A water capacitor is a device that uses water as its dielectric insulating medium.
A water capacitor is a device that uses water as its dielectric insulating medium.
A capacitor is a device in which electrical energy is introduced and can be stored for a later time. A capacitor consists of two conductors separated by a non-conductive region. The non-conductive region is called the dielectric or electrical insulator. Examples of traditional dielectric media are air, paper, and certain semiconductors. A capacitor is a self-contained system, isolated with no net electric charge. The conductors must hold equal and opposite charges on their facing surfaces. [1]
Conventional capacitors use materials such as glass or ceramic as their insulating medium to store an electric charge. Water capacitors were created mainly as a novelty item or for laboratory experimentation and can be made with simple materials. Water exhibits the quality of being self-healing; if there is an electrical breakdown through the water, it quickly returns to its original and undamaged state. Other liquid insulators are prone to carbonization after breakdown and tend to lose their hold off strength over time. These characteristics, along with the high dielectric constant, make water an excellent choice for building large capacitors.
The drawback to using water is the short length of time it can hold off the voltage, typically in the microsecond to ten microsecond (μs) range. Deionised water is relatively inexpensive and is environmentally safe. If a way can be found to reliably increase the hold off time for a given field strength, then there will be more applications for water capacitors. [2]
Water has been shown not to be a very reliable substance to store electric charge long term, so more reliable materials are used for capacitors in industrial applications. [3] However, water has the advantage of being self-healing after a breakdown, and if the water is steadily circulated through a de-ionizing resin and filters, then the loss resistance and dielectric behavior can be stabilized. Thus, in certain unusual situations, such as the generation of extremely high voltage but very short pulses, a water capacitor may be a practical solution – such as in an experimental X-ray pulser. [4]
A dielectric material is defined as a material that is an electrical insulator. An electrical insulator is a material that does not allow the flow of charge. Charge can flow as electrons or ionic chemical species. By this definition, liquid water is not an electrical insulator, and, hence, liquid water is not a dielectric. The self-ionization of water is a process in which a small proportion of water molecules dissociate into positive and negative ions. It is this process that gives pure liquid water its inherent electrical conductivity.
Because of self-ionization, at ambient temperatures, pure liquid water has a similar intrinsic charge carrier concentration to the semiconductor, germanium, and an intrinsic charge carrier concentration three orders of magnitude greater than the semiconductor, silicon; hence, based on charge carrier concentration, water can not be considered to be a purely dielectric material or full electrical insulator but to be a limited conductor of charge. [5]
The discharge of a platinum parallel-plate capacitor placed in a vessel filled with ultrapure water has been measured. [6] The observed discharge trend could be described by a Modified Poisson-Boltzmann Equation only when the voltage was very low and the system capacitance showed a dependence on the spacing between the two platinum plates. The permittivity of water, calculated considering the system as a plane capacitor, appeared to be very high. This behavior may be explained by the theory of super dielectric materials. [7] The theory of super dielectric materials and simple tests demonstrated that material on the outside of a parallel plate capacitor dramatically increases capacitance, energy density, and power density. Simple parallel plate capacitors with only ambient air between the plates behaved as per standard theory. Once the same capacitor was partially submerged in deionised water (DI), or DI with low dissolved NaCl concentrations, still with only ambient air between the electrodes, the capacitance, energy density, and power density, at low frequency, increased by more than seven orders of magnitude. Notably, conventional theory precludes the possibility that material outside the volume between the plates will in any fashion impact capacitive behavior.
An examination was made of the effect of applying voltages from 0.1 to 0.82V on pure water between metal electrodes. [8] The movement of hydronium ions away from and hydroxide ions towards the anode was followed. This movement resulted in the formation of an ion double-layer with a steeply rising electric field and a maximum pH of approximately 12. At the cathode, the opposite occurred and the pH reaches a minimum of approximately 1.7. Thus pure water in a static electric field is not a homogeneous substance but may be considered to have three zones:
(i) a zone containing excess positively charged aqueous hydrogen ions through which the electric field strength changes
(ii) an intermediate zone containing pure water in which there is no significant electric field
(iii) a zone containing excess negatively charged aqueous hydroxide ions through which the electric field strength changes.
There is an opinion that a Helmholtz double layer is not responsible for the observed electrokinetic phenomena and that the classical formulae are not strictly applicable when there exists unbalanced dissolved charges at interfaces. [9]
The transition from conductive to dielectric screening of electric fields by a tube of pure water has been investigated using a parallel plate capacitor that was used to generate a uniform electric field. [10] Two concentric acrylic plexiglass tubes passed perpendicularly through the electric field generated between the plates. The region between the tubes was filled with air or water. An electrode, suspended within the inner plexiglass tube, was used to sense the electric potential at its location. The sensor was designed so that it could be rotated to measure the potential at a second symmetric position. From the difference in the two potentials, the frequency dependence of the magnitude and phase of the electric field could be determined. With deionised water between the tubes, the magnitude and phase of the interior electric field was measured from 100 Hz to 300 kHz. The high-pass filter frequency response expected for a dielectric tube with non-negligible conductivity was observed. Fits to the data yielded a very reasonable experimental value for the ratio of the water’s conductivity to its dielectric constant. The model also predicted that at zero frequency (a static electric field) pure water would be expected to behave as a Faraday cage.
