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PHYSICAL

QUANTITY

Besaran fisika: pengamatan danpengukuran

Gatut Yudoyono

Physics department,MIPA-ITS

Physical Measurement Method

(Metode Pengukuran Fisika)

SF 0913061

PhysicalQuantity

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Like all other sciences,physicsphysics is based on experimentalbased on experimental

observations and quantitative measurementsobservations and quantitative measurements.

The main objective of physics is to find the limited

number of fundamental laws that govern natural

phenomena and to use them to develop theories that

can predict the results of future experiments.

The fundamental laws used in developing theories are expressed in the

language of mathematics, the tool that provides a bridge betweenthe tool that provides a bridge between

theory and experiment.theory and experiment.

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The need for assigning numerical values to various measured physical

quantities was expressed by Lord KelvinLord Kelvin (William Thomson) as

follows:

I often say that when you can measure what you are

speaking about, and express i t in numbersexpress i t in numbers, you

should know something about it, but when you

cannot express it in numbers, your knowledge

is of a meager and unsatisfactory kind. It may

be the beginning of knowledge but you have scarcely

in your thoughts advanced to the state of science.

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The laws of physics are expressed as mathematical

relationships among physical quantities physical quantities.

Most of these quantities are derived quantitiesderived quantities,

in that they can be expressed as combinations of

a small number of basic quantities.

In mechanicsmechanics, the three basic quantities are

LengthLength

MassMass

TimeTime.

All other quantities in mechanics can be expressed in terms of these

three.

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Standards of Length, Mass, and TimeStandards of Length, Mass, and Time

If we are to report the results of a measurement to someone who wishes to

reproduce this measurement, aa standard must be defined.standard must be defined.

Likewise, if we are told that a person has a mass of 75 kilograms and our

unit of mass is defined to be 1 kilogram, then that person is 75 times as

massive as our basic unit.

It would be meaningless if a visitorfrom another planet were to

talk to us about a length of 8 glitches if we do not know the

meaning of the unit glitch.

Whatever is chosen as a standard must be readily accessiblereadily accessible andpossess some property that can be measured reliably.measured reliably.

Measurements taken by different people in different places

must yield the same result.must yield the same result.

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A unit is a particular physical quantityA unit is a particular physical quantity, defined and adopted by

convention, with which other particular quantities of the same kind are

compared to express their value.

The value of a physical quantity is the quantitative expressionquantitative expression of a

particular physical quantity as the product of a numberand a unit, the

number being its numerical value.

For example, the circumference of the earth around the equator is given by:

Ce = 40.074.10

3

mwhere Ce is the physical quantity and the number 40.074. 10

3 is the

numericalvalue of this quantity expressed in the unit "meter".

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The basic units a physical materialization should be available

that enables a comparison of the associated quantity with other

instruments: a standard.

The value of the quantity that is represented by such a

standard is exactly 1 by convention.

For instance the unit of the meter was the distance between two small

scratches on a particular platinum bar that was kept under thesupervision of the Bureau International des Poids et Mesures (BIPM)

in Srvres, France.

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For other units too (primary) standards have been constructed, and they

have been used for a long time to compare the unit value with other

standards, called secondary standards.

The unit value for the quantities mass, length and time have been

chosen with a view to practical applicability. For instance, the

circumference of the earth could be a proper standard but is highly

impractical; therefore the meter was originally defined (1791) as one

ten millionth of a quarter of the earth's meridian passing through

Paris, which is a much more practical measure for daily life usage.

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LengthLength

In A.D. 1120 the king of England decreed that the standard of length in his

country would be named the yard (3 ft) and would be precisely equal to

the distance from the tip ofhis nose to the end of his outstretched arm.

Similarly, the original standard for the foot adopted by the French

was the length of the royal foot of King Louis XIV (until 1799).

The legal standard of length in France became the meter,meter,defined as

one ten-millionth the distance from the equator to the North

Pole along one particular longitudinal line that passesthrough Paris.

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Evidently, these physical standards were much more stable and

reproducible than those defined earlier on the basis of human properties.

Length of a foot, defined as the average of the feet of 12 men

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As recently as 1960, the length of the meter was defined as

In the 1960s and 1970s, the meter was defined as

the distance between two lines on a specific platinumiridium bar

stored under controlled conditions in France.

1 650 763.73 wavelengths of orange-red light emitted from

a krypton-86 lamp.

However, in October 1983, the meter (m) was redefined as

the distance traveled by light in vacuum during a time of

1/299 792458 second.

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MassMass

The SI unit of mass, the kilogramkilogram(kg), is defined as the mass of a

specific platinumiridium alloy cylinder kept at the International Bureau of

Weights and Measures at Svres, France.

This mass standard was established in

1887 and has not been changed since that

time because platinumiridium is anunusually stable alloy.

A duplicate of the Svres cylinder is

kept at the National Institute of

Standards and Technology (NIST) in

Gaithersburg, Maryland

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TimeTime

Before 1960, the standard of time was defined in terms of the mean

solar day for the year 1900.

(A solar day is the time interval between successive

appearances of the Sun at the highest point it reaches in the

sky each day.)

The second was defined as of a mean solar day.

