Source:Kyotoku
1、 Inductance
1. The resistance of inductance to alternating current
When the alternating current passes through the inductance coil, because the current is changing all the time, the self induced electromotive force will be generated in the coil, and the self induced electromotive force always hinders the change of the original current, so the inductance coil will hinder the alternating current. As we have learned before, the size of the self induced electromotive force is related to the self induction coefficient of the coil and the speed of the current change, and the larger the self induction coefficient is, The higher the frequency of the alternating current, the greater the self induced electromotive force, and the greater the blocking effect on the alternating current. The physical quantity of the blocking effect of the inductance on the alternating current is called inductive reactance, which is expressed by XL, and XL = 2 π fL。 The size of the inductive reactance is determined by the self inductance L of the coil and the frequency f of the alternating current.
2. The function of inductance coil in the circuit
(1) For two different types of current, DC and AC resistance are used. Because the current of constant current does not change, it can not cause self induction phenomenon, so it has no blocking effect on constant current. When the current of alternating current changes at any time, there must be self induction electromotive force to block the change of current, so it has blocking effect on AC.
(2) The higher the frequency of the alternating current, the faster the current changes, the greater the inductive reactance and the greater the obstruction to the current.
(3) Choke: an inductive coil made by using inductance to block alternating current.
Low frequency choke: the coil is wound on the iron core, with many turns, large self inductance coefficient and small resistance, which has the function of "connecting DC and resisting AC".
High frequency choke: small number of turns, small self inductance coefficient; It has the function of "connecting low frequency and blocking high frequency".
2、 Capacitance
1. Why can the capacitor be "connected to AC"
After the AC power supply is connected to the two plates of the capacitor, when the power supply voltage rises, the power supply charges the capacitor, and the charges gather on the plates of the capacitor, forming a charging current in the circuit; When the power supply voltage decreases, the capacitor discharges, and the charge accumulated on the original plate releases again, forming a discharge current in the circuit. The capacitor charges and discharges alternately, and there is a current in the circuit. It seems that the AC "passes" the capacitor, but in fact, the free charge does not pass through the insulating medium between the two plates of the capacitor.
2. How does the capacitor block the alternating current
We know that constant current cannot pass through the capacitor because the two plates of the capacitor are separated by an insulating medium. When connected to the AC power supply, the power supply makes the free charge in the wire move in a certain direction and "charge and discharge" the capacitor. In this process, the two plates of the capacitor generate electromotive force due to the accumulation (or reduction) of charge, thus resisting the continuous movement of the charge, which forms the obstruction of the capacitor to the alternating current.
The resistance of capacitor to alternating current is expressed by "capacitive reactance", that is, the larger the capacitance is, the higher the frequency is, the smaller the resistance of capacitor to alternating current is, and the smaller the capacitive reactance is.
3. The role of capacitor in the circuit
(1) The capacitor used for "connecting AC and isolating DC" is called isolating DC capacitor. Its capacitance is generally large, and it is often connected in series between two-stage circuits to make the AC component in the current pass through.
(2) In electronic technology, the AC output from a certain device often has both high-frequency and low-frequency components. If a capacitor is paralleled at the input end of the next stage circuit, only the AC signal of low-frequency components can be transmitted to the next stage device.
3、 The difference between resistance, inductance and capacitance
Resistance, inductance coil and capacitor play different roles in AC circuit. Resistance has the same blocking effect on all currents. As a result, electric energy is transformed into internal energy. The blocking effect of inductor coil (its internal resistance is neglected) and capacitor on AC is related to frequency. As a result, electric field energy and magnetic field energy are transformed into each other. When solving specific problems, do not think that the inductance coil L is always "on DC, blocking AC", and the capacitor C is always "on AC, blocking DC". Consider the frequency of alternating current, and carefully check the applicable conditions of inductance and capacitor.