household outlet with a frequency of 50 Hertz changes its direction from positive to negative and back exactly fifty times in one second. One complete change in the direction (polarity) of an electric current from positive to negative and back to positive is called - period of electric current oscillation. During the period T alternating electric current changes its direction twice.
For visual observation sinusoidal alternating current usually use . To prevent electric shock and protect the oscilloscope from mains voltage at the input, isolation transformers are used. To measure a period, it makes no difference which equivalent (equal amplitude) points to measure it at. You can use the maximum positive or negative vertices, or you can use the zero value. This is explained in the figure.
From a physics textbook we know that alternating electric current is generated using electric machine– generator. The simplest model The generator is a magnetic frame rotating in the magnetic field of a permanent magnet.
Let's imagine a rectangular wire frame with several turns, uniformly rotating in a uniform magnetic field. The emf arising in this frame. induction changes according to a sinusoidal law. Oscillation period T alternating electric current is one full revolution of the magnetic frame around its axis.
magnetic frame One of important characteristics electric current are two values of alternating electric current - the maximum value and the average value.
Maximum value of electric current voltage Umax is the voltage value corresponding to the maximum value of the sinusoid.
Average value of electric current voltage Usr is a voltage value equal to 0.636 of the maximum. Mathematically it looks like this:
U av = 2 * U max / π = 0.636 U max
Sine wave maximum voltage can be monitored on the oscilloscope screen. Understand what it is average value of variable electrical voltage You can conduct an experiment according to the figure and description below.
Using an oscilloscope, connect a sinusoidal voltage to its input. Use the vertical sweep offset knob to move the sweep “zero” to the lowest line of the oscilloscope screen scale. Stretch and shift the horizontal scan so that one half-wave of sinusoidal voltage fits into ten (five) cells of the oscilloscope screen. Using the vertical scan (gain) knob, stretch the scan so that the maximum amplitude of the half-wave fits exactly ten (five) cells on the oscilloscope screen. Determine the amplitude of the sinusoid in ten sections. Sum up all ten values and divide by ten - find his “average score”. As a result, you will get a voltage value approximately equal to 6.36 of its maximum value — 10.
Measuring instruments– voltmeters, meters, multimeters for measuring alternating voltage have a rectifier and a smoothing capacitor in their circuit. This chain “rounds” the multiplier of the difference between the maximum and measured voltage to 0.7. Therefore, if you observe a voltage sinusoid with an amplitude of 10 volts on the oscilloscope screen, then the voltmeter (tseshka, multimeter) will show not 10, but about 7 volts. Do you think that your home outlet has 220 volts? It is true, but not entirely true! 220 volts is the average voltage value of a household outlet, averaged by a measuring device - a voltmeter. The maximum voltage follows from the formula:
U max = U meas / 0.7 = 220 / 0.7 = 314.3 volts
That is why, when you are “shocked” by a current from a 220-volt electrical outlet, know that this is your illusion. In fact, you are shaking at about 315 volts.
Three phase current
Along with simple sinusoidal alternating current, the so-called three phase alternating current. Moreover, three-phase electric current is the main type of energy used throughout the world. Three-phase current has gained popularity due to the less costly transfer of energy to long distances. If ordinary (single-phase) electric current requires two wires, then three-phase current, which has three times more energy, requires only three wires. Physical meaning You will find out later in this article.
Imagine if not one, but three identical frames rotate around a common axis, the planes of which are rotated relative to each other by 120 degrees. Then the sinusoidal emfs arising in them. will also be out of phase by 120 degrees (see figure).
Such three coordinated alternating currents are called three-phase current. A simplified arrangement of wire windings in a three-phase current generator is illustrated in the figure.
The connection of the generator windings along three independent lines is shown in the figure below.
This connection with six wires is quite cumbersome. Since for phenomena in electrical circuits Only potential differences are important, then one conductor can be used for two phases at once, without reducing the load capacity for each phase. In other words, in the case of connecting the generator windings in a “star” configuration using a “zero”, energy is transferred from three sources through four wires (see figure), in which one is common - the neutral wire.
Three wires can transmit energy from three (virtually independent) sources of electric current connected by a “triangle” at once.
