Electric quantities, Fundamental constants and Standard units.

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Electric quantities

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Fundamental constants

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Electrical relationships

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Dimensions of physical properties

Length: metre [L]. Mass: kilogram [M]. Time: second [T]. Quantity of electricity: coulomb [Q]. Area: square metre [L2]. Volume: cubic metres [L3].

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Fundamental units

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Standard units

Ampere Unit of electric current, the constant current which, if maintained in two straight parallel conductors of infinite length of negligible circular cross-section and placed one metre apart in a vacuum, will produce between them a force equal to 2 x 10~7 newton per metre length.
Ampere-hour Unit of quantity of electricity equal to 3,600 coulombs. One unit is represented by one ampere flowing for one hour.
Candela Unit of luminous intensity. It is the luminous intensity, in the perpendicular direction, of a surface of 1/600,000 m 2 of a full radiator at the temperature of freezing platinum under a pressure of 101,325 newtons m'2.
Coulomb Unit of electric charge, the quantity of electricity transported in one second by one ampere.

Decibel (dB) Unit of acoustical or electrical power ratio. Although the bel is officially the unit, this is usually regarded as being too large, so the decibel is preferred. The difference between two power levels, P1 and P2, is given as:

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Farad Unit of electric capacitance. The capacitance of a capacitor between the plates of which there appears a difference of potential of one volt when it is charged b/ one coulomb of electrici . Practical umts are the microfarad oo· farad), the nanofarad oo· ) and the picofarad (10·12 farad).

Henry Unit of electrical inductance. The inductance of a closed circuit in which an electromotive force of one volt is produced when the electric current in the circuit varies uniformly at the rate of one ampere per second. Practical units are the microhenry (Io-6 henry) and the millihenry (Io-3 henry).

Hertz Unit of frequency. The number of repetitions of a regular occurrence in one second.

Joule Unit of energy, including work and quantity of heat. The work done when the point of application of a force of one newton is displaced through a distance of one metre in the direction of the force.

Kilovolt-ampere 1,000 volt-amperes.

Kilowatt 1,000 watts.

Lumen m·2, lux Unit of illuminance of a surface.

Mho Unit of conductance, see Siemens.

Newton Unit of force. That force which, applied to a mass of one kilogram, gives it an acceleration of one metre per second per second.

Ohm Unit of electric resistance. The resistance between two points of a conductor when a constant difference of potential of one volt, applied between these two points, produces in the conductor a current of one ampere.

Pascal Unit of sound pressure. Pressure is usually quoted as the root mean square pressure for a pure sinusoidal wave.

Siemens Unit of conductance, the reciprocal of the ohm. A body having a resistance of 4 ohms would have a conductance of 0.25 siemens.

Tesla Unit of magnetic flux density, equal to one weber per square metre of circuit area.

Volt Unit of electric potential. The difference of electric potential between two points of a conducting wire carrying a constant current of one ampere, when the power dissipated between these points is equal to one watt.

Volt-ampere The product of the root-mean-square volts and root­ mean-square amperes.

Watt Unit of power, equal to one joule per second. Volts times amperes equals watts.

Weber Unit of magnetic flux. The magnetic flux which, linking a circuit of one tum, produces in it an electromotive force of one volt as it is reduced to zero at a uniform rate in one second.

Light, velocity of Light waves travel at 300,000 kilometres per second (approximately). Also the velocity of radio waves.

S·Jund, velocity of Sound waves travel at 332 metres per second in air (approximately) at sea level.

Greek alphabet

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Volt-ampere The product of the root-mean-square volts and root­ mean-square amperes.

Watt Unit of power, equal to one joule per second. Volts times amperes equals watts.

Weber Unit of magnetic flux. The magnetic flux which, linking a circuit of one tum, produces in it an electromotive force of one volt as it is reduced to zero at a uniform rate in one second.

Light, velocity of Light waves travel at 300,000 kilometres per second (approximately). Also the velocity of radio waves.

S·Jund, velocity of Sound waves travel at 332 metres per second in air (approximately) at sea level.

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