Trystan Casey
Electric discharge is the process of exciting atomic states in a gaseous medium by passing an electric current through it. It is observed in everyday sources such as household fluorescent lamps, where a high voltage creates a low-pressure mercury arc that emits UV radiation, which is then absorbed and re-emitted as visible light by fluorescent phosphors. High-intensity discharge lamps, such as mercury-vapour, metal-halide, or high-pressure-sodium lamps, are used in various applications and can produce high-intensity light. Electric discharge can also occur in gases when electric current flows through a gaseous medium due to the ionization of the gas, resulting in the emission of visible light. This phenomenon has implications for the design of lighting sources and high-voltage electrical equipment. Understanding the behaviour of electric discharge is crucial for various applications, including spark gaps in internal combustion engines, arc welding, and the production of alloys.
| Characteristics | Values |
|---|---|
| Definition | Electric discharge is the process of exciting atomic states in a gaseous medium by passing an electric current through it. |
| Occurrence | Electric discharge occurs when electric current flows through a gaseous medium due to ionization of the gas. |
| Visual Appearance | Depending on several factors, the discharge may radiate visible light. |
| Applications | Electric discharge has applications in fluorescent lamps, electric discharge machining, arc welding, and the production of alloys and other products. |
| Types | Types of electric discharge include Townsend discharge, surface discharges, and electrical treeing. |
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What You'll Learn
High-intensity discharge lamps
High-intensity discharge (HID) lamps are a type of electrical gas-discharge lamp that produces light through an electric arc. This electric arc forms between tungsten electrodes inside a transparent or translucent fused quartz or alumina arc tube. The tube is filled with noble gas and often contains metal or metal salts. The noble gas allows the arc to strike initially, and once it does, the arc heats and vaporises the metallic mixture. The presence of this metallic mixture in the arc plasma significantly increases the intensity of visible light produced by the arc for a given power input. This is because the metals have many emission spectral lines in the visible part of the spectrum.
HID lamps produce more visible light per unit of electric power consumed compared to fluorescent and incandescent lamps. This is because a greater proportion of their radiation is visible light, rather than infrared. The light-producing element of these lamp types is a well-stabilized arc discharge contained within a refractory envelope arc tube with wall loading in excess of 3 watts per square centimetre (19 W/in2).
HID lamps have been used in automotive headlamps since the early 1990s. Xenon HID lighting provides brighter headlights and increases the visibility of peripheral objects, such as street signs and pedestrians, which would otherwise be left in the shadows by standard halogen lighting. HID lamps are also used in high-performance bicycle headlamps, flashlights, and other portable lights. They are also common in aircraft as replacements for traditional landing and taxi lights, as well as in underwater diving lamps.
There are various types of HID lamps, including mercury-vapour, sodium-vapour, and metal-halide lamps. The desired characteristics of light intensity, colour temperature, colour rendering index (CRI), energy efficiency, and lifespan determine the chemistry used in the arc tubes of HID lamps. For example, metal-halide and ceramic metal-halide lamps emit neutral white light, which is useful for applications where normal colour appearance is critical, such as in TV and movie production, indoor or nighttime sports games, and automotive lighting.
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Townsend discharge
In electromagnetism, a Townsend discharge is an ionization process for gases where free electrons are accelerated by an electric field, leading to a chain reaction of increasing numbers of electrons being freed. This process is also known as the Townsend avalanche, named after John Sealy Townsend, who discovered the mechanism in 1897 at the Cavendish Laboratory in Cambridge.
The Townsend discharge occurs when the voltage between two electrodes exceeds the self-breakdown voltage by a small percentage. It is a form of breakdown that can only be sustained over a limited range of gas pressure and electric field intensity. The breakdown can be characterized by the Paschen curve, which relates the separation of the electrodes to the breakdown voltage.
The process begins with free electrons being accelerated by the electric field. These electrons then collide with gas molecules, freeing additional electrons in a process known as impact ionization. The newly freed electrons are also accelerated and continue the process, resulting in an avalanche multiplication that significantly increases electrical conduction through the gas. Townsend discharges are fundamental to the operation of gaseous ionization detectors such as the Geiger-Müller tube and proportional counters, which are used to detect and measure ionizing radiation.
The Townsend discharge has a range of current densities and is influenced by the presence of a magnetic field. The phenomenon of Penning discharge, for example, can increase the likelihood of an avalanche discharge occurring under high vacuum conditions. The Townsend discharge also sets the upper limit for the blocking voltage, also known as the ignition voltage, that a glow discharge gas-filled tube can withstand. This limit is known as the Townsend discharge breakdown voltage.
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Electric discharge machining
Electrical Discharge Machining (EDM) is a non-conventional metal fabrication process that uses electrical discharges (sparks) to shape conductive workpieces. EDM is also known as spark machining, spark eroding, die sinking, wire burning, or wire erosion. The process involves rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage.
