Time dilation in special relativity is a direct consequence of the postulate that the speed of light in vacuum is the same in all directions and in all inertial reference frames. 11. if sam on earth takes a spaceship and travels to outer galaxy with speed of 0.9c, sam himself can feel that his body's biological metabolism slows down compared to on earth 12. the first law of thermodynamics is valid both in reversible and irreversible processes. 13. a perfect black-body radiator emits radiation 4 times more, when its body temperature is doubled. 14. ideal gas systems do not have 'heat of transformation' (latent heat) 15. an ideal gas system initially at 20°c is doing work to the outside environment by expanding 0.1 m3. the work done by the system following an isothermal process is always larger than that followinga constant pressure process. 16. for a diatomic ideal gas system following the maxwell's speed distribution law, the root-mean square speed is the highest, followed by the average speed and then by the most probable speed. 17. the second law of thermodynamics suggests that the entropy of an ideal gas system is always increasing during an irreversible process. 18. the change in entropy of a thermodynamic system can be calculated even for an irreversible process, such as a free expansion.