Units

Introduction

A unit is a standard quantity used to measure physical quantities. To ensure uniformity, the International System of Units (SI) is followed globally, including in India.
In Bihar, units are crucial in healthcare diagnostics, agriculture sensors, meteorology, and civil works (length, area, mass measurements).

Types of Units

1. Fundamental (Base) Units

Used to measure basic physical quantities:

Length → metre (m)
Mass → kilogram (kg)
Time → second (s)
Electric current → ampere (A)
Temperature → kelvin (K)
Luminous intensity → candela (cd)
Amount of substance → mole (mol)

📝 PYQ (BPSC Teacher): SI unit of current? → Ampere
📝 Expected MCQ: SI unit of temperature? → Kelvin

2. Derived Units

Formed from combinations of base units:

Speed → metre/second (m/s)
Force → newton (N) = kg·m/s²
Work/Energy → joule (J) = N·m
Power → watt (W) = J/s
Pressure → pascal (Pa) = N/m²
Electric charge → coulomb (C) = A·s
Potential difference → volt (V) = W/A
Frequency → hertz (Hz) = 1/s

📝 PYQ: SI unit of force? → Newton
📝 Expected MCQ: Unit of electric charge? → Coulomb

3. Supplementary Units

Plane angle → radian (rad)
Solid angle → steradian (sr)

Non-SI but Commonly Used Units

Time: Minute, hour, day
Length: Angstrom (1 Å = 10⁻¹⁰ m), nanometer
Mass: Tonne = 1000 kg
Energy: Calorie (1 cal = 4.18 J)
Pressure: bar, mmHg (used in meteorology and medicine)
Temperature: Celsius (used in weather reports)

📝 Expected MCQ: 1 calorie is equal to how many joules? → 4.18 J
📝 Expected MCQ: 1 tonne = ? → 1000 kg

Unit Conversions

1 km = 1000 m
1 cm = 10⁻² m
1 g = 10⁻³ kg
1 hour = 3600 seconds
1 hp (horsepower) ≈ 746 W
1 newton = 10⁵ dyne

📝 Expected MCQ: 1 newton = ? → 10⁵ dyne
📝 Expected MCQ: 1 hp = ? → 746 W

Unit Systems Before SI

CGS System → cm, gram, second
MKS System → metre, kg, second
FPS System → foot, pound, second (used in USA)
SI system is derived from MKS and adopted in 1960 by CGPM

📝 Expected MCQ: SI system is based on which old system? → MKS system
📝 PYQ (BPSC): Full form of SI? → Système International d’Unités

Applications in Bihar

Weather forecasts: Units of pressure (Pa), temperature (°C), humidity (%)
Construction projects: Use of metre, cubic metre, newton, tonne
Solar energy: Measurement in watts, kWh, joules
Medical diagnostics: BP in mmHg, temperature in °C, glucose in mg/dL
Agriculture sensors: Use kelvin, lux, radiation units (W/m²)

PYQs + Expected MCQs

📝 PYQ (BPSC Teacher): SI unit of current? → Ampere
📝 PYQ (BPSC 60–62): Full form of SI? → Système International d’Unités
📝 Expected MCQ: Unit of pressure in SI system? → Pascal
📝 Expected MCQ: Unit of frequency? → Hertz
📝 Expected MCQ: 1 horsepower = ? → 746 W
📝 Expected MCQ: SI unit of electric potential? → Volt

Quick Revision Points

SI System: Adopted in 1960, based on MKS
7 Base Units: m, kg, s, A, K, mol, cd
Derived Units: N, J, W, Pa, Hz, C, V
1 cal = 4.18 J
1 hp = 746 W
Pascal for pressure, Hertz for frequency
SI avoids confusion in international standards

Sources Used

NCERT Class 9 Science – Measurement
Lucent GK – Physics Units Section
Drishti Science Quick Book
EduTeria Science PYQs
BPSC PYQs – Ghatna Chakra
Bihar School Lab Manual + Govt Tender Guidelines