Physical Quantities, Measurements Standard and Units GK
Physical quantities, Measurements standard and units general knowledge (general science gs) for UPSC, IAS, Banking, Railway SSC and other competitive examinations.
Contents:
Physical Quantities
Standard and Units
Physical Quantities With Their Symbols And Units in SI & c.g.s System
To explain the natural phenomena we take help of physics. Physics enable us to understand logically as well as mathematically all natural phenomena. That’s why we introduced Physical quantity and unit.
Physical Quantities
All the laws of physics are generally expressed in terms of Physical Quantities. As an example if you go to school or college from your home by walk, you need to know your speed and time. If you start to go at 9:30 AM and reach at 10 AM, you spend 30 minutes by walk. Again distance between your school and home is 6 Km then you can easily calculate your walking speed which is [Distance/Time] = 200m/minute. Thus from the above example, time, speed and distance are Physical Quantities. Some other kinds of physical quantities are force, momentum, temperature, density, area, pressure, acceleration etc.
We need to measure the physical quantities to obtain physical meaningful results to understand physics. So measurement is necessary in physics.
Classification of Physical Quantities:
Generally Physical Quantities are classified into two classes such as fundamental and derived quantities.
Fundamental Quantities: They are not defined in terms of other physical quantities. Example: Length, Mass and Time.
Derived Quantities: Their definition derived from mainly fundamental physical quantities. Example: speed, area, acceleration, momentum, density etc. In case of speed to define it, you need to two fundamental quantities like Length and time.
Classification of Physical quantities in terms of Direction:
Physical quantities are also classified into two types one is Scalar and other is Vector quantity.
Scalar Quantity: Physical quantities those have only magnitude NOT direction. Speed, density, mass, work, energy, power etc are scalar.
Vector Quantity: A vector quantity is one which has BOTH magnitude and direction. Force, momentum, displacement, acceleration, velocity etc are vector quantities.
Explanation: Vector quantities are denoted by putting a ‘bar’ (—) or ‘arrow’ (→) sign. Like force is a vector quantity and denoted by –
Now if you push a table along north direction by applying force 5 Newton, then according to vector rule, it is written as 5N-North. Here 5 is a scalar and if you put its direction (here North), it become vector.
Standard and Unit:
To measure the physical quantities we need to introduce standards and units. Measurement of physical quantities consists of two steps –
- one is choice of the standard and
- other is comparison of the standard to the quantity to be measured.
Here choice of the standard is known as Unit. Comparison of the standard to the quantity to be measured provides the total measurement of that quantity. Consider the length of a pen, it is about 10 cm long. It means that pen’s length is 10 times the unit of length, centimeter.
Units depend on choice. Each choice of units leads to a new system or set of units. You may consider any length as a unit of length. But it is not standard. Earlier, people from various countries used different systems of units. In the year 1960 GCWM recommended that a metric system of measurements called the International System of Units or SI Unit (System Internationale).
Classification of Units:
Units are also classified into two types
Fundamental Units: can not be derived from other unit. Three fundamental units are Meter, Kilogram and Second.
Basic Units: There are seven basic units – meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mol, Candela (Cd).
Supplementary units: Plane angle and Solid angle are considered as Supplementary units.
Derived Units: can be derived from other units.
Unit of Length: In SI system the unit of length is meter. One meter is defined as the distance between two lines on a particular platinum-iridium rod at 0° C. This rod is kept in the IOWM office located near Paris. In modern physics it is also defined as the path travel by light in free space during a time interval of 1/299792458 second. In c.g.s and fps system the unit of length is centimeter and foot respectively. For large distance we used Kilometer, Megameter mile etc. To measure the distance in space we used astronomical unit or AU, light-year and parsec. 1 AU is the distance between earth and sun.
1 parsec = 3.2615 light-years = 206264.8 AU = 3.085×1016 m. 1 Light-year = 63241.1 AU = 9.461 trillion km = 9460730472580800 m. 1 AU = 149597870 km. = 149597870700 m. 1 km = 1000 m. = 103 m = 105 cm. 1 m = 100 cm.
