Technology and education

We present experimentally determined values of dielectric function of GaAs following fs laser excitation. The data at photon energies of 2.2 and 4.4 eV show that the response of the dielectric function to the excitation is dominated by changes in the electronic band structure and not by the optical susceptibility of the excited free carriers. The behavior of the dielectric function indicates a drop in the average bonding-antibonding splitting of GaAs following the excitation, which leads to a collapse of the band gap. The changes in the electronic band structure result from a combination of electronic screening as well as structural deformation of the lattice caused by the destabilization of the covalent bond. The broadband measurement dielectric function from 1.5-3.5 eV reveals the ultrafast laser-induced heating, disordering and semiconductor to metal transition on a picosecond time scale.

We discuss student participation in an online social annotation forum over two semesters of a flipped, introductory physics course at Harvard University. We find that students who engage in high-level discussion online, especially by providing answers to their peers’ questions, make more gains in conceptual understanding than students who do not. This is true regardless of students’ physics background. We find that we can steer online interaction towards more productive and engaging discussion by seeding the discussion and managing the size of the sections. Seeded sections produce higher quality annotations and a greater proportion of generative threads than unseeded sections. Larger sections produce longer threads; however, beyond a certain section size, the quality of the discussion decreases.

In the kinetic theory of non-equilibrium phenomena in dilute polyatomic gases there is a characteristic difference between the treatment of non- spherical and spherical particles. This difference stems from the fact that in a polyatomic gas, i.e. a gas of non-spherical particles, in the non-equilibrium state not only the distribution of molecular velocities becomes anisotropic, but that also the distribution of the orientations of the molecules is affected by macroscopic thermodynamic forces. These deviations from an isotropic distribution of angular momenta, or polarizations, are generally complicated in nature and may depend on both velocity and angular momentum. Their presence was first realized by Pidduck in 1922. In their book on non-uniform gases Chapman and Cowling were aware of the anisotropies in both the velocity and the rotational angular momentum, but in subsequent calculations they neglected the effects of the angular momentum dependent terms. Later in 1961, Kagan and Afanasiev showed that for a simplified classical model such terms give rise to sizable contributions to the transport properties. Experimental information on polarizations are obtained in various ways. First of all, field effects on transport phenomena, which as Kagan and Maksimov showed are a direct consequence of the existence of such polarizations, yield a wealth of data. Secondly, information is obtained by measuring the non-equilibrium birefringence caused by the anisotropy in the orientational distribution of the molecules. Additional information on certain aspects, e.g. relaxation times of polarizations, can be obtained from a study of phenomena which are determined by (equilibrium) fluctuations, such as the depolarized Rayleigh line broadening and nuclear magnetic resonance. Polarizations can be thought of as consisting of two parts: a tensorial factor (of rank one of higher) depending on the orientation of the molecule and the direction in which the molecule is moving, and a scalar factor depending on the magnitudes of both the molecular velocity and the rotational angular momentum. From studies of the dependence of field effects on the orientation of the field with respect to the gradient, the tensorial factors of polarizations produced by various macroscopic thermodynamic forces have been determined unambiguously. On the scalar structure, however, no information can be obtained from these experiments separately. An analogous situation exists with respect to the importance of higher order Sonine polynomials in the molecular velocity when one considers transport phenomena of noble gases. Measurement of one transport property by itself does not allow any conclusions to be drawn in this regard. One can, however, resort in this case to the verification of specific relations involving the Eucken factor. In a similar way internal consistency checks and a comparison of field effects with optical measurements might lead to more detailed information about the structure of polarization in polyatomic gases. So far such comparisons have not or only partially been carried out. In this thesis two experiments are described which enable some decisive consistency checks to be performed. In chapter II experiments on the influence of external electric and magnetic fields on the viscosity of some polar gases are described. Next, in chapter III, experiments on the influence of a magnetic field on diffusion in N2-noble gases are presented. These experiments–apart from yielding new numerical results–show very clearly that the structure of polarizations is much more complicated than was usually assumed so far, and that it is not possible to get detailed quantitative information on the scalar factor from experiments on field effects alone. For this reason a detailed analysis and comparison between the results of field effects and the results of optical measurements has been carried out. As we will see in chapter IV this comparison indeed clarifies the structure of polarizations. In order to present the results in a uniform and unambiguous way the kinetic theory of rotation molecules, which for this purpose has been modified and extended, is formulated at the beginning of this thesis in chapter I.

In this chapter, we introduce a new technology for facilitating and measuring learner engagement. The system creates a learning experience for students based on frequent feedback, which is critical to learning. We open by problematizing traditional approaches to learner engagement that do not maximize the potential of feedback and offer a research-based solution in a new classroom response system (CRS) two of the authors developed at Harvard University – Learning Catalytics. The chapter includes an overview of cognitive science principles linked to student learning and how those principles are tied to Learning Catalytics. We then provide an overview of the limitations of existing CRSs and describe how Learning Catalytics addresses those limitations. Finally, we describe how we used Learning Catalytics to facilitate and measure learner engagement in novel ways, through a pilot implementation in an undergraduate physics classroom at Harvard University. This pilot was guided by two questions: How can we use Learning Catalytics to help students engage with subject matter in ways that will help them learn? And how can we measure student engagement in new ways using the analytics built into the system? The objective of this chapter is to introduce Learning Catalytics as a new instructional tool and respond to these questions.