An anode is an electrode through which electric charge 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 incorrectly 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. As can be seen from the following examples, in a power-consuming device, the anode is positive, and in a power-releasing device, the anode is negative. Examples:
* In a discharging battery or galvanic cell (drawing) the anode is the negative terminal, where the hypothetic charges constituting a conventional current flow in, and electrons out. Since this inwards charge is carried externally by electrons moving outwards, the negative charge moving one way amounts to positive charge flowing 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 charge from an external generator. The electric charge 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 charge 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 charge 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 charge flows in.
An electrode through which charge flows the other way (out of the device) is termed a cathode.
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