Capacitor

context: A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store electrical energy temporarily in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e. an insulator that can store energy by becoming polarized). The conductors can be thin films, foils or sintered beads of metal or conductive electrolyte, etc. The nonconducting dielectric acts to increase the capacitor's charge capacity. Materials commonly used as dielectrics include glass, ceramic, plastic film, air, vacuum, paper, mica, and oxide layers. Capacitors are widely used as parts of electrical circuits in many common electrical devices. Unlike a resistor, an ideal capacitor does not dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field between its plates.
qas: answers: text: a condenser
question: What was a capacitor originally known as?

answers: text: two electrical conductors
question: How many electrical conductors, or plates, do all capacitors contain at a minimum?

answers: text: a dielectric
question: What is the name of the insulator in all capacitors that can store energy by becoming polarized?

answers: text: the form of an electrostatic field
question: In what form does a capacitor store energy?

answers: text: between its plates
question: Where is the energy stored by a capacitor located?

context: When there is a potential difference across the conductors (e.g., when a capacitor is attached across a battery), an electric field develops across the dielectric, causing positive charge +Q to collect on one plate and negative charge −Q to collect on the other plate. If a battery has been attached to a capacitor for a sufficient amount of time, no current can flow through the capacitor. However, if a time-varying voltage is applied across the leads of the capacitor, a displacement current can flow.
qas: answers: text: When there is a potential difference across the conductors
question: When does an electric field develop across the dielectric?

answers: text: if a time-varying voltage is applied across the leads
question: Under what condition can a displacement current flow in a capacitor?

answers: text: when a capacitor is attached across a battery
question: What is an example of when there exists a potential difference across the conductors? 

answers: text: no current can flow through the capacitor
question: What happens to the current when a battery has been attached to a capacitor for an adequate amount of time?

answers: text: positive charge +Q to collect on one plate and negative charge −Q to collect on the other
question: How do charges accumulate on each plate of a capacitor after an electric field has developed across the dielectric?

context: In October 1745, Ewald Georg von Kleist of Pomerania, Germany, found that charge could be stored by connecting a high-voltage electrostatic generator by a wire to a volume of water in a hand-held glass jar. Von Kleist's hand and the water acted as conductors, and the jar as a dielectric (although details of the mechanism were incorrectly identified at the time). Von Kleist found that touching the wire resulted in a powerful spark, much more painful than that obtained from an electrostatic machine. The following year, the Dutch physicist Pieter van Musschenbroek invented a similar capacitor, which was named the Leyden jar, after the University of Leiden where he worked. He also was impressed by the power of the shock he received, writing, "I would not take a second shock for the kingdom of France."
qas: answers: text: Ewald Georg von Kleist
question: Who first discovered the basic properties of capacitors?

answers: text: In October 1745
question: When were the basic properties of capacitors first discovered?

answers: text: a hand-held glass jar
question: In the original experiment in which the properties of capacitors were discovered, what component acted as the dielectric? 

answers: text: the water acted as conductors
question: Besides the scientist's hand, what other component of the experiment in which the basic properties of capacitors were discovered acted as the second conductor?

answers: text: Pieter van Musschenbroek
question: What was the name of the Dutch physicist who invented the Leyden Jar?

context: Daniel Gralath was the first to combine several jars in parallel into a "battery" to increase the charge storage capacity. Benjamin Franklin investigated the Leyden jar and came to the conclusion that the charge was stored on the glass, not in the water as others had assumed. He also adopted the term "battery", (denoting the increasing of power with a row of similar units as in a battery of cannon), subsequently applied to clusters of electrochemical cells. Leyden jars were later made by coating the inside and outside of jars with metal foil, leaving a space at the mouth to prevent arcing between the foils.[citation needed] The earliest unit of capacitance was the jar, equivalent to about 1.11 nanofarads.
qas: answers: text: Daniel Gralath
question: Who was the first person to connect several Leyden jars in parallel?

answers: text: stored on the glass
question: Where did Benjamin Franklin believe the charge was stored in Leyden jars?

answers: text: Benjamin Franklin
question: Who coined the term "battery"?

answers: text: 1.11 nanofarads
question: How many nanofarads did the earliest unit of capacitance equate to? 

answers: text: to prevent arcing between the foils
question: Why was a gap left at the mouth of Leyden jars?

context: Since the beginning of the study of electricity non conductive materials like glass, porcelain, paper and mica have been used as insulators. These materials some decades later were also well-suited for further use as the dielectric for the first capacitors. Paper capacitors made by sandwiching a strip of impregnated paper between strips of metal, and rolling the result into a cylinder were commonly used in the late 19century; their manufacture started in 1876, and they were used from the early 20th century as decoupling capacitors in telecommunications (telephony).
qas: answers: text: glass
question: Besides porcelain, paper and mica, what other non conductive material was used as an insulator? 

answers: text: as the dielectric
question: For what use were non conductive materials used in the first capacitors?

answers: text: strip of impregnated paper
question: What was layered between strips of metal in order to create paper capacitors?

answers: text: in 1876
question: When were paper capacitors first manufactured?

answers: text: as decoupling capacitors
question: What other use did paper capacitors serve in the telecommunications industry?

