electrode

electrode

[ih-lek-trohd]
electrode, terminal through which electric current passes between metallic and nonmetallic parts of an electric circuit. In most familiar circuits current is carried by metallic conductors, but in some circuits the current passes for some distance through a nonmetallic conductor. For example, in electrolysis current passes through a liquid electrolyte; in a fluorescent lamp current passes through a gas. An electrode is usually in the form of a wire, rod, or plate. It may be made of a metal, e.g., copper, lead, platinum, silver, or zinc, or of a nonmetal, commonly carbon. The electrode through which current passes from the metallic to the nonmetallic conductor is called the anode, and that through which current passes from the nonmetallic to the metallic conductor, the cathode. (Electron flow is in a direction opposite that of conventionally defined current.) In most familiar electric devices, current flows from the terminal at higher electric potential (the positive electrode) to the terminal at lower electric potential (the negative electrode); therefore, the anode is usually the positive electrode and the cathode the negative electrode. In some electric devices, e.g., an electric battery, nonelectric energy is converted to electric energy, causing current to flow within the device from the negative electrode to the positive electrode, so that the anode is the negative electrode and the cathode is the positive electrode.

Electric conductor, usually metal, used as one of two terminals to conduct electric current through a conducting medium. A simple voltaic cell, or battery, consists of two electrodes, usually one zinc and one copper, immersed in an electrolytic solution (see electrolyte). When a chemical reaction occurs in the solution, electrons gather on the zinc electrode, or cathode, which becomes negatively charged. At the same time, electrons are drawn from the copper electrode, the anode, giving it a positive charge. The difference in charge sets up a potential difference, or voltage, between the two electrodes. When they are connected by a conducting wire, electrons flow from the cathode to the anode, producing a current.

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An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte or a vacuum). The word was coined by the scientist Michael Faraday from the Greek words elektron (meaning amber, from which the word electricity is derived) and hodos, a way.

Anode and cathode in electrochemical cells

An electrode in an electrochemical cell is referred to as either an anode or a cathode, words that were also coined by Faraday. The anode is now defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode as the electrode at which electrons enter the cell and reduction occurs. Each electrode may become either the anode or the cathode depending on the voltage applied to the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell.

Primary cell

A primary cell is a special type of electrochemical cell in which the reaction cannot be reversed, and the identities of the anode and cathode are therefore fixed. The anode is always the negative electrode. The cell can be discharged but not recharged.

Secondary cell

A secondary cell, for example a rechargeable battery, is one in which the reaction is reversible. When the cell is being charged, the anode becomes the positive (+) electrode and the cathode the negative (−). This is also the case in an electrolytic cell. When the cell is being discharged, it behaves like a primary or voltaic cell, with the anode as the negative electrode and the cathode as the positive. The chemical change, which converts chemical energy into electrical energy, is reversible. This cell can be recharged by simply passing electrons in the opposite direction, so it is also called a storage or accumulator cell.

Other anodes and cathodes

In a vacuum tube or a semiconductor having polarity (diodes, electrolytic capacitors) the anode is the positive (+) electrode and the cathode the negative (−). The electrons enter the device through the cathode and exit the device through the anode. Many devices have other electrodes to control operation, e.g., base, gate, control grid.

In a three-electrode cell, a counter electrode, also called an auxiliary electrode, is used only to make a connection to the electrolyte so that a current can be applied to the working electrode. The counter electrode is usually made of an inert material, such as a noble metal or graphite, to keep it from dissolving.

Welding electrodes

In arc welding an electrode is used to conduct current through a workpiece to fuse two pieces together. Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding. For a direct current system the weld rod or stick may be a cathode for a filling type weld or an anode for other welding processes. For an alternating current arc welder the welding electrode would not be considered an anode or cathode.

Alternating current electrodes

For electrical systems which use alternating current the electrodes are the connections from the circuitry to the object to be acted upon by the electrical current but are not designated anode or cathode since the direction of flow of the electrons changes periodically, usually many times per second.

Uses of electrodes

Electric currents are run through nonmetal objects to alter them in numerous ways and to measure conductivity for numerous purposes. Examples include:

See also

References

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