How to determine resistance capacitance. Capacitance formula

Reactance – electrical resistance alternating current due to energy transfer magnetic field in inductances or by an electric field in capacitors.

Elements that have reactance are called reactive.

Reactance of the inductor.

When leaking alternating current I in a coil, a magnetic field creates an EMF in its turns, which prevents the current from changing.
When the current increases, the EMF is negative and prevents the current from increasing; when it decreases, it is positive and prevents its decrease, thus resisting the change in current throughout the entire period.

As a result of the created counteraction, a voltage is formed at the terminals of the inductor in antiphase U, suppressing EMF, equal to it in amplitude and opposite in sign.

When the current passes through zero, the amplitude of the emf reaches maximum value, which forms a divergence in time of current and voltage of 1/4 of the period.

If you apply voltage to the terminals of the inductor U, the current cannot start instantly due to the counter-emf equal to -U, therefore, the current in the inductance will always lag behind the voltage by an angle of 90°. The shift at the lagging current is called positive.

Let us write down the expression for the instantaneous voltage value u based on EMF ( ε ), which is proportional to the inductance L and the rate of change of current: u = -ε = L(di/dt).
From here we express the sinusoidal current.

Integral of a function sin(t) will -cos(t), or an equal function sin(t-π/2).
Differential dt functions sin(ωt) will leave the integral sign with a factor of 1 /ω .
As a result, we obtain the expression for the instantaneous current value with a shift from the stress function by an angle π/2(90°).
For RMS values U And I in this case we can write .

As a result, we have a dependence of sinusoidal current on voltage according to Ohm’s Law, where in the denominator instead of R expression ωL, which is the reactance:

The reactance of inductances is called inductive.

Capacitor reactance.

The electric current in a capacitor is a part or a set of processes of its charge and discharge - the accumulation and release of energy by the electric field between its plates.

In an AC circuit, the capacitor will charge to a certain maximum value until the current reverses direction. Consequently, at the moments of the amplitude value of the voltage on the capacitor, the current in it will be equal to zero. Thus, the voltage across the capacitor and the current will always have a time difference of a quarter period.

As a result, the current in the circuit will be limited by the voltage drop across the capacitor, which creates an alternating current reactance that is inversely proportional to the rate of change of current (frequency) and the capacitance of the capacitor.

If you apply voltage to a capacitor U, the current will instantly start from the maximum value, then decrease to zero. At this time, the voltage at its terminals will increase from zero to maximum. Consequently, the voltage on the capacitor plates lags the current in phase by an angle of 90 °. This phase shift is called negative.

The current in a capacitor is a derivative function of its charge i = dQ/dt = C(du/dt).
Derivative of sin(t) will cos(t) or an equal function sin(t+π/2).
Then for sinusoidal voltage u = U amp sin(ωt) Let's write the expression for the instantaneous current value as follows:

i = U amp ωCsin(ωt+π/2).

From here we express the ratio of the root-mean-square values .

Ohm's law dictates that 1 /ωC is nothing more than reactance for a sinusoidal current:

Capacitor reactance in technical literature often called capacitive. It can be used, for example, in organizing capacitive dividers in alternating current circuits.

Online reactance calculator

You need to enter the values ​​and click in the table.
When switching multipliers, the result is automatically recalculated.

Capacitance reactance X C = 1 /(2πƒC)

In which an alternator produces a sinusoidal voltage. Let's look at what happens in the circuit when we close the key. We will consider the initial moment when the generator voltage is zero.

In the first quarter of the period, the voltage at the generator terminals will increase, starting from zero, and the capacitor will begin to charge. A current will appear in the circuit, but at the first moment of charging the capacitor, despite the fact that the voltage on its plates has just appeared and is still very small, the current in the circuit (charge current) will be the greatest. As the charge on the capacitor increases, the current in the circuit decreases and reaches zero at the moment when the capacitor is fully charged. In this case, the voltage on the capacitor plates, strictly following the generator voltage, becomes at this moment maximum, but of the opposite sign, i.e., directed towards the generator voltage.

Rice. 1. Change in current and voltage in a circuit with capacitance

Thus, the current rushes with the greatest force into the charge-free capacitor, but immediately begins to decrease as the capacitor plates are filled with charges and drops to zero, fully charging it.

