What is a Capacitor? | Charging & Discharging of Capacitors

Author: Saurabh Salvi

Let’s consider a ceiling fan at rest (switch is off). Now, I turn on the fan. Here, the thing is that an initial burst of energy is required to make the fan just start rotating from rest. Once it has gained momentum, it consumes much less energy. To provide that initial burst of energy, capacitors come in handy.

A picture showing a boy sleeping peacefully — peacefully sleeping boy

We can compare this situation with ourselves when we were at school. It’s time to get ready for school in the morning, and we are peacefully sleeping in bed. And out of nowhere, Mom enters the room and spanks us to wake us up! But once we wake up, we used to manage all the things smoothly. So, in this example, our mom acts as a capacitor to give us that initial boost/burst which is required to get us out of bed.

This was just one of the many uses of a capacitor. Also, combining a capacitor with other components opens up even more interesting ideas.

Capacitors as storage devices

Capacitors are mainly known for their ability to store electrical energy in the form of charge, and they provide it at once when needed. But this electrical energy is obtained by the capacitor itself from another external voltage source

A capacitor gets its electrical energy from a voltage source. The figure explains the same — *Fig.1*

Basic Structure of Capacitor includes :

Two metal plates separated by a distance
A dielectric inserted between the plates

Note: For now, assume the dielectric to be some substance that doesn’t conduct electricity easily and is something used to enhance the storing capacity of a capacitor.

A capacitor with a dielectric inserted in it

The figure shows the symbol that is used for capacitors in electrical circuits — Nomenclature: The first one is normal, and the Second one is a representation for a polarized capacitor (discussed later)

How is the charging and discharging of a Capacitor done? (Important)

Charging of Capacitor:

Let us go step-by-step to understand what exactly happens when a capacitor gets charged. Also, simultaneously, we will take an example of 2 friends, A and B; the batteries actually refer to the minds/amount of knowledge of the individuals. Friend A is analogous to a battery, while Friend B is analogous to a capacitor.

Step-1 :

The initial stage before charging the capacitor is started — Fig. We have just set up everything as shown in the figure above. It’s t=0

Initially, the battery has some potential difference (full knowledge) across it but no charge on capacitor, so no potential difference (no knowledge) across it — Fig. Initially, the battery has some potential difference (full knowledge) across it, but no charge on the capacitor, so no potential difference (no knowledge) across it

Step-2 :

Note :

Negative terminal of the battery can be considered to have a cluster of free electrons (a lot of electrons), and because they are free, they tend to move across the circuit
Wherever there is a separation of charges, a potential difference will start to build up

Now, while moving, these electrons reach the capacitor plate and start accumulating (as there is no wire as such for electrons to keep moving) on the plate, causing an excess of electrons on the plate. This makes the plate become negatively charged (-).
Simultaneously, we have electrons from the other plate getting attracted towards the positive terminal of the battery. This causes a deficiency of electrons on the capacitor plate. This makes it positively charged (+). In this process, the electrons move through the bulb as well, which makes the bulb glow (but only for a short time, as discussed below).
As mentioned in ‘Note’ above, a potential difference (P.D.) will be created across the capacitor (but still V(battery) > P.D. across the capacitor)

Gradually, there is an increase in the amount of knowledge (P.D. across the capacitor) of B. B has gained this knowledge from A (battery).

Friend A and Friend B analogy - Friend B has less knowledge than Friend A

Step-3 :

The Step-2 keeps on happening (battery keeps pushing electrons to one plate and keeps pulling electrons from another plate) until V_battery = P.D. across the capacitor. Because it implies that there is sufficient negative charge on the plate, which is capable of repelling the coming electrons (and also there is enough positive charge developed on the other plate to keep the electrons attracted to itself).

We call this condition of a capacitor as ‘saturated condition.’
This stops the electron flow in the circuit, and the bulb doesn’t glow anymore

Saturation condition for a capacitor — *Fig.* *Saturation Condition for Capacitor*

Coming to our example/analogy,

The figure shown below is an example of 2 friends (Friend A and Friend B) for step-3

Friend A and Friend B analogy - both have equal knowledge, and hence, neither of them needs any help

At this saturation condition (here), we call the capacitor to be fully charged !!

Quick Question!

Comment on the number of charges present on the whole capacitor at initial t = 0 situation and after charging.

Answer: The number of charges/electrons remains the same in both situations.

Remember that: Positive charge is nothing but a deficiency of electrons, while negative charge is just an excess of electrons.

In the example below, after charging, a charge of +2 appears on A because there is a deficiency of 2 electrons, but these electrons are added to B, causing an excess of 2 electrons. But as a whole capacitor, the total number of electrons remains the same.

after charging, charges appear on plates

Some Cases (Covers Discharging of Capacitors):

Case 1: We disconnect the battery from the circuit and leave it open

We can see that the charge still remains on the capacitor even after disconnecting the battery as the circuit is opened and there is no current flow possible across the circuit

The battery has been disconnected, and the circuit is left open

Case-2 : Discharging of Capacitors

flow of electrons while discharging a capacitor

What did we do?

