Planck's constant

Beginner

Fundamental constant of quantum physics.

Key relationh = 6.626×10⁻³⁴ J·s

h = 6.626×10⁻³⁴ J·s sets the scale of quantum effects; it's tiny, which is why quantum behaviour is hidden in daily life. ħ = h/2π.

Worked example

A 1 kg ball at 1 m/s has de Broglie wavelength 6.6×10⁻³⁴ m.

Common mistake

If h were large, everyday life would look quantum.

In more detail

Planck's constant h, about 6.63 × 10⁻³⁴ J s, sets the scale of quantum effects. Its reduced form ħ = h/2π appears widely in quantum mechanics.

Planck's constant defines the kilogram in the SI system.
It determines the energy of photons in lasers and LEDs.
Metrologists use Planck's constant to define the kilogram in the SI system, linking mass to a quantum measurement.

Why is h so small?

Quantum effects are noticeable only at very small scales, where energies are tiny.

What is ħ?

The reduced Planck constant, h divided by 2π.

Why is Planck's constant so small?

It sets the scale of quantum effects, which are only noticeable for very small objects and very short timescales.