In industrial generators and converter transformers, a phase-to-phase voltage of 220 volts is usually connected using a delta connection. In this case, there is no “neutral” wire.
"Star" is used to transmit network voltage using "zero". In this case, a voltage of 220 volts is applied in the phase relative to “zero”. The phase-to-phase voltage is 380 volts.
A common occurrence during the times of “brazenly stealing democracy” was combustion household equipment in the apartments of respectable citizens, when the common “zero” burned out due to weak wiring, then, depending on how many household appliances were turned on in the apartments, the TVs and refrigerators of the one who turned them on the least burned. This is caused by the phenomenon of “phase imbalance”, which occurs when the zero is broken. Instead of 220 volts, an interphase voltage of 380 volts rushed into the socket of respectable citizens. Until now, in many communal apartments and buildings resembling our housing Russian cities and this phenomenon has not been completely eradicated.
Alternating current , in contrast to , continuously changes both in magnitude and direction, and these changes occur periodically, that is, they are exactly repeated at equal intervals of time.
To induce such a current in a circuit, they use alternating current sources that create an alternating emf that periodically changes in magnitude and direction. Such sources are called alternating current generators.
In Fig. Figure 1 shows a diagram of the device (model) of the simplest.
Rectangular frame made of copper wire, mounted on an axle and rotates in the field using a belt drive. The ends of the frame are soldered to copper contact rings, which, rotating with the frame, slide along the contact plates (brushes).
Figure 1. Diagram of a simple alternator
Let's make sure that such a device really is source of variable EMF.
Let us assume that a magnet creates between its poles, i.e., one in which the density of magnetic lines of force in any part of the field is the same. rotating, the frame crosses the lines of force magnetic field, and in each of its sides a and b.
Sides c and d of the frame are non-working, since when the frame rotates they do not intersect the magnetic field lines and, therefore, do not participate in the creation of the EMF.
At any moment of time, the EMF arising in side a is opposite in direction to the EMF arising in side b, but in the frame both EMFs act in accordance and in total constitute the total EMF, i.e., induced by the entire frame.
This is easy to verify if we use what we know to determine the direction of the EMF rule right hand .
To do this, you need to position the palm of your right hand so that it faces the north pole of the magnet, and the bent thumb coincides with the direction of movement of that side of the frame in which we want to determine the direction of the EMF. Then the direction of the EMF in it will be indicated by the outstretched fingers of the hand.
For whatever position of the frame we determine the direction of the EMF in sides a and b, they always add up and form a total EMF in the frame. In this case, with each revolution of the frame, the direction of the total EMF in it changes to the opposite, since each of the working sides of the frame passes under different poles of the magnet in one revolution.
The magnitude of the EMF induced in the frame also changes, as the speed at which the sides of the frame intersect the magnetic field lines changes. Indeed, at the time when the frame approaches its vertical position and passes it, the speed of intersection of the force lines by the sides of the frame is greatest, and the largest EMF is induced in the frame. At those moments in time when the frame passes its horizontal position, its sides seem to slide along the magnetic lines of force without crossing them, and no emf is induced.
Thus, with uniform rotation of the frame, an EMF will be induced in it, periodically changing both in magnitude and direction.
The EMF arising in the frame can be measured with a device and used to create a current in an external circuit.
Using , you can get an alternating emf and, therefore, an alternating current.
Alternating current is for industrial purposes and is produced by powerful generators driven by steam or water turbines and internal combustion engines.
Graphic representation of direct and alternating currents
The graphical method makes it possible to visualize the process of changing a particular variable depending on time.
Graphing variables, changing over time, begin by constructing two mutually perpendicular lines, called the axes of the graph. Then, segments of time are plotted on the horizontal axis on a certain scale, and on the vertical axis, also on a certain scale, the values of the quantity whose graph is going to be plotted (EMF, voltage or current).
In Fig. 2 are graphically depicted direct and alternating currents. IN in this case we plot the current values, and vertically up from the point of intersection of the O axes we plot the current values of one direction, which is usually called positive, and down from this point - opposite direction, which is usually called negative.
