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Definition
An anode is one of two points indicating current flow on an electrical device (anode and cathode). On a battery, for instance, the anode is the “-” end and the cathode is the “+” end. On a tube circuit, the anode (plate) typically goes to + supply and the cathode (filament) typically goes to ground.
Technical
An ‘’’anode’’’ is an electrode through which ‘’(positive) electric current ‘’ flows ‘’into’’ a polarized electrical device. Mnemonic: ACID (Anode Current Into Device). Electrons flow in the opposite direction to the positive electric current.
A widespread misconception is that anode polarity is always positive. This is often inferred from the correct fact that in all electrochemical devices negatively charged anions move towards the anode and/or positively charged cations move away from it. Anode polarity is not always positive but depends on the device type, and sometimes even in which mode it operates, as determined by the above electric current direction-based universal definition.
ASExamples:
* In a discharging battery or galvanic cell (drawing) the anode is the negative terminal, where conventional current flows in, and electrons out. Since this inwards current is carried externally by electrons moving outwards, the negative charge moving one way amounts to positive current flowing the other way. At the anode, the current is continued ‘’internally’’ by positive ions (cations) moving into the electrolyte ‘’from’’ the anode, i.e., away (surprisingly) from the more negative electrode and towards the more positive one (chemical energy is responsible for this “uphill” motion). If the anode is composed of a metal, electrons which it gives up to the external circuit must be accompanied by metal atoms missing those electrons (cations) moving away from the electrode and into the electrolyte.
* In a recharging battery, or an electrolytic cell, the anode is the positive terminal, which receives current from an external generator. The electric current flow through a recharging battery is opposite to the direction of current during discharge; In other words, the electrode which was the cathode during battery discharge becomes the anode while the battery is recharging.
* In a diode, it is the positive terminal at the tail of the arrow symbol, where current flows into 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 a cathode ray tube, it is the positive terminal where electrons flow out, i.e., where positive electric current flows in.
A polarised electric device always has at least two electrodes - one through which positive electric current charge flows into the device and one through which the current flows out. The electrode that electric current flows into the device through is by definition always the anode, the current out electrode is always the cathode. Again note that the electron flow is OPPOSITE to the positive electric current charge flow, which has been a source of confusion.
Etymology
The word was coined in 1834 from the Greek language ἄνοδος (’’anodos’’), ‘way up’, by William Whewell, who had been consulted by 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 anode is where the current enters the electrolyte, on the East side: “‘’ano’’ upwards, ‘’odos’’ a way ; the way which the sun rises”.
The use of ‘East’ to mean the ‘in’ direction may appear unnecessarily contrived. Previously, as related in the first reference cited above, Faraday had used the more straightforward term “eisode” (the doorway where the current enters). His motivation for changing it to something meaning ‘the East electrode’ (other candidates had been “eastode”, “oriode” and “anatolode") 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 East electrode would not have been the ‘way in’ any more. Therefore “eisode” would have become inappropriate, whereas “anode” meaning ‘East 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 anode’s function any more, but more importantly because, as we now know, the Earth’s magnetic field direction on which the “anode” term is based is subject to reversals whereas the current direction convention on which the “eisode” 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 correct technically although historically false, etymology has been suggested: anode, from the Greek ‘’anodos’’, ‘way up’, ‘the way (up) out of the cell (or other device) for electrons’.
Flow of electrons
The flow of electrons is always from anode to cathode outside of the cell or device, regardless of the cell or device type and operating mode, with the exception of diodes, where electrode naming always assumes current flows in the forward direction (that of the arrow symbol), i.e., electrons flow in the opposite direction, even when the diode reverse-conducts either by accident (breakdown of a normal diode) or by design (breakdown of a Zener diode, photo-current of a photodiode or solar cell).
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. It also accelerates the flow of these electrons.
Diode anode
In a semiconductor diode, the anode is the P-doped layer which initially supplies ‘’’holes‘’’ to the junction. In the junction region, the holes supplied by the anode combine with electrons supplied from the N-doped region, creating a depleted zone. As the P-doped layer supplies holes to the depleted region, negative dope ions are left behind in the P-doped layer (’P’ for positive charge-carrier ions). This creates a base negative charge on the anode. When a positive voltage is applied to anode of the diode from the circuit, more ‘’’holes‘’’ 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).
edited from Wikipedia