question_answer

A thermos flask contains coffee. It is vigrously shaken. Consider the coffee as the system. (i) Has any heat been added to it? (ii) Has any work been done on it? (iii) Has its internal energy changed? (iv) Does its temperature rise?

Answer:

                (i) No. As the thermos flask is insulated, heat has not been added to the coffee \[(\Delta Q=0)\]. (ii) Yes. Some work is done by the man in shaking the coffee against the forces of viscosity i.e., \[\Delta W\] is negative. (iii) By first law of thermodynamics, \[\Delta Q=\Delta U+\Delta W\]. As \[\Delta Q=0\]and \[\Delta W\] is negative, so \[\Delta U\] is positive i.e., internal energy of the coffee increases. (iv) Because of the increase in internal energy of the coffee, the temperature of the coffee will also increase.