Cathodes and anodes are types of electrodes that conduct electrical currents either into or out of an electrical device. A cathode typically acts as a positively charged terminal while an anode usually functions as a negatively charged terminal. Anodes and cathodes will sometimes function in reverse polarity in certain types of devices. As a general rule, when a device is discharging electricity, the current flows out of the cathode terminal. When a device is being charged with electricity, the current flows into the cathode causing it to function as the anode while the anode functions as the cathode.
Electrodes used as cathodes and anodes are typically found in any device that either consumes or provides electrical current. The cathode and anode designations of electrodes are typically used as a means to identify their polarity during the most common application of a device. Cathodes and anodes with non-reversible polarity can be found in devices such as disposable batteries, and semiconductor diodes. Those with reversible polarity are typically found in rechargeable batteries and cathode ray tubes.
The cathode and anode terminals of a disposable battery are non-reversible because the device is only used to discharge electrical current. In a disposable battery, the cathode terminal is always positive and the anode is always negative. The cathodes and anodes of rechargeable batteries are reversible because this device can be used to receive as well as discharge electrical current. When this type of battery is being recharged, the usually positive cathode becomes negative and the usually negative anode becomes positive.
In a cathode ray tube, the negative cathode terminal emits rays of negative electrons inside of a glass vacuum tube, which are then attracted by the positive anode inside the tube. After reaching the anode, the electrons are then focused by another electrode known as a focusing anode. Once the electrons have been focused, the are then accelerated by yet another electrode called an accelerating anode. After the electron rays have been focused and accelerated, they are sent to the screen portion of the vacuum tube in order to create the image that is seen.
In semiconductor diodes, electrical current enters the device by means of the negative cathode terminal and then exits through the positive anode terminal. Since diodes carry electrical current in only one direction, the polarity of the cathode and diode terminals does not change. This unchanging polarity configuration applies to all types of diodes, including solar cells and zenar-type diodes.