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Challenge: Problem 1

Figure 1.7 shows a sketch of the apparatus known as Lee's Disc. Its purpose is the measurement of the thermal conductivity of a poorly conducting material. The material is in the form of a disc so that the surface area A is large, and the temperature gradient d / dx is also large. Since the rate of heat transfer is given by ( dQ / dt ) = -( (k.A.d ) / (dx) ), and the thermal conductivity k is small, making A and d / dx large enables there to be a sizeable heat flow through the material. The source of heat can be steam or electricity. The disc is sandwiched between two highly conducting slabs make of brass or copper. It is assumed that all the heat passing through the thin disc of material is lost through the surface of the lower slab. To measure this heat loss, the disc is removed and the slabs suspended one on top of the other. They are in the same conditions as before, except that they are heated to temperature about 20°C above that of the lower slab during the main experiment. By measurments on the slabs as they cool, the rate of heat loss corresponding to that of the main experiment can be determined.

Select items from the Array which indicate what is needed for the calculation of (dQ) / (dt). Also select items which indicate what properties of the slabs are important to the experiment.

1
latent heat of steam		 2
flow of mass		 3
constant temperature distribution		 4
mass		 5
unwanted condensation of vapour
6
transfer of heat from electrical element		 7
temperature gradient		 8
radiation losses		 9
transfer of heat by vapour		 10
temperature rise
11
high thermal conductivity		 12
specific heat of substance		 13
condensation of saturated vapour		 14
rate of temperature drop		 15
large surface area
16
insulation against conduction or convection		 17
pressure		 18
cooling inside the enclosure		 19
unwanted conduction of heat 		20
thermal expansion