Cathode Definition and Identification Tips

The cathode is the electrode from which electrical current departs. The other electrode is named the anode. Keep in mind, the conventional definition of current describes the direction a positive electric charge moves, while most of the time electrons are true current carries. This can be confusing, so the mnenomic CCD for cathode current departs may help reinforce the definition. Usually, current departs in the direction opposite electron movement.

The word "cathode" was coined in 1834 by William Whewell. It comes from the Greek word kathodos, which means "way down" or "descent" and refers to the setting sun. Michael Faraday had consulted Whewell for name ideas for a paper he was writing on electrolysis. Faraday explains electric current in an electrolytic cell moves through the electrolyte "from East to West, or, which will strengthen to help the memory, that in which the sun appears to move." In an electrolytic cell, the current leaves the electrolyte on the west side (moving outward). Prior to this, Faraday had proposed the term "exode", discarding "dysiode," "westode," and "occiode." In Faraday's time, the electron had not been discovered. In the modern era, one way to associate the name with current is to think of a cathode as the "way down" for electrons into a cell.

Is the Cathode Positive or Negative?

The polarity of the cathode with respect to the anode may be positive or negative.

In an electrochemical cell, the cathode is the electrode at which reduction occurs. Cations are attracted to the cathode. Generally, the cathode is the negative electrode in an electrolytic cell undergoing electrolysis or in a recharging battery.

In a discharging battery or a galvanic cell, the cathode is the positive terminal. In this situation, positive ions move from the electrolyte toward the positive cathode, while electrons move inward toward the cathode. The movement of electrons towards the cathode (which carry a negative charge) means current departs from the cathode (positive charge). So, for the Daniell galvanic cell, the copper electrode is the cathode and the positive terminal. If current is reversed in a Daniell cell, an electrolytic cell is produced, and the copper electrode remains the positive terminal, yet becomes the anode.

In a vacuum tube or cathode ray tube, the cathode is the negative terminal. This is where electrons enter the device and continue into the tube. A positive current flows out from the device.

In a diode, the cathode is indicated by the pointed end of an arrow symbol. It is the negative terminal from which current flows. Even though current may flow in both directions through a diode, naming is always based on the direction in which current flows most easily.

Mnemonics to Remember the Cathode in Chemistry

In addition to the CCD mnemonic, there are other mnemonics to help identify the cathode in chemistry:

• AnOx Red Cat stands for oxidation at the anode and reduction at the cathode.
• The words "cathode" and "reduction" both contain the letter "c." Reduction occurs at the cathode.
• It may help to associate the "cat" in cation as acceptor and "an" in anion as donor.

Related Terms

In electrochemistry, the cathodic current describes the flow of electron from the cathode into solution. The anodic current is the flow of electrons from solution into the anode.