Measure the thermal conductivity of glass.
This experiment uses steam heating. Be careful to avoid touching the hot surfaces of the steam generator, plastic tubing and the Lee's disk apparatus. Make sure that the steam outlet tube from the apparatus goes to a sink.
Lee's Disk apparatus, thermometers T1 and T2, steam generator, disk shaped poor conductor (glass), gloves, ruler, Vernier callipers.
Assuming that the heat losses to the from sides of the sample are negligible, the steady state rate of heat transfer (H) by conduction is given by: Equation 1
where the k is the thermal conductivity of the sample, A is the
cross sectional area and is the temperature
difference across the sample thickness x (see figure 1).
As the sample is an insulator, lagging at the sides will not significantly reduce the energy losses. Therefore to keep these losses small the sample is a thin disk with a large cross sectional area compared to the area exposed at the edge . Keeping A large and x small produces a large rate of energy transfer across the sample. Keeping x small also means that the apparatus reaches a steady state (when temperatures T1 and T2 are constant) more quickly.
The thin sample is sandwiched between the brass disk and brass base of the steam chest (see figure 2). The temperature of the brass base (measured by thermometer T2) is very close to the temperature of the top surface of the glass disk because the thermal conductivity of brass is about one hundred times that of glass. Similarly the temperature of the brass disk (measured by T1) is very close to the temperature of the lower glass surface. In this way the temperature difference across such a thin sample can be accurately measured.
When the apparatus is in a steady state (temperatures T1 and T2 constant), the rate of heat conduction into the brass disk must be equal to the rate of heat loss due to cooling (by air convection) from the bottom of the brass disk. The rate of heat loss can be determined by measuring how fast the brass disk cools at the previous (steady state) temperature T1 (with the top of the brass disk covered with insulation, see figure 3). If the disk cools down at a rate then the rate of heat loss is given by: Equation 2
where m is the mass of the brass disk and c is the specific heat capacity of brass.
© Mark Davison, 1997, give feedback or ask questions about this experiment.Back to the experiment menu