Trystan Casey
Electric discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas. Electric discharge machining (EDM) is a manufacturing process that enables the machining of all electrically conductive materials, independent of their hardness or strength. The process is based on the erosive effect of electrical discharges that are ignited between the tool and workpiece electrode. Electric discharge has a variety of applications, including in ionic gas lasers, household fluorescent lamps, and the production of alloys and other products.
| Characteristics | Values |
|---|---|
| Definition | The release and transmission of electricity in an applied electric field through a medium such as a gas |
| Types | Atmospheric lightning, corona discharges, internal discharges, surface discharges, partial discharges, electrical treeing |
| Applications | Electric discharge machining (EDM), spark machining, arc welding, steelmaking, alloy production, fluorescent lamps, high-intensity discharge lamps, lasers |
| Industries | Electrical, aerospace, automotive, medical, tool and die |
| History | Discovered by English scientist Joseph Priestley in the 1770s; first EDM machines developed in the early 20th century by Russian scientists; advancements in the 1940s-1960s |
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What You'll Learn
Electric discharge machining (EDM)
Electric discharge refers to the release and transmission of electricity in an applied electric field through a medium such as a gas. An example of electric discharge is lightning, which occurs when there is a buildup of positive and negative charges on ice and dust particles in the atmosphere.
There are three types of EDM: die-sinking EDM, wire EDM, and hole-drilling EDM. In die-sinking EDM, the electrode is pre-formed in the shape of the desired cut and is used to cut that specific geometry. Wire EDM follows the same principles, but the wire electrode acts as a wire cutter. Hole-drilling EDM uses a pulsing electrode to create small and deep holes without burrs. EDM is often used for applications that cannot be achieved or matched in quality by conventional machining methods, such as drilling small holes, mold and die production, and creating burr-free medical equipment.
EDM is commonly used in the medical industry to create implant devices, including pacemakers, cochlear implants, neurostimulators, hip implants, spinal fixtures, and dental implants. It is also used in aerospace applications, such as producing turbine discs, although it is not suitable for high-grade nickel alloys commonly used in this industry.
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Atmospheric lightning
Electric discharge refers to the release and transmission of electricity in an applied electric field through a medium such as a gas. Atmospheric lightning is a type of electric discharge that occurs naturally on Earth. It is caused by the frictional generation and separation of positive and negative charges on ice and dust particles. As the charge on these particles builds up, it results in a spectacular discharge of electricity known as lightning.
Lightning is a natural phenomenon that occurs when there is a near-instantaneous release of energy through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, and the second region may be on the ground. Following the lightning, the regions become partially or wholly electrically neutralized. The air around the lightning flash rapidly heats up to temperatures of about 30,000°C, causing an emission of electromagnetic radiation across a wide range of wavelengths, some of which are visible as a bright flash. Lightning also causes thunder, which is the sound of the shock wave that develops as heated gases in the vicinity of the discharge experience a sudden increase in pressure.
Lightning primarily occurs when warm air mixes with colder air masses, resulting in the atmospheric disturbances necessary for polarizing the atmosphere. These disturbances result in storms, and when lightning and thunder occur, they are called thunderstorms. Lightning can also occur during dust storms, forest fires, tornadoes, volcanic eruptions, and even in the winter, where the lightning is known as thundersnow. Hurricanes and intense forest fires can also create their own weather systems that produce lightning.
There are three primary forms of lightning distinguished by where they occur: Intra-cloud (IC) or in-cloud, which occurs within a single thundercloud; Cloud-to-cloud (CC) or inter-cloud, which occurs between two clouds; and Cloud-to-ground (CG), which occurs between a cloud and the ground and is referred to as a lightning strike. While intra-cloud and cloud-to-cloud lightning are more common, cloud-to-ground lightning has the most direct effects on humans.
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Corona discharges
An electric discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas. One example of electric discharge is the corona discharge.
Corona discharge is an electrical discharge phenomenon that occurs when a conductor carrying high voltage ionizes the surrounding fluid, which is usually air. It is often referred to as the "Corona Effect". The phenomenon is called 'Corona' because the plasma creates a lighting crown around the electrodes during the discharge. The air near the electrode becomes ionized (partially conductive), while regions more distant are not affected. This has the effect of increasing the apparent size of the conductor. The ions generated eventually pass the charge to nearby areas of lower potential, or recombine to form neutral gas molecules.
Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small-diameter wires. The high curvature causes a high potential gradient at these locations, so the air breaks down and forms plasma there first. On sharp points in the air, corona discharge can start at potentials of 2–6 kV. The critical disruptive voltage, or the voltage at which air around a conductor breaks down and becomes ionized, is typically around 30 kV.
Corona discharge can cause an audible hissing or cracking noise as it ionizes the air around the conductors. This commonly occurs in high-voltage electric power transmission lines. The phenomenon can also produce a violet glow, the production of ozone gas around the conductor, radio interference, and electrical power loss. In the air, corona discharge can generate gases such as ozone and nitric oxide, and in turn, nitrogen dioxide, and thus nitric acid if water vapour is present. These gases are corrosive, can degrade and embrittle nearby materials, and are toxic to humans and the environment.
Corona discharge is generally an unwanted side effect, causing economically significant energy losses for utilities. However, controlled corona discharges are used in a variety of processes, including air filtration, photocopiers, and ozone generators.
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Electrical treeing
An electric discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas. Several types of electric discharges occur naturally on Earth, including atmospheric lightning and corona discharges.
The morphology of the polymer material used for insulation plays a significant role in electrical treeing characteristics. For example, in semicrystalline polymers like polyethylenes, the tree channels tend to grow in regions of lower crystallinity, such as at spherulite boundaries. On the other hand, higher crystallinity structures seem to impede tree growth.
Additionally, the presence of absorbed water can influence electrical treeing. In glassy epoxy resins, significant water absorption can occur, reducing the average growth rate of the tree structures. In contrast, flexible epoxy resins are less affected by absorbed water due to their hydrophobic nature.
To mitigate the effects of electrical treeing, various measures can be implemented, such as using composite materials with improved mechanical properties and incorporating nanoparticles to enhance insulating properties and alter surface charge behavior.
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Electric discharge excitation
Electric discharge refers to the excitation of atomic states in a gaseous medium when an electric current passes through it. This occurs naturally on Earth in the form of atmospheric lightning, which is caused by the frictional generation and separation of positive and negative charges on ice and dust particles. As the charge on these particles builds up, a discharge of electricity in the form of lightning occurs.
Electric discharge in gases occurs when electric current flows through a gaseous medium due to the ionization of the gas. The properties of electric discharges in gases are studied in connection with the design of lighting sources and high-voltage electrical equipment. In cold cathode tubes, the electric discharge in gas has three regions, with distinct current-voltage characteristics:
- Townsend discharge: At low voltages, the only current is that due to the generation of charge carriers in the gas by cosmic rays or other sources of ionizing radiation.
- Glow discharge: This occurs once the breakdown voltage is reached, and the voltage across the electrodes suddenly drops while the current increases.
- Arc discharge: This occurs in the ampere range of the current, and the voltage across the tube drops with increasing current.
Glow discharge is facilitated by electrons striking gas atoms and ionizing them. The breakdown voltage for a glow discharge depends on the product of gas pressure and electrode distance according to Paschen's law.
The chemistry occurring in an electric discharge can be modelled to predict how it is influenced by discharge conditions. For example, a study used a chemical kinetics calculation to determine how the discharge parameters that increase the concentration of hydroxyl radical also increase the rate of decomposition of SO2.
Laser technology also involves the use of electric discharge excitation.
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Frequently asked questions
Electric discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas.
Some examples of electric discharge include atmospheric lightning, corona discharges, and electric arcs in fluorescent lamps.
Atmospheric lightning, a form of electric discharge, occurs naturally due to the frictional generation and separation of positive and negative charges on ice and dust particles.
Electric discharge machining (EDM) is a manufacturing process that utilizes controlled electrical discharges to precisely remove material. EDM enables the creation of complex 3D geometries and intricate features that may be challenging or impossible with traditional machining methods.
Electric discharge plays a crucial role in the operation of ionic gas lasers by readily ionizing neutral particles. In chemical lasers, the electrical discharge facilitates the formation of active molecules in an excited state.
