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Chapter 1
Units associated with basic
electrical quantities
At the end of this chapter you should be able to:
• state the basic SI units
• recognize derived SI units• understand prefixes denoting multiplication and division
• state the units of charge, force, work and power and perform simple calculations involving these units
• statethe units of electrical potential, e.m.f., resistance, conductance, power and energy and perform simple
calculations involving these units
1.1 SI units
The system of unitsused in engineering and science is
the Système Internationale d’Unités (International system
of units), usually abbreviated to SI units, and is
based on the metricsystem. Thiswas introduced in 1960
and is now adopted by the majority of countries as the
official system of measurement.
The basic units in the SI system are listed belowwith
their symbols:
Quantity Unit
length metre, m
mass kilogram, kg
time second, s
electric current ampere, A
thermodynamic temperature kelvin, K
luminous intensitycandela, cd
amount of substance mole, mol
Derived SI units use combinations of basic units and
there are many of them. Two examples are:
Velocity – metres per second (m/s)Acceleration – metres per second
squared (m/s2)
SI units may be made larger or smaller by using prefixes
which denote multiplication or division by a particular
amount.The six most common multiples, with their
meaning, are listed below:
Prefix Name Meaning
M mega multiply by 1 000 000 (i.e.×106)
k kilo multiply by 1000 (i.e. ×103)
mmilli divide by 1000 (i.e. ×10−3)
μ micro divide by 1 000 000 (i.e. ×10−6)
n nano divide by 1 000 000 000
(i.e. ×10−9)
p pico divide by 1 000 000 000 000
(i.e. ×10−12)
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