Let's compare this phenomenon with what happens with the flow of water in a pipe connecting two communicating vessels (Fig. 2), one of which is filled and the other empty. One has only to pull out the valve blocking the path of water, and water will immediately rush from the left vessel under high pressure through the pipe into the empty right vessel. However, immediately the water pressure in the pipe will begin to gradually weaken, due to the leveling of the levels in the vessels, and will drop to zero. The water flow will stop.

Rice. 2. The change in water pressure in the pipe connecting communicating vessels is similar to the change in current in the circuit during the charging of the capacitor

Similarly, the current first flows into an uncharged capacitor, and then gradually weakens as it charges.

With the beginning of the second quarter of the period, when the voltage of the generator begins slowly at first, and then decreases faster and faster, the charged capacitor will be discharged to the generator, which will cause a discharge current in the circuit. As the generator voltage decreases, the capacitor is discharged more and more and the discharge current in the circuit increases. The direction of the discharge current in this quarter of the period is opposite to the direction of the charge current in the first quarter of the period. Accordingly, the current curve, having passed the zero value, is now located below the time axis.

By the end of the first half-cycle, the voltage on the generator, as well as on the capacitor, quickly approaches zero, and the current in the circuit slowly reaches its maximum value. Remembering that the magnitude of the current in the circuit is greater, the greater the amount of charge transferred along the circuit, it will become clear why the current reaches its maximum when the voltage on the capacitor plates, and therefore the charge of the capacitor, quickly decreases.

With the beginning of the third quarter of the period, the capacitor begins to charge again, but the polarity of its plates, as well as the polarity of the generator, changes to the opposite, and the current, continuing to flow in the same direction, begins to decrease as the capacitor is charged. At the end of the third quarter of the period, when the voltages across the generator and capacitor reach their maximum, the current becomes zero.

In the last quarter of the period, the voltage, decreasing, drops to zero, and the current, changing its direction in the circuit, reaches its maximum value. This ends the period, after which the next one begins, exactly repeating the previous one, etc.

So, Under the influence AC voltage The generator charges the capacitor twice per period (the first and third quarters of the period) and discharges it twice (the second and fourth quarters of the period). But since alternating one after another is accompanied each time by the passage of charging and discharging currents through the circuit, we can conclude that .

You can verify this using the following simple experiment. Connect a capacitor with a capacity of 4-6 microfarads to the AC network through a 25 W electric light bulb. The light will light up and will not go out until the circuit is broken. This indicates that alternating current passed through the circuit with the capacitance. However, it passed, of course, not through the dielectric of the capacitor, but at each moment of time it represented either the charge current or the discharge current of the capacitor.

The dielectric, as we know, is polarized under the influence of the electric field that arises in it when the capacitor is charged, and its polarization disappears when the capacitor is discharged.

In this case, the dielectric with the bias current arising in it serves as a kind of continuation of the circuit for alternating current, and breaks the circuit for direct current. But the displacement current is generated only within the dielectric of the capacitor, and therefore no through charge transfer through the circuit occurs.

The resistance provided by a capacitor to alternating current depends on the value of the capacitor's capacitance and the frequency of the current.

The larger the capacitor's capacitance, the greater the charge transferred through the circuit during the charging and discharging of the capacitor, and therefore, the greater the current in the circuit. An increase in current in the circuit indicates that its resistance has decreased.

Hence, As the capacitance increases, the circuit's resistance to alternating current decreases.

An increase increases the amount of charge transferred through the circuit, since the charge (as well as the discharge) of the capacitor must occur faster than at a low frequency. At the same time, an increase in the amount of charge transferred per unit time is equivalent to an increase in the current in the circuit, and, consequently, a decrease in its resistance.

If we somehow gradually reduce the frequency of the alternating current and reduce the current to constant, then the resistance of the capacitor connected to the circuit will gradually increase and become infinitely large (open circuit) by the time it appears.

Hence, As the frequency increases, the capacitor's resistance to alternating current decreases.

Just as the resistance of a coil to alternating current is called inductive, the resistance of a capacitor is usually called capacitive.

Thus, The capacitance is greater, the lower the capacitance of the circuit and the frequency of the current supplying it.

Capacitance is denoted by Xc and measured in ohms.

Addiction capacitance on the frequency of the current and the capacitance of the circuit is determined by the formula Xc = 1/ωС, where ω - circular frequency, equal to the product 2π f, C-capacitance of the circuit in farads.

