In the Laboratory
Laser Measurement of the Speed of Sound in Gases: A Novel Approach to Determining Heat Capacity Ratios and Gas Composition J. Clayton Baum* Department of Chemistry, Florida Institute of Technology, Melbourne, FL 32901; *
[email protected] R. N. Compton and Charles S. Feigerle Department of Chemistry, University of Tennessee, Knoxville, TN 37996
While it is clear that laser experiments capture the interest of students, there have been few laser applications to the direct measurement of thermodynamic properties in the undergraduate laboratory (1). A standard experiment in the physical chemistry laboratory has been the determination of the heat capacity ratio (γ = Cp /CV) of gases, commonly found using adiabatic expansion (2, 3) or the velocity of sound (2, 4, 5). The latter method typically involves monitoring the output of an audio oscillator with a movable microphone and measuring the distance between microphone positions that produce nodal Lissajous patterns on an oscilloscope. This distance along with the known frequency of the oscillator allows the speed of sound to be calculated. While this procedure generally gives more precise results than the adiabatic expansion method, it is sometimes difficult for students to conceptually relate the Lissajous figures to the speed of sound. We describe an alternative technique for measuring the sound velocity that involves generation of a microspark by a laser pulse focused on a graphite rod (Figure 1). This spark is a modern equivalent of the cannon ball blast used in the velocity of sound calculations by Newton in 1713 (6). The speed of sound can be determined directly by measuring the time, t, required for the photoacoustical pulse produced to travel over the fixed distance, L, through a gas to a microphone detector. The transit time is so short (10‒3 s) over this distance that direct rubber stopper clock motor
rubber stopper
Pyrex tube
microphone needle
graphite rod
measurement is possible only with short (