context: Charles Pollak (born Karol Pollak), the inventor of the first electrolytic capacitors, found out that the oxide layer on an aluminum anode remained stable in a neutral or alkaline electrolyte, even when the power was switched off. In 1896 he filed a patent for an "Electric liquid capacitor with aluminum electrodes." Solid electrolyte tantalum capacitors were invented by Bell Laboratories in the early 1950s as a miniaturized and more reliable low-voltage support capacitor to complement their newly invented transistor.
qas: answers: text: Charles Pollak
question: Who invented the first electrolytic capacitor?

answers: text: a neutral or alkaline electrolyte
question: In what sort of electrolyte does the oxide layer on an aluminum anode remain stable?

answers: text: In 1896
question: In what year was the patent filed for an electric liquid capacitor?

answers: text: Solid electrolyte tantalum capacitors
question: What sort of capacitors were created by Bell Labs in the 1950's?

answers: text: to complement their newly invented transistor
question: Why did Bell labs create their new type of capacitor?

context: Last but not least the electric double-layer capacitor (now Supercapacitors) were invented. In 1957 H. Becker developed a "Low voltage electrolytic capacitor with porous carbon electrodes". He believed that the energy was stored as a charge in the carbon pores used in his capacitor as in the pores of the etched foils of electrolytic capacitors. Because the double layer mechanism was not known by him at the time, he wrote in the patent: "It is not known exactly what is taking place in the component if it is used for energy storage, but it leads to an extremely high capacity.".
qas: answers: text: Supercapacitors
question: What are electric double-layer capacitors called today?

answers: text: In 1957
question: When were low voltage electrolytic capacitors with porous carbon electrodes invented?

answers: text: H. Becker
question: Who invented low voltage electrolytic capacitors with porous carbon electrodes?

answers: text: in the carbon pores
question: In what part of the capacitor did Becker believe the charge was being stored?

answers: text: the pores of the etched foils
question: In what component of electrolytic capacitors did Becker believe was similar to the porous carbon electrodes?

context: A capacitor consists of two conductors separated by a non-conductive region. The non-conductive region is called the dielectric. In simpler terms, the dielectric is just an electrical insulator. Examples of dielectric media are glass, air, paper, vacuum, and even a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from any external electric field. The conductors thus hold equal and opposite charges on their facing surfaces, and the dielectric develops an electric field. In SI units, a capacitance of one farad means that one coulomb of charge on each conductor causes a voltage of one volt across the device.
qas: answers: text: a non-conductive region
question: What type of area is between the two conductors in a capacitor?

answers: text: the dielectric
question: What is the name given to the area between two conductors in a capacitor?

answers: text: no net electric charge
question: What net electric charge are capacitors assumed to have?

answers: text: equal and opposite charges
question: What type of charges do conductors hold on their facing surfaces?

answers: text: the dielectric
question: What region within a capacitor develops an electric field?

context: The current I(t) through any component in an electric circuit is defined as the rate of flow of a charge Q(t) passing through it, but actual charges—electrons—cannot pass through the dielectric layer of a capacitor. Rather, one electron accumulates on the negative plate for each one that leaves the positive plate, resulting in an electron depletion and consequent positive charge on one electrode that is equal and opposite to the accumulated negative charge on the other. Thus the charge on the electrodes is equal to the integral of the current as well as proportional to the voltage, as discussed above. As with any antiderivative, a constant of integration is added to represent the initial voltage V(t0). This is the integral form of the capacitor equation:
qas: answers: text: the rate of flow of a charge Q(t) passing through it
question: What is the definition of the current I(t) through any component in an electric circuit?

answers: text: one electron accumulates on the negative plate
question: For each electron that departs the positive plate, how many electrons build up on the negative plate?

answers: text: the integral of the current
question: To what is the charge on the electrodes of a capacitor equal to?

answers: text: proportional to the voltage
question: To what is the charge on the electrodes of a capacitor proportional to?

answers: text: the initial voltage V(t0)
question: When calculating the integral to determine the charge on the electrodes of a capacitor, what does the constant of integration that must be added represent?

context: The simplest model capacitor consists of two thin parallel conductive plates separated by a dielectric with permittivity ε . This model may also be used to make qualitative predictions for other device geometries. The plates are considered to extend uniformly over an area A and a charge density ±ρ = ±Q/A exists on their surface. Assuming that the length and width of the plates are much greater than their separation d, the electric field near the centre of the device will be uniform with the magnitude E = ρ/ε. The voltage is defined as the line integral of the electric field between the plates
qas: answers: text: permittivity ε
question: In an ideal model of a capacitor, what is the value that describes the permittivity of the dielectric?

answers: text: ±ρ = ±Q/A
question: What equation describes the charge density for an ideal model of a capacitor?

answers: text: much greater than their separation d
question: In an ideal model of a capacitor, what must be assumed about the size of the plates?

answers: text: E = ρ/ε
question: Close to the middle of the capacitor, what equation describes the magnitude of the electric field?

answers: text: The voltage
question: What does the line integral of the electric field between the plates of a capacitor represent?

