How Capacitor Works with AC

In previous post, we have seen working of capacitor with DC. This post is about How Capacitor Works with AC. In AC electronics capacitors processes AC signal to get output signal of specific frequency and amplitude.

How Capacitor Works with AC

The AC signal continuously varies with respect to time. There are different types of AC signals e.g. sine, triangular, square etc. Let us first clear two most important concepts related to capacitor in AC circuits.

  • Capacitor passes AC signal and blocks DC.

This statement is not 100% true. The capacitor blocks DC except for a time while it is charging or discharging. Capacitor also blocks AC signal to some extent. This nature of capacitor towards AC signal is refer as reactance of capacitor. Reactance works in the same way as resistance in DC circuits.

  • Capacitor is short circuit to AC and open circuit to DC.

You may thinking that if capacitor is passing AC much better than DC, then it must be acting as either short or open circuit with AC signal. The answer is No. The capacitor neither act as open nor short circuit. Then question arises, how capacitor passes AC without being short or open circuited. Let’s discuss…!

The water analogy can be compare with working of capacitor with AC. Let’s consider capacitor plates as two water tanks T1 and T2, filled same at half of their full capacity. The two pipes used to fill/emptying tanks, acting as capacitors leads. A pump is use in between these two pipes is similar to voltage source. Here tank T1 gets filled with negative voltage and emptied with positive voltage. The tank T2 works exactly opposite to T1. The emptied tank is similar to capacitor plate with negative charge. The water flowing through pipes is similar to current flowing through capacitor.

Now first consider our voltage source is DC. The DC signal is constant and it can be positive or negative. For negative voltage pump will drain water from T1 and supply it to T2. After some time water flow stops, T1 get emptied and T2 get filled completely. Hence there is no continuous flow of water from T1 to T2 with DC.

Now, the voltage is replaced with AC. The AC signal continuously varies in between positive and negative. The tanks get emptied or filled for respective voltages. But this time, signal polarity is continuously changing from positive to negative and vice-versa. Hence neither T1 nor T2 get emptied completely and water flows continuously in both directions through pipes. This exactly happens when capacitor is working with AC. The charge on capacitor plates is changing continuously with alternating current. Hence it result in flow of electrons through capacitor.

Capacitive Reactance

There is one important similarity in resistor and capacitor. Resistance of resistor opposes current flow by heat dissipation. The ability of capacitor to oppose current flow (both AC and DC) is known as reactance of capacitor. Reactance opposes current flow without heat dissipation. The resistance of capacitor to current is apparent in nature i.e. it is observe only at some point. Both resistance and reactance are measured in ohms. The term reactance comes from fact that, reaction of capacitor plates to current flow i.e. plates carries either positive or negative charge when voltage applied to capacitor.

The frequency is important parameter of AC signal. You may have read that, capacitor acts as an open circuit at low frequencies and short circuit at high frequencies. This statement is based on a face that frequency is inversely proportion to capacitive reactance. This voltage fluctuation is directly proportional to current through capacitor. The slow the input voltage fluctuates, less is the electron flow through capacitor and vice-versa.

Recall the mathematical representation of capacitor time constant.

Time (τ) = R x C

Hence increase in capacitance increase required time to charge, which implies low frequency (slow input voltage fluctuation) and less electron flow though capacitor.

The mathematical expression of capacitive reactance is,

Xc = 1 ⁄ (2πfC) . . . . . . . . . . .  (Unit – Ohm)

Ohms Law for Capacitive Reactance

As discussed earlier resistance and reactance both have same unit – ohm. There is also an ohms law for capacitive reactance. The important note is while applying ohms law for capacitive reactance frequency must be constant. Let’s find out how capacitive reactance varies in ohms law.

For C = 0.1µf, f = 100 Hz, V = 5VFor C = 0.1µf, f = 10 kHz, V = 5V
I = V ⁄ Xc = 5 ⁄(15.91k)

I = 0.314 mA

I = V ⁄ Xc = 5 ⁄(159.15)

I = 31.4 mA

These examples show that change in input signal frequency changes capacitive reactance. Hence frequency of input signal should be constant while applying ohms law for capacitor with AC.

This is it for now. I hope now you know How Capacitor works with AC. In future post, we will discuss about types of capacitor. Thanks for reading and don’t forget to leave a comment. There is lot to come about capacitors in this electronics series. Keep visiting.


About Ganesh Patil

Ganesh Gopal Patil received the Master degree in Embedded Systems. He has completed bachelor of engineering in Electronics and Telecommunication. His current interests include Microcontroller based Embedded System Design.

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