Question

A thermos flask contains coffee. It is shaken vigorously. (i) Has any heat been added to it.
(ii) Has any work been done on it.
(iii) Does it internal energy change?
(iv) Does its temp. rise?

Answer 1

(a), No, heat is not transferred as the flask is insulated from the surroundings `:. dQ=0`
(b) Yes, work is done in shaking (against the viscous), i.e., `dW` is negative.
(c ) Yes, internal energy of tea increases. As `dU=dQ-dW=0-dW`, and `dW` is negative, therefore, `dU` is positive.
(d) Yes, temperature if tea rises because of increase in internal energy.