Description of a Capacitor

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Description of a Capacitor

A capacitor in its simplest form consists of two conductingplates separated by an insulating layer called a dielectric.

When a capacitor is connected in a circuit across a voltagesource, the voltage forces electrons onto the surface ofone plate and pulls electrons off the surface of the otherplate resulting in a potential difference between the plates.Capacitors are charged and discharged as needed by itsapplication. Capacitors differ in size and arrangements ofplates and the type of dielectric materials used. Paper,ceramic, air, mica, and electrolytic materials can be used,

depending on the type of dielectric needed. Thecapacitance of a capacitor may be fixed or adustable !asin a radio tuner".

Charging of a Capacitor

When a capacitor is connected across a voltage source,such as a battery, the voltage forces electrons onto oneplate resulting in a negatively charged plate. The electronsof the other plate are pulled off by the battery resulting in apositively charged plate. #ecause the dielectric betweenthe plates is an insulator, current cannot flow through it. Acapacitor has a finite amount of capacity to store charges.When a capacitor reaches its capacity it is fully charged.

The following diagrams illustrate the charging of acapacitor. $igure % shows a circuit containing a conductor

connecting a battery, an open switch, and a capacitor. Thecapacitor in $igure % is not charged. There is no potentialdifference between the plates.


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When the switch is closed, as in $igure &, there is amomentary surge of current through the conductor to andfrom the plates of the capacitor. When the current reachesthe negative plate of the capacitor, it is stopped by thedielectric.

The surge of electric current to the capacitor induces acounter electromotive force in the conductor and theplates. This counter electromotive force is call reactance.When reactance has reached a level e'ual to the voltageof the battery, the capacitor is fully charged. There is nofurther flow of current. When the capacitor is fully charged,


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the switch may be opened and the capacitor will retain itscharge !$igure (". #ecause of the difference of charges onthe plates there is a source of potential energy in thecapacitor. The energy stored is the energy that was

re'uired to charge the capacitor.

The lines of force between the plates of the capacitorrepresent an electric force field !see $igures & and (". Thiselectric force field exists because of the une'ual charges,

positive and negative, on the inside surfaces of the plates.Current cannot flow through the electrostatic field becauseof the dielectric insulator. )n other words, the difference inpotential between the plates induces within the dielectrican electrostatic field that retains the charge.

Discharging of a Capacitor

The charged capacitor shown in $igure ( is now a sourceof potential energy. This potential energy is now availablefor its intended electronic application. )f the switch isclosed, as in $igure *, current will immediately begin toflow through from the negative plate to the positive plate.The capacitor is discharging.


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The charged capacitor is the source of voltage for thecurrent flow. The current will cease flowing when the

charges of the two plates are again e'ual, meaning thatthe capacitor is completely discharged.

We have simulated the charging and discharging of acapacitor in our Capacitor Interactive Java Tutorial.

Interactive Java Tutorial

Capacitor

Discover how a capacitor is chargedand discharged.

Dielectric Materials

The dielectric material in a capacitor prevents the flow ofcurrent between its plates. )t also serves as a medium tosupport the electrostatic force of a charged capacitor. A

variety of materials are used for dielectrics as shown in thechart below.

+ielectric materials are rated based upon their ability tosupport electrostatic forces in terms of a number called adielectric constant. The ability of the dielectric to support

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electrostatic forces is directly proportional to the dielectricconstant. A vacuum is the standard by which otherdielectrics are rated. The dielectric constant of a vacuum is. -ou can see from the chart that there is very little

difference in the dielectric constant of a vacuum and air.Therefore, air is often referred to as having a dielectricconstant of .

Material

DielectricConstant

Vacuum 1.0

Air 1.000!

"ol#st#rene $.

"aper %.

Mica .&

'lint glass !.!

Meth#lalcohol

%

(l#cerin ).$

"ure water *1

Measurement of Capacitance

Capacitance is measured in farads, which is named afterichael $araday !/0123/". The symbol for farads is $.)f a charge of coulomb is placed on the plates of acapacitor and the potential difference between them is volt, the capacitance is then defined to be farad. 4necoulomb is e'ual to the charge of 3.%* x 52 electrons.

4ne farad is an extremely large 'uantity of capacitance.icrofarads !513 $" and picofarads !51% $" are morecommonly used.

