Some points to remember on the thermal conductivity
experiment
- We must calibrate the thermistor. This means getting
4 or more data points of (temperature, voltage)
- For each point, the aluminum cup should be filled with
liquid and stirred and placed in its rigid styrofoam jacket. After 3-5
min or so of stirring the voltage should be read and the temperature read
with a thermometer.
- For reading the voltage, set the parameters box to 6V,
100 Hz, and 100 data points
- The data will be taken in one second. Then click the
graph icon. This toggles the graph on and off.
- You may have to toggle the graph a second time to see
it.
- This should give a fairly linear graph of voltage vs.
temperature.(Make this graph while you are waiting for the data run below
to finish)
- The experiment proper involves reading the temperature
of the aluminum disc while heat flows out of it through the test block
to the cup filled with ice and water (to insure a fixed temperature of
) C).
- Fill the aluminum cup with hot water from the tap and
place it on the aluminum disc for 5 min to let the aluminum disc become
relatively hot..
- Then the experiment proper begins
- The cup is filled with ice and water
- Select a test block and place it on the aluminum disc,
so it will be between the disc and the cup
- Start up the data-taking [at 0.5 hz for 1200 data points
(about 40 min)] and then place the cup with ice and water (surrounded by
its styrofoam jacket) on the test block
- At the end of taking data, one saves data to a file,
something like thcond1.xls, so Excel can open the file
- Open the file in Excel. There will be a voltage Vo (from
your graph of V vs T) corresponding to T=0 C. Subtract this voltage from
the data, and take the logarithm, making a column of ln(V-Vo).
- One wants to plot this log data against the time, and
look for a region in the middle of the data where the slope is constant.
- The slope of the ln(V-Vo) vs t graph should be - k A
/[m C x] where
- k is the thermal conductivity in W/(m-K)
- A is area in m^2
- x is thickness in m
- m is the mass of the aluminum disc in kg
- C is the specific heat (900 J/kg/K) of aluminum
- k should be somewhere in the vicinity of 0.05 W/m/K