Measurement in Physics

The standard with which things are compared.

All measurable quantities are Physical Quantities. 

Magnitude of a Physical Quantity means a number and a proper unit.

1. Base quantities

2. Derived quantities

Minimum Number of those physical quantities in terms of which other quantities can be defined are called Base Quantities. 

Derived quantities are those quantities whose definitions are based on Base Quantities. The rest of the Physical Quantities except Base Quantities are Derived Quantities.

 A complete set of units, both Base & Derived, for all kinds of Physical Quantities is called a System of Units.

There are two Unit Systems in use at present time:

1. English System (or British System) (or F-P-S System)

2. SI System (International System)

The English System (or British System or FPS System) of units is based on the following standard units:

• Foot-Pound-Second

There is also a system called British Engineering System (sometimes called the Conventional System) where the units of Length, Mass and Time are Foot, Slug and Second. 

14th International Conference on Weights and Measures in 1971 picked seven quantities as Base Quantities forming the basis of International System of Units abbreviated as SI (Systeme Internatioanle from its French name) popularly known as the Metric System.

Standard Units:

• Meter-Kilogram-Second 

also called MKS System.

Following are the seven Base Quantities with their Base Units:

1. Length (Meter) (m)
2. Mass (Kilogram) (kg)
3. Time (Second) (s)
4. Electric Current (Ampere) (A)
5. Temperature (Kelvin) (K)
6. Amount of Substance (Mole) (mol)
7. Luminous Intensity (Candela) (cd)

1 Light Year = Distance traveled by light in 1 year.

1 year = 3.16 x 10⁷ s

Speed of Light = 3x10⁸ m/s

1 Light Year = (3x10⁸ m/s) (3.16x10⁷s) 

 = 9.48 x 10¹⁵ m 

The distance to Proxima Centauri = 4 x 10¹⁶ m

= 4.2 Light Years

Light from Proxima Centauri takes 4.2 Light Years to travel to Earth.

The Kilogram ( Established in 1887) is defined as the mass of specific Platinum-Iridium alloy cylinder kept at the International Bureau of Weights & Measures at Sevres, France. 

The second is defined as 9,192,631,770 times the period of vibration of radiation from Cesium-133 atom. 

One Ampere of Current is equivalent to one Coulomb of Charge passing through the surface area in One Second

1 A = 1C/1s

The triple point of water is the single combination of Temperature and Pressure at which liquid Water, Gaseous Water and Ice (Solid Water) coexist in equilibrium. This triple point occurs at:

• Temperature: 0.01⁰ C
• Pressure: 4.58 mm of Mercury.

The Kelvin is defined to be 1/273.16 of the difference between the absolute zero and the temperature of the triple point of water.

One Mole of a substance is that amount of substance that contains as many elementary entities (atoms, molecules, or other particles) as there are atoms in exactly 12 grams of Carbon-12 isotope. 

Power emitted by a light source in a particular direction.

The Candela is defined as the Luminous Intensity in a given direction of a source that emits monochromatic radiation of frequency 540x10¹² Hertz and has a radiant intensity in that same direction of 1/683 watt per steradian.