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Hello, my name is Stephen Lanford and I am currently working on a Physics II lab.
I am solving for the thermal conductivity of four materials (glass, plexiglass, pine, and sheetrock) using data from thermal conductivity experiments. The experiments consist of a steam chamber at 100 C, a block of ice inside an ice mold (with one side exposed to allow water to run off when it melts), and one of the four materials clamped between the hot and cold surfaces, through which energy is transferred. Using the thermal conductivity formula K = (m)Lf(h) / A(TH-TC)t---where m is the mass of the melted water, Lf is the latent heat of fusion, h is the thickness of the material, A is its area TH and TC the temperatures of the hot and cold sides of the experiment, and t the time elapsed---I made a calculation of the thermal conductivity for each of the four materials.
Unfortunately, some of the thermal conductivity values I calculated were nowhere close to the stated literature values of thermal conductivity for these same materials.
I am asked to describe the potential sources of my errors. I suspect that some energy is being lost to either conduction or radiation (thus marring the accuracy of the conduction measurements), but I am not exactly sure how. Moreover, my assignment asks me two questions:
(1) What would likely be a significant source of error in performing this experiment on a material that has a very low thermal conductivity that you would not see as much for materials that have higher thermal conductivities?
(2) What would likely be a significant source of error in performing this experiment on a material that has a very high thermal conductivity that you would not see as much for materials that have lower thermal conductivities?
All I could think of for question 1 is that an experiment performed with lower conductivity materials would result in more energy being lost to the environment (since less heat would escape through a low conductivity barrier than through a high conductivity barrier), but this would make no sense: energy loss to the surrounding air would not increase just because energy loss through the material barrier has decreased!
All I could think of for question 2 is that higher conductivities would result in faster heat transfer and more chance that water could overflow or leak out (resulting in an inaccurate final measurement of the water from melted ice), but the experiment was set up with precautions so that there would be no water loss during the time measured.
Clearly, I am not thinking correctly about either of these scenarios. Can someone please help me to understand the situation better and determine the sources of error?
Also, both the pine and the sheetrock were wrapped in aluminum foil for waterproofing. Is it likely the aluminum foil also prevented much of the convection that would have otherwise occurred between the material and the surrounding air? Would this make for a more accurate final measurement of thermal conductivity? Would wrapping the glass and plexiglass in aluminum foil also be a good idea for improving accuracy---even though waterproofing is unnecessary for these materials?
Thanks so much for your help with these questions!
- Stephen Lanford
I am solving for the thermal conductivity of four materials (glass, plexiglass, pine, and sheetrock) using data from thermal conductivity experiments. The experiments consist of a steam chamber at 100 C, a block of ice inside an ice mold (with one side exposed to allow water to run off when it melts), and one of the four materials clamped between the hot and cold surfaces, through which energy is transferred. Using the thermal conductivity formula K = (m)Lf(h) / A(TH-TC)t---where m is the mass of the melted water, Lf is the latent heat of fusion, h is the thickness of the material, A is its area TH and TC the temperatures of the hot and cold sides of the experiment, and t the time elapsed---I made a calculation of the thermal conductivity for each of the four materials.
Unfortunately, some of the thermal conductivity values I calculated were nowhere close to the stated literature values of thermal conductivity for these same materials.
I am asked to describe the potential sources of my errors. I suspect that some energy is being lost to either conduction or radiation (thus marring the accuracy of the conduction measurements), but I am not exactly sure how. Moreover, my assignment asks me two questions:
(1) What would likely be a significant source of error in performing this experiment on a material that has a very low thermal conductivity that you would not see as much for materials that have higher thermal conductivities?
(2) What would likely be a significant source of error in performing this experiment on a material that has a very high thermal conductivity that you would not see as much for materials that have lower thermal conductivities?
All I could think of for question 1 is that an experiment performed with lower conductivity materials would result in more energy being lost to the environment (since less heat would escape through a low conductivity barrier than through a high conductivity barrier), but this would make no sense: energy loss to the surrounding air would not increase just because energy loss through the material barrier has decreased!
All I could think of for question 2 is that higher conductivities would result in faster heat transfer and more chance that water could overflow or leak out (resulting in an inaccurate final measurement of the water from melted ice), but the experiment was set up with precautions so that there would be no water loss during the time measured.
Clearly, I am not thinking correctly about either of these scenarios. Can someone please help me to understand the situation better and determine the sources of error?
Also, both the pine and the sheetrock were wrapped in aluminum foil for waterproofing. Is it likely the aluminum foil also prevented much of the convection that would have otherwise occurred between the material and the surrounding air? Would this make for a more accurate final measurement of thermal conductivity? Would wrapping the glass and plexiglass in aluminum foil also be a good idea for improving accuracy---even though waterproofing is unnecessary for these materials?
Thanks so much for your help with these questions!
- Stephen Lanford
