Working Principle of a capacitor

Definition of Capacitor

Capacitor is a passive electronic component or device capable of storing charge with a certain voltage level across two conducting plates or surfaces, separated by an insulating material or dielectric substance. Capacitor is constructed by using two conducting surfaces or plates and an insulating material (i.e. Dielectric like mica, paper, air etc.) between these two surfaces.

Working of Capacitor

As a capacitor is passive component, it does not generate energy. But it is able to store energy from an energy source like a battery or another charged capacitor. When a battery (DC Source) is connected across a capacitor, one surface, named plate I gets positive end of the battery and another surface, named plate II gets negative end of the battery. When battery is connected, the full voltage of that battery is applied across that capacitor. At that situation, plate I is in positive potency with respect to the plate II.


Current from the battery tries to flow through this capacitor from its positive plate (plate I) to negative plate (plate II) but cannot flow at max value due to separation of these plates with an insulating material. Rather a very small current will flow through this insulating material (dielectric) from Positive to Negative plate depending upon the value of strength of this dielectric.
An electric field will form inside the capacitor dielectric from positive to negative plate. As time goes on, positive plate (plate I) will accumulate positive charge from the battery and negative plate (plate II) will accumulate negative charge from negative end of the battery. After a certain time, the capacitor holds maximum amount of charge as per its capacitance with respect to this voltage. This time span is called charging time of this capacitor.

charging capacitorcharged capacitor

Now, after removing this battery from this capacitor, these two plates will hold positive and negative charges with respect to a certain voltage level for long time. Thus this capacitor acts as energy source. If two ends (plate I and plate II) get shorted through a load, a current will flow through this load from plate I to plate II up to all charges get vanished from both plates. This time span is known as discharging time of the capacitor.

discharged capacitor

How does a Capacitor Respond in DC?

Suppose a capacitor is connected across a battery through a switch. When switch is ON, i.e. t = 0+, a certain value of current will flow through this capacitor. After a certain time (i.e. charging time) capacitor never allow current to flow through it further. It is because of maximum number of charges are accumulated on both surfaces and capacitor acts as a battery which has positive end connected to the positive end of the battery and negative end connected to the negative end of the battery with same potency. Due to zero potential difference between battery and capacitor, no current will flow through it. So, it can be said that, initially a capacitor is short circuited and finally open circuited when it gets connected across a battery.

How does a Capacitor Respond in AC?

Suppose a capacitor is connected across an AC source. Consider, at a certain moment of positive half of this alternating voltage, plate I gets positive polarity and plate II negative polarity. Just at that moment plate I accumulates positive charges and plate II accumulates negative charges. But at the negative half of this applied AC voltage, plate I gets negative charges and plate II positive charges. And so on. There is no flow of electron between these two plates as they change their polarity with the change of source polarity. The capacitor plates get charged and discharged alternatively by the AC.

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