A cathode is an electrode through which (positive) electric charge flows out of a polarized electrical device. Mnemonic: CCD (Cathode Current Departs).
From an electrochemical point of view, positively charged cations invariably move toward the cathode and/or negatively charged anions move away from it to balance the electrons arriving from external circuitry. This does not mean that cathode polarity is always negative; cathode polarity depends on the device type, and can even vary according to the operating mode. Examples:
* In a discharging battery or galvanic cell (drawing) the cathode is the positive terminal, toward the direction of conventional current. This outward charge is carried internally by positive ions moving from the electrolyte to the positive cathode (chemical energy is responsible for this "uphill" motion). It is continued externally by electrons moving inwards, negative charge moving one way amounting to positive charge flowing the other way.
* In a recharging battery, or an electrolytic cell, the cathode is the negative terminal, which sends charge back to the external generator.
* In a diode, it is the negative terminal at the pointed end of the arrow symbol, where charge flows out of 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 negative terminal where electrons flow in from the wiring and leave for the tube's near vacuum. So from the outside, a positive charge flows out of the device.
An electrode through which charge flows the other way (into the device) is termed an anode.
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