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Heat is a form of energy. There are three ways heat can be transferred: (1) conduction, (2) convection, and (3) radiation. This lab will be focusing on conduction as a method of heat transfer. Conduction is defined as the transfer of heat energy without any bulk movement of the material. Thermal conductors are materials that conduct heat well. Those materials that do not conduct heat well are referred to as thermal insulators. Metals tend to be good conductors, while wood, glass, and plastics tend to be good insulators. Each of the following factors are involved in determining the amount of heat (Q) that is transferred through an object:

1. The conductivity of the object (k)
2. The cross-sectional area of the object (A)
3. The thickness of the object (L)
4. The amount of time that conduction has taken place (t)
5. The temperature difference between the two sides of the object (D T)

Using the above information, the amount of heat transferred through an object can be determined as follows:

Conduction plays an important part in the construction of buildings. The insulating ability of materials is very important in order to increase the energy efficiency of buildings. In this lab we will look at some building materials such as glass, wood, and sheet rock and evaluate their ability to insulate. The effectiveness of materials to insulate is usually referred to as thermal resistance or R value. R values are related to thermal conductivities by the following equation:

The higher the R value, the better the material insulates. The common units in the US for R values are ft² *° F*h/Btu. In colder parts of the US., an R value of 30 is recommended for ceilings.

At your workstation you should find a thermal conductivity apparatus (including base and steam chamber), a steam generator, two containers to collect water, three test materials (one of which should be glass), a micrometer, and a container filled with ice. Measure and record the thickness of the 3 test plates using the micrometer. Take four different measurements for each plate and use the average of the four. Be careful not to over tighten the micrometer or damage to the test materials will occur. Mount the glass plate onto the steam chamber so that the melted ice will run down the channel on the chamber. Be sure the material is flush against the channel so water will not leak. Now measure the diameter of the ice block. It is not necessary to remove the block from the mold, but be sure the block is free to move inside the mold before you begin. Place the mold upside down on the glass plate. Do not continue until water begins to flow.

The first part of the experiment is to measure the ambient melting rate of the ice. This is the rate at which the ice melts due to the temperature of the room. Find the mass of one of the containers at your work station. Place the container under the channel and collect the melted ice for about 10 minutes. Record the amount of time and the mass of the water collected. Dividing the mass of the water by the time, you can determine the ambient rate at which the ice melted. During this time fill the steam generator with water and turn it on.

Run steam into the steam chamber. Place a container at the side of the chamber to collect the water from the chamber. Let the steam run for several minutes until temperatures stabilize. Be sure to collect this water, but you do not have to know the amount of it. Now place a cup of known mass under the channel and collect water for 6-8 minutes. Record the mass of the collected water and the time. When this is complete, measure the new diameter of the block.

Repeat the above procedure for the remaining two materials at your station. You do not have to repeat the section to determine the ambient melting rate. Continue to use the same ice block.

Using the initial and final diameters of the block, find the average diameter during the experiment. Using this diameter, find the area of the block. This is just the area of the block that is in contact with the glass plate. Using the mass and time, determine the rate at which the ice melted. Subtracting the ambient rate from this you can determine the rate (R₀) at which the ice melted due to the temperature difference only. Remembering that it requires 80 cal to melt 1 gram of ice at 0°C, you can find Q using the following relationship:

Now using equation #1, calculate the value for the thermal conductivity. Since there is steam on one side of the plate and ice on the other, the value for D T should be 100 ° C. Now calculate the R value for each material and compare them to the standard value. (The standard values will be supplied by the instructor.) Use the following chart to convert the units for your R value to the same units as the standard value.

FINAL SUMMARY:

In your summary, you should report the experimental values for the conductivity and the R value for each material tested. Which material has the highest conductivity? Which material has the highest R value? What conclusions can you draw from this information?

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