The two electrodes are the tool-electrode and the workpiece-electrode. The tool-electrode is guided along a path very close to the workpiece without making physical contact, although this may occur due to the performance of the specific motion control in use. The recurring sparks cause the removal of material from both the tool and the workpiece, forming small craters. The size of these craters is determined by the technological parameters set for the task, ranging from nanoscale dimensions in micro-EDM operations to hundreds of micrometers in roughing conditions.
EDM can be used to machine extremely hard materials like carbides, ceramics, titanium alloys, and heat-treated tool steels that are challenging to machine using conventional methods. It is also used in the medical industry to create implant devices such as pacemakers, cochlear implants, neurostimulators, hip implants, spinal fixtures, and dental implants.
There are three main types of EDM: die-sinking EDM, wire EDM, and hole-drilling EDM. Die-sinking EDM involves using a pre-formed electrode in the desired cut shape to cut a specific geometry. Wire EDM follows the same principles, but the wire electrode acts like a wire cheese cutter. In hole-drilling EDM, a pulsing electrode creates small, deep holes without burrs, and most conductive materials can be machined using this process.
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Arc welding
Electric discharge is a form of electric current with the highest current density. An electric arc is a type of electric discharge that is continuous and is characterised by high-temperature emission, high current density, and high temperature. Arc welding is a welding process that uses an electric arc to join metals. The process involves using a power supply to create an electric arc between an electrode and the base metal, which heats the metals to their melting point and allows them to fuse together.
There are several types of arc welding processes, including:
- Gas metal arc welding (GMAW) or metal/inert-gas (MIG) welding: This is a semi-automatic or automatic process that uses a continuously fed consumable wire as the electrode and filler metal, along with an inert or semi-inert shielding gas to protect the weld site.
- Submerged arc welding (SAW): This process involves striking the arc beneath a covering layer of granular flux to block contaminants and improve arc quality. It is commonly used for large products and is typically automated.
- Gas tungsten arc welding (GTAW) or tungsten/inert-gas (TIG) welding: This is a manual welding process that uses a non-consumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material.
- Plasma arc welding: This process uses a tungsten electrode and plasma gas to create a more concentrated arc, making it suitable for mechanized welding.
Other arc welding processes include atomic hydrogen welding, carbon arc welding, electroslag welding, electrogas welding, and stud arc welding. Each process has its unique characteristics, and the choice of process depends on the specific application and material being welded.
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Electric discharge excitation
In a gaseous medium, such as the gas inside a fluorescent lamp, the application of a sufficiently high voltage can cause the gas to become ionized and emit light. This is the principle behind the operation of a household fluorescent lamp, which contains a mixture of a rare gas and mercury vapour. When high voltage is applied across the lamp's terminals, a low-pressure mercury arc forms, causing excited mercury (Hg) atoms to emit ultraviolet (UV) radiation. This UV radiation is then absorbed by a phosphorescent coating inside the lamp and re-emitted as visible light.
The behaviour of electric discharges in gases can be quite complex, with several distinct regions and current-voltage characteristics. For example, at low voltages, the only current present is that generated by external sources of ionizing radiation, such as cosmic rays. As the voltage increases, electrons gain enough energy to cause further ionization, leading to an "electron avalanche" and a significant increase in current. This is known as a Townsend discharge.
At higher voltages, a glow discharge occurs, where the voltage across the electrodes suddenly drops and the current increases to the milliampere range. This is used in applications such as voltage regulation and lighting. If the current continues to increase into the ampere range, an arc discharge occurs, characterized by a highly luminous core surrounded by a more diffuse aureole. The temperature of the core is high enough to completely dissociate and ionize the gas, and the current-voltage characteristic is governed by the electric power dissipated in the arc column.
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Frequently asked questions
Electrical discharge is the process of exciting atomic states in a gaseous medium by passing an electric current through it. High-level electrical discharge refers to the phenomenon where a high voltage creates a low-pressure arc that emits radiation, which can be in the form of UV radiation or high-intensity visible light.
High-intensity discharge lamps, such as mercury-vapour, metal-halide, or high-pressure-sodium lamps, are examples of high-level electrical discharge. In these lamps, the discharge is contained in arc-tubes filled with inert gas. Other examples include electric arc furnaces, which sustain arc currents in the tens of thousands of amperes, and spark gaps used in internal combustion engines to ignite fuel/air mixtures.
Electric discharge in gases occurs when electric current flows through a gaseous medium due to the ionization of the gas. The gas glows as a result of the electric current, and the intensity of the light depends on factors such as the electric field intensity and the gas density.
High-level electrical discharge has various applications, including lighting sources, high-voltage electrical equipment, and welding/machining processes. For example, in electric discharge machining, multiple tiny electric arcs are used to erode a conductive workpiece to create a finished shape.
