Unpacking the Heat and Temperature Phenomena Diagnostic Questions

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Heat and Temperature Diagnostic Questions
Heat and Temperature Opening Activities
Fall 2009-Day 11

The students first brainstormed ideas about why the four plates might have felt differently, even though they were all the same temperature.

All of the objects were the same temperature because they are all in the same room. The atmosphere is the same temperature in the room around them, causing each of the four plates to be the same temperature. However, the metal plates felt colder. The students were puzzled as to why this was happening.

One student's idea

One student's puzzled piecing together of scientific concepts about density, conductivity, and insulation, and their relationships. Note the inference that the metal does NOT absorb heat from the hand as fast as the Styrofoam because the metal is more dense. This is an unexpected inference that will need thoughtful exploration.

A second student's idea

Through discussion and exploration, one group of students came up with a novel idea: The metal was “stealing” the heat from your hand when you touched it, so that is why it feels colder. This “might” be what a conductor is. If an object is a good conductor, it might steal the heat from objects that are warmer that come into contact with them.

Energy is flowing from the hot object (in this case a person's hand) to the cold object (in this case the four room temperature plates). If the “cold” object (the plate) has a high conductivity, then the energy from the the person's hand will travel to and throughout the plate at a quick pace. The reason the plates with high conductivity will feel cold to the touch is because the person's hand is continuing to lose energy.

If the object has a low conductivity, then the energy from the hot object will travel to and throughout the plate at a very slow pace. If the hot object is a person's hand, then the area where the person is touching on the plate will begin to warm and feel warm to the touch.

Although tempting to think that the more dense material would have a higher conductivity, particularly given these four plates, a material's density is not related to the rate at which it conducts heat, its thermal conductivity. See graph of thermal conductivity vs density.

Example:
metal with low density but high conductivity: Density of aluminum is 2.70 grams per cubic centimeter. Its thermal conductivity is 2.37 Watts/centimeter degree Kelvin
metal with high density but low conductivity: Density of lead is 11.35 g/cc. Its thermal conductivity is only 0.35 W/cmK

http://www.webelements.com/periodicity/thermal_conductivity/ shows a scatter plot of thermal conductivity versus atomic number for the elements