Current flows through the capacitor. Entertaining radio technology

A capacitor in an alternating current or direct current circuit, which is often simply called a capacitor, consists of a pair of plates covered with a layer of insulation. If current is supplied to this device, it will receive a charge and retain it for some time. Its capacity largely depends on the gap between the plates.

The capacitor can be made in different ways, but the essence of the work and its main elements remain unchanged in any case. To understand the principle of operation, it is necessary to consider its simplest model.

The simplest device has two plates: one of them is positively charged, the other, on the contrary, negatively. Although these charges are opposite, they are equal. They attract with a certain force, which depends on the distance. The closer the plates are to each other, the greater the force of attraction between them. Thanks to this attraction, the charged device does not discharge.

However, it is enough to lay any conductor between the two plates and the device will instantly discharge. All electrons from the negatively charged plate will immediately transfer to the positively charged one, resulting in equalization of the charge. In other words, to remove the charge from the capacitor, you only need to short-circuit its two plates.

Electrical circuits are of two types - permanent or variables. It all depends on how the electric current flows in them. Devices on these circuits behave differently.

To consider how a capacitor will behave in a circuit direct current, need to:

1. Take the power supply DC voltage and determine the voltage value. For example, "12 Volts".
2. Install a light bulb rated for the same voltage.
3. Install a capacitor in the network.

There will be no effect: the light bulb will not light up, but if you remove the capacitor from the circuit, the light will appear. If the device is connected to an alternating current network, it simply will not close, and therefore no electric current will be able to pass here. Permanent - not able to pass through the network in which the capacitor is connected. It's all because of the plates of this device, or rather, the dielectric that separates these plates.

You can make sure that there is no voltage in the direct current network in other ways. You can connect anything to the network, the main thing is that a source of constant electric current is included in the circuit. The element that will signal the absence of voltage in the network or, conversely, its presence, can also be any electrical appliance. It is best to use a light bulb for these purposes: it will glow if there is electric current, and will not light if there is no voltage in the network.

We can conclude that the capacitor is not capable of conducting direct current through itself, but this conclusion is incorrect. In fact, an electric current appears immediately after applying voltage, but disappears instantly. In this case, it passes within only a few fractions of a second. The exact duration depends on how capacious the device is, but this is usually not taken into account.

To determine whether alternating current will flow, the device must be connected to the appropriate circuit. The main source of electricity in this case should be a device that generates alternating current.

Constant electricity does not flow through the capacitor, but the alternating current, on the contrary, flows, and the device constantly resists the electric current passing through it. The magnitude of this resistance is related to the frequency. The dependence here is inversely proportional: the lower the frequency, the higher the resistance. If to alternating current source connect the condenser, then highest value The voltage here will depend on the current strength.

A simple circuit consisting of:

• Current source. It must be variable.
• Electric current consumer. It is best to use a lamp.

However, it is worth remembering one thing: the lamp will light up only if the device has a fairly large capacity. The alternating current has such an effect on the capacitor that the device begins to charge and discharge. And the current that passes through the network during recharging increases the temperature of the lamp filament. As a result, it glows.

The recharging current largely depends on the capacity of the device connected to the AC network. The dependence is directly proportional: the greater the capacity, the greater the value characterizing the strength of the recharging current. To verify this, you just need to increase the capacity. Immediately after this, the lamp will begin to glow brighter, since its filaments will be more heated. As you can see, a capacitor, which acts as one of the elements of an alternating current circuit, behaves differently than a constant resistor.

When an AC capacitor is connected, more complex processes begin to occur. A tool such as a vector will help you understand them better. The main idea of ​​the vector in this case will be that you can represent the value of a time-varying signal as the product of a complex signal, which is a function of the axis representing time and a complex number, which, on the contrary, is not related to time.

Since vectors are represented by a certain magnitude and a certain angle, they can be drawn in the form of an arrow that rotates in the coordinate plane. The voltage on the device lags slightly behind the current, and both vectors by which they are designated rotate counterclockwise on the plane.

A capacitor in an alternating current network can be periodically recharged: it either acquires some charge, or, on the contrary, releases it. This means that the conductor and the alternating current source in the network constantly exchange electrical energy with each other. This type of electricity in electrical engineering is called reactive.