Capacitive reactance, like inductive reactance, is reactive in nature, since the capacitor does not consume the energy of the current source.

The formula for a circuit with capacitance is I = U/Xc, where I and U are effective values current and voltage; Xc is the capacitance of the circuit.

The property of capacitors to provide high resistance to low frequency currents and easily pass currents high frequency widely used in communication equipment circuits.

With the help of capacitors, for example, the separation of direct currents and low-frequency currents from high-frequency currents necessary for the operation of circuits is achieved.

If you need to block the path of low-frequency current into the high-frequency part of the circuit, a capacitor is connected in series large capacity. It offers great resistance to low-frequency current and at the same time easily passes high-frequency current.

If it is necessary to prevent high-frequency current, for example, from entering the power circuit of a radio station, then a large capacitor is used, connected in parallel with the current source. In this case, the high-frequency current passes through the capacitor, bypassing the power supply circuit of the radio station.

Active resistance and capacitor in an alternating current circuit

In practice, there are often cases when the circuit is in series with the capacitance Total resistance chain in this case is determined by the formula

Hence, the total resistance of a circuit consisting of active and capacitive resistance to alternating current is equal to the square root of the sum of the squares of the active and capacitive resistance of this circuit.

Ohm's law remains valid for this circuit I = U/Z.

In Fig. Figure 3 shows curves characterizing the phase relationships between current and voltage in a circuit containing capacitive and active resistance.

Rice. 3. Current, voltage and power in a circuit with a capacitor and active resistance

As can be seen from the figure, the current in this case leads the voltage not by a quarter of a period, but less, since the active resistance has violated the purely capacitive (reactive) nature of the circuit, as evidenced by the reduced phase shift. Now the voltage at the circuit terminals will be determined as the sum of two components: the reactive component of the voltage u c, which goes to overcome the capacitance of the circuit, and the active component of the voltage, which overcomes its active resistance.

The greater the active resistance of the circuit, the smaller the phase shift will be between current and voltage.

The power change curve in the circuit (see Fig. 3) twice during the period acquired a negative sign, which is, as we already know, a consequence of the reactive nature of the circuit. The less reactive circuit, the smaller the phase shift between current and voltage and the greater the power of the current source this circuit consumes.

A capacitor is used in circuits to separate the alternating and direct voltage components, while it conducts high-frequency signals well and poorly conducts low-frequency ones. While in the chain direct current, its impedance is assumed to be infinitely large. For alternating current, the capacitance of the capacitor does not have a constant value. Therefore, calculating this value is extremely important when designing various radio-electronic devices.

general description

Physically electronic device- capacitor - consists of two plates made of conductive material, between which there is a dielectric layer. Two electrodes are removed from the surface of the plates, intended for connection to an electrical circuit. Structurally, the device can be various sizes and shape, but its structure remains unchanged, that is, there is always an alternation of conductive and dielectric layers.

The word "capacitor" comes from the Latin "condensatio" - "accumulation". The scientific definition is that cumulative electrical appliance is a two-terminal network characterized by constant and variable values capacitance and high resistance. It is designed to store energy and charge. The unit of measurement of capacitance is the farad (F).

In the diagrams, the capacitor is depicted as two straight lines, corresponding to the conducting plates of the device, and drawn perpendicular to their centers by drawn segments - the terminals of the device.

The principle of operation of the capacitor is as follows: when the device is connected to an electrical circuit, the voltage in it will have a zero value. At this moment, the device begins to receive and accumulate charge. The electric current supplied to the circuit will be the maximum possible. After some time, positive charges will begin to accumulate on one of the electrodes of the device, and negative charges on the other.

The duration of this process depends on the capacity of the device and active resistance. The dielectric located between the terminals prevents the movement of particles between the plates. But this will happen only until the potential difference of the power source and the voltage at the capacitor terminals are equal. At this moment, the capacity will become maximum possible, and the electric current will be minimal.

If voltage is no longer supplied to the element, then when a load is connected, the capacitor begins to transfer its accumulated charge to it. Its capacity decreases, and the voltage and current levels in the circuit decrease. In other words, the storage device itself turns into a power source. Therefore, if a capacitor is connected to alternating current, it will begin to periodically recharge, that is, create a certain resistance in the circuit.

