Planck’s constant, h, appears throughout quantum mechanics and is one of the fundamental constants of physics. It has a value of:
h = 6.6260693(11) x 10-34 J s
Planck’s constant was first identified as part of Max Planck’s description of blackbody radiation. Later, it was shown by Albert Einstein to be the constant of proportionality between the energy ( E ) and frequency ( f ) of photons:
E = hf
A closely-related quantity (usually pronounced “h-bar”) is:
ħ = h/2π = 1.054571596(82) x 10-34J s
In this form, Planck’s (reduced) constant appears in the two uncertainty relationships:
|ΔxΔp ≥ ħ||and||ΔEΔt ≥ ħ|
where we have uncertainties in the measurements of a particle’s
or wave-packet’s position Δx, momentum Δp, energy ΔE and lifetime Δt.
In Planck units, or natural units, ħ = 1, c = 1.