LEE’S DISC METHOD FOR BAD CONDUCTORS
The thermal conductivity o bad conductors like ebonite or card board is determined by this methods.
The apparatus consists o circular metal disc or slab C(Lee’s Disc) by strings rom as stand. The given bad conductor (such as glass, ebonite) is taken in the orm of the disc(D).
This disc has the same diameter as that of the slab and is placed over it.
A cylindrical hollow steam chamber A having the same diameter as that of the slab is placed over the bad conductor. There are holes in the steam chamber and the slab through which thermometers T1 and T2 are inserted to record the respective temperatures.
Steam is passed through the steam chamber until the temperatures o the chamber and the slab are ready. When the thermometer show steady temperatures, there reading θ1 and θ2 are noted. The radius(r) o the disc D and its thickness (d) are also noted.
Observation and Calculation
Thickness of the bad conductor = d meter
Radius of the bad conductor = r meter
Mass of the Slab (c) = M kg
Steady temperature in the steam chamber = θ1
Steady temperature in the bad conductor = θ2
Thermal conductivity o the bad conductor = K
Rate o cooling at θ1 = R
Speciic heat capacity of the slab = S
Area o cross section A = πr2
Amount of heat conducted through the specimen per second
At this stage, all the heat conducted through the bad conductor is completely radiated by the bottom flat surface and the curved surface of the Slab C.
Amount of heat lost per second by the Slab C
Q= Mass x Specific Heat Capacity x Rate of cooling
Heat conducted through bad conductor per second = heat lost [per seond by the slab
Hence the equation (1) and (2) are equal
Determination of Rate of Cooling
The bad conductor is removed and the steam chmber is placed directly on the slab. The slab is heated to a temperature of about 5oC higher than θ2.
The steam chamber is removed and the slab alone is allowed to cool.
As the slab cools, the temperatures of the slab are noted at regular intervals of hal a minute until the temperature o the slab falls to about 5oC below θ2. The time temperature graph is drawn as shown in the igure and the rate of cooling dθ/dt at the steady temperature θ2 is determined.
During the first part of the experiment, the top surface of the slab is covered by the bad conductor. Radiation is taking place only from the bottom surface area and curved surface area.
In the second part o the experiment, heat is radiated from the top surface aream the bottom surface area and the curved sides i.e. over an area