context: The maximum energy is a function of dielectric volume, permittivity, and dielectric strength. Changing the plate area and the separation between the plates while maintaining the same volume causes no change of the maximum amount of energy that the capacitor can store, so long as the distance between plates remains much smaller than both the length and width of the plates. In addition, these equations assume that the electric field is entirely concentrated in the dielectric between the plates. In reality there are fringing fields outside the dielectric, for example between the sides of the capacitor plates, which will increase the effective capacitance of the capacitor. This is sometimes called parasitic capacitance. For some simple capacitor geometries this additional capacitance term can be calculated analytically. It becomes negligibly small when the ratios of plate width to separation and length to separation are large.
qas: answers: text: The maximum energy
question: What is a function of the amount of dielectric, the strength of dielectric and its permittivity?

answers: text: no change of the maximum amount of energy
question: If the plate area and separation distance are altered while keeping the amount of dielectric the same, what effect is had on the maximum energy of the capacitor? 

answers: text: fringing fields outside the dielectric
question: In a realistic model of a capacitor, where else besides between the dielectric between the conductors might an electric field be found?

answers: text: will increase the effective capacitance
question: When an electric field exists between the sides of the plates as well as in within the dielectric, what effect is had on the effective capacitance of the capacitor?

answers: text: negligibly small
question: When the ratios of plate length and width to separation distance are large, what size is the parasitic capacitance?

context:  Capacitors deviate from the ideal capacitor equation in a number of ways. Some of these, such as leakage current and parasitic effects are linear, or can be assumed to be linear, and can be dealt with by adding virtual components to the equivalent circuit of the capacitor. The usual methods of network analysis can then be applied. In other cases, such as with breakdown voltage, the effect is non-linear and normal (i.e., linear) network analysis cannot be used, the effect must be dealt with separately. There is yet another group, which may be linear but invalidate the assumption in the analysis that capacitance is a constant. Such an example is temperature dependence. Finally, combined parasitic effects such as inherent inductance, resistance, or dielectric losses can exhibit non-uniform behavior at variable frequencies of operation.
qas: answers: text: can be assumed to be linear
question: What characteristic can leakage current and parasitic effects be assumed to have in the equation for a realistic capacitor?

answers: text: virtual components
question: What sort of components can be added to the equivalent circuit of a capacitor in order to deal with leakage current and parasitic effects?

answers: text: The usual methods of network analysis
question: Upon dealing with leakage current and parasitic effects in a realistic model of a capacitor, what sort of methods can then be applied?

answers: text: normal (i.e., linear) network analysis cannot be used
question: What sort of analysis cannot be used in cases of voltage breakdown in modelling a capacitor?

answers: text: temperature dependence
question: What causes the assumption of constant capacitance to fail even if the effect is linear in nature?

context: For air dielectric capacitors the breakdown field strength is of the order 2 to 5 MV/m; for mica the breakdown is 100 to 300 MV/m; for oil, 15 to 25 MV/m; it can be much less when other materials are used for the dielectric. The dielectric is used in very thin layers and so absolute breakdown voltage of capacitors is limited. Typical ratings for capacitors used for general electronics applications range from a few volts to 1 kV. As the voltage increases, the dielectric must be thicker, making high-voltage capacitors larger per capacitance than those rated for lower voltages. The breakdown voltage is critically affected by factors such as the geometry of the capacitor conductive parts; sharp edges or points increase the electric field strength at that point and can lead to a local breakdown. Once this starts to happen, the breakdown quickly tracks through the dielectric until it reaches the opposite plate, leaving carbon behind and causing a short (or relatively low resistance) circuit. The results can be explosive as the short in the capacitor draws current from the surrounding circuitry and dissipates the energy.
qas: answers: text: of the order 2 to 5 MV/m
question: What order is the breakdown field strength for air dielectric capacitors of?

answers: text: 100 to 300 MV/m
question: Of what order is the breakdown field strength for mica dielectric capacitors?

answers: text: The dielectric is used in very thin layers
question: In what way is the dielectric used in order to cause the absolute breakdown voltage of capacitors to be limited?

answers: text: the geometry of the capacitor conductive parts
question: What is one factor that critically affects the breakdown voltage of the capacitor?

answers: text: the dielectric must be thicker
question: What is the difference in the physical attributes of the dielectric used in high voltage capacitors compared to low voltage capacitors?

context: Ripple current is the AC component of an applied source (often a switched-mode power supply) whose frequency may be constant or varying. Ripple current causes heat to be generated within the capacitor due to the dielectric losses caused by the changing field strength together with the current flow across the slightly resistive supply lines or the electrolyte in the capacitor. The equivalent series resistance (ESR) is the amount of internal series resistance one would add to a perfect capacitor to model this. Some types of capacitors, primarily tantalum and aluminum electrolytic capacitors, as well as some film capacitors have a specified rating value for maximum ripple current.
qas: answers: text: Ripple current
question: What name is given to the AC component of an applied source having either constant or alternating frequency?

answers: text: changing field strength
question: What is one cause of dielectric losses in a capacitor?

answers: text: current flow across the slightly resistive supply lines or the electrolyte
question: What is another cause of dielectric losses in a capacitor?

answers: text: equivalent series resistance (ESR)
question: What is the name given to the amount of internal series resistance needed to add to an ideal capacitor model in order to represent the heat generated by dielectric losses?

answers: text: aluminum electrolytic capacitors
question: What is one type of capacitor that has a specified rating value for maximum ripple current?

