I've just done Q2 from the 2009 past paper and am slightly stuck for ideas in the last part where it asks for two possible reasons for discrepancy between measured and calculated values of Cv.
Using the values of va and vb that I calculated at the beginning, and plugging them into the derived Debye heat capacity as trial velocities for the vibrational modes, I get the following two heat capacities:
Cv = 3.76 (microJK^-1m^-2)
Cv = 13.2 (microK^-1m^-2)
which are both clearly bigger than the measured value of
Cv = 2.5 (microK^-1m^-2)
Provided my calculated values are correct, it would appear that the actual velocity is higher than va and vb but I'm not sure why?
The only thing I can think to say is that the linear approximation is not a good one at this temperature but I'm not sure why this would lead to an over-estimate in heat capacity?