The capacitor does not allow direct electric current to pass through the network. In this case, it will have a resistance equal to infinity. Alternating current is capable of passing through this device. In this case, the resistance has a finite value.

>> Capacitor in AC circuit

§ 33 CAPACITOR IN AC CIRCUIT

Direct current cannot flow through a circuit containing a capacitor. Indeed, in fact, in this case the circuit turns out to be open, since the capacitor plates are separated by a dielectric.

Alternating current can flow through a circuit containing a capacitor. This can be verified through simple experiment.

Let us have sources of direct and alternating voltages, and the constant voltage at the source terminals is equal to the effective value AC voltage. The circuit consists of a capacitor and an incandescent lamp (Fig. 4.13), connected in series. When the direct voltage is turned on (the switch is turned to the left, the circuit is connected to points AA"), the lamp does not light up. But when the alternating voltage is turned on (the switch is turned to the right, the circuit is connected to points BB"), the lamp lights up if the capacitance of the capacitor is large enough.

How can alternating current flow through the circuit if it is actually open (charges cannot move between the plates of the capacitor)? The thing is that the capacitor is periodically charged and discharged under the influence of alternating voltage. The current flowing in the circuit when the capacitor is recharged heats the lamp filament.

Let us establish how the current strength changes over time in a circuit containing only a capacitor, if the resistance of the wires and plates of the capacitor can be neglected (Fig. 4.14).

Capacitor voltage

The current strength, which is the time derivative of the charge, is equal to:

Consequently, current fluctuations are ahead in phase of voltage fluctuations across the capacitor in (Fig. 4.15).

The amplitude of the current is:

I m = U m C. (4.29)

If you enter the designation

and instead of the amplitudes of current and voltage using their effective values, we get

The value X c, the reciprocal of the product C of the cyclic frequency and the electrical capacitance of the capacitor, is called capacitance. The role of this quantity is similar to the role of active resistance R in Ohm’s law (see formula (4.17)). The effective value of the current is related to the effective value of the voltage on the capacitor in the same way as the current and voltage are related according to Ohm's law for a section of a DC circuit. This allows us to consider the value of X c as the resistance of the capacitor to alternating current (capacitance).

The larger the capacitor capacity, the more current recharge. This is easy to detect by the increase in the lamp's incandescence as the capacitor's capacitance increases. While a capacitor's resistance to direct current is infinite, its resistance to alternating current has a finite value X c . As capacity increases, it decreases. It also decreases with increasing frequency.

In conclusion, we note that during the quarter period when the capacitor is charged to maximum voltage, energy enters the circuit and is stored in the capacitor in the form of electric field energy. In the next quarter of the period, when the capacitor is discharged, this energy is returned to the network.

The resistance of a circuit with a capacitor is inversely proportional to the product of the cyclic frequency and the electrical capacitance. Current fluctuations are ahead of voltage fluctuations in phase by .

1. How are the effective values ​​of current and voltage on a capacitor in an alternating current circuit related to each other?
2. Is energy released in a circuit containing only a capacitor if active resistance chains can be neglected!
3. The circuit breaker is a kind of capacitor. Why does the switch reliably open the circuit!

Myakishev G. Ya., Physics. 11th grade: educational. for general education institutions: basic and profile. levels / G. Ya. Myakishev, B. V. Bukhovtsev, V. M. Charugin; edited by V. I. Nikolaeva, N. A. Parfentieva. - 17th ed., revised. and additional - M.: Education, 2008. - 399 p.: ill.

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This can be easily confirmed by experiments. You can light a light bulb by connecting it to an AC power supply through a capacitor. The loudspeaker or handsets will continue to work if they are connected to the receiver not directly, but through a capacitor.

A capacitor consists of two or more metal plates separated by a dielectric. This dielectric is most often mica, air or ceramics, which are the best insulators. It is quite natural that direct current cannot pass through such an insulator. But why does alternating current pass through it? This seems all the more strange since the same ceramics in the form of, for example, porcelain rollers perfectly insulate alternating current wires, and mica perfectly functions as an insulator in electric irons and other heating devices that operate properly on alternating current.