The most important characteristic of a storage device is capacity. The charging time depends on it when the device is connected to a power source. The discharge time is directly related to the value of the load resistance: the higher it is, the faster the process of releasing the accumulated energy occurs. This capacity is determined by the following expression:

C = E*Eo*S / d, where E is the relative dielectric constant of the medium (reference value), S is the area of ​​the plates, d is the distance between them.

The total resistance of a capacitor (impedance) to an alternating signal is the sum of three components: capacitive, resistive and inductive reactance. All these quantities must be taken into account when designing circuits containing a storage element. Otherwise in electrical circuit, with appropriate wiring, the capacitor can behave like a choke and is in resonance. Of all three quantities, the most significant is the capacitive reactance of the capacitor, but under certain circumstances the inductive reactance also has an effect.

Impedance element is expressed in the formula Z = (R2 + (Xl-Xc) 2) ½, where

• Xl - inductance;
• Xс - capacity;
• R is the active component.

The latter arises due to the appearance electromotive force(EMF) self-induction. The inconstancy of the current leads to a change in the magnetic flux, which maintains the self-inductive emf current constant. This value is determined by the inductance L and the frequency of flowing charges W. Xl = wL = 2*p*f*L. Xc is capacitive reactance, depending on the storage capacity C and current frequency f. Xc = 1/wC = ½*p*f*C, where w is the circular frequency.

The difference between the capacitive and inductive values ​​is called the reactance of the capacitor: X = Xl-Xc. According to the formulas, you can see that as the frequency f of the signal increases, the inductive value begins to dominate, and as it decreases, the capacitive value begins to dominate. Therefore if:

• X > 0, the element exhibits inductive properties;
• X = 0, only the active value is present in the container;
• X< 0, в элементе проявляется ёмкостное сопротивление.

Active resistance R is associated with power losses, its conversion electrical energy to thermal. Reactive - with energy exchange between alternating current and electromagnetic field. Thus, the total resistance can be found using the formula Z = R +j*X, where j is the imaginary unit.

Capacitance

To understand the process, you should imagine a capacitor in an electrical circuit through which alternating current flows. Moreover, there are no other elements in this chain. The value of the current passing through the capacitor and the voltage applied to its plates changes over time. Knowing any of these values, you can find another.

Let the current vary according to a sinusoidal dependence I (t) = Im * sin (w*t+ f 0). Then the voltage can be described as U (t) = (Im/C*w) *sin (w*t+ f 0 -p/2). When taking into account the 90-degree phase shift that occurs between signals in the formula, a complex quantity j, called the imaginary unit, is introduced. Therefore, the formula for finding the current will look like I = U / (1/j*w*C). But given that the complex number only denotes the displacement of the voltage relative to the current, and on their amplitude values does not affect, it can be removed from the formula, thereby significantly simplifying it.

Since, according to Ohm’s law, resistance is directly proportional to the voltage in a section of the circuit and inversely proportional to the current, then transforming the formulas, you can get the following expression:

• Xc = 1/w*C = ½*p*f*C. The unit of measurement is ohm.

It becomes clear that capacitive reactance depends not only on capacitance, but also on frequency. Moreover, the higher this frequency, the less resistance the capacitor will provide to the current passing through it. In relation to capacity, this statement will be the opposite. That is why for direct current, the frequency of which is zero, the resistance of the drive will be infinitely large.

Inductive component

When an alternating signal passes through the drive, it can be represented as an inductor connected in series with the power source. This coil is characterized by greater resistance in the AC signal circuit than in the DC one. The current value at a certain point in time is found as I = I 0 * sinw.

Taking into account that the instantaneous value of voltage U 0 is opposite in sign to the instantaneous value of the self-inductive emf E 0, and also using Lenz’s rule, we can obtain the expression E = L * I, where L is the inductance.

Therefore: U = L*w * I 0 *cosw*t = U 0 *sin (wt + p /2), and the current lags behind the voltage by p /2. Using Ohm's law and assuming that the coil resistance is w * L, we get a formula for a section of an electrical circuit that has only an inductive component: U 0 = I 0 / w * L.

Thus, the inductive reactance will be equal to Xl = w * L, it is also measured in ohms. From the resulting expression it can be seen that the higher the frequency of the signal, the stronger the resistance to the passage of current.

