Cathode polarity with respect to the anode can be positive or negative depending on how the device is being operated. Although positively charged cations always move towards the cathode (hence their name) and negatively charged anions move away from it, cathode polarity depends on the device type, and can even vary according to the operating mode. In a device which absorbs energy of charge (such as recharging a battery), the cathode is negative (electrons flow out of the cathode, and charge flows into it), and in a device which provides energy (such as battery in use), the cathode is positive (electrons flow into it and charge flows out): °A battery or galvanic cell in use has a cathode that is the positive terminal since that is where the current flows out of the device. 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). It is continued externally by electrons moving into the battery which constitutes positive current flowing outwards. For example, the Daniell galvanic cell's copper electrode is the positive terminal and the cathode. °A battery that is recharging or an electrolytic cell performing electrolysis has its cathode as the negative terminal, from which current exits the device and returns to the external generator as charge enters the battery/ cell. For example, reversing the current direction in a Daniell galvanic cell converts it into an electrolytic cell[1] where the copper electrode is the positive terminal and also the anode. °In a diode, the cathode is the negative terminal at the pointed end of the arrow symbol, where current flows out of the device. 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 vacuum tubes (including cathode ray tubes) it is the negative terminal where electrons enter the device from the external circuit and proceed into the tube's near-vacuum, constituting a positive current flowing out of the device. An electrode through which current flows the other way (into the device) is termed an anode.
Contents
1 Etymology 2 Flow of electrons 3 In chemistry
3.1 Electrolytic cell 3.2 Galvanic cell 3.3 Electroplating metal cathode (electrolysis)
4 In electronics
4.1 Vacuum tubes
4.1.1 Hot cathode 4.1.2 Cold cathode
4.2 Diodes
5 See also 6 References 7 External links
Etymology[edit]
The word was coined in 1834 from the Greek κάθοδος (kathodos),
'descent' or 'way down', by William Whewell, who had been consulted[2]
by
Michael Faraday
Michael Faraday over some new names needed to complete a paper on
the recently discovered process of electrolysis. 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".[3][4]
The use of 'West' to mean the 'out' direction (actually 'out' →
'West' → 'sunset' → 'down', i.e. 'out of view') 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. 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. 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.
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'.
Flow of electrons[edit]
The flow of electrons is almost always from anode to cathode outside
of the cell or device, regardless of the cell or device type and
operating mode. An exception is when a diode reverse-conducts, either
by accident (breakdown of a normal diode) or by design (breakdown of a
Zener diode, photo-current of a photodiode).
In chemistry[edit]
In chemistry, a cathode is the electrode of an electrochemical cell at
which reduction occurs; a useful mnemonic to remember this is AnOx
RedCat (Oxidation at the
Anode
Anode = Reduction at the Cathode). Another
mnemonic is to note the cathode has a 'c', as does 'reduction'. Hence,
reduction at the cathode. Perhaps most useful would be to remember
cathode corresponds to cation (acceptor) and anode corresponds to
anion (donor). The cathode can be negative like when the cell is
electrolytic (where electrical energy provided to the cell is being
used for decomposing chemical compounds); or positive as when the cell
is galvanic (where chemical reactions are used for generating
electrical energy). 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).
The cathodic current, in electrochemistry, is the flow of electrons
from the cathode interface to a species in solution. The anodic
current is the flow of electrons into the anode from a species in
solution.
Electrolytic cell[edit]
In an electrolytic cell, the cathode is where the negative polarity is
applied to drive the cell. 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[edit]
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.
Electroplating metal cathode (electrolysis)[edit]
When metal ions are reduced from ionic solution, 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.
In electronics[edit]
In physics or electronics, a cathode is an electrode that emits
electrons into the device. This contrasts with an anode, which accepts
electrons.
Vacuum tubes[edit]
Glow from the directly heated cathode of a 1 kW power tetrode tube in a radio transmitter. The cathode filament is not directly visible
In a vacuum tube or electronic vacuum system, the cathode is a metal surface which emits free electrons into the evacuated space. Since the electrons are attracted to the positive nuclei of the metal atoms, they normally stay inside the metal and require energy to leave it; this is called the work function of the metal.[5] Cathodes are induced to emit electrons by several mechanisms:[5]
Thermionic emission: The cathode can be heated. The increased thermal motion of the metal atoms "knocks" electrons out of the surface, an effect called thermionic emission. This technique is used in most vacuum tubes. Field electron emission: A strong electric field can be applied to the surface by placing an electrode with a high positive voltage near the cathode. The positively charged electrode attracts the electrons, causing some electrons to leave the cathode's surface.[5] This process is used in cold cathodes in some electron microscopes,[6][7][8] and in microelectronics fabrication,[7] Secondary emission: An electron, atom or molecule colliding with the surface of the cathode with enough energy can knock electrons out of the surface. These electrons are called secondary electrons. This mechanism is used in gas-discharge lamps such as neon lamps. Photoelectric emission: Electrons can also be emitted from the electrodes of certain metals when light of frequency greater than the threshold frequency falls on it. This effect is called photoelectric emission, and the electrons produced are called photoelectrons.[5] This effect is used in phototubes and image intensifier tubes.