Through some experiments we could “prove” an even stranger fact: if in a capacitor a dielectric with comparatively poor insulating properties is replaced by another dielectric that is a better insulator, then the properties of the capacitor will change so that the passage of alternating current through the capacitor will not be hindered, but rather on the contrary, it is facilitated. For example, if you connect a light bulb to an alternating current circuit through a capacitor with a paper dielectric and then replace the paper with such an excellent insulator; like glass or porcelain of the same thickness, the light bulb will begin to burn brighter. Such an experiment will lead to the conclusion that alternating current not only passes through the capacitor, but that it also passes the more easily the better the insulator its dielectric is.

However, despite all the apparent convincingness of such experiments, electric current - neither direct nor alternating - does not pass through the capacitor. The dielectric separating the plates of the capacitor serves as a reliable barrier to the path of current, whatever it may be - alternating or direct. But this does not mean that there will be no current in the entire circuit in which the capacitor is connected.

A capacitor has a certain physical property that we call capacitance. This property consists of the ability to accumulate electrical charges on the plates. A source of electric current can be roughly likened to a pump that pumps electrical charges into a circuit. If the current is constant, then electrical charges are pumped all the time in one direction.

How will a capacitor behave in a DC circuit?

Our “electric pump” will pump charges onto one of its plates and pump them out from the other plate. The ability of a capacitor to hold on its plates (plates) a certain difference number of charges and is called its capacity. The larger the capacitance of the capacitor, the more electrical charges can be on one plate compared to the other.

At the moment the current is turned on, the capacitor is not charged - the number of charges on its plates is the same. But the current is on. The “electric pump” started working. He drove the charges onto one plate and began pumping them out from the other. Once the movement of charges begins in the circuit, it means that current begins to flow in it. Current will flow until the capacitor is fully charged. Once this limit is reached, the current will stop.

Therefore, if there is a capacitor in a DC circuit, then after its closure, the current will flow in it for as long as necessary for full charge capacitor.

If the resistance of the circuit through which the capacitor is charged is relatively small, then the charging time is very short: it lasts an insignificant fraction of a second, after which the current flow stops.

The situation is different in the alternating current circuit. In this circuit, the “pump” pumps electrical charges in one direction or the other. Having barely created an excess of charges on one plate of the capacitor compared to the number on the other plate, the pump begins to pump them in the opposite direction. Charges will circulate continuously in the circuit, which means that, despite the presence of a non-conducting capacitor, there will be a current in it - the charge and discharge current of the capacitor.

What will the magnitude of this current depend on?

By current magnitude we mean the number of electrical charges flowing per unit time through the cross section of a conductor. The greater the capacitance of the capacitor, the more charges will be required to “fill” it, which means the stronger the current in the circuit will be. The capacitance of a capacitor depends on the size of the plates, the distance between them and the type of dielectric separating them, its dielectric constant. Porcelain has a greater dielectric constant than paper, so when replacing paper with porcelain in a capacitor, the current in the circuit increases, although porcelain is a better insulator than paper.

The magnitude of the current also depends on its frequency. The higher the frequency, the greater the current will be. It is easy to understand why this happens by imagining that we fill a container with a capacity of, for example, 1 liter with water through a tube and then pump it out from there. If this process is repeated once per second, then 2 liters of water will flow through the tube per second: 1 liter in one direction and 1 liter in the other. But if we double the frequency of the process: we fill and empty the vessel 2 times per second, then one tube per second it will already pass 4 liters of water - an increase in the frequency of the process with a constant container capacity led to a corresponding increase in the amount of water flowing through the tube.

From all that has been said, the following conclusions can be drawn: electric current - neither direct nor alternating - does not pass through the capacitor. But in the circuit connecting the AC source to the capacitor, the charge and discharge current of this capacitor flows. The larger the capacitance of the capacitor and the higher the frequency of the current, the stronger this current will be.

This feature of alternating current is extremely widely used in radio engineering. The emission of radio waves is also based on it. To do this, we excite a high-frequency alternating current in the transmitting antenna. But why does current flow in the antenna, since it is not a closed circuit? It flows because there is capacitance between the antenna and counterweight wires or ground. The current in the antenna represents the charge and discharge current of this capacitor, this capacitor.

