Plus 1 : UNITS AND MEASUREMENTS

CHAPTER 1 :UNITS AND MEASUREMENTS

1. WHAT IS A PHYSICAL QUANTITY?  GIVE EXAMPLES?

Any thing that can be measured is a physical quantity.  Example : length, mass, time

Velocity, density

2. What is a Unit?  Give examples?

Units are certain reference standards used for making measurements.

Example:  m/s, kg/ m³,  Km, Kg

3. What are fundamental quantities and fundamental Units?

   The physical quantities mass, length and time are known as fundamental quantities.

They are called so because, Units of all other quantities in mechanics can be derived from fundamental units.  Also they can not be sub divided in to smaller units

Units of fundamental quantities are known as fundamental units.

 (meter , kilogram, second )

4. What are derived quantities and derived units?

      Physical quantities that can be expressed in terms of fundamental quantities are

     known as derived quantities.

      (example : m/s  , m/s², m³ )

      Note:   Two parts for measured result:

     Any measurement has two parts

1. Its magnitude
2. Its Unit

If we decrease the size of the unit, the numerical; value will increase.

1 Km  = 1000 m

Here Km is a large unit of length, while magnitude is 1

But, meter is a small unit of length and its magnitude is 1000

Smaller the size of the unit, greater is its numerical value.

5. What are the characteristics of a unit , that is chosen for making measurement ?

The unit that is chosen for making a measurement should have the following properties:

1. It must be well defined.
2. It should be of suitable size
3. It should be accepted internationally
4. It should not change with change in temperature, pressure ,etc

6. What are the different system of units ?

CGS – Centimeter, gram , second

       FPS – Foot, Pound, Second

       MKS – Meter, Kilogram, second

7. What are the physical quantities and units in SI system?

7 Basis physical quantities  of SI unit system are

Length	meter	M
Mass	Kilogram	Kg
Time	Second	S
Temperature	Kelvin	K
Electric current	Ampere	A
Luminous intensity	Candela	Cd
Quantity of matter	Mole	mol

2 Supplementary quantities of SI system are

Plane angle	Radian	rad
Solid angle	Steradians	sr

8. Define 1 meter in 3 ways ?

1)      It is 1 / (10⁷) th of the distance from north pole to equator of earth

1 meter - old definition

2)      1 meter is the distance between two lines marked on a Platinum-Iridium rod

Kept at a constant temperature of 273.16 K and at  1 bar pressure in the

International bureau of weights and measures at severs near Paris in France.

3)      1 meter is  equal to 1650763.73 times the wavelength of orange red light

Emitted by Kr-86 source, kept at triple point of Nitrogen.

 

Many units to measure length

1 Km = 10³ m

1 deci meter = 10^-1 m

1 centi meter = 10^-2 m

1 milli meter = 10^-3 m

1 micro meter = 10^-6 m

1 angstrom = 10^-10 m

1 fermi = 10^-15 m

9 .   Define 1 Kg

1 Kg is equal to the mass of Platinum – Iridium cylinder of diameter equal to its height, kept at international bureau of weights and measures.

Definition of 1 Kg

Many units to measure mass

1 Kg = 1000 gm

1 Ton = 1000 Kg

1 Quintal = 100 Kg

1 gm = 1000 mg

       10. Define 1 second

(a)    1 second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of  Cesium – 133 atom in ground state.

(b)   1 second is defined as 1 / (86,400) th  part of a mean solar day.

Mean solar day is the average of all solar days in a single year.

    

11. Define  1 Ampere

1 Ampere is that constant current which if maintained in two straight parallel conductors of infinite length , of negligible cross section and placed 1 m apart in vacuum , would produce between these conductors a force equal to  2 x 10 ^-7 N

12. Define 1 Radian

1 Radian is the plane angle between two radii of a circle which cutts off on the circumference an arc equal to the radius of the circle.

1 radian

360 degree  =  2 ∏  radian

There for,  1 radian  = 180 / ∏  degree

13. Define 1 Steradian

1 Steradian is the solid angle which with its vertex at the center of a sphere cuts off an area on the surface of the sphere equal to that of a square having sides of length equal to the radius of the sphere.

Order of Magnitude

To estimate how big or small a given physical quantity , we can use the idea of

Order of magnitude.

The given magnitude is expressed as    a x 10 ^b

Where  ‘a’ is a number between 1 and 10

Number  ‘b’ can be +ve or –ve

If  a >  5 , then order of magnitude = b+1

If  a < =5, then order of magnitude = b

For example :

    Diameter of earth 1.28 x 10⁷ m

     Order of magnitude = 7 and it is of the Order of 10⁷

 

     Distance between two points is measured as  7.25 x 10^-10 m

      Its order of magnitude is 10^-9  and It is of the Order of 10^-9