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.
Test Your Knowledge (Quiz)
Physics Constants Quiz
Do You Know? (Interesting Facts)
- The speed of light is the fastest speed possible in the universe.
- The concept of absolute zero represents a temperature where atomic motion nearly stops.
- The Avogadro number is named after Amedeo Avogadro, who proposed that equal volumes of gases contain equal numbers of molecules.
Worksheet for Practice
- Define the term ‘universal gravitational constant’ and provide its value.
- Calculate the energy of a photon with a frequency of 5 × 10¹⁴ Hz using Planck’s constant.
- Convert 1 atmosphere into Pascals.
- If the density of air at STP is 1.293 kg/m³, find the mass of 2 m³ of air.
- Explain why the permeability and permittivity of free space determine the speed of light.
Test Paper (Marks Distribution: 20 Marks)
Section A: Conceptual Questions (2 × 3 = 6 Marks)
- Explain the significance of the Boltzmann constant in thermodynamics.
- Why is the value of ‘g’ not the same everywhere on Earth?
Section B: Numerical Problems (3 × 3 = 9 Marks)
- Calculate the energy of a photon with a wavelength of 500 nm.
- Find the pressure exerted by 2 moles of gas at 300K in a 10L container (use R = 8.314 J/mol-K).
Section C: MCQs (1 × 5 = 5 Marks)
- Select the correct answers from the given multiple-choice questions.
Important Points for Quick Revision
- Speed of light (c) is 3 × 10⁸ m/s.
- Planck’s constant (h) is 6.626 × 10⁻³⁴ Js.
- Avogadro’s number (N) is 6.023 × 10²³/mol.
- Gas constant (R) is 8.314 J/mol-K.
- Universal gravitational constant (G) is 6.67 × 10⁻¹¹ Nm²/kg².
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