h links a photon's energy to its frequency, E = hν. Since 2019 it is exact by definition: h = 6.62607015 × 10⁻³⁴ J s.
Value of h in different units
| Value | Use it when |
|---|---|
| 6.626 × 10⁻³⁴ J s | SI. Most numericals. |
| 4.136 × 10⁻¹⁵ eV s | When energy is in eV. |
| 6.626 × 10⁻²⁷ erg s | CGS units. |
| 1.055 × 10⁻³⁴ J s (ħ = h/2π) | Angular momentum, nh/2π in Bohr's model. |
| 1240 eV nm (hc) | Photon energy straight from wavelength: E = 1240/λ(nm) eV. |
Formulas that use h
- E = hν = hc/λ
- λ = h/p = h/mv (de Broglie)
- mvr = nh/2π
- Δx Δp ≥ h/4π
- hν = φ + KEₘₐₓ
Its dimensional formula is worked out on the dimensional formula of planck's constant h page.
Other constants
- Gas constant (R): 8.314 J mol⁻¹ K⁻¹
- Planck's constant (h): 6.626 × 10⁻³⁴ J s
- Avogadro's number (Nₐ): 6.022 × 10²³ mol⁻¹
- Speed of light (c): 3 × 10⁸ m/s
- Charge of an electron (e): 1.602 × 10⁻¹⁹ C
- Mass of an electron (mₑ): 9.109 × 10⁻³¹ kg
- Mass of a proton (mₚ): 1.673 × 10⁻²⁷ kg
- Mass of a neutron (mₙ): 1.675 × 10⁻²⁷ kg
- Universal gravitational constant (G): 6.674 × 10⁻¹¹ N m² kg⁻²
- Acceleration due to gravity (g): 9.8 m/s²
- Boltzmann constant (k): 1.381 × 10⁻²³ J/K
- Faraday constant (F): 96 485 C/mol
- Permittivity of free space (ε₀): 8.854 × 10⁻¹² C² N⁻¹ m⁻²
- Permeability of free space (μ₀): 4π × 10⁻⁷ T m/A
- Rydberg constant (R): 1.097 × 10⁷ m⁻¹
- Stefan–Boltzmann constant (σ): 5.67 × 10⁻⁸ W m⁻² K⁻⁴
- Wien's constant (b): 2.898 × 10⁻³ m K
- Bohr radius (a₀): 0.529 Å
- Molar volume of a gas at STP (Vₘ): 22.4 L/mol
The full list, searchable, is on the physical constants page.
Questions students ask
What is the value of Planck's constant?
h = 6.626 × 10⁻³⁴ J s. In other units: 6.626 × 10⁻³⁴ J s; 4.136 × 10⁻¹⁵ eV s; 6.626 × 10⁻²⁷ erg s; 1.055 × 10⁻³⁴ J s (ħ = h/2π); 1240 eV nm (hc).
What is h in physics and chemistry?
h links a photon's energy to its frequency, E = hν. Since 2019 it is exact by definition: h = 6.62607015 × 10⁻³⁴ J s.
Which value of h should I use?
6.626 × 10⁻³⁴ J s: SI. Most numericals. 4.136 × 10⁻¹⁵ eV s: When energy is in eV. 6.626 × 10⁻²⁷ erg s: CGS units. 1.055 × 10⁻³⁴ J s (ħ = h/2π): Angular momentum, nh/2π in Bohr's model. 1240 eV nm (hc): Photon energy straight from wavelength: E = 1240/λ(nm) eV.
What is the dimensional formula of Planck's constant?
See the dimensional formula of Planck's constant, worked out step by step on its own page.