Cathodes can be divided into two types: Hot cathode[edit] Main article: Hot cathode
Two indirectly-heated cathodes (orange heater strip) in ECC83 dual triode tube
Cutaway view of a triode vacuum tube with an indirectly-heated cathode (orange tube), showing the heater element inside
Schematic symbol
Schematic symbol used in circuit diagrams for vacuum tube, showing
cathode
A hot cathode is a cathode that is heated by a filament to produce electrons by thermionic emission.[5][9] The filament is a thin wire of a refractory metal like tungsten heated red-hot by an electric current passing through it. Before the advent of transistors in the 1960s, virtually all electronic equipment used hot-cathode vacuum tubes. Today hot cathodes are used in vacuum tubes in radio transmitters and microwave ovens, to produce the electron beams in older cathode ray tube (CRT) type televisions and computer monitors, in x-ray generators, electron microscopes, and fluorescent tubes. There are two types of hot cathodes:[5]
Directly heated cathode: In this type, the filament itself is the cathode and emits the electrons directly. Directly heated cathodes were used in the first vacuum tubes, but today they are only used in fluorescent tubes, some large transmitting vacuum tubes, and all X-ray tubes. Indirectly heated cathode: In this type, the filament is not the cathode but rather heats the cathode which then emits electrons. Indirectly heated cathodes are used in most devices today. For example, in most vacuum tubes the cathode is a nickel tube with the filament inside it, and the heat from the filament causes the outside surface of the tube to emit electrons.[9] The filament of an indirectly heated cathode is usually called the heater. The main reason for using an indirectly heated cathode is to isolate the rest of the vacuum tube from the electric potential across the filament. Many vacuum tubes use alternating current to heat the filament. In a tube in which the filament itself was the cathode, the alternating electric field from the filament surface would affect the movement of the electrons and introduce hum into the tube output. It also allows the filaments in all the tubes in an electronic device to be tied together and supplied from the same current source, even though the cathodes they heat may be at different potentials.
In order to improve electron emission, cathodes are treated with chemicals, usually compounds of metals with a low work function. Treated cathodes require less surface area, lower temperatures and less power to supply the same cathode current. The untreated tungsten filaments used in early tubes (called "bright emitters") had to be heated to 1400 °C (~2500 °F), white-hot, to produce sufficient thermionic emission for use, while modern coated cathodes produce far more electrons at a given temperature so they only have to be heated to 425–600 °C (~800–1100 °F) ()[5][10][11] There are two main types of treated cathodes:[5][9]
Cold cathode
Cold cathode (lefthand electrode) in neon lamp
Coated cathode – In these the cathode is covered with a coating of alkali metal oxides, often barium and strontium oxide. These are used in low-power tubes. Thoriated tungsten – In high-power tubes, ion bombardment can destroy the coating on a coated cathode. In these tubes a directly heated cathode consisting of a filament made of tungsten incorporating a small amount of thorium is used. The layer of thorium on the surface which reduces the work function of the cathode is continually replenished as it is lost by diffusion of thorium from the interior of the metal.[12]
Cold cathode[edit] Main article: Cold cathode This is a cathode that is not heated by a filament. They may emit electrons by field electron emission, and in gas-filled tubes by secondary emission. Some examples are electrodes in neon lights, cold-cathode fluorescent lamps (CCFLs) used as backlights in laptops, thyratron tubes, and Crookes tubes. They do not necessarily operate at room temperature; in some devices the cathode is heated by the electron current flowing through it to a temperature at which thermionic emission occurs. For example, in some fluorescent tubes a momentary high voltage is applied to the electrodes to start the current through the tube; after starting the electrodes are heated enough by the current to keep emitting electrons to sustain the discharge. Cold cathodes may also emit electrons by photoelectric emission. These are often called photocathodes and are used in phototubes used in scientific instruments and image intensifier tubes used in night vision goggles. Diodes[edit]
