Cathode

From Wikipedia, the free encyclopedia

Image:Copper cathode.png 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.

1 Flow of electrons
2 Chemistry cathode
3 Electronics and physics cathode
4 See also
5 External links

[edit] Flow of electrons

The flow of electrons is always from anode–to–cathode outside of the cell or device, and from cathode–to–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 cathode–to–anode inside the cell.

[edit] Chemistry cathode

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).

[edit] 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.

[edit] 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.

[edit] 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.

[edit] Electronics and physics cathode

In physics or electronics, a cathode is an electrode that emits electrons into the device.

[edit] 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 effect is called photoelectric emission.

[edit] 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.

[edit] Diodes

Image:Diode symbol.svg

In a semiconductor diode, the cathode is the N–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.