Page:Zur Theorie der Strahlung in bewegten Körpern.djvu/16

 Now, when our system was initially at rest, yet the surfaces $$A$$ and $$B$$ were not hindered to emanate, then the energy density $$\epsilon_0$$ is initially present in $$R$$, and it is the question, what is happening with the energy quantity when the system is suddenly brought to velocity $$w$$. The previous consideration indicates that the energy amount has to be completely absorbed by the surfaces $$A$$ and $$B$$; no part of it may be transformed into mechanical work. Incidentally, also a direct calculation leads to this result, which starts from the values (given in the previous section) for the quantities $$p_1$$ and $$p_2$$. This calculation is only a special case of the following one, and therefore I don't want to execute it here, since this already happened at another place.

On the other hand, if we would bring our system (with the energy given above in the cavity) suddenly from the state of motion into that of rest, then the whole energy quantity $$h\epsilon'$$ (generally the whole surplus of energy in $$R$$ caused by motion) would be absorbed by $$A$$ and $$B$$; because at the now resting boundary surfaces of space $$R$$, no work can indeed be performed. Thus if the motion of our system is suddenly accelerated, then work must be performed which is transformed into (radiating) heat; this work won't be regained at a sudden delay of motion, its energy amount rather remains in the form of heat. Such rapid changes of the velocity of motion are thus accompanied with irreversible processes. On the other hand, the energy transformation is completely reversibly, as I will show soon. The relations are quite similar, for example, as at the compression and expansion of a gas. The compression work is always transformed into heat, independently on whether the compression takes place slowly or rapidly; yet this heat quantity only then is transformed into work again, when the expansion takes place quite slowly, i.e., "reversible".

Thus we want to investigate now, what is happening when