Cathode
A
cathode is the
electrode at which
electrons go into a cell, tube or diode, whether driven externally or internally. It comes from the
Greek word
κάθοδος meaning, 'descent'. The other charged electrode in the same cell or device is the
anode.
The flow of electrons is
always from
anodeâ€"toâ€"cathode outside of the cell or device, and from
cathodeâ€"toâ€"anode inside the cell or device, regardless of the cell or device type. Inside a chemical cell, ions are carrying the electrons but the flow is still from
cathodeâ€"toâ€"anode inside the cell.
In
chemistry, a
cathode is the
electrode of an
electrochemical cell at which
reduction occurs (electrons are added to
cations to complete the valence shell or bond).
Electrolytic cell
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.
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.
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.
In
physics or
electronics, a
cathode is an electrode that emits electrons into the device.
Vacuum tubes
In a
vacuum tube or other electronic vacuum system, the cathode emits free electrons. 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. Electrons can also be emitted from the
electrodes of certain metals when light of
frequency greater than the threshold frequency falls on it. This is called
photoelectric emission.
Cold cathodes and hot cathodes
Cathodes used for
field emission in vacuum tubes are called
cold cathodes. Heated electrodes or
hot cathodes, frequently called
filaments, are much more common. 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. Hot electron emitters are also are used as the electrodes in
fluorescent lamps.
Diodes
In a
semiconductor diode, the cathode is the Nâ€"doped layer of the PN junction. Initially, the N-doped layer supplies 'holes' to flow into the junction. The holes given by the N-doped layer combine with
electrons supplied from the P-doped layer. The electrons and holes combining creates a 'depleted' zone at the junction, leaving behind in the cathode a layer of negative ions which gives a base negative charge to the cathode side of device (N-doped for negative charge carrier ions). (The
anode side has a base positive charge at this point, since it supplied electrons to the recombinant region and the doped ions are short of a full valence shell of electrons). As a negative charge is applied to the cathode from the circuit external to the diode, more N-doped ions are able to supply 'holes' 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 N-doped to P-doped when the bias is overcome). Unlike a typical diode, there is no fixed anode or cathode in a zener diode.
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Anode*
Electrolytic cell*
Electrode*
Battery*
Cathode ray tube*
Oxidation-reduction*
Electron tube*
Electrolysis*
The Cathode Ray Tube site*
How to define anode and cathode
