1879 – Austria

## STEFAN-BOLTZMANN CONSTANT

‘The total energy radiated from a blackbody is proportional to the fourth power of the temperature of the body’

(A blackbody is a hypothetical body that absorbs all the radiation falling on it)

Stefan discovered the law experimentally, but Boltzmann discovered it theoretically soon after.

## BOLTZMANN CONSTANT

‘Heat at the molecular level’

Shortly after JAMES CLERK MAXWELL’s analysis of molecular motion, Ludwig Boltzmann gave a statistical interpretation of CLAUSIUS’s notion of entropy.

Boltzmann’s formula for entropy is

*S***=** ** k** log

*W* ** S** is entropy,

**is now known as Boltzmann’s constant and**

*k***is a measure of the number of states available to the system whose entropy is being measured.**

*W*The notion that heat flows from hot to cold could be phrased in terms of molecular motions. Molecules in a container collide with one another and the faster ones slow down while the slower ones speed up. Thus the hotter part becomes cooler and the colder part becomes hotter – thermal equilibrium is reached.

The Boltzmann constant is a physical constant relating energy at the individual particle level with temperature. It is the gas constant **R** divided by the Avogadro constant **N _{A}** :

** k** =

**R**/

**N**

_{A}It has the same dimension (energy divided by temperature) as entropy.

(In rolling a dice, a seven may be obtained by throwing a six and a one, a five and a two or a four and a three, while three needs only a two and a one. Seven has greater ‘entropy’ – more states.)

###### Related articles

- The Boltzmann Mystery (Part 1) (drvitelli.typepad.com)
- The Boltzmann Mystery (Part 2) (drvitelli.typepad.com)