Capacitance and Charging

Capacitance, or shallow, capacity is the ability of an object to store electrical charge. Every electrically charged object is shallow.

Capacitors store electrical energy in their plates in the form of electric charge.

Capacitors consist of two parallel conductive plates (usually a metal) that are prevented from contacting (separating) each other with an insulating material called "dielectric".When a voltage is applied to these plates, an electric current flows by charging one plate positively according to the feed voltage and the other plate with an equal and opposite negative charge.

Next, a capacitor is capable of storing the electrical charge of electrons Q (units in Coulomb).Onewhen the capacitor is fully charged, there is a potential difference between the plates, the larger the area of the PD and plates and/or the smaller the distance between them (known as separation),the greater the load the capacitor can carry, and the greater its capacity.

The ability of capacitors to store this electrical charge (Q) between the plates isthe voltage applied to the capacitor is proportional to V.Note that C capacitance is DAMA positive and never negative.

The larger the voltage applied,the greater the load stored on the plates of the capacitor.Likewise, the smaller the voltage applied, the smaller the charge.Therefore, the actual Q load on the plates of the capacitor and can be calculated as follows:

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Charging in capacitor

Where: Q (Charge, in Coulomb) = C (capacitance, in Persian) x V (Voltage, volts)

Sometimes it's easier to remember this relationship using pictures.Here, three amounts of Q, C and V are placed in a triangle that gives off loads, with capacitance at the top and voltage at the bottom.This arrangement represents the actual position of each quantity in capacitor load formulas.

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and the displacement of the above equation gives us the following combinations of the same equation:

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Where: Q measured in Coulomb, V in volts and C in Farad.

Then from above we can define the Capacitance unit as a constant of proportion equal to Coulomb/volt, also called Farad.

Since capacitance represents the ability (capacity) of capacitors to store an electrical charge on their plates, we can first define a Farad as "the capacitance of a capacitor that requires a coulomb load to make a potential difference of one volt between its plates".Identified by Michael Faraday.Therefore, the larger the capacitance, the higher the amount of load stored in a capacitor for the same amount of voltage.

The ability of a capacitor to store a load on conductive plates is called capacitance value. Capacitance can also be determined from the dimensions or area of the plates, the A plates and the characteristics of the dielectric material between the plates.A measure of dielectric material is given with permeability (ε ) or dielectric constant.So another way to express the capacitance of a capacitor is:

Air-Used Capacitor as Dielectric

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DielectricAlly Solid UsedCapacitor

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The smaller the distance between the two plates, the higher the load storage capability of the plates, since -Q has a greater effect on the loaded plate -and its load has a greater effect on the plate loaded with +Q, which causes more electrons to be pushed from +.

ε 0 (epsilon) is the permeability value for air with 8.84 x 10 -12 F/m, and ε r is the permeability of the dielectric environment used between the two plates.

Parallel Plate Capacitor

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We have previously said that the capacitance of a capacitor with parallel plates is proportional to the surface area A between the two plates and inversely proportional to distance d, and this applies to the dielectric air environment.However, the capacitance value of a capacitor can be increased by placing a solid medium between conductive plates whose dielectric constant is larger than that of air.

Typical epsilon ε values for various commonly used dielectric materials are: Air = 1.0, Paper = 2.5 – 3.5, Glass = 3 – 10, Mika = 5 – 7, etc.

Dielectric material or insulation, compared to airthe factor for increasing the capacitance of the capacitor is known as the Dielectric Constant ( k ). "k" is the ratio of the permeability of the dielectric environment used to the permeability of the free space, otherwise known as vacuum.

Therefore, all capacitance values are related to the permeability of the vacuum.A dielectric material with a high dielectric constant is a better insulator than a dielectric material with a lower dielectric constant.A dielectric constant is a sizeless quantity because it is relative to free space.

Capacitance Question Example 1

A parallel plate capacitor consists of two plates with a total surface area of 100 cm2.The plate separation is 0.2 cm and if the dielectric environment used is air, what will be the capacitance of the capacitor in pico-Farads (pF)?

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then the capacitor is 44pF.

Charging and Discharge of the Capacitor

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Suppose the capacitor is completely discharged and the switch connected to the capacitor has just been moved to position A.The voltage on the 100uf capacitor is zero at this point, and a charging current (i) starts to flow by charging the capacitor until the voltage between the plates is equal to the 12v supply voltage.The charging current stops flowing and the capacitor is said to be "fully charged."Then Vc = Vs = 12v.

