A cathode is an electrode through which (positive) electric current flows out of a polarized electrical device. An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. Mnemonic: CCD (Cathode Current Departs).
To dispel a common misconception, often incorrectly inferred from the correct fact that in all electrochemical devices positively charged cations move towards the cathode and/or negatively charged anions move away from it, cathode polarity is not always negative but depends on the device type, and sometimes even in which mode it operates, as determined by the above current direction based universal definition. Examples:
- In a discharging battery or galvanic cell (drawing) the cathode is the positive terminal, where conventional current flows out. The Galvanic cell, named after Luigi Galvani, consists of two different metals connected by a Salt bridge or a porous disk between the individual half-cells Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. This outward current is carried internally by positive ions moving from the electrolyte to the positive cathode (chemical energy is responsible for this "uphill" motion). An electrolyte is any substance containing free Ions that behaves as an electrically conductive medium It is continued externally by electrons moving inwards, negative charge moving one way amounting to positive current flowing the other way.
- In a recharging battery, or an electrolytic cell, the cathode is the negative terminal, which sends current back to the external generator. An electrolytic cell decomposes chemical compounds by means of electrical energy in a process called Electrolysis; the Greek word Lysis means to break up
- In a diode, it is the negative terminal at the pointed end of the arrow symbol, where current flows out of the device. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes Note electrode naming for diodes is always based on the direction of the forward current (that of the arrow, in which the current flows "most easily"), even for types such as zener diodes or solar cells where the current of interest is the reverse current.
- In a cathode ray tube, it is the negative terminal where electrons flow in, i. The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or e. , where current flows out of the device.
An electrode through which current flows the other way (into the device) is termed an anode. An anode is an Electrode through which Electric current flows into a polarized electrical device
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Etymology
The word was coined in 1834 from the Greek κάθοδος (kathodos), 'descent' or 'way down', by William Whewell, who had been consulted[1] by Michael Faraday over some new names needed to complete a paper on the recently discovered process of electrolysis. Year 1834 ( MDCCCXXXIV) was a Common year starting on Wednesday (link will display the full calendar of the Gregorian Calendar (or a Common Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly William Whewell ( May 24, 1794 &ndash March 6, 1866) was an English Polymath, Scientist, Anglican Priest Michael Faraday, FRS ( September 22 1791 – August 25 1867) was an English In that paper Faraday explained that when an electrolytic cell is oriented so that electric current traverses the "decomposing body" (electrolyte) in a direction "from East to West, or, which will strengthen this help to the memory, that in which the sun appears to move", the cathode is where the current leaves the electrolyte, on the West side: "kata downwards, `odos a way ; the way which the sun sets" ([2], reprinted in [3]).
The use of 'West' to mean the 'out' direction (actually 'out' → 'West' → 'sunset' → 'down') may appear unnecessarily contrived. Previously, as related in the first reference cited above, Faraday had used the more straightforward term "exode" (the doorway where the current exits). His motivation for changing it to something meaning 'the West electrode' (other candidates had been "westode", "occiode" and "dysiode") was to make it immune to a possible later change in the direction convention for current, whose exact nature was not known at the time. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. The reference he used to this effect was the Earth's magnetic field direction, which at that time was believed to be invariant. He fundamentally defined his arbitrary orientation for the cell as being that in which the internal current would run parallel to and in the same direction as a hypothetical magnetizing current loop around the local line of latitude which would induce a magnetic dipole field oriented like the Earth's. A solenoid is a three-dimensional Coil. In Physics, the term solenoid refers to a loop of wire often wrapped around a Metallic core which In physics there are two kinds of dipoles ( Hellènic: di(s- = two- and pòla = pivot hinge An electric dipole is a This made the internal current East to West as previously mentioned, but in the event of a later convention change it would have become West to East, so that the West electrode would not have been the 'way out' any more. Therefore "exode" would have become inappropriate, whereas "cathode" meaning 'West electrode' would have remained correct with respect to the unchanged direction of the actual phenomenon underlying the current, then unknown but, he thought, unambiguously defined by the magnetic reference. In retrospect the name change was unfortunate, not only because the Greek roots alone do not reveal the cathode's function any more, but more importantly because, as we now know, the Earth's magnetic field direction on which the "cathode" term is based is subject to reversals whereas the current direction convention on which the "exode" term was based has no reason to change in the future. A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere.
