Anode

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Image:Zinc anode.png

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.

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Flow of electrons

Outside the cell, where the current consists mostly of electrons flowing in wires, the flow of electrons is always from the cell's anode to its cathode. Inside chemical cells, there are two currents: negative ions (anions) flowing from cathode to anode, and positive ions (cations) 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.

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Electrolytic anode

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.

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Battery or galvanic cell anode

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 of the cell.

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Vacuum tube anode

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.

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Diode anode

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).

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Sacrificial anode

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.

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Related antonym

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.

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See also

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External links

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