context: The capacitance of certain capacitors decreases as the component ages. In ceramic capacitors, this is caused by degradation of the dielectric. The type of dielectric, ambient operating and storage temperatures are the most significant aging factors, while the operating voltage has a smaller effect. The aging process may be reversed by heating the component above the Curie point. Aging is fastest near the beginning of life of the component, and the device stabilizes over time. Electrolytic capacitors age as the electrolyte evaporates. In contrast with ceramic capacitors, this occurs towards the end of life of the component.
qas: answers: text: The capacitance
question: What value of some capacitors decreases with age?

answers: text: degradation of the dielectric
question: What causes the decrease of capacitance in ceramic capacitors as they age?

answers: text: The type of dielectric
question: What is one of the most important aging factors in capacitors?

answers: text: ambient operating and storage temperatures
question: What is another important factor which governs how a capacitor ages?

answers: text: the Curie point
question: At what point can the aging effect of a capacitor be reversed if the component is heated beyond?

context: Capacitors, especially ceramic capacitors, and older designs such as paper capacitors, can absorb sound waves resulting in a microphonic effect. Vibration moves the plates, causing the capacitance to vary, in turn inducing AC current. Some dielectrics also generate piezoelectricity. The resulting interference is especially problematic in audio applications, potentially causing feedback or unintended recording. In the reverse microphonic effect, the varying electric field between the capacitor plates exerts a physical force, moving them as a speaker. This can generate audible sound, but drains energy and stresses the dielectric and the electrolyte, if any.
qas: answers: text: microphonic effect
question: What is the effect called when capacitors absorb sound waves?

answers: text: the capacitance
question: What value of a capacitor is varied when vibration moves the conducting plates? 

answers: text: AC current
question: What sort of current is induced when the capacitance of a capacitor is altered?

answers: text: audio applications
question: In what type of applications is piezeoelectricity especially problematic?

answers: text: the reverse microphonic effect
question: What is it called when a changing electric field existing between the conductive plates of a capacitor physically moves them?

context: In DC circuits and pulsed circuits, current and voltage reversal are affected by the damping of the system. Voltage reversal is encountered in RLC circuits that are under-damped. The current and voltage reverse direction, forming a harmonic oscillator between the inductance and capacitance. The current and voltage will tend to oscillate and may reverse direction several times, with each peak being lower than the previous, until the system reaches an equilibrium. This is often referred to as ringing. In comparison, critically damped or over-damped systems usually do not experience a voltage reversal. Reversal is also encountered in AC circuits, where the peak current will be equal in each direction.
qas: answers: text: DC circuits
question: What is one type of circuit in which voltage and current reversal are affected by damping?

answers: text: pulsed circuits
question: What is another type of circuit in which voltage and current reversal are affected by damping?

answers: text: under-damped
question: Under what condition does voltage reversal occur in RLC circuits?

answers: text: a harmonic oscillator
question: What is formed between the inductance and the capacitance when the current and the voltage switch direction?

answers: text: a voltage reversal
question: When a system is over-damped, what does it typically not experience?

context: For maximum life, capacitors usually need to be able to handle the maximum amount of reversal that a system will experience. An AC circuit will experience 100% voltage reversal, while under-damped DC circuits will experience less than 100%. Reversal creates excess electric fields in the dielectric, causes excess heating of both the dielectric and the conductors, and can dramatically shorten the life expectancy of the capacitor. Reversal ratings will often affect the design considerations for the capacitor, from the choice of dielectric materials and voltage ratings to the types of internal connections used.
qas: answers: text: 100% voltage reversal
question: What percentage of voltage reversal does an AC circuit experience?

answers: text: excess electric fields
question: What does voltage reversal create in the dielectric of a capacitor?

answers: text: less than 100%
question: What percentage of voltage reversal will an under damped DC circuit experience?

answers: text: heating of both the dielectric and the conductors
question: What does excess electric field present in the dielectric lead to?

answers: text: Reversal ratings
question: When designing a capacitor, what is an important rating to consider?

context: Capacitors made with any type of dielectric material will show some level of "dielectric absorption" or "soakage". On discharging a capacitor and disconnecting it, after a short time it may develop a voltage due to hysteresis in the dielectric. This effect can be objectionable in applications such as precision sample and hold circuits or timing circuits. The level of absorption depends on many factors, from design considerations to charging time, since the absorption is a time-dependent process. However, the primary factor is the type of dielectric material. Capacitors such as tantalum electrolytic or polysulfone film exhibit very high absorption, while polystyrene or Teflon allow very small levels of absorption. In some capacitors where dangerous voltages and energies exist, such as in flashtubes, television sets, and defibrillators, the dielectric absorption can recharge the capacitor to hazardous voltages after it has been shorted or discharged. Any capacitor containing over 10 joules of energy is generally considered hazardous, while 50 joules or higher is potentially lethal. A capacitor may regain anywhere from 0.01 to 20% of its original charge over a period of several minutes, allowing a seemingly safe capacitor to become surprisingly dangerous.
qas: answers: text: hysteresis in the dielectric
question: After discharging and disconnecting a capacitor, what causes it to develop a voltage?

answers: text: time-dependent
question: What does the process of dielectric absorption in a capacitor depend on?

answers: text: very high absorption
question: What level of dielectric absorption would a tantalum electrolytic capacitor display? 