Since the later discovery of the electron, an easier to remember, and more durably technically correct (although historically false), etymology has been suggested: cathode, from the Greek kathodos, 'way down', 'the way (down) into the cell (or other device) for electrons'. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J
Flow of electrons
The flow of electrons is always from anode to cathode outside of the cell or device, regardless of the cell or device type and operating mode, with the exception of diodes where electrode naming always assumes current flows in the forward direction (that of the arrow symbol), i. e. , electrons flow in the opposite direction, even when the diode reverse-conducts either by accident (breakdown of a normal diode) or by design (breakdown of a Zener diode, photo-current of a photodiode or solar cell).
Chemistry cathode
In chemistry, a cathode is the (negative or positive, depending on whether the cell is electrolytic or galvanic) electrode of an electrochemical cell at which reduction occurs. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e An electrochemical cell is a device used for generating an Electromotive force ( Voltage) and current from chemical reactions. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state The cathode supplies electrons to the positively charged cations which flow to it from the electrolyte (even if the cell is galvanic, i. e. , when the cathode is positive and therefore would be expected to repel the positively charged cations; this is due to electrode potential relative to the electrolyte solution being different for the anode and cathode metal/electrolyte systems in a galvanic cell). In Electrochemistry, the standard electrode potential, abbreviated Eo E0 or EO (with a superscript Plimsoll character pronounced The Galvanic cell, named after Luigi Galvani, consists of two different metals connected by a Salt bridge or a porous disk between the individual half-cells
Electrolytic cell
In an electrolytic cell, the cathode is where the negative polarity is applied to drive the cell. An electrolytic cell decomposes chemical compounds by means of electrical energy in a process called Electrolysis; the Greek word Lysis means to break up Common results of reduction at the cathode are hydrogen gas or pure metal from metal ions. When discussing the relative reducing power of two redox agents, the couple for generating the more reducing species is said to be more "cathodic" with respect to the more easily reduced reagent.
Galvanic cell
In a galvanic cell, the cathode is where the positive pole is connected to allow the circuit to be completed: as the anode of the galvanic cell gives off electrons, they return from the circuit into the cell through the cathode. The Galvanic cell, named after Luigi Galvani, consists of two different metals connected by a Salt bridge or a porous disk between the individual half-cells
Electroplating metal cathode
When metal ions are reduced from ionic solution onto the cathode, they form a pure metal surface on the cathode. Items to be plated with pure metal are attached to and become part of the cathode in the electrolytic solution.
Electronics and physics cathode
In physics or electronics, a cathode is an electrode that emits electrons into the device. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Electronics refers to the flow of charge (moving Electrons through Nonmetal conductors (mainly Semiconductors, whereas electrical
Vacuum tubes
In a vacuum tube or electronic vacuum system, the cathode emits free electrons. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. Electrons are extracted from metal electrodes either by heating the electrode, causing thermionic emission, or by applying a strong electric field and causing field emission. Thermionic emission is the flow of Charge carriers from a surface or over some other kind of Electrical potential barrier caused by thermal vibrational energy Field emission (FE is the emission of electrons from the surface of a condensed phase into another phase due to the presence of high electric fields Electrons can also be emitted from the electrodes of certain metals when light of frequency greater than the threshold frequency falls on it. An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e Frequency is a measure of the number of occurrences of a repeating event per unit Time. This effect is called photoelectric emission. Introduction When a Metallic surface is exposed to Electromagnetic radiation above a certain threshold Frequency, the light is absorbed and Electrons
Cold cathodes and hot cathodes
Cathodes used for field emission in vacuum tubes are called cold cathodes. Field emission (FE is the emission of electrons from the surface of a condensed phase into another phase due to the presence of high electric fields A cold cathode is an element used within some Nixie tubes Gas discharge lamps Gas filled tubes and Vacuum tubes Cold cathodes do not Heated electrodes or hot cathodes, frequently called filaments, are much more common. Hot cathode is also a name for a Hot filament ionization gauge, a vacuum measuring device Most radios and television sets prior to the 1970s used filament-heated-cathode electron tubes for signal selection and processing; to this day, a hot cathode forms the source of the electron beam(s) in cathode ray tubes in many television sets and computer monitors. The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or Hot electron emitters are also used as the electrodes in fluorescent lamps and in the source tubes of X-ray machines. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor. X-radiation (composed of X-rays) is a form of Electromagnetic radiation.