Calculation example

Capacitive and inductive reactance They are classified as reactive, that is, those that do not consume power. Therefore, Ohm's law for the section of the circuit with capacitance has the form I = U/Xc, where current and voltage denote effective values. It is because of this that capacitors are used in circuits to separate not only direct and alternating currents, but also low and high frequencies. Moreover, the lower the capacitance, the higher the frequency the current can pass. If an active resistance is connected in series with the capacitor, then the total impedance of the circuit is found as Z = (R 2 +Xc 2) ½.

Practical use formulas can be considered when solving the problem. Let there be an RC circuit consisting of a capacitance C = 1 μF and a resistance R = 5 kOhm. It is necessary to find the impedance of this section and the circuit current if the signal frequency is f = 50 Hz and the amplitude is U = 50 V.

First of all, you will need to determine the resistance of the capacitor in the AC circuit for a given frequency. Substituting the data into the formula, we find that for a frequency of 50 Hz the resistance will be

Xc = 1/ (2*p*F*C) = 1/ (2*3.14*50*1* 10 −6) = 3.2 kOhm.

Using Ohm's law you can find the current: I = U /Xc = 50 /3200 = 15.7 mA.

The voltage is taken to be variable according to the sine law, therefore: U (t) = U * sin (2*p*f*t) = 50*sin (314*t). Accordingly, the current will be I (t) = 15.7* 10 −3 + sin (314*t+p/2). Using the results obtained, you can plot the current and voltage at this frequency. We find the total resistance of the circuit section as Z = (5000 2 +3200 2)½ = 5,936 Ohm = 5.9 kOhm.

Thus, it is not difficult to calculate the total resistance at any part of the circuit. In this case, you can also use the so-called online calculators, where you enter initial data, such as frequency and capacity, and all calculations are performed automatically. This is convenient, since there is no need to memorize formulas and the probability of error tends to zero.

DEFINITION

Capacitor, in the simplest case, consists of two metal conductors (plates), which are separated by a dielectric layer. Each of the capacitor plates has its own terminal and can be connected to an electrical circuit.

A capacitor is characterized using a number of parameters (capacitance, operating voltage, etc.), one of these characteristics is resistance. The capacitor practically does not allow direct electric current to pass through. That is, the capacitor resistance is infinitely large for direct current, but this is the ideal case. A very small current can flow through a real dielectric. This current is called leakage current. Leakage current is an indicator of the quality of the dielectric used in the manufacture of the capacitor. With modern capacitors, the leakage current is several fractions of a microampere. The resistance of the capacitor in this case can be calculated using Ohm's law for a section of the circuit, knowing the voltage to which the capacitor is charged and the leakage current. But usually when deciding educational tasks The capacitor's resistance to direct current is considered infinitely large.

Capacitor resistance to alternating voltage

When a capacitor is connected to an alternating current circuit, current flows freely through the capacitor. This can be explained very simply: a process of constant charging and discharging of the capacitor occurs. In this case, they say that the circuit contains capacitive reactance of the capacitor, in addition to active resistance.

And so, a capacitor, which is connected to an alternating current circuit, behaves as a resistance, that is, it affects the current flowing in the circuit. We denote the value of capacitance as , its value is related to the frequency of the current and is determined by the formula:

where is the frequency of alternating current; - angular frequency of current; C is the capacitance of the capacitor.

If a capacitor is connected to an alternating current circuit, then no power is expended in it, because the phase of the current is shifted relative to the voltage by . If we consider one period of current oscillation in the circuit (T), then the following happens: when the capacitor is charged (this amounts to ), energy is stored in the capacitor field; in the next period of time (), the capacitor discharges and releases energy into the circuit. Therefore, capacitive reactance is called reactive (watt-free).

It should be noted that in every real capacitor, real power (loss power) is still spent when alternating current flows through it. This is caused by changes occurring in the state of the dielectric of the capacitor. In addition, there is some leakage in the insulation of the capacitor plates, so a small active resistance appears, which is, as it were, connected in parallel with the capacitor.

Examples of problem solving

EXAMPLE 1

Exercise	The oscillatory circuit has a resistance (R), an inductor (L) and a capacitor C (Fig. 1). Connected to it external voltage, the amplitude of which is , and the frequency is . What is the amplitude of the current in the circuit?
Solution	The circuit resistance in Fig. 1 consists of the active resistance R, the capacitance of the capacitor and the resistance of the inductor. The total resistance of a circuit (Z) that contains the above elements is found as: Ohm's law for our section of the circuit can be written as: Let us express the desired current amplitude from (1.2), substitute the right-hand side of formula (1.1) instead of Z, and we have:
Answer

Capacitors, like resistors, are among the most numerous elements of radio engineering devices. The main property of capacitors is ability to accumulate electrical charge . The main parameter of a capacitor is its capacity .