Connected to a resistor, the current and voltage in the circuit at any point in the timing diagram will be proportional to each other. This means that the current and voltage waveforms will reach their "peak" value at the same time. In this case, we say that the current and voltage are in phase.

Let us now consider how a capacitor will behave in an alternating current circuit.

If a capacitor is connected to an AC voltage source, the maximum voltage across it will be proportional to the maximum current flowing in the circuit. However, the peak of the voltage sine wave will not occur at the same time as the peak of the current.

In this example, the instantaneous value of the current reaches its maximum value a quarter of a period (90 electric degrees) earlier than the voltage does. In this case, they say that “the current leads the voltage by 90◦.”

Unlike the situation in a DC circuit, the V/I value here is not constant. However, the V ratio is a very useful quantity and in electrical engineering is called the capacitance reactance (Xc) of a component. Since this value still represents the ratio of voltage to current, i.e. V physical sense is resistance, its unit is ohm. The value of Xc of a capacitor depends on its capacitance (C) and the frequency of alternating current (f).

Since an RMS voltage is applied to a capacitor in an AC circuit, the same AC current flows in that circuit, which is limited by the capacitor. This limitation is caused by the capacitor.

Therefore, the value of the current in a circuit containing no other components other than a capacitor is given by alternative version Ohm's Law

I RMS = U RMS / X C

Where U RMS is the root mean square (rms) voltage value. Note that Xc replaces the value of R in the version of Ohm's law for

Now we see that a capacitor in an alternating current circuit behaves completely differently than a constant resistor, and the situation here is, accordingly, more complicated. In order to better understand the processes occurring in such a chain, it is useful to introduce such a concept as a vector.

The basic idea of ​​a vector is the idea that the complex value of a time-varying signal can be represented as the product (which is independent of time) and some complex signal that is a function of time.

For example, we can represent the function A cos(2πνt + θ) simply as a complex constant A∙e jΘ .

Since vectors are represented by a magnitude (or magnitude) and an angle, they are represented graphically by an arrow (or vector) rotating in the XY plane.

Taking into account the fact that the voltage on the capacitor “lags” in relation to the current, the vectors representing them are located in the complex plane as shown in the figure above. In this figure, the current and voltage vectors rotate in the opposite direction to the clockwise movement.

In our example, the current on the capacitor is due to its periodic recharging. Since a capacitor in an AC circuit has the ability to periodically accumulate and reset electric charge, between it and the power source there is a constant exchange of energy, which in electrical engineering is called reactive.

We continue to study electronics, and next we have an analysis of how a capacitor behaves in an alternating current, direct current circuit, what it is needed for, as well as several examples of practical application.

The capacitor is a passive element electronic circuit, consisting of two conductive plates, which are separated by some kind of dielectric.

Properties and functions performed

The main task of a capacitor is to accumulate a certain amount of electrostatic charge on the plates after connecting it to a live circuit. When the power is turned off, the capacitor retains the resulting charge.

• If the capacitor is connected to a closed circuit, but without power, or the voltage in it is lower than what is accumulated in the capacitor, then a complete or partial discharge of the element will occur, releasing the accumulated energy.

• Let us immediately introduce the concept of capacitance. In simple words, this is the number electrical energy, which an element connected to the network is capable of accumulating. This parameter is designated by the Latin letter “C”, and it is measured in Farads (F).

Interesting to know! AC capacitors large capacity capable of creating very powerful impulses during rapid discharge. They can be used, for example, in powerful photo flashes.

• The capacitance is calculated using the following formula: C=q/U, where q is the charge on one plate in Coulombs (the amount of energy passed through the conductor in 1 second at a current of 1 Ampere); and U – Voltage in Volts between shells.

• The body of any capacitor contains data about its main parameters, including capacity. In the photo above it is highlighted in red, this is the designation. There you can also find out the operating voltage and temperature.
• Everything is simple, but it is worth considering that the indicated capacity is nominal, while its actual value can differ quite greatly, which is influenced by many factors.
• The capacitance of a capacitor can vary from units of picofarads to tens of farads, which depends on the area of ​​the electrode (usually aluminum foil).

