Electrical Blog

UNIT OF WORK

noreply@blogger.com (eddyh) — Mon, 18 Nov 2013 15:03:00 +0000

The unit of work or energy is the joule (J) where one joule is one newton metre. The joule is deﬁned as the work done or energy transferred when a force of one newton is exerted through a distance of onemetre in the direction of the force. Thus work done on a body, in joules

W = Fs where F is the force in newtons and s is the distance in metres moved by the body in the direction of the force. Energy is the capacity for doing work.

UNIT OF FORCE

noreply@blogger.com (eddyh) — Mon, 18 Nov 2013 15:02:00 +0000

The unit of force is the newton (N) where one newton is one kilogram metre per second squared. The newton is deﬁned as the force which, when applied to a mass of one kilogram, gives it an acceleration of one metre per second squared. Thus, force, in newtons
F = ma

where m is the mass in kilograms and a is the acceleration inmetres per second squared. Gravitational force, or weight,ismg,where g=9.81m/s^2.

ELECTRICAL UNIT OF CHARGE

noreply@blogger.com (eddyh) — Fri, 15 Nov 2013 13:55:00 +0000

The unit of charge is the coulomb (C) where one coulomb is one ampere second. (1coulomb = 6.24×10^18 electrons). The coulomb is deﬁned as the quantity of electricity which ﬂows past a given point in an electric circuit when a current of one ampere is maintained for one second. Thus, charge, in coulombs Q = It

where I is the current in amperes and t is the time in seconds.

Problem 1. If a current of 5A ﬂows for 2 minutes, ﬁnd the quantity of electricity transferred.

Quantity of electricity Q = It coulombs

I = 5A, t = 2×60 = 120s

Hence Q = 5×120 = 600 C

SI UNITS

noreply@blogger.com (eddyh) — Fri, 15 Nov 2013 13:52:00 +0000

The system of units used in engineering and science is the Système Internationale d’Unités (International system of units), usually abbreviated to SI units, and is based on themetric system. 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 with their symbols, in Table 1.1. 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/s^2)

SI unitsmay bemade 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 in Table 1.2.