pa

Pa is a term with several meanings in science and everyday usage. The two most common are the pascal, the SI unit of pressure, and the chemical element protactinium, which has the symbol Pa.

A pascal (Pa) is defined as one newton per square meter. It is used to measure pressure in physics, meteorology, engineering, and fluid dynamics. Because many pressures are large, submultiples like kilopascals (kPa) and hectopascals (hPa) are often used. For reference, standard atmospheric pressure is about 101,325 Pa (approximately 101.3 kPa).

Protactinium (Pa) is a radioactive element in the actinide series with atomic number 91. It occurs only in trace amounts in uranium ore and is not found in pure form. It was discovered in 1917 by Kasimir Fajans and Oswald Helm and was historically called protoactinium before the name Protactinium was adopted. Protactinium exhibits several oxidation states, with +5 common in compounds; its isotopes are radioactive and are studied mainly in nuclear and actinide chemistry. Because of its scarcity and radioactivity, protactinium has no large-scale commercial applications and is used primarily in scientific research with appropriate safety precautions.

Other uses of the term Pa appear in abbreviations and place names. For example, PA is the postal abbreviation for the U.S. state of Pennsylvania and is used in contexts such as public address systems (PA). The internet country code for Panama is .pa. The string Pa may also appear in various acronyms or as part of names, depending on context.