In a semiconductor diode, the cathode is the N–doped layer of the PN junction with a high density of free electrons due to doping, and an equal density of fixed positive charges, which are the dopants that have been thermally ionized. In the anode, the converse applies: It features a high density of free "holes" and consequently fixed negative dopants which have captured an electron (hence the origin of the holes). When P and N-doped layers are created adjacent to each other, diffusion ensures that electrons flow from high to low density areas: That is, from the N to the P side. They leave behind the fixed positively charged dopants near the junction. Similarly, holes diffuse from P to N leaving behind fixed negative ionised dopants near the junction. These layers of fixed positive and negative charges are collectively known as the depletion layer because they are depleted of free electrons and holes. The depletion layer at the junction is at the origin of the diode's rectifying properties. This is due to the resulting internal field and corresponding potential barrier which inhibit current flow in reverse applied bias which increases the internal depletion layer field. Conversely, they allow it in forwards applied bias where the applied bias reduces the built in potential barrier. Electrons which diffuse from the cathode into the P-doped layer, or anode, become what are termed "minority carriers" and tend to recombine there with the majority carriers, which are holes, on a timescale characteristic of the material which is the p-type minority carrier lifetime. Similarly, holes diffusing into the N-doped layer become minority carriers and tend to recombine with electrons. In equilibrium, with no applied bias, thermally assisted diffusion of electrons and holes in opposite directions across the depletion layer ensure a zero net current with electrons flowing from cathode to anode and recombining, and holes flowing from anode to cathode across the junction or depletion layer and recombining. 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 flow from anode to cathode) if its breakdown voltage or "Zener voltage" is exceeded. See also[edit]
Battery
Cathode
Cathode bias
Electrolysis
Electrolytic cell
Gas-filled tube
Oxidation-reduction
PEDOT
Vacuum tube
References[edit]
^ [1] Archived 4 June 2011 at the Wayback Machine.,
Daniell cell
Daniell cell can
be reversed to, technically, produce an electrolytic cell.
^ Ross, S, Faraday Consults the Scholars: The Origins of the Terms of
Electrochemistry
Electrochemistry in Notes and Records of the Royal Society of London
(1938–1996), Volume 16, Number 2 / 1961, Pages: 187–220,
[2][permanent dead link] consulted 2006-12-22
^ Faraday, Michael, Experimental Researches in Electricity. Seventh
Series, Philosophical Transactions of the Royal Society of London
(1776–1886), Volume 124, 1 January 1834, Page 77, [3][permanent dead
link] 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.org
transcript [4] consulted 2007-01-11
^ a b c d e f g h Avadhanulu, M.N.; P.G. Kshirsagar (1992). A Textbook
Of Engineering
Physics
Physics For B.E., B.Sc. S. Chand. pp. 345–348.
ISBN 8121908175. Archived from the original on 2 January
2014.
^ "Field emission". Encyclopædia Britannica online. Encyclopædia
Britannica, Inc. 2014. Archived from the original on 2 December 2013.
Retrieved 15 March 2014.
^ a b Poole, Charles P. Jr. (2004). Encyclopedic Dictionary of
Condensed Matter Physics, Vol. 1. Academic Press. p. 468.
ISBN 0080545238. Archived from the original on 24 December
2017.
^ Flesch, Peter G. (2007). Light and Light Sources: High-Intensity
Discharge Lamps. Springer. pp. 102–103. ISBN 3540326855.
Archived from the original on 24 December 2017.
^ a b c Ferris, Clifford "
Electron
Electron tube fundamentals" in Whitaker,
Jerry C. (2013). The
Electronics
Electronics Handbook, 2nd Ed. CRC Press.
pp. 354–356. ISBN 1420036661. Archived from the original
on 2 January 2014.
^ Poole, Ian (2012). "
Vacuum tube
Vacuum tube electrodes". Vacuum Tube Theory
Basics Tutorial. Radio-Electronics.com, Adrio Communications. Archived
from the original on 4 November 2013. Retrieved 3 October 2013.
^ Jones, Martin Hartley (1995). A Practical Introduction to Electronic
Circuits. UK: Cambridge Univ. Press. p. 49. ISBN 0521478790.
Archived from the original on 2 January 2014.
^ Sisodia, M. L. (2006). Microwave Active Devices Vacuum and Solid
State. New Age International. p. 2.5. ISBN 8122414478.
Archived from the original on 2 January 2014.
External links[edit]
The
Cathode
Cathode Ray Tube site
How to define anode and cathode
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