The rotation of the Earth is now known to vary slightly with time,

however, and therefore this motion is not a good one to use for defining

a time standard.

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In 1967, the second was redefined to take advantage of the high

precision attainable in a device known as an atomic clockatomic clock, which uses

the characteristic frequencyof the cesium-133 atom as the reference

clock.

The second (s) is now

defined as 9 192 631770

times the period of

vibration of radiation from

the cesium atom.

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Quantities andproperties

A quantity is a property ascribed to phenomena, processes or objects, to

which a value or a class can be assigned.

Quantities can be classified in many ways into groups.

Quantities that have magnitude only are called scalarquantities;

a quantity with magnitude and direction is called a vectorquantity.

We distinguish energy related quantitiesenergy related quantities (associated with energetic

phenomena, for instance force, electric current) and quantities that are notquantities that are notassociated with energyassociated with energy(static quantities or simply properties, for instance

resistivity, length).

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Properties that are independent of the dimensions or the amount of

matter arepure material properties; others have a value that is

determined not only by the substance but also by the size or the

construction layout.

For instance, the resistivity (m) is a pure material

property, where as the resistance R( ) depends on the

material as well as the dimensions of the resistor body.

Variables characterizing the state of a system are related by physical

laws. Variables acting upon the system are called independent variablesindependent variables..

Variables describing the system's state are related to these input

variables, hence they are called dependent variablesdependent variables.

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A particular variable can be dependent or independent, according to its

purport in the system.

The classification of quantities, based on an energetic consideration:

through-variables and across-variables (the geometric, electrical,the geometric, electrical,

magnetic, thermal, mechanical, and optical domainsmagnetic, thermal, mechanical, and optical domains).

The geometric domain

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The electrical and magnetic domain

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In the electrical domain, some particular quantities associated with time

apply.

The duty cycle is defined as the high-low ratio of one period

in a periodic pulse signal:

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The thermal domain

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The mechanical domain

Quantities in the mechanical domain describe state properties related to

distance, force and motion.

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The optical domain

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en

d

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The seven standard units are defined as follows:

The metermeteris the length of the path travelled by light in vacuum during a

time interval of 1/299 792 458 of a second [17th CGPM (1983), Res.1].

CGPM = Conference Generale des Poids et Mesures

A former standard meter," left: end view of the British copy of the International

Meter; right: rulings on the polished facet; the two thick vertical lines indicate the

length at 0~ and 20~ taking into account the thermal expansion [National Physical

Laboratory, Teddington, Middlesex]

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The kilogramkilogramis the unit of mass; it is equal to the mass of the international

prototype of the kilogram [ 1 st CGPM (1889)].

The secondsecondis the duration of 9 192 631 770 periods of the radiation

corresponding to the transition between the two hyperfine levels of the

ground state of the cesium 133 atom [13th CGPM (1967), Res.1].

The ampereampereis that constant current which, if maintained in two straight

parallel conductors of infinite length, of negligible circular cross-section, and

placed 1 meter apart in vacuum, would produce between these conductors a

force equal to 2 x 10 -7 newton per meter of length [9th CGPM (1948)].

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The KelvinKelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the

thermodynamic temperature of the triple point of water [ 13th CGPM (1967),

Res.4].

The candelacandelais the luminous intensity, in a given direction, of a source that

emits monochromatic radiation of frequency 540x1012 hertz and that has a

radiant intensity in that direction of 1/683 watt per steradian [16th CGPM(1979), Res.3].

The molemoleis the amount of substance of a system which contains

as many elementary entities as there are atoms in 0.012

kilogram of carbon 12 [14th CGPM (1971), Res.3].

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Physical quantity is the numerical value of a measurableproperty that

describes a physical system's state at a moment in time.

A physical property is any measurable property the value of

which describes a physical system's state at any given

moment in time.

For that reason the changes in the physical quantities of a system

describe its transformation (or evolution between its momentary states).

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International System of Units base quantities

Name Symbol forquantity

Symbol fordimension

SI base unit Symbol for unit

Length l, x, r, etc. L meter m

Time t T second s

Mass m M kilogram kg

Electric current I, i I ampere A

Thermodynamic temperatureT kelvin K

Amount ofsubstance

n N mole mol

Luminous intensity Iv J candela cd

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absorptionalbedoareabrittlenessboiling pointcapacitancecolor

concentrationconductancedensitydielectricductilitydistribution

efficacy

electric chargeelectric fieldelectric potentialemissionflow ratefluidity

frequencyimpedanceinductanceintensityirradiancelength

locationluminancelustermalleabilitymagnetic fieldmagnetic fluxmass

melting pointmomentmomentumpermeabilitypermittivitypressure

radiancesolubilityspecific heatresistancereflectivityspinstrength

temperaturetensionthermal transfervelocityviscosity

volume

The physical propertiespropertiesof an object are defined traditionally in a Newtonian sense;the physical properties of an object may include, but are not limited to:

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A quantity is called:extensive when its magnitude is additive for subsystems (volume,

mass, etc.)

intensive when the magnitude is independent of the extent of thesystem (temperature, pressure, etc.)

There are also physical quantities that can be classified as neither

extensive nor intensive, for example angular momentum, area, force,length, and time.