Why Capacitors block DC and Passes Ac?

Last time we understood the working of capacitors in DC. In this post, we will try to understand how capacitors work in Ac.

But before going into detail, let's just revise some concepts of the last post. In the last post, we deal with the charging & discharging of capacitors in Dc which gives us the clear idea that once the capacitor is charged fully with dc supply it won't collect electrons on its plate
i.e.[When input voltage (Vin) = (Vc) Voltage of capacitor]
Due to this property of the capacitor, the capacitors block Dc once it is fully charged.

@ Then the question arises does the capacitor also block the Ac?
Let's Go into detail about how capacitors work in Ac.
The dc voltage is steady voltage i.e. electrons move only in one direction either forward or in a backward direction depending upon the terminals of the voltage source while in Ac voltage the electrons keep on moving back & forth (backward and forward) due to this factor known as FREQUENCY comes into existence in an ac signal.

Since the Ac signal is alternating in nature, therefore, capacitor charges and discharges very quickly. Let's understand it in more detail. Consider that the applied voltage is sinusoidal in nature 10v AC with load resistor RL connected across the capacitor.

Now we will divide the work into 2 cases

# Case:-1

When electrons move in a forward direction i.e. during the positive half cycle the capacitors start charging quickly from 1v to 6v (ASSUMED). Now as the voltage is increased further the acceleration of electrons moving in a forward direction towards the capacitor decreases this is because the

electrons accommodated in the capacitor's plate repel the electrons coming towards it. Due to this after 6v the capacitors charge slowly from 7v-10v (ASSUMED). When the AC signal reaches to its peak value of +10v the charging stops. Since the applied voltage is sinusoidal in nature. Therefore the input voltage starts decreasing from 10v-0v. Due to this, the electrons which were accommodated in plates of capacitors start moving out of the plate which in short leads to the discharging of the capacitor & electrons move towards the RL load resistor.

The Concept Would Be More Clear By Watching The Video given Below

VIDEO:-

# Case:-2

When electrons move in a backward direction i.e. during the negative half cycle the capacitors start charging quickly from 1v to 6v (ASSUMED). Now as the voltage is increased further the acceleration of electrons moving in a backward direction towards the capacitor decreases this is because the electrons accommodated in the capacitor's plate repels the electrons coming towards it. Due to this after 6v the capacitors charge slowly from 7v-10v (ASSUMED). When the AC signal reaches its peak value -10v the charging stops. Since the applied voltage is sinusoidal in nature. Therefore the input voltage starts decreasing from -10v to 0v. Due to this, the electrons which were accommodated in plates of capacitors start moving out of the plate which in short leads to the discharging of the capacitor & electrons move towards RL load resistor.

The Concept Would Be More Clear By Watching The Video given Below

VIDEO:-

In the end, I will conclude this in mathematical form. The reactance of the capacitor is given as

Xc = 1/2 * pi *f * c

were Xc is reactance also known as impedance or resistance of the capacitor

         f is frequency

         and c is the capacitance

@ Conclusion:-

# For DC:-

The voltage is constant and the current flows only in one direction, therefore, frequency is zero, since the frequency is zero the reactance or resistance offered by the capacitor will be more (infinite) due to this reason we can say that capacitors block dc.

# For AC:-

Here the voltage is varying in nature therefore it has some frequency since it has some frequency, therefore, the reactance or resistance offered by the capacitor is less If the frequency increases reactance of the capacitor decreases & the capacitor can conduct more current very easily &

If frequency decreases reactance of the capacitor increases & the capacitor cannot conduct the current it tries to oppose it.

NOTE:-DC capacitors are polarized which means they have positive and negative terminals while AC  capacitors are non-polarized.

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