For a class demonstration project, I placed an empty stainless-steel pan (k =15 W/m K) onto a hot plate with its temperature set at 150°C as shown in Fig. P.3.6. After a few minutes, I asked the students whether I could touch the bottom of the pan with my bare finger. A few said “yes, do it!” (I wonder if they did not like the last test…), but most said an emphatic

“no.” I then removed the pan from the hot plate and filled it with water at room temperature. I also placed a magnetic stirrer in the pan, which kept the water well mixed in the pan. After a few minutes on the hot plate, I asked the students again whether I could touch the bottom of the pan. Few answered this time and there seemed to be some surprised gasps when I stuck my finger in and touched the bottom of the pan without getting burned. I repeated this a few times, but only until the water got hot, of course. The bottom of the pan is 5 mm thick.

(a) Explain what happened in this demonstration.

(b) Estimate what the temperature at the bottom surface of the pan was when there was only air at 22°C.

(c) Estimate what the temperature at the bottom surface of the pan was after a few minutes when it was full of water at 22°C.

Figure P.3.6:

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150 C 0 rpm OK 150 C 300 rpm