Diodes
In a semiconductor diode, the cathode is the N–doped layer of the PN junction. Semiconductor devices are Electronic components that exploit the electronic properties of Semiconductor materials principally Silicon, Germanium Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes Initially, the N-doped layer supplies electrons to flow into the junction (N-doped for negative charge carriers). The electrons given by the N-doped layer combine with 'holes' supplied from the P-doped layer. The electrons and holes combining create a 'depleted' zone at the junction, leaving behind in the cathode a thin layer of positive ions which gives a base positive charge to the junction's cathode side of the device. (The anode side has a base negative charge at the junction, since it supplied 'holes' to the recombinant region and the doped ions have one electron more than their full valence shell of electrons). An anode is an Electrode through which Electric current flows into a polarized electrical device As a negative charge is applied to the cathode from the circuit external to the diode, more N-doped ions are able to supply electrons to the recombinant region and the diode becomes conductive, which allows electrons to flow though the diode from the cathode to the anode (electrons flow from the N-doped to the P-doped side when the bias is overcome). Like a typical diode, there is a fixed anode and cathode in a zener diode, but it will conduct current in the reverse direction (electrons from anode to cathode) if its breakdown or Zener voltage is exceeded.
See also
- Anode
- Electrode
- Electrolytic cell
- Battery
- Cathode ray tube
- Oxidation-reduction
- Electron tube
- Electrolysis
- Primary cell terminology
- Cathode bias
References
- ^ Ross, S, Faraday Consults the Scholars: The Origins of the Terms of Electrochemistry in Notes and Records of the Royal Society of London (1938-1996), Volume 16, Number 2 / 1961, Pages: 187 - 220, [1] consulted 2006-12-22
- ^ Faraday, Michael, Experimental Researches in Electricity. An anode is an Electrode through which Electric current flows into a polarized electrical device An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e An electrolytic cell decomposes chemical compounds by means of electrical energy in a process called Electrolysis; the Greek word Lysis means to break up In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state In chemistry and manufacturing electrolysis is a method of separating chemically bonded elements and compounds by passing an Electric current Primary cell terminology offers much scope for confusion When a Primary cell is in use there are two circuits The external circuit e In order for a Vacuum tube (or valve to operate in a fairly linear region of its characteristic curve the grid element must be maintained at a bias voltage more negative than the Seventh Series, Philosophical Transactions of the Royal Society of London (1776-1886), Volume 124, 01 Jan 1834, Page 77, [2] consulted 2006-12-27 (in which Faraday introduces the words electrode, anode, cathode, anion, cation, electrolyte, electrolyze)
- ^ Faraday, Michael, Experimental Researches in Electricity, Volume 1, 1849, reprint of series 1 to 14, freely accessible Gutenberg. An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e An anode is an Electrode through which Electric current flows into a polarized electrical device An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge An electrolyte is any substance containing free Ions that behaves as an electrically conductive medium In chemistry and manufacturing electrolysis is a method of separating chemically bonded elements and compounds by passing an Electric current org transcript [3] consulted 2007-01-11
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