Electrical polarity

The following outline is provided as an overview of and topical guide to electrical polarity.

Positive and negative polarity

In electrical engineering, electrical polarity defines the direction in which the electrical current would flow once a source is connected; usually used for the direct current sources, where terminals are traditionally labeled with polarity symbols + and -, with the conventional current chosen to flow from the positive to negative terminal.
* By analogy, when in electronics a signal is observed across two terminals, the measurement of voltage between the terminals yields opposing signs for the positive and negative polarity.
In physics and chemistry, electric polarity defines the electric charge separation into positive and negative charges within a system or molecule. The quantitative measure of this separation is called an electric dipole moment.
In biology, electrical polarity refers to the sign of the difference in electric potential between the parts of a living organism. For example, the inner surface of a cell membrane is usually negatively charged with respect to the outer surface. When this polarity briefly reverses in a nerve, an opposite action potential is communicated over long distances. The potential is maintained by a sodium–potassium pump. While sodium and potassium ions are both positively charged, their unequal concentration inside and outside of a cell causes the difference in potential.

Many electrical devices, from power sources to loudspeakers, operate in parallel. For proper operation, the connectors of these devices are usually polarized.

Anode and cathode

Some electrical components are non-polar and function in the same way regardless of the direction of current through them. For example, properties of a resistor are unaffected if the wires on its terminals are swapped. Many other components, however, require a particular direction of current to operate. For terminals of such polarized electrical devices, the anode/cathode terminology is used, with anode being the connection from which the conventional current is flowing inside the component. Anode/cathode terminology is not directly tied to the electric potential of the terminals. Generally, in a battery anode has a negative potential, while in an electric load − positive. The cathode has the opposite potential:

Inside an electric battery, positive charges are flowing away from the anode to the cathode.
In a diode, the operating current typically flows from the anode to cathode, and an arrow on the diode symbol indicates the direction. There are, however, exceptions, like zener diodes, that are connected in a reverse polarity, with operating current flowing from the cathode to anode. The anode/cathode terminology is also used for a similar device, a thyristor.
Many capacitors are non-polar, but the electrolytic ones have anodes and cathodes, with anode potential required to be positive with respect to the cathode to avoid damage, even though DC current will not be flowing during the operation.
In electrochemistry, by convention, anode is always the place for oxidation, and cathode for reduction. There are two types of cells: galvanic, where spontaneous chemical reactions produce electricity and electrolytic, where an external electricity source causes chemical reactions. In a galvanic cell, potential on the cathode is positive with respect to the anode; in electrolytic cells cathode is negative relative to the anode.
Transistors

While a bipolar junction transistor can be simplistically thought of as two diodes with a shared terminal, the transistor polarity is usually expressed based on the prevalence of charge carriers in the parts of the device: N-type for the regions where the charge flow is primarily due to the electrons, and P-type for the regions where the flow is mostly due to electron holes.

BJT uses both types of regions and comes in either PNP or NPN polarity. The polarity is indicated by an arrow depicting the conventional current direction from the emitter to the base.
Field-effect transistor uses the region of just one type and it can be an either N-channel or P-channel device. The wide variety of FET devices causes an elaborate set of polarity marks :
History

The binary nature of electrical phenomena was known for a very long time; its similarities to the magnetic polarity were driving research on electromagnetism, with Ørsted finally succeeding in finding a link between electricity and magnetism in 1820. The use of plus and minus signs for the opposing electrical charges was introduced by Georg Christoph Lichtenberg in the 18th century. The terms positive and negative were introduced by Benjamin Franklin in 1747. Franklin compared electricity to fluid, with positive indicating the excess of it, and negative identifying the deficit. Prior to Franklin, nomenclature varied, for example, du Fay called the positive charge vitreous, and negative resinous.
Berzelius, in his early 19th-century work on electrochemistry, used the term electrical polarity to explain the chemical reactions. Per Berzelius, while all atoms possessed both positive and negative polarities, the balance depended on an element, and the reactions were caused by the electrical attraction between the atoms.
The terms anode and cathode, roughly meaning, respectively, way up and way down in Greek, were introduced by Faraday. Knowing well the Earth's magnetic field stretching North to South and assuming that it was generated by a conventional current, the direction of this current, per Ampère's circuital law, should be East to West. Sun in the East goes up and in the West down, hence the terminology.

Electrical polarity

Positive and negative polarity

Anode and cathode

Transistors

History