The larger the area of ​​its plates and the thinner the dielectric layer between them, the greater the capacitance of the capacitor. The basic unit of electrical capacitance is the farad (abbreviated F), named after the English physicist M. Faraday. However, 1 F is a very large capacity. The globe, for example, has a capacity of less than 1 F. In electrical and radio engineering, a unit of capacity equal to a millionth of a farad is used, which is called microfarad (abbreviated uF) .

The capacitance of a capacitor to alternating current depends on its capacitance and current frequency: the greater the capacitance of the capacitor and the frequency of the current, the lower its capacitance.

Ceramic capacitors have relatively small capacitances - up to several thousand picofarads. They are placed in those circuits in which high-frequency current flows (antenna circuit, oscillatory circuit), for communication between them.

The simplest capacitor consists of two conductors electric current, for example: - two metal plates, called capacitor plates, separated by a dielectric, for example: - air or paper. The larger the area of ​​the capacitor plates and the closer they are located to each other, the greater the electrical capacitance of this device. If a direct current source is connected to the capacitor plates, a short-term current will arise in the resulting circuit and the capacitor will be charged to a voltage equal to the voltage of the current source. You may ask: why does current occur in a circuit where there is a dielectric? When we connect a current source to a capacitor, the electrons in the conductors of the resulting circuit begin to move towards the positive pole of the current source, forming a short-term flow of electrons throughout the circuit. As a result, the plate of the capacitor, which is connected to the positive pole of the current source, is depleted of free electrons and is charged positively, and the other plate is enriched in free electrons and, therefore, is charged negatively. Once the capacitor is charged, the short-term current in the circuit, called the capacitor charging current, will stop.

If the current source is disconnected from the capacitor, the capacitor will be charged. The dielectric prevents the transfer of excess electrons from one plate to another. There will be no current between the plates of the capacitor, and the electrical energy accumulated by it will be concentrated in the electric field of the dielectric. But as soon as the plates of a charged capacitor are connected with some kind of conductor, the “excess” electrons of the negatively charged plate will pass through this conductor to another plate where they are missing, and the capacitor will be discharged. In this case, a short-term current also arises in the resulting circuit, called the capacitor discharge current. If the capacity of the capacitor is large and it is charged to a significant voltage, the moment it is discharged is accompanied by the appearance of a significant spark and crackling sound. The property of a capacitor to accumulate electrical charges and discharge through conductors connected to it is used in the oscillatory circuit of a radio receiver.

Capacitor(from lat. condensare- “compact”, “thicken”) - a two-terminal network with a certain capacitance value and low conductivity; a device for accumulating charge and energy of an electric field. A capacitor is a passive electronic component. In its simplest form, the design consists of two plate-shaped electrodes (called linings), separated by a dielectric whose thickness is small compared to the dimensions of the plates (see figure). Practically used capacitors have many layers of dielectric and multilayer electrodes, or strips of alternating dielectric and electrodes, rolled into a cylinder or parallelepiped with four rounded edges (due to winding). A capacitor in a DC circuit can conduct current at the moment it is connected to the circuit (charging or recharging of the capacitor occurs); at the end of the transient process, no current flows through the capacitor, since its plates are separated by a dielectric. In an alternating current circuit, it conducts alternating current oscillations through cyclic recharging of the capacitor, closing with the so-called bias current.

From the point of view of the complex amplitude method, the capacitor has a complex impedance

,

Where j - imaginary unit, ω - cyclic frequency ( rad/s) flowing sinusoidal current, f - frequency in Hz, C - capacitor capacity ( farad). It also follows that the reactance of the capacitor is equal to: . For direct current, the frequency is zero, therefore the reactance of the capacitor is infinite (ideally).

The resonant frequency of the capacitor is

At f > f p A capacitor in an AC circuit behaves like an inductor. Therefore, it is advisable to use a capacitor only at frequencies f< f p , where its resistance is capacitive in nature. Typically, the maximum operating frequency of a capacitor is approximately 2-3 times lower than the resonant frequency.

A capacitor can store electrical energy. Energy of a charged capacitor:

Where U - voltage (potential difference) to which the capacitor is charged.