Learning About Capacitors | Article on Basic Electronics

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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 the momentum, it consumes much lesser energy. To provide that initial burst of energy, capacitors come handy !!

Ceiling fan 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 the bed. And out of nowhere, mom enters the room and spanks, 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 the bed !!

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

Topics Covered :

Capacitors as storage devices
How is charging and discharging done? (*Imp)
I oppose change in voltage !
How does AC pass through?
How to read capacitors ?

1. 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

Fig.1

Basic Structure of Capacitor :

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

Note:

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

First one is normal & Second one is representation for polarized capacitor (discussed later)

2. How is Charging and Discharging of a Capacitor done? **

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 battery while Friend B is analogous to capacitor.

Step-1 :

Fig. We have just set up everything as shown in the figure above. It’s t=0

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

Step-2 :

Note :

Negative terminal of battery can be considered to have a cluster of free electrons (lot of electrons) and because they are free, they tend to move across the circuit
Wherever there is a separation of charges, 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 excess of electrons on the plate. This makes the plate to become negatively charged (-).

Simultaneously, we have electrons from the other plate getting attracted towards the positive terminal of battery. This causes 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 to glow !! (but only for short time (discussed below))

As mentioned in ‘Note’ above, potential difference (p.d.) will get created across the capacitor (but still Vbattery > p.d. across the capacitor)

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

Step-3 :

The Step-2 keeps on happening (battery keeps pushing electrons to one plate and keeps pulling electrons from another plate) until Vbattery = p.d. across capacitor. Because it implies that there is sufficient negative charge on the plate which is capable enough to repel 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 capacitor as ‘saturated condition’
This stops the electron flow in the circuit and the bulb doesn’t glow anymore

Fig. Saturation Condition for Capacitor

Coming to our example,

Fig. Example of 2 friends (Friend A and Friend B) for step-3

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 situation .

Answer :

The number of charges/electrons remain the same in both the situation. Remember that : Positive charge is nothing but deficiency of electrons while negative charge is just excess of electrons.

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

Some Cases :

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

Case-2: Discharging of Capacitor

What we did ?

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

What happens ?

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

3. Capacitor says : "I oppose voltage change !"

Capacitors are someone who 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 battery. It does take some time for building up of the charge
Similarly, on replacing the battery with wire, i.e. discharging of capacitor, the potential difference across capacitor doesn’t become 0 immediately. It took some time for that to happen

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

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

4. How does AC pass through !?

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

Fig. DC current flow through capacitor

Now, coming to AC, our main goal is to study, how capacitors deal with 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 case of DC source, the knowledge of A was always full (constant) but now since we are dealing with AC, for the example purpose, the knowledge of A will also vary !

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

We divide this thing into 4 parts :

(i) 0 to +peak

(ii) +peak to 0

(iii) 0 to -peak

(iv) -peak to 0

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

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

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

Step-2 : AC voltage from +peak to 0

Now after reaching the peak, the source voltage is starting to decrease BUT as discussed already, capacitor oppose voltage change so the p.d. across capacitor is still at the same value as it ended in the step-1.
A time will come when the source voltage will become lesser than the p.d. across capacitor.
At that time, the positive plate of capacitor will have more strength than positive of AC source. This will cause electrons to get attracted more towards + of capacitor. This will eventually cause discharging of 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 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 AC Source get reversed. This will make the capacitor to immediately get discharged completely (THINK !)
Once discharged completely, charges will start to get developed in an opposite manner (the plate which was positive earlier becomes negatively charged this time while the plate which was negative earlier becomes positively charged now).
Basically, the capacitor is again ‘Recharging’.
And still, as there is flow of electrons through the bulb –> it will keep glowing !!

Fig. Recharging

Friend B is about to get discharged and as soon as Vsource reaches some negative value, 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 capacitor will become same. After this instant, once the voltage further reduces to approach zero, the capacitor gets ‘more strength’ than AC source.
This will cause electrons to get more attracted towards positive plate of capacitor. This will cause discharging of capacitor plates (as the electrons start neutralizing everything)

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 continuous flow of electrons through the bulb. (In order 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, bascially, the continuous cycle of Charging, Discharging, Recharging, discharging keeps the electron flow happening in the circuit .

5. How to read Capacitors ?

Capacitance is the ability of the body to store charge.

But this ability itself depends on 3 things:

Surface area of the metal plates
Type of dielectric which we are using between the plates
Distance between the plates

Unit of capacitance : farad (F)

1F implies that the capacitor has the ability to store 1C of charge on each plate (+1C and -1C) when 1V of potential difference is applied across it.

For example, we have 2 capacitors. 1st has a capacitance of 2F and 2nd has a capacitance of 10F. Which has more capacitance ?

For the 2F capacitor, we can store 2C charge on plates with the potential difference of 1V. But for the same potential difference of 1V, we can store 10C of charge in case of 10F capacitor. So obviously, 10F is a capacitor of higher capacitance than 2F

Note:

We can observe that the area of the plates, distance between the plate and the dielectric, are different for both. It is not possible to have different capacitances with the same 3 parameters. Atleast one of them should differ !

For example,

You need to store 5 laddoos in a given container. On keeping all the laddoos, You find the container is fully filled.
Next, You are asked to fill 10 laddoos in a container of your choice. It’s obvious that you won’t choose the previous container; instead you would choose the one with larger dimensions –> since it gives more capacity !!

Reading Values :

We have three-digit marking system for mica disc capacitors. Refer to the picture below.

Checking Working Voltage/Rated Voltage

Often, you will also see one more value in addition to the capacitance. It is usually given in V (volts). This is nothing but the ‘Working voltage’ and also known as ‘Rated Voltage’.

Your capacitor should atleast have a working voltage which is 10-15 % higher than the supply voltage in your circuit. This keeps your circuit safe and also keeps you safe !!

Take Care of Polarity !

Some Capacitors are polarized which means that, the way you insert them/connect them to the circuit matters a lot !

The positively marked terminal should be kept at a higher voltage than the negative terminal

Usually, the manufacturers keep a separate band which directs the user towards the negative terminal of polarised capacitor.

Conclusion :

In this article, we got know the basics about the capacitors. But the fact that it is basic doesn’t make it less important.

Similar to the Resistors, Capacitors can also be combined together in order to get a single capacitance out of the given network/circuit. In addition to this, there are also some important things to be covered about Dielectrics in capacitor !!

Keep Learning !!

All the Best !!

Learning about Capacitors !