Circuit Theory

Capacitor

WHY CAPACITOR BLOCKS DC BUT ALLOWS AC???

Direct Current (DC):

  • Steady flow of electric charges in one direction constitutes direct current.
  • DC current always flows from positive terminal to negative terminal.
  • DC is always constant over time.
  • DC voltage has constant polarity.
  • DC has zero frequency.
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DC source:
    • Batteries
    • Solar Panels
    • DC generators
    • Rectifiers, etc.,

Alternating Current (AC):

  •  
  • Bi-directional flow of electrons constitute alternating current.
  • AC is alternating in nature over time.
  • AC voltage changes its polarity continuously on its own.
  • AC has frequency ranges from $1$ Hz to infinity.
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AC source:
    • Alternators
    • AC generators
    • Inverters, etc.,

Why Capacitor blocks DC???

Consider a battery (DC voltage source) connected to a capacitor as shown below,

Once battery is connected, the positive terminal begins to attract electrons from the metal plate $1$ and the negative terminal begins to supply electrons to metal plate $2$.

(Assume these metal plates are infinitely large, having high conductivity and it can supply or accept any number of electrons.)

e-e-e-e-e-e-e-e-e-e-e-e-Electron flowCurrent flow

This makes metal plate $1$ as positively charged and metal plate $2$ as negatively charged.

As capacitor is getting charged, the voltage across the capacitor will increase. 

-----+--++++++++

Once the capacitor is fully charged, the capacitor voltage will be equal to the battery voltage.

Hence, the potential difference between the battery and the capacitor will become zero. Therefore electron flow or current flow will stop. 

$V_{battery}=V_{capacitor}$ implies $I=0$

Once the capacitor is fully charged, current flow will become zero.

Now, if we want to attract more number of electrons from the metal plates,

  • We need to make the potential difference between the battery and the capacitor as non-zero.$$V_{battery} \neq V_{capacitor}$$

But,

  • As long as the battery is connected as it is, the capacitor will remain fully charged. Hence $V_{battery}=V_{capacitor}$ remains same.
  • As the capacitor is fully charged, we have no way to attract more electrons from the metal plates other than discharging the capacitor.
  • To discharge the capacitor, we need to change the polarity of the battery. But DC source won’t change the polarity on its own. 

Hence we can conclude that, if we connect a DC power supply to a capacitor, initially we can see a sudden spike in current flow (as capacitor charges) and it ceases to zero after sometime (when capacitor is fully charged). After that there will be no current flow.

Hence we can say, Capacitor blocks DC.

Why Capacitor allows AC???

Once capacitor is fully charged $V_{battery}$ will be equal to $V_{capacitor}$, hence the current flow will become zero.

To attract more electrons from the metal plates, we need to discharge the capacitor first. For that we need to change the polarity of the battery. This polarity change creates a potential difference between the battery and the capacitor, results in a current flow.$$V_{battery}\neq V_{capacitor}$$

Discharging gives a current flow opposite to that of charging current.

-----+--++++++++e-e-e-e-e-e-e-e-e-e-e-e-e-Electron flowCurrent flow

Once capacitor is fully discharged, it begins to charge in the other way (i.e.,) metal plate$1$ gets negatively charged and metal plate$2$ gets positively charged. 

+++++++++---------

Again, once capacitor is fully charged, the potential difference between the battery and the capacitor will become zero, $$V_{battery}=V_{capacitor}$$ and hence current flow will cease to zero.

Again to make the current to flow through, we need to discharge the capacitor. This charging and discharging cycle goes on and on.

in opposite directionChargingChargingDischargingDischargingFully chargedFully charged(No current flow)(No current flow)(current flow)(current flow)in opposite direction(current flow)(current flow)

From the above discussion, we can say, 

  • If we charge and discharge the capacitor continuously, current will flow through the capacitor continuously (but direction of the current changes accordingly).
  • For continuous charging and discharging, we need a voltage source that can change its polarity continuously.
  • The voltage source that can change its polarity continuously is nothing but an AC voltage source.
Hence, we can say, capacitor allows AC.

Summarizing,

For continuous flow of current in a capacitor,

  • Capacitor has to be charged and discharged continuously.
  • For that we need a voltage source that can change its polarity continuously.
  • Source that can change its polarity continuously is nothing but an AC source.
  • This continuous polarity change cannot be achieved by using a DC source.
 
Hence, we can say, Capacitor allows AC but blocks DC.