Interesting to know! To increase the useful capacity, the foil is rolled into rolls - this is how cylindrical capacitors are obtained.

If the circuit requires a large capacitor capacity, then they are connected in parallel. In this case, the operating voltage is maintained, but the capacitance will increase in direct proportion, that is, it will be the sum of the capacitances of the connected capacitors.

If the capacitors are connected in series, the capacitance will not change; more precisely, it will be slightly less than the minimum capacitance included in the circuit. Why is such a connection needed? With it, the probability of breakdown of one of the capacitors is reduced to a minimum, that is, they seem to distribute the load.

• Capacitors are also characterized by such a parameter as specific capacitance. This is the direct ratio of the capacitance of an electrical part to the mass or volume of the dielectric. Maximum values This parameter can be achieved with the smallest thickness of the dielectric spacer, however, for the breakdown of such a capacitor, a lower voltage is required, which we will talk about now.
• The part marking also indicates the voltage rating. Everything here is extremely simple - this value shows the maximum voltage level in the circuit at which the radio component can work out its entire service life without significantly changing its specified parameters.
• Hence the simple conclusion - the voltage on the capacitor should not exceed the nominal value, otherwise it may break through.
• The rated voltage level is affected by the materials from which the capacitor is assembled.

The concept of polarity for capacitors and their failure

Interesting to know! For many types of capacitors, the permissible voltage will decrease as it heats up, so the maximum operating temperature is also indicated on the product cases.

Failure of capacitors is a very common failure in electrical engineering. They can “die” quietly, simply by swelling, or under the cannonade of a heavy explosion, flooding all nearby parts with electrolyte, under “stage smoke” and other effects.

That is why the failure of this element can be diagnosed purely visually, without the use of test equipment, but not always.

Many electrolytic capacitors (with an oxide dielectric), due to the peculiarities of the interaction between the dielectric and the electrolyte, are capable of operating only if a certain polarity is observed, as indicated by the corresponding marking on the part body.

• When you try to connect them to a circuit in reverse polarity, the capacitors usually immediately fail - the dielectric is destroyed, the electrolyte boils, resulting in the same explosion.
• Capacitors explode quite often, especially in pulse devices. This happens due to overheating, leakage or an increase in equivalent series resistance as the part ages.
• It is no secret that a damaged part in any circuit can be replaced with a new one, and the device will function as before, however, the consequences of an explosion can be quite serious - neighboring elements will be damaged, which will greatly complicate the repair, plus its price will increase.

To reduce the consequences, a valve is installed on the housings of large-capacity capacitors or a notch is made at the end in the form of the letters “X, K, and T.” Such capacitors explode very rarely, due to the fact that either the valve or the housing that has collapsed along the notch releases the electrolyte in the form of caustic fumes, that is, the pressure inside the housing decreases.

Other parameters

In addition to the parameters that we have already discussed, capacitors have inductance and their own resistance, so the circuit of a real capacitor can be represented as follows.

These include (denoted as in the diagram above):

Types of capacitors

Capacitors are classified, first of all, by the type of dielectric used in them, which determines everything electrical parameters element.

• Vacuum capacitors– their structure is such that several coaxial cylinders, which are built into one, are located in an outer glass cylinder. These devices are characterized by the highest power per unit volume.

• Air or gas condensers– there are constant and variable capacities. They are used mainly in electrical measuring equipment, radio receivers and transmitters, as they allow you to configure oscillatory circuits.
• Capacitors with liquid dielectric;

• Capacitors with solid inorganic dielectrics– these include models on glass enamels, glass ceramics, glass films, mica, ceramics, etc. Such capacitors are characterized by a very large capacitance, despite their modest dimensions.

• Capacitors with solid organic dielectrics– here the variety is also great: paper and metal, film and combined.

• Separately, we can distinguish electrolytic and oxide-semiconductor capacitors, since they are distinguished by a large specific capacity. They use an oxide layer around a metal anode as a dielectric. The second plate in it is either an electrolyte, in the first case, or a semiconductor, in the second. The anode, depending on the capacitor, can be made of tantalum, niobium or aluminum foil, as well as sintered powder.

This classification is not the only one and distinguishes between capacitors and, if possible, changing their capacitance:

• Constant capacitors are capacitors whose capacitance is constant throughout their service life, not counting changes associated with the aging of the part.

