Anderson Sec Sch 2007 Sec 4 CA1 Q13
The figure below shows two identical glass cylinders containing equal volumes of water, initially at room temperature. Identical electric heaters were placed in the cylinders, near the top in cylinder A and near the bottom in cylinder B.
The heaters were switched on at the same time. The readings of the thermometers then changed as shown in the following figure.
(a) Explain why the thermometer reading in cylinder A initially shows a slower rate of increase than that in B.
(b) Two metal cans are identical except that one has a blackened outer surface and the other has a polished outer surface. They are both filled with a hot liquid at the same initial temperature and then placed in a shaded room. The freezing point of the liquid is 70°C. The following figure is a temperature-time graph for the blackened can.
Draw on the figure, a temperature-time graph for the polished can.
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Answer:
(a) In A, the heater is placed nearer to the top. This heats up the water around the heater (nearer the top). Hot water, being less dense, remains above and the cold denser water remains near the bottom. This makes it difficult to heat up water by convection. The thermometer, being at the bottom, thus takes a longer time to register a temperature increase.
In B, the heater is placed near to the base. Warm water, being less dense, rises to the top, and the cold denser water sinks. This cycle continues, resulting in a convection current current in the water. Heat transfer is faster here than in A. The thermometer, being at the top, measures the temperature increase faster.
(b)
