physical quantity lesson 1  (physics)

PHYSICAL QUANTITY (Lesson 1)

Physical quantity is any measurable property of a substance. For instance, the properties of a human being that is measurable are his/ her weight, temperature, height, blood pressure, etc. these are referred to as physical quantities in physics. Physical quantities are specified and identified by their units e.g., Mass is measured in grams/kilograms /tonnes. However, there is a standard unit of measurement that is acceptable globally for each quantity. It is called the Systeme’ International Unit (SI unit). Physical quantities can also be classified into two groups based on their dependency i.e. the fundamental quantities and the derived quantities.

Fundamental Quantities

These are physical quantities that do not depend on other quantities for their derivation. There are seven fundamental quantities which I gave the acronym “METTALL” as expressed in the table below:

Derived quantities

Derived quantities are quantities that depend on fundamental quantities for their derivation. There are many examples of derived quantities, some of which are given in the table below:

Standard Prefixes

These are factor of 10n used to express a relatively large or small unit of measurement in concise or simple form

e.g. 10, 000m = 10 x 103m (Since 103 represents the prefix ‘KILO denoted K’)

Therefore, 10, 000m = 10 x 103m = 10Km

The table below shows the standard prefixes, their symbol and value

Conversion of Other units to S.I unit

Example: Convert the units of the following quantities:

(a)    speed, 18km/h (b) density, 2000g/cm3  (c) frequency, 3.5GHz (d) capacitor, 200 µF

Solution

a) Speed, 18k/h

  The S.I unit of Speed is m/s, so:

Speed = 18km/h = 18km/1h= (18x1000)m/(3600)s = 18000m/3600s

Speed = 5m/s

NB: (K= 103 = 1000 and 1 hour = 3600 seconds)

b) Density, 2000 g/cm3  

  The S.I unit of Density is kg/m3, so:

   Density = 2000g/cm3 = (2000g) / (1cm x 1cm x 1cm) = (2000 ÷ 1000) Kg / (10-2m x 10-2m x 10-2m)

                  = 2Kg / (10-6m3) = 2 x 106Kgm-3

      NB: (1000g = 1kg, c =10-2)

(c) Frequency, 3.5GHz;    F= 3.5 x 109 Hz

(b)   Capacitor, 200 µF;  C= 200 x 10-6F = 2.0 x 10-4 F (mathematic standard form)

 NB: (For Qa &Qb, G = 109 and µ=10-6)

Evaluation 

Convert the units of the following quantities:

(a)    speed, 1800cm/h (b) density, 20g/mm3  (c) frequency, 1.5MHz (d) capacitor, 4 nC