How Do You Select Laboratory Experiments to Perform?

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Selecting Laboratory Experiments 135orand-b + V^-4ac2o_-b - VV^ac2aHow Do You Select Laboratory Experiments to Perform?Albert G. KriegerJackson Community College, Jackson^ Michigan 49201Every science teacher encounters the problem of selecting experi-ments for students to perform in the laboratory. Usually, the problemis that of elimination, as there have been countless numbers of ex-periments published and suggested. But with a limited amount oflaboratory time available, the problem really reduces down to select-ing the experiments that will display a variety of phenomenon, de-velop the maximum skills, and may be performed with readily avail-able laboratory equipment.Another criteria for the selection is to insure that the overall pur-pose of the experiment is not lost in a maze of tedious manipulationsor theoretical corrections. Yet, many "straight-forward^ laboratorymeasurements leave much to be desired as to the accuracy of the finaldesired constant or relationship. The student in either case may leavethe laboratory wondering about a "so-called law." This latter criteriacan be evaluated only after close scrutiny of the experiment and theperformance of the student investigator.To demonstrate the various aspects of selection, let us suppose thatan experiment is to be selected that will demonstrate wave motion ofsound and will include a determination of the velocity of sound. Aperusal of laboratory manuals and science education journals willlead to a list of experiments that determine the velocity of sound bythe following methods:(a) Kundts Tube (Resonance)(b) Kundts Tube (Standing Waves)(c) Water Column (Resonance)(d) Youngs Experiment (Interference)(e) Time-of-Flight (Oscilloscope)(f) Lissajous Figures (Phase Difference)(g) Standing Waves in a Large Tube.Let^s take a close look at each of these methods.136 School Science and MathematicsKUNDTS TUBE (RESONANCE)This method is a well established method and most laboratoriesseem to be equipped with the apparatus. The nodal points are easilyvisible and the half-wave lengths can be measured directly using ameter stick. A limitation of the method is that resonance at the fre-quency studied must be achieved in order for there to be sufficientenergy to move the node indicating dust. Space becomes limited andfew measurements can be made at any given frequency.In increasing the sound energy to drive the dust to the nodalpoints, the sound intensity may become uncomfortable to the in-vestigator. The results are generally good and the expense is notlarge.KUNDTS TUBE (STANDING WAVES) 1This is a recent method introduced by Hastings. The nodal indi-cating dust is removed from the Kundts tube and a thermistor isused to probe the tube volume for maxima and minima sound energy.The sound energy changes the thermistor resistance which is thenmeasured on a wheatstone bridge. The results appear good but theequipment needed is more elaborate and expensive per studentstation.WATER COLUMN (RESONANCE)This method has been well established in high school laboratories.. -The length of the whaler column is adjusted until the air columnreaches resonance when driven by a tuning fork. The need for reso-nance reduces the number of measurements possible at any given fre-quency is inexpensive. The results vary and require corrections.YOUNGS EXPERIMENT (INTERFERENCE)23The apparatus is simple and is an excellent teaching device asstudents can readily detect the nodal lines with their ears. However,points on the nodal line may not have zero amplitude as there are re-flections from walls and nearby objects. A microphone attached to thevertical deflection input of an inexpensive oscilloscope works well toidentify the maxima points.The wavelength determinations are poor unless very large roomsare available. A serious objection to the method is that sound travelsand bothers everyone in the building.1 Hastings, R. B., "Thermistor Explorations in a Kundt Tube," American Journal of Physics, 37 (July 1969),709-12.1 Vickery, Jon P., "Determination of the Velocity of Sound in Air," The Physics Teacher, 3 (April 1965),170-1. Amend, John R., "The Velocity of Sound," The. Science. Teacher, 31 (February 1964;, 20-3.Selecting Laboratory Experiments 137If an oscilloscope is not available, both maxima and minima canbe .detected satisfactorily by plugging one ear with a finger andpointing the other ear towards the sound source. Although the ex-periment leaves much to be desired as to the accuracy of the results,it is inexpensive. Though not to be recommended as a laboratoryexperiment, the method does have much to be recommended as ademonstration device. Students arc almost always impressed withthe fact that they can actually detect nodal points and this writercan testify that he has seen former students who have deliberatelysought out the presence of nodal lines in the sound pattern of a two-speaker system of pop electric guitar combos. Adults may readilyunderstand the motive.TIME-OF-FLIGHT (OSCILLOSCOPE)4The method requires oscilloscopes of the Tektronix variety andthe results are good. However, Tektronix oscilloscopes are expensiveand therefore not available for most student investigations.LISSAJOUS FIGURES (PHASE DIFFERENCE)56The method is easy, inexpensive, the wavelengths are well-defined,and the students learn to use the inexpensive oscilloscope. PSSCPhysics high school courses and those courses influenced by PSSC,study phase changes in water ripple tanks. The concept may thenbe applied to study the wave motion of sound. Unfortunately, thewavelengths of several trials at the same frequency vary considerably.Reflections from surroundings including those from the investigatoraffect the measurements and other sounds in the room are picked upand tend to distort the oscilloscope display.The computed velocity appears to vary with frequency and thenecessary sound level for satisfactory measurements is objectionableto others in the same building.STANDING WAVES IN A LARGE TUBE7The method was discovered by this writer in the Taylor Manual.The varieties of difficulties, expense, etc., in the many methodsavailable led this writer from one experimental approach to anotheruntil the Taylor Manual was finally consulted. The method basically4 Manka, C. K., "A Direct Measurement of the Speed of Sound in Air," American Journal of Physics, 37(February 1969), 223-4.6 Christensen, F. E., "Determination of the Velocity of Sound in Air," The Physics Teacher, 2 (November1964), 390-1.6 Woods, Frank, Jr., "Experimentally Determining the Velocity of Sound," Radio-TV Experimenter, (Spring1963), 86-8."1 Brown, Thomas B. (Ed.), Taylor Manual of Advanced Undergraduate Experiments in Physics, Reading,Massachusetts: Addison-Wesley Publishing Co. 1959, 162.138School Science and Mathematicsis to probe the volume of a large tube with a button type microphoneprobe to determine positions of sound maxima and minima. SeeFigure 1. The resulting signal is then amplified and displayed as avertical line on an inexpensive student type oscilloscope. The tubeused by this writer was a blueprint paper mailing tube which wasabout six inches in diameter.osc-6-FIG. 1. Standing waves in a large tube.Resonance need not be achieved and the sound does not carrymore than five feet. Because the probe and the sound is inside of thetube, extraneous room noise and movement of the investigator doesnot interfere with the measurements. The probe does interfere withthe sound by many measurements (the tube may be five feet long)are possible and the measured wavelengths smooth out.The determined velocity does not vary with frequency and com-pares favorably with the velocity as determined by other methods.In summary, once the teacher has decided upon a particular typeof measurement for the laboratory, it is necessary to evaluate themany methods available to make the measurement. In each case, theteacher should evaluate fa) the expense, (b) the good points and badpoints, (c) the accuracy of the measurements, (d) the ease with whichthe measurements are made, and most importantly (e) will the stu-dent really learn something from the experience.It would be of great value to borrow parts of the various experi-mental methods to be used as demonstration material. It is of greatimportance that the student understand the properties of sound aswell as know how to make the measurements of frequency, wave-length, and velocity.


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