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A water capacitor is a device that uses water as its dielectric insulating medium.
Contents
Theory of operation
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, is isolated with no net electric charge. The conductors must hold equal and opposite charges on their facing surfaces.
Water as a dielectric
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.
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. Deionized water is relatively inexpensive and is environmentally safe. These characteristics, along with the high dielectric constant, make water an excellent choice for building large capacitors. 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.
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. 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 Xray pulser.
Applications
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 100mm for the same voltage in dry air).
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.
Hazards and benefits
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 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.
History
Capacitors can originally be traced back to a device called a Leyden jar, created by the Dutch physicist Pieter van Musschenbroek. 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.
Although the design and materials used in capacitors have changed greatly throughout history, the basic 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 Nicola Tesla described capacitors as the "electrical equivalent of dynamite".