Similarly very small distance is measured by millimeter (mm) , micrometer (μm), angstrom (Å), nano-meter (nm) and femtometer (fm).
1 m = 106 μm = 109 nm = 1010 Å = 1015 fm.
Units are also classified into various types such as C.G.S , M.K.S, F.P.S etc. cgs is for small unit and mks is for larger.
Unit of Mass: In SI system the unit of mass is Kilogram (Kg). One kg is defined as the mass of a particular solid cylinder of platinum-iridium alloy kept a t Sevres. To measure the large masses we used tonne. In c.g.s and f.p.s system the unit of mass are gram and pound respectively.
1 tonne is equal to 103 kg.
Unit of Time: In SI system the unit of time is second. It is represented by small later “s”. Previously it was defined as the 1/86400th part of a mean solar day. But in modern physics 1 second is redefined as atomic clock such as time taken to complete 9192631770 periods of transition of the two hyper levels of the ground state of the Caesium 133 atom.
Physical Quantities With Their Symbols And Units in SI & c.g.s System
| Physical Quantity | Symbol* | Unit in SI | SI Unit Symbol | Unit in c.g.s | c.g.s Unit Symbol |
| Length | l | Meter | m | Centimeter | cm |
| Mass | M | Killogram | kg | gram | g |
| time | t | Second | s | second | s |
| electric current | I | ampere | A | biot | biot |
| temperature | T | kelvin | K | kelvin | K |
| amount of substance | n | Mole | mol | ||
| luminous intensity | Iv | Candela | Cd | ||
| area | A | square metre | m2 | square centimeter | cm2 |
| volume | V | Cubic meter | m3 | Cubic centimeter | cm3 |
| speed, velocity | v, s | meter/second | m/s | cm/second | cm/sec |
| acceleration | a | meter/second2 | m/s2 | cm/second2 | cm/s2 |
| Angular momentum | L | kg-m2/s | g-cm2/s | ||
| Angular speed (or angular velocity) | ω | rad s−1 | – | rad s−1 | – |
| Capacitance | C | Farad | F | statfarad | statF |
| wavenumber | k | reciprocal metre | m-1 | reciprocal cm | cm-1 |
| Current density | J | ampere/meter2 | A/m2 | ||
| density, mass density | ρ | kg/cubic metre | kg/m3 | gram/metre3 | g/m3 |
| Electric charge | Q | coulomb | C | statcoulomb | e.s.u |
| Electric potential | V | Volt | V | statVolt | statV |
| Electrical resistance | R | ohm | Ω | – | – |
| Energy | E | Joule | j | Erg | erg |
| Force | F | Newton | N | Dyne | dyn |
| Frequency | f | Hertz | Hz | Cycle/sec | |
| magnetic field | B | tesla | tesla | gauss | gauss |
| refractive index | μ | unit less | – | unit less | – |
| Inductance | L | henry | H | abhenry | abH |
| Momentum | p | kg-m/s | kg-m/s | g-m/s | g-m/s |
| Permeability | μ | henry/meter | H m−1 | abhenry/cm | abH/cm |
| Permittivity | ε | farad/meter | C2N-1m-2 | ||
| plane angle | Θ | radian | rad | – | – |
| solid angle | Ω | steradian | sr | – | – |
| pressure | P | pascal (N/m2) | Pa | bar | Dyne/cm2 |
| power | P | watt or (Joule / second) | W | Erg/second | |
| Specific heat capacity | c | J kg−1 K−1 | |||
| Wavelength | λ | Meter | m | Centimeter | cm |
| Entropy | S | Joule/Kelvin | J K−1 | – | – |
| dynamic viscosity | Pascal-second | Pa·s | poise or [g/(cm·s)] | P | |
| Heat | Q | Joul | J | Erg | erg |
| Surface tension | Y | N m−1 or J m−2 | Dyn cm−1/erg m−2 | ||
| Chemical potential | μ | Joule/ mol | J mol−1 |
* The bolt symbols represent vector quantities.