Point O itself serves simultaneously as the beginning of the countdown of current values (vertically down and up) and time (horizontally to the right). In other words, this point corresponds to the zero value of the current and the initial moment in time from which we intend to trace how the current will change in the future.
Let us verify the correctness of what is constructed in Fig. 2, and a graph of a constant current of 50 mA.
Since this current is constant, i.e., does not change its magnitude and direction over time, the same current values, i.e., 50 mA, will correspond to different moments in time. Therefore, at a time equal to zero, i.e. at the initial moment of our observation of the current, it will be equal to 50 mA. By plotting upward on the vertical axis a segment equal to the current value of 50 mA, we get the first point of our graph.
We must do the same for the next moment in time, corresponding to point 1 on the time axis, i.e., set aside a segment vertically upward from this point, also equal to 50 mA. The end of the segment will determine the second point of the graph.
Having carried out a similar construction for several subsequent moments in time, we will obtain a series of points, the connection of which will give a straight line, which is graphic image direct current value 50 mA.
Let's now move on to studying variable emf graph. In Fig. 3 at the top shows a frame rotating in a magnetic field, and at the bottom is a graphical representation of the emerging EMF variable.
Figure 3. Plotting a graph of the variable EMF
Let's begin to uniformly rotate the frame clockwise and follow the progress of the change in the EMF in it, taking the horizontal position of the frame as the initial moment.
At this initial moment, the EMF will be zero, since the sides of the frame do not intersect the magnetic lines of force. On the graph, this zero EMF value corresponding to the moment t = 0 will be represented by point 1.
With further rotation of the frame, an emf will begin to appear in it and will increase in value until the frame reaches its vertical position. On the graph, this increase in EMF will be depicted as a smooth upward curve that reaches its peak (point 2).
As the frame approaches horizontal position The emf in it will decrease and drop to zero. On the graph this will be depicted as a descending smooth curve.
Consequently, during the time corresponding to half a revolution of the frame, the EMF in it managed to increase from zero to its maximum value and again decrease to zero (point 3).
With further rotation of the frame, an emf will again arise in it and will gradually increase in value, but its direction will already change to the opposite, which can be verified by applying the right-hand rule.
The graph takes into account the change in the direction of the EMF in that the curve depicting the EMF intersects the time axis and is now located below this axis. The EMF increases again until the frame takes a vertical position.
Then the EMF will begin to decrease, and its value will become equal to zero when the frame returns to its original position, having completed one full revolution. On the graph this will be expressed by the fact that the EMF curve, having reached its peak in the opposite direction (point 4), then meets the time axis (point 5)
This ends one cycle of changing the EMF, but if we continue to rotate the frame, a second cycle immediately begins, exactly repeating the first, which, in turn, will be followed by a third, and then a fourth, and so on until we stop the rotation framework.
Thus, for each revolution of the frame, the EMF arising in it makes full cycle of your change.
If the frame is closed to any external circuit, then an alternating current will flow through the circuit, the graph of which will be the same in appearance as the EMF graph.
The wave-like curve we obtained is called a sine wave, and the current, emf or voltage that changes according to this law is called sinusoidal.
The curve itself is called a sine wave because it is a graphical representation of a variable trigonometric quantity called sine.
The sinusoidal nature of current change is the most common in electrical engineering, therefore, when speaking about alternating current, in most cases we mean sinusoidal current.
To compare different alternating currents(EMF and voltage) there are quantities that characterize a particular current. They're called AC parameters.
Period, amplitude and frequency - parameters of alternating current
Alternating current is characterized by two parameters - period and amplitude, knowing which we can judge what kind of alternating current it is and build a current graph.
The period of time during which a complete cycle of current change occurs is called a period. The period is designated by the letter T and is measured in seconds.
The period of time during which half of the complete cycle of current change occurs is called a half-cycle. Consequently, the period of change of current (EMF or voltage) consists of two half-cycles. It is quite obvious that all periods of the same alternating current are equal to each other.
As can be seen from the graph, during one period of its change the current reaches twice its maximum value.
The maximum value of an alternating current (emf or voltage) is called its amplitude or amplitude current value.
Im, Em and Um are generally accepted designations for the amplitudes of current, EMF and voltage.
