Anode
An
anode (from the
Greek άνοδος = 'going up') is the
electrode in a device that electrons flow out of to return to the circuit. Literally, the path through which the
electrons ascend out of an
electrolyte solution. The other charged electrode in the same cell or device is the
cathode.
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.
Inside chemical cells, there are two currents: negative ions () flowing from cathode to anode, and positive ions () flowing from anode to cathode, resulting in a net movement of electrons from cathode to anode. Inside solid state or transistor type cells the flow of electrons is also from cathode to anode within the cell.
Note that most
electronic circuit diagrams, and their symbols for diodes and transistors, show "conventional" current, which flows from positive to negative. However, in a metallic conductor, the current consists of electrons that flow from negative to positive.
In
electrochemistry, the
anode is where
oxidation occurs, and is the positive polarity contact in an
electrolytic cell. At the anode, anions are forced by the electrical potential to react chemically and give off electrons (oxidation) which then flow up and into the driving circuit.
In a
battery or
galvanic cell, the anode is the negative contact from which electrons flow towards the circuit. Internally the anions are flowing to the anodic material inside the cell which is connected to the negative contact of the cell; but, external to the cell in the circuit, electrons are being pushed out through the negative contact and thus through the circuit by the voltage potential.
It should be noted that in the United States, many battery manufacturers regard the positive electrode as the anode, particularly in their technical literature. Though technically incorrect, it does resolve the problem of which electrode is the anode in a secondary (or rechargeable) cell. Using the traditional definition, the anode switches ends between charge and discharge cycles.
In electronic vacuum devices such as a
cathode ray tube, the anode is the positively charged electron collector. In a tube, the anode is a charged positive plate that collects the electrons emitted by the cathode through electric attraction.
In a
semiconductor diode, the anode is the P-doped layer which initially supplies
electrons to the junction. In the junction region, the electrons supplied by the anode combine with holes supplied from the N-doped region, creating a depleted zone. As the P-doped layer supplies electrons to the depleted region, positive dope ions are left behind in the P-doped layer ('P' for positive charge-carrier ions). This creates a base positive charge on the anode. When a positive voltage is applied to anode of the diode from the circuit, more
electrons are able to be transferred to the depleted region, and this causes the diode to become conductive, allowing current to flow through the circuit. The terms anode and cathode should not be applied to a zener diode, since it allows flow in either direction, depending on the polarity of the applied potential (i.e. voltage).
In
cathodic protection, a metal anode that is more reactive to the corrosive environment of the system to be protected is electrically linked to the protected system, and partially
corrodes or dissolves, which protects the metal of the system it is connected to. As an example, an
iron or
steel ship's hull may be protected by a zinc sacrificial anode, which will dissolve into the seawater and prevent the hull from being corroded. Sacrificial anodes are particularly needed for systems where a
static charge is generated by the action of flowing liquids, such as pipelines and watercraft.
The opposite of an anode is a
cathode. When the charge on the system is reversed, the
electrodes switch functions, so
anode becomes
cathode, while
cathode becomes
anode, as long as the reversed charge is applied.
*
Cathode*
Anodising (a method of enhancing the surface properties of
aluminium)
*
Battery*
Cathodic protection*
Electron tube*
Electrolysis*
Galvanic cell*
Redox (oxidation-reduction)
*
The Cathode Ray Tube site*
How to define anode and cathode*
Valence Technologies Inc. battery education page
