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1.2 Unit of Measurement

Introduction

Physics is an experimental science. Theories are useful only if their predictions agree with the results of experiments. So measurement plays an important part in physics.Together with other scientists , physicists have agreed on a single system of units for the measurement of physical quantities .

A physical quantity is clearly defined with a numerical value and a unit. A physical quantity can be measured using a standard size called the unit.

Units identify the quantity that has been measured.

The standard size used must

(i)            be easily reproduced

(ii)           not have its magnitude changed

(iii)          be internationally accepted

Although various systems of units have been used over the years, scientists have generally agreed to use the International System of Units (S.I. Unit).

Units for the base quantities are known as base units, i.e. metre , kilogram, second, kelvin and ampere.

The definition of base units.

The metre is the length of the path traveled  by light in vacuum during a time interval of   1299,792,458  of a second

The kilogram is the mass of a prototype cylinder of platinum-iridium alloy that  is kept

at  the International Bereau of Weights and Measures.

The second is  9192631770 periods of a specified radiation from  Cesium-133 atoms

The kelvin is  1/ 273.16 of  the  thermodynamic temperature of the triple point of water

The ampere is the current carried by two long parallel wires placed one meter apart,

when the attractive force per unit length between two wires is 2 x 10-7 Nm-1.

Prefixes

Prefix  is a scientific notation word  before the base units  and have certain value  uses powers of 10.

Factor

Name

Symbol

1024

yotta

Y

1021

zetta

Z

1018

exa

E

1015

peta

P

1012

tera

T

109

giga

G

106

mega

M

103

kilo

k

102

hecto

h

101

deka

da

100

One

10-1

deci

d

10-2

centi

c

10-3

milli

m

10-6

micro

µ

10-9

nano

n

10-12

pico

p

10-15

femto

f

10-18

atto

a

10-21

zepto

z

10-24

yocto

y

Standard Form

The radius of earth 6400000000 mm and the diameter a metal wire is  0.00000045 km. What is the best way to write these numbers ?

The best way is try to write the numbers in shorthand form.

The shorthand form of writing numbers is called standard form.

In general, the standard form number is

a x 10n

where   image

and   n  are positive integers or  negative integers.

so

6 400 000 000 mm  becomes 6.4 x 109mm

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