answers: text: very small levels of absorption
question: What level of dielectric absorption would a Teflon capacitor exhibit?

answers: text: over 10 joules of energy
question: What amount of energy must a capacitor contain in order to be generally considered dangerous? 

context: Leakage is equivalent to a resistor in parallel with the capacitor. Constant exposure to heat can cause dielectric breakdown and excessive leakage, a problem often seen in older vacuum tube circuits, particularly where oiled paper and foil capacitors were used. In many vacuum tube circuits, interstage coupling capacitors are used to conduct a varying signal from the plate of one tube to the grid circuit of the next stage. A leaky capacitor can cause the grid circuit voltage to be raised from its normal bias setting, causing excessive current or signal distortion in the downstream tube. In power amplifiers this can cause the plates to glow red, or current limiting resistors to overheat, even fail. Similar considerations apply to component fabricated solid-state (transistor) amplifiers, but owing to lower heat production and the use of modern polyester dielectric barriers this once-common problem has become relatively rare.
qas: answers: text: Leakage
question: With what value is a resistor in parallel with a capacitor equal to?

answers: text: Constant exposure to heat
question: What can cause excessive leakage in a capacitor?

answers: text: interstage coupling capacitors
question: What sort of capacitors are used to carry a varying signal from the conductive plate of one tube to the grid circuit of the next stage?

answers: text: A leaky capacitor
question: What type of capacitor can cause signal distortion in the downstream tube?

answers: text: cause the plates to glow red
question: If the grid circuit voltage is increased beyond its regular bias setting, what can happen to the conductive plates in power amplifiers?

context: Most types of capacitor include a dielectric spacer, which increases their capacitance. These dielectrics are most often insulators. However, low capacitance devices are available with a vacuum between their plates, which allows extremely high voltage operation and low losses. Variable capacitors with their plates open to the atmosphere were commonly used in radio tuning circuits. Later designs use polymer foil dielectric between the moving and stationary plates, with no significant air space between them.
qas: answers: text: a dielectric spacer
question: What do many capacitors contain that raises the capacitance?

answers: text: a vacuum
question: What sort of dielectric is used in low capacitance devices?

answers: text: Variable capacitors
question: What sort of capacitors have traditionally been used in radio tuning circuits?

answers: text: use polymer foil dielectric between the moving and stationary plates
question: How did the capacitor traditionally used in radio tuning circuits change over time? 

answers: text: no significant air space
question: What did the later designs of capacitors used in radio tuning circuits no longer have?

context: Several solid dielectrics are available, including paper, plastic, glass, mica and ceramic materials. Paper was used extensively in older devices and offers relatively high voltage performance. However, it is susceptible to water absorption, and has been largely replaced by plastic film capacitors. Plastics offer better stability and ageing performance, which makes them useful in timer circuits, although they may be limited to low operating temperatures and frequencies. Ceramic capacitors are generally small, cheap and useful for high frequency applications, although their capacitance varies strongly with voltage and they age poorly. They are broadly categorized as class 1 dielectrics, which have predictable variation of capacitance with temperature or class 2 dielectrics, which can operate at higher voltage. Glass and mica capacitors are extremely reliable, stable and tolerant to high temperatures and voltages, but are too expensive for most mainstream applications. Electrolytic capacitors and supercapacitors are used to store small and larger amounts of energy, respectively, ceramic capacitors are often used in resonators, and parasitic capacitance occurs in circuits wherever the simple conductor-insulator-conductor structure is formed unintentionally by the configuration of the circuit layout.
qas: answers: text: relatively high voltage performance
question: What benefit is offered by solid state paper capacitors?

answers: text: plastic film capacitors
question: What type of capacitor has mostly replaced solid state paper capacitors?

answers: text: timer circuits
question: What sort of circuit are plastic capacitors especially useful for?

answers: text: Glass
question: What is one type of reliable capacitor that is resistant to temperature and voltage changes?

answers: text: mica capacitors
question: What is another type of reliable capacitor that is resistant to temperature and voltage changes?

context: Electrolytic capacitors use an aluminum or tantalum plate with an oxide dielectric layer. The second electrode is a liquid electrolyte, connected to the circuit by another foil plate. Electrolytic capacitors offer very high capacitance but suffer from poor tolerances, high instability, gradual loss of capacitance especially when subjected to heat, and high leakage current. Poor quality capacitors may leak electrolyte, which is harmful to printed circuit boards. The conductivity of the electrolyte drops at low temperatures, which increases equivalent series resistance. While widely used for power-supply conditioning, poor high-frequency characteristics make them unsuitable for many applications. Electrolytic capacitors will self-degrade if unused for a period (around a year), and when full power is applied may short circuit, permanently damaging the capacitor and usually blowing a fuse or causing failure of rectifier diodes (for instance, in older equipment, arcing in rectifier tubes). They can be restored before use (and damage) by gradually applying the operating voltage, often done on antique vacuum tube equipment over a period of 30 minutes by using a variable transformer to supply AC power. Unfortunately, the use of this technique may be less satisfactory for some solid state equipment, which may be damaged by operation below its normal power range, requiring that the power supply first be isolated from the consuming circuits. Such remedies may not be applicable to modern high-frequency power supplies as these produce full output voltage even with reduced input.
qas: answers: text: aluminum
question: What is one type of metal which the conducting plates in electrolytic capacitors are commonly made of?