We first of all paid attention to , however, as can be seen from the graph, there are countless intermediate values that are smaller than the amplitude.
The value of alternating current (EMF, voltage) corresponding to any selected moment in time is called its instantaneous value.
i, e and u are generally accepted designations for instantaneous values of current, emf and voltage.
The instantaneous current value, as well as its amplitude value, can be easily determined using a graph. To do this, from any point on the horizontal axis corresponding to the moment of time we are interested in, we draw a vertical line to the point of intersection with the current curve; the resulting segment of the vertical straight line will determine the value of the current in this moment, i.e. its instantaneous value.
It is obvious that the instantaneous value of the current after time T/2 from the starting point of the graph will be equal to zero, and after time T/4 its amplitude value. The current also reaches its amplitude value; but in the opposite direction, after a time equal to 3/4 T.
So, the graph shows how the current in the circuit changes over time, and that each moment in time corresponds to only one specific value of both the magnitude and direction of the current. In this case, the value of the current at a given moment in time at one point in the circuit will be exactly the same at any other point in this circuit.
The number of complete periods completed by a current in 1 second is called AC frequency and is designated Latin letter f.
To determine the frequency of alternating current, i.e. find out how many periods of change does the current complete within 1 second?, it is necessary to divide 1 second by the time of one period f = 1/T. Knowing the frequency of the alternating current, you can determine the period: T = 1/f
It is measured in a unit called the hertz.
If we have alternating current, the frequency of which is equal to 1 hertz, then the period of such current will be equal to 1 second. And, conversely, if the period of current change is 1 second, then the frequency of such current is 1 hertz.
So we have defined AC parameters - period, amplitude and frequency, - which make it possible to distinguish different alternating currents, emfs and voltages from each other and to construct their graphs when necessary.
When determining the resistance of various circuits to alternating current, use another auxiliary quantity characterizing alternating current, the so-called corner or circular frequency .
Circular frequency denoted related to frequency f by the relation 2пif
Let us explain this dependence. When constructing a graph of the variable EMF, we saw that during one full revolution of the frame, a complete cycle of EMF changes occurs. In other words, in order for the frame to make one revolution, i.e., turn 360°, it takes time equal to one period, i.e. T seconds. Then in 1 second the frame makes a 360°/T revolution. Therefore, 360°/T is the angle through which the frame rotates in 1 second, and expresses the speed of rotation of the frame, which is commonly called angular or circular speed.
But since the period T is related to the frequency f by the ratio f = 1/T, the circular speed can be expressed in terms of frequency and will be equal to 360°f.
So we came to the conclusion that 360°f. However, for the convenience of using the circular frequency in all kinds of calculations, the angle of 360° corresponding to one revolution is replaced by a radial expression equal to 2pi radians, where pi = 3.14. Thus, we finally get 2pif. Therefore, to determine the circular frequency of alternating current (), it is necessary to multiply the frequency in hertz by constant The number is 6.28.
Today, if you look around, almost everything you see is powered by electricity in one form or another.
Alternating current and direct current are the two main forms of charge that power our electrical and electronic world.
What is AC? Alternating current can be defined as a stream electric charge, which changes its direction at regular intervals.
The period/regular intervals at which AC changes its direction is its frequency (Hz). Marine vehicles, spacecraft, and military equipment sometimes use 400 Hz AC. However, for most of the time, including indoor use, the AC frequency is set to 50 or 60 Hz.
What is DC? (Symbol on electrical appliances) D.C is a current (flow of electric charge or electrons) that flows in only one direction. Subsequently, there is no frequency associated with DC. DC or direct current has zero frequency.
AC and DC power sources:
AS: Power plants and alternating current generators produce alternating current.
DC: Solar panels, fuel cells, and thermocouples are the main sources for DC production. But the main source of DC current is AC conversion.
Application of AC and DC current:
AC is used to power refrigerators, home fireplaces, fans, electric motors, air conditioners, televisions, food processors, washing machines, and almost all industrial equipment.
DC is mainly used to power electronics and other digital technology. Smartphones, tablets, electric cars, etc. LED and LCD TVs also operate on DC, which is converted from a regular alternating current network.
