CAPACITORS

INTRODUCTION:

Capacitor is a basic component of electronic circuits and basically it is used to store electrical energy for small periods of time. Capacitor is a passive component which store charge on its plates when connected to a source, and Capacitor stores the energy in the form of electric field.  The charged capacitor produces a potential difference (static voltage) across its plates just like a small rechargeable battery.

CONSTRUCTION:

Capacitors are of different sizes and power ratings but the basic construction of all of the capacitors it simple same.

Capacitor consists of two parallel metallic plates placed close to each other but not physically/electrically connected but are separated by Dielectric. Dielectric is some medium like air, paper, mica, ceramic or some liquid gel used in electrolytic capacitors. Because of this dielectric, capacitor blocks the DC as it doesn't gives any conducting path for the current to flow. So it could be a logical reasoning of the fact that capacitor blocks DC, and we will prove this fact later mathematically also.       

                                                                (Figure.1.1)

The above figure shows a basic construction of a capacitor, two parallel plates are separated by a dielectric medium, one is positively charged and other one is negatively charged. When the plates are connected to a DC source, the capacitor starts charging within fractions of seconds and stops when the capacitor is fully charged. In the beginning, the plates were discharged and so voltage V across the plates would be Zero ideally and as we switched on the DC source, the current starts flowing. So initially current I would be at maximum value. As the plates starts charging, the current decreases and the voltage across the plates increases and finally the voltage across the plates would be equal to the applied DC source and the current would be Zero ideally. 

So by this, we can say that both voltage and charging current are inversely proportional to each other.

Charging Current of a capacitor is the amount of current a capacitor draws when the source is connected to the capacitor.

We can see, charge on the plates will increase as we increase the potential difference across the plates so Q is directly proportional to V. 

The amount of charge on a capacitor is Q and can be given by the equation

  Q=CV                .......( 1 )

Here C is the constant of proportionality and called capacitance of a capacitor  and  V is the applied voltage. 

(Figure.1.2)

Every capacitor has its own charge storing capacity called the capacitance of the capacitor and is given by the relation.

A is the area of plates, d is the distance between the plates and  εo is known as Permittivity of free space.

From the above equation, we can conclude that charge in the plates increases as we increase the area of plates and decreases as we increase the distance between the plates. 

Capacitance of the capacitor the measure of ability of a capacitor to store electric charge in its electric field and its Unit is Farad named after the physicist Michael Faraday. However farad is a large unit of measurement so we use smaller units like Micro Farad and Nano Farad.

Capacitance will be One Farad if One Coulomb charge is stored on plates if capacitor by applying One Volt potential difference.

Task: Make a simple capacitor by yourself taking two metallic conducting small plates or simply use aluminium foil as plate and a paper as a dielectric.

BEHAVIOR WITH AC/DC:

As we know that we have two type of sources AC and DC. Capacitors can be used in both the circuits but they have different behavior.

Capacitors in AC Circuits behaves like a short circuit and allows the current to flow through it. logically it is because of the fact that in AC (50Hz),  the supply voltage is 50 times positive and 50 times negative in one second. So this doesn't allows the capacitor to charge and capacitor always remain in charging and discharging condition, hence passing the current through it.

Capacitors in DC Circuits behaves like an open circuit when capacitor is fully charged. As discussed earlier, when the capacitor is fully charged the voltage across the plates of capacitor becomes equal to the voltage applied by the source and the current stops flowing. Hence  the capacitor blocks DC to flow through it.

CAPACITOR CONNECTIONS:

As we know that we have two basic type of connections Series and Parallel. we will now see the behavior of capacitors if connected in series or in parallel.

Capacitor In Series simply behaves like resistors in parallel. if we connect capacitors in series, the equivalent capacitance of the circuit would be less than the smallest capacitance of among all.

Figure(1.3)

 If we have three capacitors of same capacitance and connected in series as shown in the figure.1.3.

 So                                             

Q1=C1/V1,       Q2=C2/V2,       Q3=C3/V3

 Qt= Ct/Vcc      also   Qt = Q1+Q2+Q3

 Vcc = Qt/Ct,  V1= Qt/C1,    V2= Qt/C2,   V3=Qt/C3

 Qt/Ct = Qt/C1 + Qt/C2 + Qt/C3      

  Qt/Ct= Qt( 1 / C1+C2+C3)     

  1/Ct = 1/( C1+C2+C3)       

If we have n number of capacitors connected in series then their total capacitance would be

1/Ct=1/C1_C2+C3+.......+Cn

Capacitors In Parallel acts as the resistors behaves in series, which means the total capacitance will be simply the sum of all the capacitance connected.

 Ct=C1+C2+C3+....+Cn

Reactance is the basically the amount of resistance whist a capacitor provides to the flow of current, it is denoted by Xc and is given by the formula 

 Xc=1/2*3.14*f*C

 From the above equation we can conclude a very important result that if frequency is zero (In case of DC supply) the opposition provided by the capacitor to the flow of current would be infinity and as the frequency increases the opposition will become lesser and lesser.

APPLICATIONS:

Capacitors have a lot of applications, and we can't find any of the electronic circuit without capacitors.

Capacitors are used to remove the ripples from a fluctuating DC.

Capacitors are used to store charge for high speed use.

Capacitors are used in power supplies, buck boost converters, filters, inverters and different electronic equipment.

Power Capacitors are used in industries and power plants or sub stations to improve the power factor of the system.

We must know that in capacitor current leads voltage by an angle of 90 degree and because of this factor we use capacitors to improve the lagging current (power factor).A capacitor provides reactive power to counter the inductive load in the system.

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