Posts
(Reviewed and updated on 30/05/2016)
Quality and Quantity
Quality and Quantity
1.
Quality 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.
2.
Quantity:
· Quantity
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 (International System of Units)
has chosen and 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 base quantities and others as derived
quantities. What are their differences?
Base Quantities &
Base Units
1.
Base
quantities are fundamental physical quantities that are not defined in terms of other physical quantities and upon which 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.
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 (newton, N)
·
Pressure (P) Force/Area kg ms-2/m2 (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)
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 (International System of Units). State these derived quantities (that you know of) and their
special names.
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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