Thermodynamics, Statistical Physics, and Quantum Mechanics

THERMODYNAMICS AND STATISTICAL PHYSICS 183

The total radiation energy density over all frequencies is

The integral with factor is just a number which we can take (in

fact it is So

The energy flux per a unitsolid angle is

The flux that illuminates the Earth is proportional to the solid angle

subtended by the Sun’s surface at the Earth:

The radiant energy flux at the Earth is therefore

where is the temperature of the Sun’s surface in K. Now we may estimate

(The actual temperature is about 6000 K; see Problem 4.73.)

4.73 Radiation Force (Princeton, Moscow Phys-Tech,

MIT)

a) The total radiation flux from the Sun is