When the capacitor is in theory "full charge", the voltage will maintain the charging state even when the supply voltage is interrupted, as they act as a kind of temporary storage device.However, while this is true for an "ideal" capacitor, a truethe capacitor will slowly empty itself for a long time due to internal leakage currents flowing through the dielectric.

This is a large value that connects to high voltage sourcesSince capacitors can maintain a significant amount of charge even when the supply voltage is "OFF", it is an important point to remember.

If the key is disconnected at this point,the capacitor will retain its charge indefinitely, but due to internal leakage currents flowing through the dielectric, electrons willthe capacitor will begin to empty itself very slowly.The time it takes for the capacitor to discharge up to 37% of the supply voltage is known as the Time Constant.

If the switch is now moved from position A to position B , the full-charge capacitor lights the lamp until the capacitor is completely discharged, since the element of the lamp has a resistant value, it begins to discharge through the lamp that is now connected to it.

The brightness and lighting time of the lamp will ultimately depend on the capacitance value of the capacitor and the resistance of the lamp (t = R*C).The larger the value of the capacitor, the brighter the lamp will be, the more active it will be, and the longer it burns.

Capacitor Charging Question Example 2

Calculate the load on the capacitor circuit above.

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the calculated load on the capacitor is 1.2 milicoulomb.

Current Passing Through capacitor

The electric current cannot flow through a capacitor, such as a resistance or inductor, due to the insulating properties of the dielectric material between the two plates.However, charging and discharge of the two plates gives the effect that the current flows.

The current passing through a capacitor is directly related to the load on the plates, as it is the load flow rate over time.Since the load ( Q) storage capability between the plates of capacitors is proportional to the applied voltage ( V ), the relationship between the current and voltage applied to the plates of a capacitor is as follows:

Current-Voltage (IV) Relationship

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As the voltage on the plates increases (or decreases) over time, the current flowing through the capacitance accumulates (or removes) the load on the plates with the amount of load proportional to the applied voltage.Then both current and voltage applied to a capacitance are functions of time and are indicated by the symbols i (t) and v (t).

However, from the equation above, we can also see that if the voltage remains constant, the load will be constant, and therefore the current will be zero!…In other words, there is no change in voltage, no load movement and no current flow.Therefore, when a capacitor is connected to a constant DC voltage, it appears to "block" the current flow.

Farad

We now know that the load storage capability of a capacitor gives it a C of capacitance, which is Farad unit F.But Farad is an extremely large unit in its own right and its use is impractical, so the lower layers or fractions of the standard Farad unit are used.

To get an idea of how large a Farad really is, the surface area of the plates required to produce a capacitor of only one Farad value, with a reasonable plate separation of only 1 mm running in the vacuum.If we rearrange the capacitance equation above, this gives us a plate area:

A = Cd ÷ 8.85pF/m = (1 x 0.001) ÷ 8.85×10 -12 = 112,994,350 m 2

or 113 million m 2, which is very large.

Capacitors with a Farad or more value tend to be a solid dielectric, and because the use of "A Farad" is a very large unit, reduced values are used for capacitor values.

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Sub-Units of Farad

As an example, you can check out these conversions:

a) 22nF = 0.022μF

b) 0.2μF = 200nF

c) 550pF = 0.00055μF

Although a Farad is a great value in itself, capacitors are now widely available with capacitance values of hundreds of Farads and have the names "Super capacitors" or "Ultra capacitors" to reflect this.

These capacitors are electrochemical energy storage devices that use the high surface area of carbon dielectrics to provide much higher energy densities than conventional capacitors, and since capacitance is proportional to the surface area of carbon, the thicker the carbon, the greater the capacitance.

Low-voltage supercapacitors are widely used in portable handheld devices to replace large, expensive and heavy lithium-type batteries, as they provide battery-like storage and discharge capabilities, making them ideal for use as an alternative power source or for battery backup.Supercapacitors used in handheld devices are usually charged using solar cells installed in the device.

Ultracapacitors are being developed for use in hybrid electric cars and alternative energy applications instead of large conventional batteries, as well as DC softening applications in vehicle audio and video systems.Ultracapacitors can be recharged quickly and have very high energy storage densities, making them ideal for use in electric vehicle applications.

Energy in capacitor

When a capacitor charges from its connected power supply,an electrostatic field is formed in the capacitor that stores energy.The amount of energy in Joule stored in this electrostatic field isequals the energy applied by the capacitor to maintain the load on its plates and is given with the following formula:

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therefore the above 100uFThe energy stored in the capacitor circuit is calculated as follows:

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In the next tutorial in our section on capacitors, we will look at the Capacitor Color Codes.