• Variables - this type is capable of changing its capacity while the equipment is operating. Such capacitors are controlled through mechanics, electrical voltage, and temperature.

• Trimming - the capacitance of these capacitors can also change, but this does not happen while the equipment is operating, but one-time, during installation or configuration. They are used mainly for leveling the initial capacitances of mating circuits, as well as for adjusting the parameters of circuit circuits.

Application of capacitors

Concluding the first part of the article, we cannot help but draw attention to the areas of application of these elements of electrical circuits. And they are used everywhere.

• They are combined with inductors and resistors to produce circuits in which the properties of the current will depend on its frequency, for example, a frequency filter or circuit feedback oscillatory circuit.
• In systems that require the creation of a powerful pulse, which we have already mentioned today - camera flashes, pulsed lasers, Marx generators, etc.
• Capacitors are also used as memory elements, as they are able to retain a charge for quite a long time. The same property is used in devices designed to store energy.
• If we talk about industrial-level electrical engineering, capacitors are used to compensate reactive power and as filters for higher harmonics.

And this is not all areas, but we think that this is enough for now. Let's better move on to experiments and see what happens to the current when it passes through a capacitor.

Capacitor in electric current circuits

So, we roughly understand what a capacitor is, but we haven’t really figured out how this element works yet.

DC circuit

If we talk in simple words, then a capacitor, or “conder”, as it is popularly called, is a small element that, like a battery, is capable of accumulating a certain charge, which it is ready to discharge in a matter of seconds

Interesting to know! Unlike a battery, there is no source of EMF in a capacitor.

In order for the conductor to discharge, it needs to close the contacts directly or through a circuit. It seems that everything is clear, but how does the current flow in the capacitor when it is connected to the network?

• Let's start with direct current and conduct one small experiment. To do this, we need the capacitor itself, a 12-volt DC source and a light bulb with wires, also 12 volts.

• We connect all this together, as shown in the photo above, and we see that nothing happens - the light does not light.

• We change the position of the “crocodile” so as to allow current to bypass the capacitor. And, lo and behold! The light came on! Why is this happening?
• It's simple, just remember that current flows through a capacitor only when it is charging and discharging, and the voltage will always lag behind the current.
• A discharged capacitor is akin to short circuit in a circuit - when it is connected to a voltage source, at the first moment of time there is no voltage in it, but there is a current, which at this moment in time is maximum (that's the lag).
• Current flows through the capacitor, and it begins to accumulate charge, increasing its internal voltage until it is equal to the voltage of the power source and the capacitor fills its entire capacity.
• At this moment in time, the current stops flowing, and since the capacitor cannot discharge, then, accordingly, the light bulb will not light.
• This process can be compared with a water system in the form of a communicating vessel, separated by a valve, with one part empty and the other full. Remove the obstacle, and water will flow into the second vessel until the pressures equalize, that is, the pressure drops to zero.
• What would happen if the capacitor were disconnected from the circuit and shorted? Yes, everything is the same! At the first moment of time, the current will be maximum at a constant voltage. The current will run forward, and the voltage will follow it, until all the charge is gone.
• Again, as an example, we take a water system consisting of a full tank, which will act as a condenser, and a faucet on it through which water can be drained. We open the tap and see that water immediately flows, while the pressure (voltage) will drop smoothly as the container is emptied.

The same patterns are characteristic of sinusoidal current, which we will talk about now.

AC circuit

Let's first carry out some experiment, and then explain it in simple language.

We will need: a capacitor with a capacity of 1 microfarad, a regular 100 Ohm resistor and a frequency generator. We connect it all, as shown in the next photo.

Next, according to the diagram, we connect a digital oscilloscope, which will operate in two-channel mode in order to see the signals at the input and output: the first channel (red) is what the generator produces, and the second (yellow) is what is removed from the load, that is, from the resistor .

• So, we have already seen that a capacitor does not allow direct current (current with zero frequency) to pass through. What happens if you apply a frequency of 100 Hz?