A simple type of water capacitor is created by using water-filled glass jars and some form of insulating material to cover the ends of the jar. Water capacitors are not widely used in the industrial community due to their large physical size for a given capacitance. The conductivity of water can change very quickly and is unpredictable if left open to atmosphere. Many variables such as temperature, pH levels, and salinity have been shown to alter conductivity in water. As a result, there are better alternatives to the water capacitor in the majority of applications.
The pulse-withstand voltage of carefully purified water can be very high – over 100kV/cm (comparing to about 10 cm for the same voltage in dry air). [11]
A capacitor is designed to store electric energy when disconnected from its charging source. Compared to more conventional devices, water capacitors are currently not practical devices for industrial applications. Capacitance can be increased by the addition of electrolytes and minerals to the water, but this increases the self leakage, and cannot be done beyond its saturation point. [12]
Modern high voltage capacitors may retain their charge long after power is removed. This charge can cause dangerous, or even potentially fatal, shocks if the stored energy is more than a few joules. At much lower levels, stored energy can still cause damage to connected equipment. Water capacitors, being self-discharging, (for totally pure water, only thermally ionized, at 25 °C (77 °F) the ratio of conductivity to permittivity means that self-discharge time is circa 180μs, faster with higher temperatures or dissolved impurities) usually cannot be made to store enough residual electrical energy to cause serious bodily injury.
Unlike many large, industrial, high voltage capacitors, water capacitors do not require oil. Oil found in many older designs of capacitors can be toxic to both animals and humans. If a capacitor breaks open and its oil is released, the oil often finds its way into the water table, which can cause health problems over time. [13]
Capacitors can originally be traced back to a device called a Leyden jar, created by the Dutch physicist Pieter van Musschenbroek. [14] The Leyden jar consisted of a glass jar with tin foil layers on the inside and outside of the jar. A rod electrode was directly connected to the inlayer of foil by means of a small chain or wire. This device stored static electricity created when amber and wool where rubbed together. [15] [1]
Although the design and materials used in capacitors have changed greatly throughout history, the fundamentals remain the same. In general, capacitors are very simple electrical devices which can have many uses in today's technologically advanced world. A modern capacitor usually consists of two conducting plates sandwiched around an insulator. Electrical researcher Nikola Tesla described capacitors as the "electrical equivalent of dynamite". [16]
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An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge carriers, which may be one of several types of particles, depending on the conductor. In electric circuits the charge carriers are often electrons moving through a wire. In semiconductors they can be electrons or holes. In an electrolyte the charge carriers are ions, while in plasma, an ionized gas, they are ions and electrons.
In electronics, the metal–oxide–semiconductor field-effect transistor is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The term metal–insulator–semiconductor field-effect transistor (MISFET) is almost synonymous with MOSFET. Another near-synonym is insulated-gate field-effect transistor (IGFET).
In electromagnetism, a dielectric is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor, because they have no loosely bound, or free, electrons that may drift through the material, but instead they shift, only slightly, from their average equilibrium positions, causing dielectric polarisation. Because of dielectric polarisation, positive charges are displaced in the direction of the field and negative charges shift in the direction opposite to the field. This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarised, but also reorient so that their symmetry axes align to the field.
In physics, the term dielectric strength has the following meanings:
The relative permittivity is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulator measures the ability of the insulator to store electric energy in an electrical field.
Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related notions of capacitance: self capacitance and mutual capacitance. An object that can be electrically charged exhibits self capacitance, for which the electric potential is measured between the object and ground. Mutual capacitance is measured between two components, and is particularly important in the operation of the capacitor, an elementary linear electronic component designed to add capacitance to an electric circuit.
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp, chemiluminescence. Corona discharges can also happen in weather, such as thunderstorms, where objects like ship masts or airplane wings have a charge significantly different from the air around them.
In electronics, electrical breakdown or dielectric breakdown is a process that occurs when an electrically insulating material, subjected to a high enough voltage, suddenly becomes a conductor and current flows through it. All insulating materials undergo breakdown when the electric field caused by an applied voltage exceeds the material's dielectric strength. The voltage at which a given insulating object becomes conductive is called its breakdown voltage and, in addition to its dielectric strength, depends on its size and shape, and the location on the object at which the voltage is applied. Under sufficient voltage, electrical breakdown can occur within solids, liquids, or gases. However, the specific breakdown mechanisms are different for each kind of dielectric medium.
