Beatrice Lang
Electric discharge is the release and transmission of electricity through a medium such as a gas. This occurs when an electric current flows through a medium due to the ionization of the gas. There are two types of electric discharge: self-sustaining and non-self-sustaining. Non-self-sustaining discharges occur at low currents and are induced by irradiating the gas between two electrodes to produce initial ionization. Examples of non-self-sustaining discharges include sparks, with lightning being the longest known spark. Self-sustaining discharges include arc discharges, which are used in high-pressure lamps, gas lasers, welding, and arc heaters.
| Characteristics | Values | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Definition | The release and transmission of electricity in an applied electric field through a medium such as a gas | ||||||||
| Uses | - Detecting ionizing radiation in a Geiger-Müller tube | - Illuminating and regulating voltage in a neon lamp | - Generating short pulses of intense light in photography using a flashtube | - Copying in photocopiers | - Conveying substantial energy to electrodes | - Igniting fuel/air mixture in internal combustion engines | - Switching heavy currents in a Marx generator | - Protecting electrical apparatus | - Machining conductive workpieces into finished shapes |
| Types | - Self-sustaining | - Non-self-sustaining | |||||||
| Parameters | Frequency and energy of impulses | ||||||||
| Examples | - Townsend discharges | - Arc discharges | - Spark | - Lightning |
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What You'll Learn
Electric discharge in gases
To study the phenomenon, the air inside the tube is pumped out, creating a vacuum, and a desired pressure is maintained. It was initially believed that gases could not conduct electricity, even at high electrical potentials. However, William Crookes discovered that gases could conduct electricity at low pressures. When the pressure inside the discharge tube is reduced, a glow is observed surrounding the cathode (negative electrode), and this glow expands until it fills the tube. The colour of the glow depends on the nature of the gas and the colour of the glass used for the tube.
At even lower pressures, light emission occurs from the residual air in the tube, opposite to the cathode. At this stage, cathode rays are emitted from the cathode. Cathode rays are so named because they are released from the cathode and are composed of electrons. These electrons are released when gaseous ions collide with the cathode.
Gas-discharge lamps are electric lights that use gas-filled tubes, including fluorescent lamps, metal-halide lamps, sodium-vapour lamps, and neon lights. These lamps can be used for illumination and voltage regulation. Additionally, gas discharges have applications in photocopiers, high-pressure lamps, gas lasers, welding, arc heaters, and current-interruption devices.
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Self-sustaining and non-self-sustaining discharges
Electric discharges have historically been divided into two categories: self-sustaining and non-self-sustaining. The transition between these two forms is abrupt and occurs through the formation of a spark. Non-self-sustaining discharges occur at relatively low currents (approximately 10−8A). Townsend discharges, which occur below the breakdown voltage, are an example of a non-self-sustaining discharge. At low voltages, the only current is generated by the creation of charge carriers in the gas by cosmic rays or other sources of ionizing radiation. As the voltage increases, the free electrons carrying the current gain enough energy to cause further ionization, resulting in an electron avalanche. In this phase, the current increases from femtoamperes to microamperes. Such discharges may be induced by irradiating the gas between two electrodes to produce the initial ionization. They are non-sustaining because the current flow stops as soon as the ionizing radiation is removed.
On the other hand, self-sustained Townsend discharges are observed at higher voltages, such as those exceeding the Paschen breakdown voltage. Arc discharges are an example of self-sustaining discharges. They are characterized by low voltages (10–50 V) and high currents (1–100 A). To initiate an arc discharge, a voltage higher than the voltage drop at steady-state operation is required. Alternatively, it can be produced by first establishing contact between the two electrodes, which heats them up and initiates the thermionic emission necessary for arc formation. In this case, the applied voltage does not need to exceed the voltage drop. A spark, such as lightning, is a non-self-sustaining arc discharge that occurs over a specific distance, known as the spark distance, which is typically around 20 cm for a voltage of 105 V in air.
