Cathode

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A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD (Cathode Current Departs).

A widespread misconception is that cathode polarity is always negative. This is often incorrectly inferred from the correct fact that in all electrochemical devices positively charged cations move towards the cathode (hence their name) and/or negatively charged anions move away from it. In fact cathode polarity depends on the device type, and can even vary according to the operating mode, as per the above electric current direction-based universal definition. Consequently, as can be seen from the following examples, in a device which consumes power the cathode is negative, and in a device which provides power the cathode is positive:

  • In a discharging battery or a galvanic cell the cathode is the positive terminal since that is where the current flows out of the device (see drawing). This outward current 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 constituting positive current flowing the other way.
  • In a recharging battery, or an electrolytic cell, the cathode is the negative terminal, which sends current back to the external generator.
  • In a diode, it is the negative terminal at the pointed end of the arrow symbol, where current 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 current flows "most easily"), even for types such as Zener diodes or solar cells where the current of interest is the reverse current.
  • In vacuum tubes (including cathode ray tubes) it is the negative terminal where electrons flow in from the wiring and through the tube's near vacuum, constituting a positive current flowing out of the device.

An electrode through which current flows the other way (into the device) is termed an anode.

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