We replaced the battery with the wire -> This basically closes the circuit

What happens?

Electrons start flowing from the negative plate (excess of electrons) to the positive plate.
While moving through the circuit, the electrons pass through the bulb, which causes the bulb to light up.
But this goes on only till the positive charge gets vanished (due to neutralization done by electrons), and simultaneously, the negative charge on the negative plate also reduces as electrons leave the plate
When there’s no charge on the plates -> no potential difference -> no current -> the bulb goes off.

3. Capacitor says: “I oppose voltage change!”

Capacitors are those that don’t adapt to changes very quickly. They take some time!
As seen in the above section, a capacitor doesn’t charge up immediately on connecting it to a battery. It does take some time to build up the charge
Similarly, on replacing the battery with a wire, i.e., discharging the capacitor, the potential difference across the capacitor doesn’t become 0 immediately. It took some time for that to happen

Overall, the inference that we can take is that Capacitors oppose voltage change. It takes time for the capacitor to reach the target voltage applied across it.

But for Resistor, the case is different. It adapts to the change very quickly, unlike capacitors.

The figure shows that the potential difference across the resistor changes instantly when the voltage source is changed

4. How does Alternating Current (AC) pass through capacitor?

In the case of DC, the current flows through the capacitor only for a short interval of time

Now, coming to AC, our main goal is to study how capacitors deal with an AC source exactly.

We will again go step-by-step to understand the procedure, and also have our 2 friends’ example alongside for better understanding.

But note one important difference that in the case of a DC source, the knowledge of A was always full (constant), but now, since we are dealing with AC, for example, the knowledge of A will also vary!

An AC source can be represented as a sine curve (just to show one of many AC curves):

We divide this thing into 4 parts :

0 to +peak
+peak to 0
0 to -peak
-peak to 0

Step-1: starting at t=0, the AC voltage is going from zero towards the peak, & Capacitor is uncharged

The voltage of the AC Source is going on increasing. This causes electron flow in the circuit as charges start developing on the plates of the capacitor.
At V= +peak, the capacitor might be fully charged or charged to some extent (let’s consider the second case(i.e., charged to some extent))

The flow of electrons is shown. This makes the light bulb glow

Coming to our 2 friends example. Again, reminding: Friend A represents the power source (AC Source here) while Friend B represents the capacitor. Initially, both have zero knowledge here (as the AC voltage is zero, and also the capacitor is uncharged initially).

Initial stage - friend analogy for explaining condition of the the capacitor

Step-2 : AC voltage from +peak to 0

Now, after reaching the peak, the source voltage is starting to decrease, BUT as discussed already, the capacitor opposes voltage change, so the P.D. across the capacitor is still at the same value as it ended in the step-1.
A time will come when the source voltage will become less than the P.D. across the capacitor.
At that time, the positive plate of the capacitor will have more strength than the positive of the AC source. This will cause electrons to get attracted more towards the positive end of the capacitor. This will eventually cause the discharge of the capacitor.
Due to discharging, the current flow reverses, but electron flow is still there through the bulb, which makes the bulb continue to glow.

Coming to the Friends’ example

Step-3 : AC voltage goes from 0 to -peak

As soon as the AC voltage enters negative y, it implies that the polarities of the AC Source get reversed. This will cause the capacitor to immediately get discharged completely (THINK!)
Once discharged completely, charges will start to get developed in an opposite manner (the plate that was positive earlier becomes negatively charged this time, while the plate that was negative earlier becomes positively charged now).
Basically, the capacitor is again ‘Recharging’.
And still, as there is a flow of electrons through the bulb -> it will keep glowing.

Flow of electrons while recharging — *Fig. Recharging*

Friend B is about to get discharged, and as soon as V_source reaches some negative value, the capacitor gets completely discharged, after which, Recharging starts!

Step-4 : AC Voltage goes from -peak to 0

The source is again returning to zero, which means that the strength of the source is reducing. And again at some instant, the source voltage and P.D. across the capacitor will become the same. After this instant, once the voltage further reduces to approach zero, the capacitor gets ‘more strength’ than the AC source.
This will cause electrons to get more attracted to the positive plate of the capacitor. This will cause the discharging of the capacitor plates (as the electrons start neutralizing everything)

Flow of electrons while discharging — *Fig. Discharging*

Observation and Inference:

From the above steps, we can observe that the bulb never really stops glowing when AC voltage is applied. This shows that there was a continuous flow of electrons through the bulb. (To glow, the bulb just needs electrons to flow through it; it literally doesn’t care about the direction in which the electrons are flowing through it.)
So, basically, the continuous cycle of Charging, Discharging, and recharging keeps the electron flow happening in the circuit.

What is a Capacitor? | Charging & Discharging – Part 1