answers: text: an oxide dielectric layer
question: What type of dielectric layer is used in electrolytic capacitors?

answers: text: drops at low temperatures
question: What happens to the conductivity of the electrolyte at low temperatures?

answers: text: by gradually applying the operating voltage
question: How can electrolytic capacitors be safely used after having been stored unused for a long time?

answers: text: these produce full output voltage even with reduced input
question: Why might the method gradually applying the operating voltage not be suitable to protect electrolytic capacitors in modern devices?

context: Several other types of capacitor are available for specialist applications. Supercapacitors store large amounts of energy. Supercapacitors made from carbon aerogel, carbon nanotubes, or highly porous electrode materials, offer extremely high capacitance (up to 5 kF as of 2010[update]) and can be used in some applications instead of rechargeable batteries. Alternating current capacitors are specifically designed to work on line (mains) voltage AC power circuits. They are commonly used in electric motor circuits and are often designed to handle large currents, so they tend to be physically large. They are usually ruggedly packaged, often in metal cases that can be easily grounded/earthed. They also are designed with direct current breakdown voltages of at least five times the maximum AC voltage.
qas: answers: text: Supercapacitors store large amounts of energy
question: What is the main feature of a supercapacitor?

answers: text: carbon nanotubes
question: What is one type of material with which a supercapacitor may be constructed?

answers: text: up to 5 kF
question: As of 2010, what is the highest capacitance a supercapacitor has achieved?

answers: text: Alternating current capacitors
question: What type of capacitor is commonly used in electric motor circuits?

answers: text: at least five times the maximum AC voltage
question: How many times the maximum AC voltage are AC capacitors designed to withstand?

context: If a capacitor is driven with a time-varying voltage that changes rapidly enough, at some frequency the polarization of the dielectric cannot follow the voltage. As an example of the origin of this mechanism, the internal microscopic dipoles contributing to the dielectric constant cannot move instantly, and so as frequency of an applied alternating voltage increases, the dipole response is limited and the dielectric constant diminishes. A changing dielectric constant with frequency is referred to as dielectric dispersion, and is governed by dielectric relaxation processes, such as Debye relaxation. Under transient conditions, the displacement field can be expressed as (see electric susceptibility):
qas: answers: text: driven with a time-varying voltage that changes rapidly
question: Under what condition might the polarization of the dielectric be unable to follow the voltage?

answers: text: the internal microscopic dipoles
question: What type of dipoles which add to the dielectric constant cannot move instantly if a rapidly changing time varying voltage is driving the capacitor?

answers: text: dielectric dispersion
question: What is it called when a dielectric constant changes with frequency?

answers: text: dielectric relaxation processes
question: What regulates dielectric dispersion?

answers: text: Debye relaxation
question: What is an example of a dielectric relaxation process?

context: where a single prime denotes the real part and a double prime the imaginary part, Z(ω) is the complex impedance with the dielectric present, Ccmplx(ω) is the so-called complex capacitance with the dielectric present, and C0 is the capacitance without the dielectric. (Measurement "without the dielectric" in principle means measurement in free space, an unattainable goal inasmuch as even the quantum vacuum is predicted to exhibit nonideal behavior, such as dichroism. For practical purposes, when measurement errors are taken into account, often a measurement in terrestrial vacuum, or simply a calculation of C0, is sufficiently accurate.)
qas: answers: text: Z(ω)
question: How is the complex impedance with dielectric represented mathematically?

answers: text: C0
question: How is the complex capacitance without dielectric represented mathematically?

answers: text: even the quantum vacuum is predicted to exhibit nonideal behavior
question: Why is the value of C0 unattainable in reality?

answers: text: in terrestrial vacuum
question: Under what condition is the approximation C0 sufficiently accurate for calculation purposes?

answers: text: Ccmplx(ω)
question: How is the complex capacitance mathematically represented with the dielectric present?

context: The arrangement of plates and dielectric has many variations depending on the desired ratings of the capacitor. For small values of capacitance (microfarads and less), ceramic disks use metallic coatings, with wire leads bonded to the coating. Larger values can be made by multiple stacks of plates and disks. Larger value capacitors usually use a metal foil or metal film layer deposited on the surface of a dielectric film to make the plates, and a dielectric film of impregnated paper or plastic – these are rolled up to save space. To reduce the series resistance and inductance for long plates, the plates and dielectric are staggered so that connection is made at the common edge of the rolled-up plates, not at the ends of the foil or metalized film strips that comprise the plates.
qas: answers: text: microfarads and less
question: What order of magnitude of capacitance is suitable for capacitors comprised of ceramic disks with metallic coatings?

answers: text: multiple stacks of plates and disks
question: How are larger value capacitors often constructed?

answers: text: impregnated paper or plastic – these are rolled up to save space
question: What do larger value capacitors often use as the dielectric medium?

answers: text: To reduce the series resistance and inductance
question: Why are the plates and dielectrics often staggered in larger value capacitors?

answers: text: metal film layer deposited on the surface of a dielectric film
question: How are the conductive plates often made for larger value capacitors?

