A simple, rapid and entertaining method to measure specific heat of metals using liquid nitrogen and Leidenfrost effect

Abstract

Taking advantage of the fascination of ‘playing’ with liquid nitrogen, this article proposes a didactical experiment involving the use of this liquid to measure the heat capacity of cylindrical metallic blocks (copper, aluminum, brass, and stainless steel; volume ≈ 20 cm3). It also permits to demonstrate qualitatively and quantitatively the Leidenfrost effect. The experimental setup consists of a low-cost recipient adequate for liquid N2 storage, a multimeter and a weighing scale. The experience starts by a rapid immersion of metallic block in liquid N2 and by measuring simultaneously the block’s temperature and the mass of the evaporated liquid along the cooling process. Knowing the latent heat of the liquid L , the evaporated mass Δm during a temperature variation ΔT , the heat capacity is obtained by Cp=L Δm/ΔT . This method allowed to measure the specific heat in the 100–270 K temperature range in less than 3 min and the results are in quite good agreement with the literature data. On the other hand, the cooling rate, measured by temperature and liquid N2 change, clearly shows two regimes as expected by the Leidenfrost effect and the critical heat flux calculated is in good agreement with previous experiments. Such an experiment can be adapted to various student levels and present a didactical approach to important thermodynamical concepts in parallel with appealing phenomena.