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Ice Melting Blocks


Heat conduction.  Two identical looking black squares sit on the table.  I ask a student to feel them with the back of her hand.  The student says one feels colder than the other.  I ask the class if the cold one has a lower temperature than the warm one. (When does the zeroth law of thermodynamics apply?)  There is some discussion and no agreement.  I ask the student to use an electronic thermometer to measure the temperature of each.  A video camera allows all students to observe the temperature.  The temperature stays the same, so I ask the student to measure the temperature of her hand.  The temperature rises.  So we talk about why one block feels cold and the other not cold.  It has to do with the temperature falling in the skin due to the larger heat conduction of the “cold” block.  Now I ask, if an ice cube is put onto each block at the same time, which cube will melt first.  Now there is real grinding and confusion and certainty murmuring in the classroom.  The blocks are the same temperature.  One block feels cold. Which block will melt an ice cube first?  The student puts on ice cubes.  The “cold” block melts the ice cube immediately.  The warm block has a slow melt.  The difference in heat conduction explains all this. You may check the temperatures with the IR thermometer.