__(Reviewed and updated on 30/05/2016)__

__Quality and Quantity__

1.

**refers to a non-quantitative characteristic of a matter or phenomenon that can be described. For examples: inverted or upright; virtual or real; red, green, blue, etc.; opaque or transparent; sweet, salty, sour or bitter; etc.**__Quality__

2.

__Quantity:__

·

**Q****uantity**refers to a characteristic of a matter or phenomenon that can be quantified.**To quantify**means to measure and give it a**numerical value**and a**unit of measurement**.
·
A

**unit of measurement**indicates the size of that unit based on a measurement standard and**together with**the**numerical value**, they**express the size**of the**physical quantity**.

·
Examples of

**physical quantities**:
o Mass,
weight

o Length,
area, volume

o Time

o Temperature

o Electric
current, voltage, resistance, charge

o Number
of particles in a matter

o Brightness
of light

o Angle
(of reflection, refraction…), etc.

·
In the past, for the same physical quantity,
different units of measurement were used depending on the cultural backgrounds of the users. For examples:

o For

**mass**:**kilograms**, tonnes, pounds, ounces, grams…
o For

**length**: inches, feet, millimeters, centimeter,**metres**, etc.
o For

**time**:**seconds**, minutes, hours, days, etc.
o For

**temperature**: Celsius (or Centigrades), Fahrenheits,**kelvins**
This has caused

**difficulty in comparison and communication**.
·
To overcome the difficulty, the

**SI**(**) has chosen and***International System of Units***standardized the units of measurement**for all physical quantities – and, these chosen units of measurement are known as the**SI units**.
·

**SI**has recognized some physical quantities as**and others as***base quantities***. What are their differences?***derived quantities*

__Base Quantities__

**& Base Units**

1.

**Base quantities**are**fundamental physical quantities**that**and upon which**__are not__defined in terms of other physical quantities**other physical quantities**- known as derived quantities -**are****derived**. The following physical quantities, units and symbols are**chosen by the SI**(International System of Units) as**base quantities, base units**and**unit symbols**:

**Base Quantity**(Symbol)

**Base Unit**(**Symbol**)
Length (l) metre (m)

Mass (m) kilogram (kg)

Time (t) second (s)

**Electric current (I)**

**ampere (A) (2013 P1 Q1)**

Temperature (T) kelvin (K)

**(2006 P1 Q1 Pg. 48)**2. The

**SI**has recognised

**7**quantities as

**base quantities**and defined their

**base units**as: metre, kilogram, second, ampere, Kelvin, mole and candela.

[

__Students' Common Misunderstanding / Error on "__:

**Electric Current**"1) Students - because they can easily remember electric current as I = V/R (Ohm's Law) or I = Q/t (rate of flow of electric charges) - tend to think that "electric current", I is a derived quantity: This is wrong!

**Electric current (I)**was chosen by SI to be

**a base quantity -**

**a**

**fundamental physical quantity**

**.**A

**base**

**quantity**- though

**-**

__is not__defined in terms of other quantities**can**however

**be expressed in terms of other quantities.**For example: the base quantity length (l) can be expressed in terms of "square root of the area A of a square" and, that does not make length a derived quantity! Similarly,

**electric current (I) -**though can be expressed in terms of

**I = V/R**or,

**I = Q/t**- is

**a base quantity**as

**chosen by SI!**

2)

**SI**defines

**electric current**of

**one ampere**as the current that flows through 2 straight conductors of infinite length of negligible cross-sectional area placed 1 metre apart in vacuum that

**produces**between the conductors

**a force**of

**2 x 10**

^{-7 }**newton per metre length of the conductors**]

__Derived Quantities__

**& Derived Units**
1.

**Derived quantities**are physical quantities which are**derived from**the**base quantities**by multiplication or division or both. For example, speed is a derived quantity of length (distance travelled) over time.

2.

**Derived units**are units of measurements (for derived quantities) which are**derived from base units**of the component base quantities by multiplication or division or both. In the case of the derived quantity, speed, its derived unit is metre/time (with unit symbol, m/s or ms^{-1}).
3.

**Some****derived units**have been**given special names**by**SI**. For examples:

**Derived Quantity**(Symbol)

**Formula**

**Derived Unit**(**Special Name**)
·

**Force**(F) Mass x Acceleration kg ms^{-2}(**n****ewton**, N)
·

**Pressure**(P) Force/Area kg ms^{-2}/m^{2}(**pascal**, Pa)
·

**Frequency**(f) 1/Period 1/s = s^{-1}(**hertz**, Hz)
·

**Work**(W) Force x Displacemt N x m (**joule**, J)
·

**Power**(P) Work/Time J/s (**watt**, W)**Electric charge**(Q) Current x Time A s (**coulomb**, C)

**(2011 P1 Q3 at pg. 280****- Weight**being a measure of gravitational

**force**is a derived quantity

__2012 P1 at pg. 332__- What is the

**S.I.**unit for density? Answer:

**D**kg m

^{-3}- avoid

**C)**

------------------------------------------------------------------------------------------------

__Segment Review Questions:__

A)

__Base Quantity and Derived Quantity__
1.
What is a base quantity? Name 5 base quantities.

2.
Define derived quantity. And, state 5 derived
quantities.

3.
State two main advantages of standardization of all
units of measurement for physical quantities.

4.
Give the name and symbol of the SI unit of measurement
for each of the following physical quantities:

a.
Length

b.
Mass

c.
Time

d.
Temperature

e.
Current

f.
Force

g.
Energy

h.
Power

5.
Some derived quantities have been given special names
by SI (

**). State these derived quantities (that you know of) and their special names.***International System of Units*

Reviewed and updated on 30/05/2016 by tutortan1@gmail.com

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## 2 comments:

it is so easy,i like very much,thankssssssss

Identify the base quantity in the following:

(a) Speed (b) Area (c) Force (d) Distance

??????????????????

plz send me answer at hafizadnan088@gmail.com

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