The capacitance of a capacitor is proportional to the'uantity of charge that can be stored in it for each volt


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difference in potential between its plates. athematicallythis relationship is written as6

C + ,-V

Where C is capacitance in farads, , is the 'uantity ofstored electrical charge in coulombs, and V is thedifference in potential in volts.

Therefore, stored electric charge can be calculated usingthe formula6

, + CV

The difference in potential or voltage of the capacitor canbe calculated using the formula6

V + ,-C

'actors Affecting Value of Capacitance

The capacitance of a capacitor is affected by three factors6

. The area of the plates%. The distance between the plates&. The dielectric constant of the material between the

plates

7arger plates provide greater capacity to store electriccharge. Therefore, as the area of the plates increase,capacitance increases.

Capacitance is directly proportional to the electrostaticforce field between the plates. This field is stronger whenthe plates are closer together. Therefore, as the distancebetween the plates decreases, capacitance increases. As


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the distance between the plates increases, capacitancedecreases.

As discussed above, the ability of the dielectric to support

electrostatic forces is directly proportional to the dielectricconstant. Therefore, as the dielectric constant increases,capacitance increases.

Ta8ing into account each of the above three factors, thecapacitance of a capacitor with two parallel plates can becalculated using the formula6

C + *.*K A/ d

Where C is capacitance in picofarads, K is the dielectricconstant, A is the area of one plate in m2 , and d is thedistance between plates in m.

4ur 'actors Affecting Capacitance Interactive JavaTutorial demonstrates changes of capacitance as platesize, distance, and dielectric constants are adusted.


'actors AffectingCapacitance

Discover the factors affectingcapacitance.

esistive2Capacitive 3eries Circuits and Time

Constant

As a capacitor becomes charged, the current flowdecreases because the voltage developed by the capacitor increases over time and opposes the source voltage.Therefore, the rate of charge of a capacitor is reduced over

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time. The amount time re'uired to charge and discharge acapacitor is a very important factor in the design ofelectronic circuits. 9esistors are often used in combinationwith capacitors in order to control the charge and

discharge time necessary for the intended application.9esistance directly affects the time re'uired to charge acapacitor. As resistance increases, it ta8es more time tocharge a capacitor. The amount of time for the capacitor tobecome fully charged in a resistive1capacitive !9C" circuitdepends on the values of the capacitor and resistor.

The following graph shows the rate of charge of a

capacitor in a 9C circuit. :ote that the rate of chargegreatly decreases over time. The latter part of its chargingtime is many times longer than the first part. )n fact, acapacitor reaches 3&.%; of its charge in one fifth of thetime it ta8es to become fully charged. #ecause of this,capacitors in actual applications are generally not fullycharged. Capacitors in circuits are generally charged to

ust 3&.%; of full capacity. The time re'uired for a

capacitor to charge to 3&.%; of its full capacity is referredas its 9C !resistive1capacitive" time constant.


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)t is important to 8now how to calculate 9C time constantsin order to design many different 8inds of electroniccircuits. The 9C time constant of a circuit can becalculated by using the following formula6

t + C 4

Where t is time in seconds, C is capacitance in farads, and

is resistance in ohms.

4ur C Time Constant Interactive Java Tutorial demonstrates changes in the 9C time constant as valuesof resistance and capacitance are adusted.


C Time Constant

56serve changes in the C timeconstant for different values ofresistance and capacitance.

Capacitors in "arallel Circuits

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Capacitance can be increased in a circuit by connectingcapacitors in parallel as shown in the following diagram6

We 8now that capacitance of a capacitor can be increased

by increasing the size of its plates. Connecting two or morecapacitors in parallel in effect increases plate size.)ncreasing plate area ma8es it possible to store morecharge and therefore creating greater capacitance. Todetermine total capacitance of several parallel capacitors,simply add up their individual values. The following is theformula for calculating total capacitance in a circuitcontaining capacitors in parallel6

CT + C1 7 C$ 7 C% . . .

Capacitors in 3eries Circuits

Capacitance can be decreased in a circuit by capacitors inseries as shown in the following diagram6


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We 8now that capacitance of a capacitor can be decreasedby placing the plates further apart. Connecting two or morecapacitors in series in effect increases the distance

between the plates and thic8ness of the dielectric, therebydecreasing the amount of capacitance.

