Physics constants
The 2019 SI values, with a note on where each one actually shows up in a class 9–12 syllabus.
c
2.998 × 10⁸ m/s
speed of light in vacuum
g
9.8 m/s²
standard gravity
G
6.674 × 10⁻¹¹ N·m²/kg²
gravitational constant
e
1.602 × 10⁻¹⁹ C
elementary charge
Mechanics & gravitation
| Constant | Symbol | Value | Where it's used |
|---|---|---|---|
| Standard gravity | g | 9.8 m/s² (9.80665 exact) | Free fall, projectile motion, weight = mg. |
| Gravitational constant | G | 6.674 × 10⁻¹¹ N·m²/kg² | Newton's law of gravitation, F = Gm₁m₂/r²; orbital mechanics. |
| Standard atmosphere | atm | 101 325 Pa | Pressure problems, barometers, fluid statics. |
Electricity & magnetism
| Constant | Symbol | Value | Where it's used |
|---|---|---|---|
| Elementary charge | e | 1.602 × 10⁻¹⁹ C | Charge quantisation, electrolysis, the electron/proton charge. |
| Coulomb's constant | k | 8.988 × 10⁹ N·m²/C² | Coulomb's law, F = kq₁q₂/r². |
| Permittivity of free space | ε₀ | 8.854 × 10⁻¹² F/m | Capacitance, electric fields, k = 1/(4πε₀). |
| Permeability of free space | μ₀ | 1.257 × 10⁻⁶ N/A² | Magnetic field of a current-carrying wire, solenoids. |
| Faraday constant | F | 96 485 C/mol | Electrolysis — charge per mole of electrons. |
Thermodynamics
| Constant | Symbol | Value | Where it's used |
|---|---|---|---|
| Universal gas constant | R | 8.314 J/(mol·K) | Ideal gas law, PV = nRT. |
| Boltzmann constant | kᵇ | 1.381 × 10⁻²³ J/K | Average kinetic energy of gas molecules, statistical mechanics. |
| Stefan–Boltzmann constant | σ | 5.670 × 10⁻⁸ W/(m²K⁴) | Black-body radiation, P = σAT⁴. |
| Absolute zero | 0 K | −273.15 °C | Kelvin/Celsius conversion; the coldest possible temperature. |
Modern & atomic physics
| Constant | Symbol | Value | Where it's used |
|---|---|---|---|
| Speed of light in vacuum | c | 2.998 × 10⁸ m/s | E = mc², all of relativity and optics. |
| Planck constant | h | 6.626 × 10⁻³⁴ J·s | Photon energy E = hf; the photoelectric effect. |
| Reduced Planck constant | ħ | 1.055 × 10⁻³⁴ J·s | Angular momentum quantisation, ħ = h/2π. |
| Avogadro constant | Nᴸ | 6.022 × 10²³ /mol | Particles per mole — links atomic and macroscopic scales. |
| Electron mass | mᵉ | 9.109 × 10⁻³¹ kg | Atomic structure, mass-energy problems. |
| Proton mass | mᴵ | 1.673 × 10⁻²⁷ kg | Nuclear physics, mass defect calculations. |
| Atomic mass unit | u | 1.6605 × 10⁻²⁷ kg | Atomic and molecular mass values. |
| Rydberg constant | R∞ | 1.097 × 10⁹ /m | Hydrogen atom spectral lines. |
Frequently asked questions
Why do some sources round g to 9.81 and others to 9.8?
Standard gravity is exactly 9.80665 m/s² by definition. Most school problems round to 9.8; a few textbooks use 9.81 or, for quick mental estimates, 10.
What's the difference between G and g?
G is the universal gravitational constant (the same everywhere in the universe); g is the acceleration due to gravity at Earth's surface, which follows from G, Earth's mass and Earth's radius.
Do I need to memorise all of these?
No — exams almost always give you the constants you need. Knowing roughly what each one means and which formula it belongs to matters far more than the digits.