# Units & Measurement (GA Physics) Notes for SSC CGL, CHSL & Railways

## Table of Contents

- Physical Quantity
- SI Units
- SI Base Units
- SI Derived Units
- Other Units
- Practical Units
- Measuring Devices

## Physical Quantity

A physical quantity is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.

- A physical quantity can be expressed as the combination of a
*magnitude*(expressed by a number – usually a real number) and a*unit*.

## Unit of measurement

A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity.

## International System of Units (SI Units)

The International System of Units (SI, abbreviated from the French *Système international (d’unités))* is the modern form of the metric system, and is the most widely used system of measurement.

- It comprises a coherent system of units of measurement built on
**7 base units**and a set of**20 prefixes**to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. - The system also specifies names for
**22 derived units**.

## SI Base Units

Symbol | Name | Quantity |
---|---|---|

A | ampere | electric current |

K | kelvin | temperature |

s | second | time |

m | metre | length |

kg | kilogram | mass |

cd | candela | luminous intensity |

mol | mole | amount of substance |

## Important SI Derived Units

Symbol | Name | Quantity |
---|---|---|

rad | radian | angle |

Hz | hertz | frequency |

N | newton | force, weight |

m | metre | length |

Pa | pascal | pressure, stress |

J | joule | energy, work, heat |

W | watt | power, radiant flux |

C | coulomb | electric charge |

V | volt | voltage (electric potential), emf |

F | farad | capacitance |

Ω | ohm | resistance, impedance, reactance |

S | siemens | electrical conductance |

Wb | weber | magnetic flux |

T | tesla | magnetic flux density |

H | henry | inductance |

°C | degree Celsius | temperature |

lm | lumen | luminous flux |

lx | lux | illuminance |

Bq | becquerel | radioactivity |

## Other Important Units

intensity of Sound | decibel (dB) |

acceleration | m s^{-2} |

entropy | J K^{-1} |

jerk | m s^{-3} |

magnetic field strength | A m^{-1} |

relative density | no units |

Torque | N m |

momentum | kg m s^{-1} |

## Important Practical Units

### Length

- Astronomical Unit (au): Distance Between the centre of Earth and the centre of the Sun (1 au = 1.5 x 10
^{11}m) - Light Year (ly): Distance travelled by light in vacuum in one year.
- 1 Angstrom = 10
^{-10}m

### mass & weight

- The largest practical unit of mass is Chandrasekhar limit (CSL).
- Atomic or nuclear masses are made in terms of a unit called amu (atomic mass unit).

1 CSL = 1.4 times the mass of sun. 1 metric ton = 1000 kg Quintal = 100 kg Slug = 14.57 kg

## Important Measuring Devices

**tensiometer**: To measure soil-moisture tension (matric water potential)**hygrometer**: To measure humidity in the atmosphere**hydrometer**: To measure the density of liquids**fathometer**: To measure depth of oceans**anemometer**: To measure the speed of the wind, or of any current of gas**pyrometer**: To measure high temperatures, especially in furnaces and kilns.**barometer**: To measure atmospheric pressure**lactometer**: To measure the density of milk**sphygmomanometer**: To measure blood pressure**Karatmeter**: To measure the purity of gold**luxmeter**: To measure intensity of luminescence/ light**seismograph (seismometer)**: To measure motion of the ground (based on Richter magnitude scale)**polygraph (lie detector)**: To measure and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity while a person is asked and answers a series of questions.

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