The following is the formula for calculating totalcapacitance in a circuit containing two capacitors in series6

CT + C1 4 C$4 C%/ - C1 7 C$7 C%/

Voltage ating of Capacitors

)n selecting an appropriate capacitor for a givenapplication, consideration must be made not only for valueof capacitance, but also for the amount of voltage thecapacitor will be subect to. Capacitors are designed towithstand a certain maximum voltage.


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Varia6le Capacitors

There are two maor types of capacitors6 fixed andvariable. The fixed capacitor has a specific value of

capacitance. A variable capacitor allows for a range ofcapacitance. >ariable capacitors are designed so thatcapacitance can be changed through a mechanical meanssuch as adusting a screw or turning a shaft. >ariablecapacitors are used when the application re'uires anadustment of capacitance such as in a radio tuner.

#elow is a typical variable capacitor. )t has two sets of

plates. 4ne set is called the rotor and the other the stator.The rotor is usually connected to a 8nob outside thecapacitor. The two sets of plates are close together but nottouching. Air is the dielectric in a variable capacitor. As the8nob is turned, the sets of plates become more or lessmeshed, increasing or decreasing the distance betweenthe plates. As the plates become more meshed,capacitance increases. As the plates become less meshed,

capacitance decreases.

4ur Varia6le Capacitor Interactive Java Tutorial demonstrates the mechanics of a variable capacitor.


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Varia6le Capacitor 56serve the mechanics of a varia6lecapacitor.

Capacitors in Action

Computer Memor#

)n most cases, the main memory of a computer is a high1speed random1access memory !9A". Two types of mainmemory are possible with 9A circuits, static random1access memory !?9A" and dynamic random1access

memory !+9A". A single memory chip is made up ofseveral million memory cells. )n a ?9A chip, eachmemory cell consists of a resistor circuit flip1flop for storingthe binary digits or 5. )n a +9A chip, each memory cellconsists of a capacitor rather than a resistor circuit flip1flop.When a capacitor is electrically charged, it is said to storethe binary digit , and when discharged, it represents 5.$igure 5 below shows a portion of a memory chipcontaining 3 memory cells.

http://micro.magnet.fsu.edu/electromag/java/varcapacitor/index.html


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alternating flows of current provide wea8 electronic signalswhich travel to a mixer, then to an amplifier, and finally to aloudspea8er. -ou can observe the operation of acondenser microphone at our Condenser Microphone

Java Tutorial.


Condenser Microphone Discover how a microphone wor8s.

adio eceiver

>ariable capacitors are used in tuning circuits of radios. )n$igure , a variable capacitor is connected to an antenna1transformer circuit. Transmitted radio waves cause aninduced current to flow in the antenna through the primarycoil to ground.

A secondary current in the opposite direction is induced inthe secondary coil. This current flows to the capacitor. We

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8now that the surge of current to the capacitor induces acounter electromotive force. This counter electromotiveforce is call reactance. The induced flow of current throughthe coil also induces a counter electromotive force. This is

called inductive reactance. ?o we have both capacitive andinductive reactance.

At higher fre'uencies, inductive reactance is greater andcapacitive reactance is smaller. At lower fre'uencies theopposite is true. A variable capacitor is used to e'ualizethe inductive and capacitive reactances. The condition inwhich the reactances are e'ualized is called resonance.

The particular fre'uency that is isolated by the e'ualizedreactances is called the resonant fre'uency.

A radio circuit is tuned by adusting the capacitance of avariable capacitor to e'ualize the inductive and capacitivereactance of the circuit for the desired resonant fre'uency,or in other words, to tune in the desired radio station. 4uradio eceiver Interactive Java Tutorial demonstrates

how a variable capacitor is used to tune in radiofre'uencies.


adio eceiver Discover how a varia6le capacitor isused to tune in radio fre9uencies.

"ulsed Magnets At the :ational =igh agnetic $ield 7aboratory, magnetsare used for research in all areas of science, includingbiology, chemistry, geology, engineering, material science,and physics. 9esearch in high magnetic fields is critical

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"ulsed Magnets 3ee how pulsed magnets are powered6# capacitors.

7in8s to more information about pulsed magnets6
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Description of a Capacitor