Why AC is used to transmit electricity. It is cheaper and easier to produce. AC at high voltage can be transported hundreds of kilometers without much power loss. Power plants and transformers reduce the voltage to (110 or 230 V) to transmit it to our homes.
Which is more dangerous? AC or DC?
DC is believed to be less dangerous than AC, but there is no definitive proof. There is a misconception that contact with high voltage AC is more dangerous than contact with low voltage DC. Actually, it's not about tension, we're talking about about the amount of current passing through the human body. Direct and alternating current can be fatal. Do not insert fingers or objects into outlets or gadgets and high power equipment.
Direct electric current is the movement of particles with a charge in a certain direction. That is, its tension or force (characterizing quantities) have the same value and direction. This is how direct current differs from alternating current. But let's consider everything in order.
The history of the appearance and “war of currents”
Direct current used to be called galvanic current due to the fact that it was discovered as a result of a galvanic reaction. I tried to transmit it through electrical transmission lines. At that time, there were serious disputes between scientists on this issue. They even received the name “war of currents.” The question of choice as the main one, variable or permanent, was being decided. The “fight” was won by the variable species, since the constant one incurs significant losses, transmitted over a distance. But transform variable view is not difficult, this is how direct current differs from alternating current. Therefore, the latter is easy to transmit even over long distances.
Sources of direct electric current
The sources can be batteries or other devices where it occurs through a chemical reaction.
These are generators, where it is obtained as a result and then rectified by the collector.
Application
IN various devices Direct current is used quite often. For example, many household appliances work with it, charging device and car generators. Any portable device is powered from a source that produces permanent view.
On an industrial scale, it is used in engines and batteries. And in some countries they are equipped with high-voltage power lines.
In medicine, healing procedures are carried out using direct electric current.
On railway(for transport) both variable and constant types are used.
Alternating current
Most often, however, this is what is used. Here is the average value of force and stress for certain period are equal to zero. It constantly changes in size and direction, and at regular intervals.
To generate alternating current, generators are used in which electromagnetic induction occurs. This is done using a magnet rotated in a cylinder (rotor) and a stator made in the form of a stationary core with a winding.
Alternating current is used in radio, television, telephony and many other systems due to the fact that its voltage and power can be converted with almost no loss of energy.
It is widely used in industry, as well as for lighting purposes.
It can be single-phase or multi-phase.
Which varies according to a sinusoidal law, is single-phase. It changes over a certain period of time (period) in magnitude and direction. AC frequency is the number of cycles per second.
In the second case, the three-phase version is most widespread. This is a system of three electrical circuits that have the same frequency and emf, shifted in phase by 120 degrees. It is used to power electric motors, furnaces, and lighting fixtures.
Many developments in the field of electricity and their practical applications, as well as the impact on alternating current high frequency humanity owes its debt to the great scientist Nikola Tesla. Until now, not all of his works left to descendants are known.
How does direct current differ from alternating current and what is its path from source to consumer?
So, a variable is a current that can change in direction and magnitude over a certain time. The parameters that are paid attention to are frequency and voltage. In Russia in household electrical networks supply alternating current having a voltage of 220 V and a frequency of 50 Hz. The frequency of alternating current is the number of times the direction of particles of a certain charge changes per second. It turns out that at 50 Hz it changes its direction fifty times, in which direct current differs from alternating current.
Its source is the sockets to which household appliances under different voltages are connected.
Alternating current begins its movement from power stations, where available powerful generators, from where it comes out with a voltage of 220 to 330 kV. Then it goes to which are located near houses, businesses and other structures.
The current entering the substation is 10 kV. There it is converted into a three-phase voltage of 380 V. Sometimes with this indicator, the current passes directly to the facilities (where powerful production is organized). But basically it is reduced to the usual 220 V in all houses.
Conversion
It is clear that in sockets we receive alternating current. But often for electrical appliances a permanent look is required. Special rectifiers are used for this purpose. The process consists of the following steps:
- connecting a bridge with four diodes having the required power;
- connecting a filter or capacitor to the bridge output;
- connecting voltage stabilizers to reduce ripple.
Conversion can occur either from alternating current to direct current or vice versa. But the latter case will be much more difficult to implement. You will need inverters, which, among other things, are not cheap.