# The purpose of Plank's constant

1. Dec 17, 2004

### Gamish

What is the purpose or meaning of Plank's constant? I keep on hearing about it, but I never learned what it is. Maybe it has to do with the size of everything on the quantum level.

2. Dec 17, 2004

### dextercioby

Though the original number was $h$ and it was called "Planck's constant",phyisicists did't like that number,so,instead they used the so-called:"reduced Planck constant",which is $\frac{h}{2\pi}$,which is more commonly seen and named:$\hbar$,i.e.HBAR.
In the original theory of Max Planck,it was a constant linking the energy of the quanta of (blackbody/thermal) radiation and its frequancy:
$$E=h\nu$$.
In the theory of Bohr,$\hbar$ it was regarded as the 'quanta for the angular momentum' of the electron orbiting the nucleus.From there on,in physics,$\hbar$ took the place of $h$.
In QM is is still regrarded as a 'quanta of angular momentum',this time on the (preferrentially chosen) Oz/Ox_{3} axis.
In QFT,though it's formulated in a sytem of units in which $\hbar=1$,$c=1$,it has the meaning of 'quanta of (lagrangian/hamiltonian) action'.

Daniel.

Last edited by a moderator: Dec 18, 2004
3. Dec 18, 2004

### Gamish

Can you please explaine what this means, in lamem terms

4. Dec 18, 2004

### dextercioby

Planck was computing the average energy per vibration mode for an isotropic stationary electromagnetic radiation and he came to an expression in which,in the LHS,it was average energy (measured in Joules) and in the RHS it was some adimensional constant (1 over some exponential) multiplied with frequency (of the radiation).So he invented a dimensional constant (known later as Planck's constant "h") to set the equation dimensionally correct.

Daniel.