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Other Units in Physics Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 29, 2011

The Coulomb

The standard unit of electric charge quantity is the coulomb , symbolized C. This is the electric charge that exists in a congregation of approximately 6.241506 × 10 18 electrons. It also happens to be the electric charge contained in that number of protons, antiprotons, or positrons (antielectrons). When you walk along a carpet with hard-soled shoes in the winter or anywhere the humidity is very low, your body builds up a static electric charge that can be expressed in coulombs (or more likely a fraction of one coulomb). Reduced to base units in SI, one coulomb is equal to one ampere-second (1 A · s).

The Volt

The standard unit of electrical potential or potential difference , also called electromotive force (emf), is the volt , symbolized V. One volt is equivalent to one joule per coulomb (1 J/C). The volt is, in real-world terms, a moderately small unit of electrical potential. A standard dry cell of the sort you find in a flashlight (often erroneously called a battery ), produces about 1.5 V. Most automotive batteries in the United States produce between 12 and 13.5 V.

The Ohm

The standard unit of electrical resistance is the ohm , symbolized by the uppercase Greek letter omega (Ω). When one volt is applied across a resistance of ohm, the result is one ampere of current flow. The ohm is thus equivalent to one volt per ampere (V/A).

The Siemens

The standard unit of electrical conductance is the siemens , symbolized S. It was formerly called the mho , and in some papers and texts you’ll still see this term. Conductance is the reciprocal of resistance. One siemens can be considered the equivalent of one ampere per volt (A/V). If R is the resistance of a component in ohms and G is the conductance of the component in Siemens, then

G = 1/ R

R = 1/ G

The Hertz

The standard unit of frequency is the hertz , symbolized Hz. It was formerly called the cycle per second or simply the cycle . The hertz is a small unit in the real world, and 1 Hz represents an extremely low frequency. Usually, frequency is measured in thousands, millions, billions, or trillions of hertz. These units are called kilohertz (kHz), megahertz (MHz), gigahertz (GHz), and terahertz (THz), respectively. In terms of SI units, the hertz is mathematically simple, but the concept is esoteric for some people to grasp: It is an inverse second (s −1 ) or per second (/s).

The Farad

The standard unit of capacitance is the farad , which is symbolized F. The farad is equivalent to one coulomb per volt (1 C/V). This is a large unit in real-world applications. Most values of capacitance that you will find in electrical and electronic circuits are on the order of millionths, billionths, or trillionths of a farad. These units are called microfarads (μF), nano-farads (nF), and picofarads (pF).

The Henry

The standard unit of inductance is the henry , symbolized H. One henry is equivalent to one volt-second per ampere (V · s/A or V · s · A −1 ). This is a large unit in practice but not quite as gigantic as the farad. In electrical and electronic circuits, most values of inductance are on the order of thousandths or millionths of a henry. These units are called millihenrys (mH) and microhenrys (μH).

The Weber

The standard unit of magnetic flux is the weber , symbolized Wb. This is a large unit in practical applications. One weber is equal to one ampere-henry (1 A · H). This is represented in the real world as the amount of magnetism produced by a constant direct current of 1 A flowing through a coil having an inductance of 1 H.

The Tesla

The standard unit of magnetic flux density is the tesla , symbolized T. One tesla is equivalent to one weber per meter squared (1 Wb/m 2 or Wb · m −2 ) when the flux is perpendicular to the surface under consideration. Sometimes magnetic flux density is spoken of in terms of the number of “lines of flux” per unit cross-sectional area; this is an imprecise terminology unless we are told exactly how much magnetic flux is represented by a line.

Practice problems of these concepts can be found at: Units And Constants Practice Test

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