• A signal is supplied from the generator with an amplitude of 2 Volts and a frequency of 100 Hz. On the second channel we see the same frequency, but a much smaller amplitude of 136 millivolts. In this case, the signal is distorted by interference that is picked up from the surrounding space.
• The yellow graph has moved to the left, ahead of the red one. Before you is the same phase shift.

Advice! Here it is worth understanding that only the phase is ahead, not the signal. Otherwise, we would have a simple time machine in front of us, and everything is within the limits of understanding.

• That is, we mean the difference between the initial phases of voltages that have the same frequency.

• Now let's increase the frequency to 500 Hz. We see that the signal amplitude has increased to 560 millivolts, and the phase shift has become smaller.

• We increase the frequency to 2 kHz - the trend continues.

• Now we set the frequency to 10 kHz, and we see that the amplitude is almost equal, and the phase shift is almost unnoticeable.

• We set the maximum frequency on the generator and see that the channel indicators are almost equal.

What does this all mean? The higher the frequency, the lower the resistance of a capacitor in an alternating current circuit. At the same time, the phase shift also disappears.

Interesting to know! When connecting direct current, the frequency of which is zero, the phase shift is π/2 or 90 degrees.

But is it only frequency that affects the resistance of capacitors in an AC circuit? Let's repeat our experiment, but with a capacitor of smaller capacity, say 0.1 microfarad.

• We start, as last time, with a frequency of 100 Hz. It is immediately noticeable that the amplitude has decreased to 101 millivolts, whereas previously it was 136.

• The amplitude is still smaller.

• At maximum frequencies the resistance is already low, but the phase shift and lower amplitude remain.

We draw simple conclusions and understand that the resistance of a capacitor also depends on its capacitance - the larger it is, the lower the resistance.

In an attempt to answer the question of how to calculate the resistance of a capacitor to alternating current, mathematicians and physicists have derived the following formula:

Put the frequency equal to zero into this formula and you get zero, or infinite resistance. In practice, we have an actual high-pass filter - solder a capacitor in front of the speaker and you will hear that it only reproduces high frequencies. It is easy to install such a filter with your own hands - instructions are only needed when calculating the resistance parameters.

Well, what is happening inside the capacitor itself at this moment?

We remember that there is a sinusoidal current. Such a current consists of a repeating period, the first half of which it flows in one direction, and the second in the opposite direction. The periods are divided into half-cycles, each of which has phases of increasing, peaking and decreasing voltage.

• So, we actually analyzed the first quarter period using direct current as an example - the capacitor charges until its voltage reaches a peak value.
• At the beginning of the second quarter period, the voltage on the generator begins to decrease, accelerating. The resulting voltage difference causes the capacitor to discharge, giving current in the direction of the generator, that is, in the opposite direction than it flowed during charging - it provides resistance.
• At the moment when the first half-cycle ends, the voltage in the circuit and capacitor becomes zero, while the current, on the contrary, becomes maximum (we analyzed this dependence above).
• The third quarter begins, and the capacitor charges again, only in reverse polarity. In this case, the current continues to flow in the same direction, beginning to decrease as the voltage inside the capacitor increases.
• The fourth quarter is similar to the second - the capacitor is discharged and the current flows in the opposite direction. That is, the two half-cycles are literally mirror copies of each other.

As a result, we have that in one period the capacitor manages to charge and discharge twice, which indicates the constant passage of charging and discharging currents in the circuit, that is, that the current here is variable.

If we had used a light bulb instead of a resistor in our experiment, we would have seen its glow. However, the current feeding it would be a charge and discharge current, and not passing through the dielectric of the capacitor.

The greater the capacitance of the capacitor, the more charge is transferred to the circuit during the charge and discharge cycles of this element, and, consequently, the resistance becomes less. Increasing the frequency gives the same effect, but due to the amount of charge transferred in the same time, which is why the current also increases. It's like two businessmen - one receives income by making a big markup by selling a one-time item, and the second has the same thing, but due to a larger turnover with a smaller markup.

Because of this simple relationship, the resistance that a capacitor provides to the current in a circuit is called capacitive.

We'll probably end here. We have popularly explained what it is electrical circuit AC with real capacitor. Yes, the material is not easy to master, but if you figure it out, it’s not so scary. In addition, be sure to watch the video we have selected to completely answer all possible questions.