Detailed Explanation of Important Physical Constants for JEE, NEET & CBSE Class 11
Understanding fundamental physical constants is crucial for students preparing for competitive exams like JEE, NEET, and CBSE Class 11. These constants serve as the backbone for solving numerical problems in Physics. This article provides a detailed explanation of these constants, along with interactive questions, conceptual clarity, FAQs, MCQs, and worksheets for practice.
List of Important Physical Constants
Constant
Symbol
Value
Velocity of light in vacuum
c
3 × 10⁸ m/s
Velocity of sound in air at STP
–
331 m/s
Acceleration due to gravity
g
9.81 m/s²
Avogadro number
N
6.023 × 10²³/mol
Density of water at 4°C
–
1000 kg/m³ or 1 g/cc
Absolute zero
–
-273.15°C or 0 K
Atomic mass unit
amu
1.66 × 10⁻²⁷ kg
Quantum of charge
e
1.602 × 10⁻¹⁹ C
Stefan’s constant
σ
5.67 × 10⁻⁸ W/m²K⁴
Boltzmann’s constant
K
1.381 × 10⁻²³ J/K
One atmosphere
atm
76 cm Hg = 1.013 × 10⁵ Pa
Mechanical equivalent of heat
J
4.186 J/cal
Planck’s constant
h
6.626 × 10⁻³⁴ Js
Universal gas constant
R
8.314 J/mol-K
Permeability of free space
μ₀
4π × 10⁻⁷ H/m
Permittivity of free space
ε₀
8.854 × 10⁻¹² F/m
Density of air at STP
–
1.293 kg/m³
Universal gravitational constant
G
6.67 × 10⁻¹¹ Nm²/kg²
Conceptual Understanding: Question-Answer Format
Q1. Why is the velocity of light in vacuum a constant?
A: The velocity of light in vacuum (c = 3 × 10⁸ m/s) is constant due to Maxwell’s equations, which show that light propagates as an electromagnetic wave with a speed determined by permittivity and permeability of free space.
Q2. What is the significance of Avogadro’s number?
A: Avogadro’s number (N = 6.023 × 10²³/mol) represents the number of atoms or molecules present in one mole of a substance and is fundamental in stoichiometric calculations.
Q3. Why is Planck’s constant important in quantum mechanics?
A: Planck’s constant (h = 6.626 × 10⁻³⁴ Js) defines the quantization of energy in photons, forming the basis of quantum mechanics.
MCQs (Multiple Choice Questions)
Q1. What is the SI unit of Planck’s constant?
(a) J/m² (b) Js (c) J/m (d) J/K Answer: (b) Js Explanation: Planck’s constant is defined as energy per unit frequency, and its SI unit is Joule-second (Js).
Q2. What is the value of the Universal Gas Constant (R) in J/mol-K?
(a) 4.186 (b) 6.626 (c) 8.314 (d) 9.81 Answer: (c) 8.314 J/mol-K Explanation: The universal gas constant R appears in the ideal gas equation PV = nRT.
Q3. What is the numerical value of the charge of an electron?
(a) 1.602 × 10⁻¹⁹ C (b) 9.81 m/s² (c) 6.67 × 10⁻¹¹ Nm²/kg² (d) 8.314 J/mol-K Answer: (a) 1.602 × 10⁻¹⁹ C Explanation: The fundamental charge on an electron is -1.602 × 10⁻¹⁹ C.
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