A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD (Cathode Current Departs).
Cathode polarity is not always negative. Although positively charged cations always move towards the cathode (hence their name) and/or negatively charged anions move away from it, cathode polarity depends on the device type, and can even vary according to the operating mode. In a device which consumes power, the cathode is negative, and in a device which provides power, the cathode is positive:
An electrode through which current flows the other way (into the device) is termed an anode.
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