What is the approximate value of the Planck constant in joule-seconds?

The approximate value of the Planck constant is 6.626 × 10⁻³⁴ joule-seconds. Usually written as h, it is a fundamental constant of quantum mechanics and sets the scale at which energy and matter exhibit quantum behavior.

Max Planck introduced the constant in 1900 while studying the spectrum of black-body radiation. He found that energy had to be exchanged in discrete packets rather than in an entirely continuous stream. In 1905, Albert Einstein used the same idea to explain the photoelectric effect, linking a photon's energy to its frequency through E = hf.

The joule-second unit can also be written as kilogram-metre squared per second. In the modern SI, h has the exact defined value 6.62607015 × 10⁻³⁴ J·s; the quiz's rounded value is therefore appropriate. The related reduced Planck constant, ħ, equals h divided by 2π and appears frequently in quantum equations.

The distractors describe other famous constants: the speed of light, the electron mass, and the elementary charge. None has the same units or role as h.