context: Capacitors may have their connecting leads arranged in many configurations, for example axially or radially. "Axial" means that the leads are on a common axis, typically the axis of the capacitor's cylindrical body – the leads extend from opposite ends. Radial leads might more accurately be referred to as tandem; they are rarely actually aligned along radii of the body's circle, so the term is inexact, although universal. The leads (until bent) are usually in planes parallel to that of the flat body of the capacitor, and extend in the same direction; they are often parallel as manufactured.
qas: answers: text: axially
question: What is one type of configuration in which a capacitor may have its connecting leads organized?

answers: text: radially
question: What is another type of configuration in which a capacitor may have its connecting leads arranged?

answers: text: Radial
question: Which type of configuration is often manufactured with the leads parallel to the body of the capacitor?

answers: text: on a common axis
question: How are the leads of an axially configured capacitor arranged?

answers: text: as tandem
question: How could radial leads be more correctly described?

context: Small, cheap discoidal ceramic capacitors have existed since the 1930s, and remain in widespread use. Since the 1980s, surface mount packages for capacitors have been widely used. These packages are extremely small and lack connecting leads, allowing them to be soldered directly onto the surface of printed circuit boards. Surface mount components avoid undesirable high-frequency effects due to the leads and simplify automated assembly, although manual handling is made difficult due to their small size.
qas: answers: text: the 1930s
question: Since when have inexpensive ceramic disc capacitors existed?

answers: text: the 1980s
question: Since when have surface mount packages for capacitors been commonly in use?

answers: text: connecting leads
question: What do surface mount packages lack that allows their use on the surface of printed circuit boards?

answers: text: simplify automated assembly
question: What is one benefit of using surface mount components?

answers: text: manual handling is made difficult due to their small size
question: What is one difficulty when using surface mount components?

context: Mechanically controlled variable capacitors allow the plate spacing to be adjusted, for example by rotating or sliding a set of movable plates into alignment with a set of stationary plates. Low cost variable capacitors squeeze together alternating layers of aluminum and plastic with a screw. Electrical control of capacitance is achievable with varactors (or varicaps), which are reverse-biased semiconductor diodes whose depletion region width varies with applied voltage. They are used in phase-locked loops, amongst other applications.
qas: answers: text: the plate spacing
question: What do mechanically controlled variable capacitors enable to be modified?

answers: text: with a screw
question: How do cheap variable capacitors vary the separation distance between layers of a plastic and aluminum?

answers: text: reverse-biased semiconductor diodes
question: Of what type of diodes are varactors or varicaps comprised?

answers: text: depletion region
question: What varies as a function of voltage in varactors?

answers: text: phase-locked loops
question: What is an application of varactors?

context: Most capacitors have numbers printed on their bodies to indicate their electrical characteristics. Larger capacitors like electrolytics usually display the actual capacitance together with the unit (for example, 220 μF). Smaller capacitors like ceramics, however, use a shorthand consisting of three numeric digits and a letter, where the digits indicate the capacitance in pF (calculated as XY × 10Z for digits XYZ) and the letter indicates the tolerance (J, K or M for ±5%, ±10% and ±20% respectively).
qas: answers: text: display the actual capacitance together with the unit (for example, 220 μF)
question: How do larger capacitors present their electrical characteristics?

answers: text: three numeric digits and a letter
question: What does the abbreviated electrical characteristics of smaller capacitors consist of?

answers: text: the digits indicate the capacitance
question: What part of the electrical characteristics of smaller capacitors do the digits of the abbreviated notation represent ?

answers: text: the letter indicates the tolerance
question: What does the letter of the abbreviated notation for the electrical characteristics of smaller capacitors represent?

answers: text: in pF
question: In what SI unit is the capacitance reported on smaller capacitors?

context: Capacitors are connected in parallel with the power circuits of most electronic devices and larger systems (such as factories) to shunt away and conceal current fluctuations from the primary power source to provide a "clean" power supply for signal or control circuits. Audio equipment, for example, uses several capacitors in this way, to shunt away power line hum before it gets into the signal circuitry. The capacitors act as a local reserve for the DC power source, and bypass AC currents from the power supply. This is used in car audio applications, when a stiffening capacitor compensates for the inductance and resistance of the leads to the lead-acid car battery.
qas: answers: text: to provide a "clean" power supply for signal or control circuits
question: Why are capacitors connected in parallel with the power circuits of many devices and large systems?

answers: text: as a local reserve for the DC power source
question: How do the capacitors connected in parallel act with currents from the power supply?

answers: text: the inductance
question: What is one value a stiffening capacitor accounts for when used in for car audio purposes?

answers: text: resistance
question: What is another value a stiffening capacitor accounts for when used in for car audio purposes?

answers: text: conceal current fluctuations
question: What do parallel capacitors do to fluctuating current in order to deliver "clean" power for control circuits?

context: In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as three capacitors connected as a three phase load. Usually, the values of these capacitors are given not in farads but rather as a reactive power in volt-amperes reactive (var). The purpose is to counteract inductive loading from devices like electric motors and transmission lines to make the load appear to be mostly resistive. Individual motor or lamp loads may have capacitors for power factor correction, or larger sets of capacitors (usually with automatic switching devices) may be installed at a load center within a building or in a large utility substation.
qas: answers: text: In electric power distribution
question: In what scenario are capacitors used for power factor correction?

answers: text: as three capacitors connected as a three phase load
question: How are capacitors used for for power factor correction prepared?

