What is the name of the constant with a value of 6.626×10^-34 joule-seconds?

The constant with a value of 6.626×10^-34 joule-seconds is the Planck constant. It is symbolized by h and sets the scale at which quantum effects become important.

Max Planck introduced it in 1900 while studying black-body radiation, proposing that electromagnetic energy is emitted or absorbed in discrete amounts. The constant appears in the equation E = hf, where a photon's energy equals its frequency multiplied by h. It also connects a particle's momentum with its wavelength through p = h/λ.

The number in the question is rounded. Since 2019, the SI system defines the Planck constant exactly as 6.62607015×10^-34 J·s, helping define the kilogram. A common mix-up is the reduced Planck constant, ħ, which equals h/(2π) and is approximately 1.054571817×10^-34 J·s. The fine-structure constant is dimensionless, while the Avogadro and Boltzmann constants describe particle counts and thermal energy.