Consider the device illustrated in Figure 21.3. A string is attached to a paddle wheel that is immersed in a viscous liquid in a container. As the string is pulled, the paddle wheel rotates at constant speed, increasing the temperature of the liquid from \(T_{i}\) to \(T_{f}\).

(a) Draw an energy inputoutput diagram for the system comprising the paddle wheel and the liquid. Do the energy and entropy of the system comprising the paddle wheel and the liquid increase, decrease, or stay the same?

(b) Assume the paddle wheel stops turning at the instant shown on the right in Figure 21.3, and we now wait until the liquid cools back down to its initial temperature \(T_{1}\). Draw an energy inputoutput diagram for the system, but this time over the time interval from the instant the wheel begins turning to the instant the liquid temperature is again \(T_{1}\). Do the energy and entropy of the system increase, decrease, or stay the same over this new time interval?

Data from Figure 21.3

viscous liquid T>T Mechanical work done by paddle wheel on liquid raises temperature.