answers: text: reactive power in volt-amperes reactive (var)
question: In what what unit is the capacitance reported for capacitors used in power factor correction?

answers: text: to counteract inductive loading
question: What is the reason for using capacitors in power factor correction?

answers: text: at a load center within a building
question: How are power factor correction capacitors sometimes installed?

context: When an inductive circuit is opened, the current through the inductance collapses quickly, creating a large voltage across the open circuit of the switch or relay. If the inductance is large enough, the energy will generate a spark, causing the contact points to oxidize, deteriorate, or sometimes weld together, or destroying a solid-state switch. A snubber capacitor across the newly opened circuit creates a path for this impulse to bypass the contact points, thereby preserving their life; these were commonly found in contact breaker ignition systems, for instance. Similarly, in smaller scale circuits, the spark may not be enough to damage the switch but will still radiate undesirable radio frequency interference (RFI), which a filter capacitor absorbs. Snubber capacitors are usually employed with a low-value resistor in series, to dissipate energy and minimize RFI. Such resistor-capacitor combinations are available in a single package.
qas: answers: text: the current through the inductance collapses quickly
question: What happens to the current when an inductive circuit is opened?

answers: text: a large voltage across the open circuit of the switch
question: What happens to the voltage when an inductive circuit is opened?

answers: text: A snubber capacitor
question: What type of capacitor is used to make a path to bypass the contact points?

answers: text: contact breaker ignition systems
question: What is a typical system in which a snubber capacitor is found?

answers: text: a filter capacitor
question: What type of capacitor absorbs radio frequency interference?

context: In single phase squirrel cage motors, the primary winding within the motor housing is not capable of starting a rotational motion on the rotor, but is capable of sustaining one. To start the motor, a secondary "start" winding has a series non-polarized starting capacitor to introduce a lead in the sinusoidal current. When the secondary (start) winding is placed at an angle with respect to the primary (run) winding, a rotating electric field is created. The force of the rotational field is not constant, but is sufficient to start the rotor spinning. When the rotor comes close to operating speed, a centrifugal switch (or current-sensitive relay in series with the main winding) disconnects the capacitor. The start capacitor is typically mounted to the side of the motor housing. These are called capacitor-start motors, that have relatively high starting torque. Typically they can have up-to four times as much starting torque than a split-phase motor and are used on applications such as compressors, pressure washers and any small device requiring high starting torques.
qas: answers: text: a rotational motion on the rotor
question: What is the main winding on a squirrel cage motor capable of withstanding?

answers: text: series non-polarized starting capacitor
question: What sort of capacitors are used on the second winding of a squirrel cage motor?

answers: text: introduce a lead in the sinusoidal current
question: What does the capacitor on the second winding of a squirrel cage motor do?

answers: text: disconnects the capacitor
question: What does the centrifugal switch do the capacitor when the rotor achieves operating speed?

answers: text: to the side of the motor housing
question: Where is the start capacitor commonly mounted?

context: Capacitors may retain a charge long after power is removed from a circuit; this charge can cause dangerous or even potentially fatal shocks or damage connected equipment. For example, even a seemingly innocuous device such as a disposable-camera flash unit, powered by a 1.5 volt AA battery, has a capacitor which may contain over 15 joules of energy and be charged to over 300 volts. This is easily capable of delivering a shock. Service procedures for electronic devices usually include instructions to discharge large or high-voltage capacitors, for instance using a Brinkley stick. Capacitors may also have built-in discharge resistors to dissipate stored energy to a safe level within a few seconds after power is removed. High-voltage capacitors are stored with the terminals shorted, as protection from potentially dangerous voltages due to dielectric absorption or from transient voltages the capacitor may pick up from static charges or passing weather events.
qas: answers: text: over 15 joules of energy
question: How much energy could a capacitor in a disposable camera contain?

answers: text: over 300 volts
question: To what voltage could a capacitor from a disposable camera be charged to?

answers: text: a Brinkley stick
question: What device can be used to discharge large voltage capacitors?

answers: text: with the terminals shorted
question: How are the terminals of high voltage capacitors stored?

answers: text: from static charges
question: What type of charge could a capacitor pick up a transient charge from?

context: Capacitors may catastrophically fail when subjected to voltages or currents beyond their rating, or as they reach their normal end of life. Dielectric or metal interconnection failures may create arcing that vaporizes the dielectric fluid, resulting in case bulging, rupture, or even an explosion. Capacitors used in RF or sustained high-current applications can overheat, especially in the center of the capacitor rolls. Capacitors used within high-energy capacitor banks can violently explode when a short in one capacitor causes sudden dumping of energy stored in the rest of the bank into the failing unit. High voltage vacuum capacitors can generate soft X-rays even during normal operation. Proper containment, fusing, and preventive maintenance can help to minimize these hazards.
qas: answers: text: when subjected to voltages or currents beyond their rating
question: What could cause the failure of a capacitor?

answers: text: overheat
question: What can happen to capacitors used in high current applications?

answers: text: violently explode
question: What can happen to capacitors used in high energy capacitor banks?

answers: text: High voltage vacuum capacitors
question: What type of capacitors can generate soft x-rays?

answers: text: preventive maintenance
question: What is one way to help minimize capacitor dangers?