Electric discharge in gases occurs when an electric current flows through a gaseous medium due to ionization of the gas. The properties of these discharges are important in the design of lighting sources and high-voltage electrical equipment. Cold cathode tubes, for instance, exhibit three distinct regions with unique current-voltage characteristics: Townsend discharge, glow discharge, and arc discharge. Glow discharges occur once the breakdown voltage is attained, causing a sudden drop in voltage across the electrodes and an increase in current to the milliampere range. At lower currents, the voltage is nearly independent of the current, making it useful for voltage regulation. As the current increases, the normal glow transitions to an abnormal glow, and the voltage gradually rises.
Corona discharges, which occur in high-voltage transmission lines, can result in continuous power loss, especially over long distances, leading to economic inefficiencies. They can also cause deterioration of insulating materials through ion bombardment and the formation of chemical compounds like ozone and nitrogen oxides. In contrast, arc discharges find applications in high-pressure lamps, gas lasers, welding, and arc heaters, where stable arc sustenance is crucial. Electric discharges have a wide range of practical applications, from detecting ionizing radiation in Geiger–Müller tubes to illustrating the gas spectrum in gas-filled tubes. They are also used in neon lamps for illumination and voltage regulation, and in flashtubes for photography, where a heavy current is sent through a gas arc discharge to generate a short pulse of intense light.
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Corona discharge
A corona discharge is a phenomenon of partial electrical discharge caused by the ionization of a fluid, usually air, surrounding a conductor carrying a high voltage. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages.
The phenomenon gets its name from the fact that the plasma creates a lighting crown around the electrodes during the discharge. It is also sometimes referred to as a ""single-electrode discharge", as opposed to a "two-electrode discharge" or an electric arc. A corona discharge occurs when the strength of the electric field (potential gradient) around a conductor exceeds the dielectric strength of the air.
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Electric discharge milling
Electric discharge refers to the release and transmission of electricity through a medium such as a gas. This process has a wide range of applications, including the detection of ionizing radiation in a Geiger-Müller tube, illustrating the gas spectrum in a gas-filled tube, and providing illumination through a neon lamp. Electric discharge machining (EDM) is a specific application of electric discharge, where multiple tiny electric arcs are utilised to erode a conductive workpiece, shaping it into the desired form.
- Electrode Selection: An electrode, typically made of graphite or copper, is chosen based on the specific material being milled and the desired shape. The electrode is connected to a power source, creating an electric potential between the electrode and the workpiece.
- Dielectric Fluid: A dielectric fluid, such as deionized water or hydrocarbon-based oil, is circulated between the electrode and the workpiece. This fluid serves multiple purposes, including providing insulation, flushing away debris, and facilitating the formation of controlled electrical discharges.
- Controlled Discharges: A controlled electrical discharge occurs between the electrode and the workpiece, creating a spark that removes a small amount of material from the workpiece. This process is repeated at a rapid pace, with the electrode moving along a pre-programmed path, gradually milling the workpiece to the desired shape.
- Debris Removal: The dielectric fluid helps flush away the debris created by the electrical discharges, ensuring a clean and precise milling process. The debris, along with the fluid, is filtered and recycled or disposed of safely.
- Precision Machining: Electric discharge milling allows for extremely precise machining, capable of achieving complex shapes and intricate details. This precision is due to the ability to control the electrical discharges, their frequency, and their intensity, allowing for the removal of material in a highly controlled manner.
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Arc discharge
The process of arc discharge involves the electrical breakdown of a gas, resulting in a prolonged electrical discharge. This breakdown occurs when the current passing through a normally non-conductive medium, such as air, produces a plasma that emits visible light. The arc is initiated by either thermionic emission or field emission, and it occurs between two conductive electrodes, often made of tungsten or carbon. The distance between the electrodes and the type of gas surrounding them also influence the breakdown voltage.
There are two types of arc discharge: hot cathode arc discharge and cold cathode arc discharge. In hot cathode arc discharge, the cathode is heated, emitting thermal electrons that generate plasma for lamps and other light sources. Cold cathode arc discharge, on the other hand, involves the direct emission of electrons from the cathode surface by a strong electric field. Fluorescent lights are a common application of cold cathode arc discharge.
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