Likewise, capacitors exhibit many properties , when used in AC or DC circuits and hence they play important role in electrical and electronic circuits.
As said before , there are different types of capacitors. These different types will have different type of construction. A Parallel plate capacitor is the simplest capacitor. Let us understand the construction of this capacitor. It consists of two metal plate separated by a distance. The space between these two plates is filled with a dielectric material. The two leads of the capacitor are taken from these two plates.
The capacitance of the capacitor depends on the distance between the plates and area of the plates. Capacitance value can be changed by varying any of these parameters. Dielectric acts as an insulating material between the plates. Dielectric can be any non conducting material such as ceramic, waxed paper, mica, plastic or some form of a liquid gel.
Dielectric also plays an important in deciding the value of capacitance. As the dielectric is introduced between the plates of the capacitor ,its value increases. The expression for the complex permittivity is given as follows,. The value of dielectric constant or complex permittivity varies from one dielectric material to another. As said before capacitor consists of two conductor separated by a dielectric , when there is any potential difference between the two conductors electric potential is developed.
This causes the capacitor to charge and discharge. Let us understand this in a practical way. When the capacitor is connected to a battery a DC source , current starts flowing through the circuit. Thus negative charge is accumulated on one plate and positive charge is accumulated on the other plate. This process continuous until the capacitor voltage reaches supply voltage.
When the charging voltage is equal to the supply voltage capacitor stops charging further even though the battery is connected. When the battery is removed two plates will be accumulated with positive and negative charges. Thus the charge is stored in the capacitor. But when the supply voltage is from an AC source it charges and discharges continuously.
The rate of charging and discharging depends on the frequency of the source. Working can be understood using simple example here. Below circuit shows two switches A and B. When switch 1 is closed , current starts flowing from from the battery to the capacitor. When the capacitor voltage reaches the supply voltage ,it stops charging further.
Now connect the switch to position B. Now you can observe the LED starts glowing and this slowly fades out as the capacitor is discharging. Capacitance is the property of the capacitor that defines the maximum amount of electrical charge stored in it. Capacitance may vary depending on the shape of the capacitor. Capacitance can be calculated by using the geometry of the conductors and dielectric material properties.
Let us see the capacitance of a parallel plate capacitor. Capacitance is defined as the ratio of charge Q on the either plates to the potential difference V between them ,. From the above definition we can observe that capacitance is directly proportional to the charge Q and is inversely proportional to the voltage V. Capacitors consist of conducting surfaces separated dielectric insulator. The effect of this is that when a voltage is applied, charge flows into the capacitor and is stored. When an external circuit is connected to the capacitor, this stored charge will flow from the capacitor into the circuit.
Capacitance is a measure of amount of charge which can be stored within a capacitor. The SI unit of capacitance is the farad F. The farad is the ratio of electrical charge stored by the capacitor to the voltage applied:.
Simpler geometries can also be solved using other methods the example shows an example for a parallel plate capacitor. Capacitor shown and assume the dielectric is a vacuum. Electrostatic theory suggests that the ratio of electric flux density to electric field strength is the permittivity of free space:. The above equations can be combined and solved to give the capacitance of a parallel plate capacitor with a free air dielectric as:.
For more real dielectrics the capacitance will be increased directly in proportion to the the relative permittivity and given by:. Charging and discharging of capacitors follows an exponential law. Consider the circuit which shows a capacitor connected to a d. The resistor represents the leakage resistance of the capacitor, resistance of external leads and connections and any deliberately introduced resistance.
When the switch is closed, the initial voltage across the capacitor C is zero and the current i is given by:. From Kirchhoff's voltage law, the d.
The voltage across the capacitor will increase from zero to that of the d. Note: increasing the value of resistance R, will increase the time constant resulting in a slower charge or discharge of the capacitor. When discharging the current behaves the same as that for charging, but in the opposite direction. Voltage across the capacitor will decay exponentially to zero.
Equations for both current and voltage discharge can be determined in a similar way to that shown above and are summarized as:. By integrating the instantaneous energy as the capacitor voltage rises, we can find the total energy stored:. It is worth noting that when connecting capacitors in series, the total capacitance reduces but the voltage rating increases. Connecting in parallel keeps the voltage rating the same, but increases the total capacitance.
Either way the total energy storage of any combination is simply the sum of the storage capacity of each individual capacitor.
In charging an ideal capacitor there are no losses. However, should a capacitor be charged via a resistor then it should be understood that half of the charging energy will be lost and dissipated as heat across the capacitor. It can be seen that the energy loss is the same as that stored within the capacitor.
On discharging, there will also be half the store energy lost within the resistor. Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author. Trackback from Notes Capacitors have numerous applications in electrical and electronic applications.
This note, examines the use of capacitors to store electrical energy. The sidebar shows details of a typical commercially available energy storage module.
The other day I came across an article in Technology Review on the development of a smart transformer. A professor at North Carolina State University is Reading is a bit of a hobby of mine and I"ve done a few off-topic posts in the past on this. Rather than continue doing the occasional post I thought I have been thinking recently that there appears to be less professional integrity around than when I first started my career in electrical engineering It wasn't so long ago I was telling someone that I don't use rules of thumb as most things are easily calculated anyhow.
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