In mesoscopic physics, a Coulomb blockade (CB), named after Charles-Augustin de Coulomb's electrical force, is the decrease in electrical conductance at small bias voltages of a small electronic device comprising at least one low-capacitance tunnel junction. Because of the CB, the conductance of a device may not be constant at low bias voltages, but disappear for biases under a certain threshold, i.e. no current flows.
Capacitors are manufactured in many styles, forms, dimensions, and from a large variety of materials. They all contain at least two electrical conductors, called plates, separated by an insulating layer (dielectric). Capacitors are widely used as parts of electrical circuits in many common electrical devices.
In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term still encountered in a few compound names, such as the condenser microphone. It is a passive electronic component with two terminals.
Differential capacitance in physics, electronics, and electrochemistry is a measure of the voltage-dependent capacitance of a nonlinear capacitor, such as an electrical double layer or a semiconductor diode. It is defined as the derivative of charge with respect to potential.
A lithium-ion capacitor is a hybrid type of capacitor classified as a type of supercapacitor. It is called a hybrid because the anode is the same as those used in lithium-ion batteries and the cathode is the same as those used in supercapacitors. Activated carbon is typically used as the cathode. The anode of the LIC consists of carbon material which is often pre-doped with lithium ions. This pre-doping process lowers the potential of the anode and allows a relatively high output voltage compared to other supercapacitors.
Quantum capacitance, also known as chemical capacitance and electrochemical capacitance, was first theoretically introduced by Serge Luryi (1988), and is defined as the variation of electrical charge with respect to the variation of electrochemical potential , i.e., . In the simplest example, if you make a parallel-plate capacitor where one or both of the plates has a low density of states, then the capacitance is not given by the normal formula for parallel-plate capacitors, . Instead, the capacitance is lower, as if there was another capacitor in series, . This second capacitance, related to the density of states of the plates, is the quantum capacitance and is represented by . The equivalent capacitance is called electrochemical capacitance .
A supercapacitor (SC), also called an ultracapacitor, is a high-capacity capacitor, with a capacitance value much higher than solid-state capacitors but with lower voltage limits. It bridges the gap between electrolytic capacitors and rechargeable batteries. It typically stores 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerates many more charge and discharge cycles than rechargeable batteries.
Dielectric absorption is the name given to the effect by which a capacitor, that has been charged for a long time, discharges only incompletely when briefly discharged. Although an ideal capacitor would remain at zero volts after being discharged, real capacitors will develop a small voltage from time-delayed dipole discharging, a phenomenon that is also called dielectric relaxation, "soakage", or "battery action". For some dielectrics, such as many polymer films, the resulting voltage may be less than 1–2% of the original voltage, but it can be as much as 15% for electrolytic capacitors. The voltage at the terminals generated by the dielectric absorption may possibly cause problems in the function of an electronic circuit or can be a safety risk to personnel. In order to prevent shocks, most very large capacitors are shipped with shorting wires that need to be removed before they are used and/or permanently connected bleeder resistors. When disconnected at one or both ends, DC high-voltage cables can also "recharge themselves" to dangerous voltages.
In physics, the field effect refers to the modulation of the electrical conductivity of a material by the application of an external electric field.
Double-layer capacitance is the important characteristic of the electrical double layer which appears at the interface between a surface and a fluid. At this boundary two layers of electric charge with opposing polarity form, one at the surface of the electrode, and one in the electrolyte. These two layers, electrons on the electrode and ions in the electrolyte, are typically separated by a single layer of solvent molecules that adhere to the surface of the electrode and act like a dielectric in a conventional capacitor. The amount of charge stored in double-layer capacitor depends on the applied voltage.
Aluminium electrolytic capacitors are (usually) polarized electrolytic capacitors whose anode electrode (+) is made of a pure aluminium foil with an etched surface. The aluminum forms a very thin insulating layer of aluminium oxide by anodization that acts as the dielectric of the capacitor. A non-solid electrolyte covers the rough surface of the oxide layer, serving in principle as the second electrode (cathode) (-) of the capacitor. A second aluminum foil called "cathode foil" contacts the electrolyte and serves as the electrical connection to the negative terminal of the capacitor.
Electromagnetism is one of the fundamental forces of nature. Early on, electricity and magnetism were studied separately and regarded as separate phenomena. Hans Christian Ørsted discovered that the two were related – electric currents give rise to magnetism. Michael Faraday discovered the converse, that magnetism could induce electric currents, and James Clerk Maxwell put the whole thing together in a unified theory of electromagnetism. Maxwell's equations further indicated that electromagnetic waves existed, and the experiments of Heinrich Hertz confirmed this, making radio possible. Maxwell also postulated, correctly, that light was a form of electromagnetic wave, thus making all of optics a branch of electromagnetism. Radio waves differ from light only in that the wavelength of the former is much longer than the latter. Albert Einstein showed that the magnetic field arises through the relativistic motion of the electric field and thus magnetism is merely a side effect of electricity. The modern theoretical treatment of electromagnetism is as a quantum field in quantum electrodynamics.
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