Michael J. Harrison

Computer-assisted assignments in a large physics class

Abstract The CAPA system, a software tool to implement a computer-assisted personalized approach for homework assignments and examinations, was used in a large introductory physics class for the first time. The students rated the system extremely favorably even though they spent significantly more time on the assignments compared to traditional classes. Fewer teaching assistants were needed and their time could be diverted from grading to more interactive contact with their students.

Modification of the velocity of sound in water by solute ions

Abstract The effect on the velocity of sound in water of a uniform distribution of compensating positive and negative solute ions is investigated in the hydrodynamic approximation. Coulomb interactions between ions are represented by a self-consistent electric field in the water, which is determined by Poissons equation and the distribution of ions. Three coupled equations of motion for the water molecules, and positive and negative ions, are respectively linearized and combined with Poissons equation to study the normal modes of the system. A secular equation for the frequencies of traveling waves is solved in the long-wavelength limit to study ionic hydration effects.

Interplay of Orbital Paramagnetism and Diamagnetism in Nanometer Scale 2DEG Strips

The role of boundary corrections to Landau diamagnetism for macroscopic free electronic systems has been discussed [1–8], and orbital paramagnetic effects have been experimentally observed in small systems [9]. The correction terms can theoretically be either paramagnetic or diamagnetic depending on the increase in potential energy at the surface which defines the perimeter of the electron system. In this work calculations of orbital magnetism have been performed for nanometer scale 2DEG strips laterally confined by an attractive transverse parabolic potential. The emergence of orbital paramagnetism depends sensitively on areal electron density, strip width, and parabolic potential strength such as may be produced by a split gate with negative voltage applied. Selected graphical results display considerable variation in the orbital paramagnetic contribution compared to the Landau diamagnetic susceptibility.

Thank You Walter for All You Have Taught Us

How many physicists do you know whose late-night reading includes 16th century French literature in the original? I know one; Walter Kohn! It was early in 1981, and my wife Ann and I had come to Santa Barbara, whereWalter had kindly permitted me to occupy a desk during my sabbatical at the Institute for Theoretical Physics at UCSB. I had been doing administrative work as a dean at Michigan State University, and I very much wanted to do physics again and learn what were the interesting research problems that people were studying. Walter and Mara were most hospitable, and when Walter learned that Ann taught medieval and post-medieval French literature at MSU their conversation turned to Montaigne and his work. As it happened, Ann had a second copy of Montaigne’s ESSAIS, and was quite happy to present the volumes to Walter as a gift. And we know that he read them in the weeks that followed.

The cubic growth of AIDS cases: General dependence on early infection rates and distribution of times to appearance of clinical symptoms

An observed cubic dependence on time of the cumulative growth in numbers of AIDS patients is shown to be the leading term of a power series in general. The cubic leading term follows from a nearly model-independent description of the rate of HIV infection, provided only that there is a distribution of incubation times to the appearance of clinically diagnosed AIDS symptoms with a finite initial derivative, that none of those infected exhibit immediate indications of illness, and that the epidemic begins with a finite initial rate of infection.

Magnetoelastic wave instability from drifted conduction electrons

Abstract The normal modes of a current-carrying ferromagnetic or ferrimagnetic semiconductor are studied by combining the equations of motion of the magnetization, the lattice, and the itinerant charge with Maxwells equations. The dispersion relation for the resulting magnetoelastic waves indicates an instability when the conduction electron drift velocity becomes greater than the phase velocity of the magnetoelastic wave.