Physics
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Item COMPARING AND CONTRASTING DIFFERENT METHODS FOR PROBING STUDENT EPISTEMOLOGY AND EPISTEMOLOGICAL DEVELOPMENT IN INTRODUCTORY PHYSICS(2009) McCaskey, Timothy Lee; Redish, Edward F; Elby, Andrew R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, I perform and compare three different studies of introductory physics students' epistemological views - their views about the nature of knowledge and how it is learned. Physics education research (PER) shows that epistemological views affect how students learn, so they are important to understand and diagnose. The first study uses a Likert-scale instrument, adapted from the Maryland Physics Expectation Survey, designed to assess to what extent students see physics knowledge as coherent (rather than piecemeal), conceptual (rather than just formulas), and constructed (rather than absorbed). Using this survey, I documented several results, including that (i) a large lecture class can produce favorable changes in students' epistemological views, at least in the context of the class, and (ii) teaching a rushed modern physics unit at the end of an introductory sequence can lead to negative epistemological effects. The second study uses the Force Concept Inventory with modified instructions: students indicated both the answer they think a scientist would give and the answer that makes the most sense to them personally. A "split" between these two answers shows that the student does not think she has reconciled her common sense with the formal physics concepts. This study showed that attention to reconciliation in a course allows students to see initially-counterintuitive ideas as making sense. Finally, I did a detailed study of one student by (i) watching video of her in tutorial, where she and three other students answered a structured series of conceptual and quantitative physics questions, (ii) formulating interviews based largely on what I observed in the video, and (iii) interviewing her while the tutorial was still fresh in her head. I repeated this cycle every week for a semester. I found that her tendency to focus on the multiple and ambiguous meanings of words like "force" hampered her ability to reconcile physics concepts with common sense. This last method is time-consuming, but it produces rich data and allows for a fine-grained analysis of individual students. The first two survey methods are best suited for measuring the effect of epistemologically-centered course reforms on large groups of students.Item The Dynamics of Variability in Introductory Physics Students' Thinking: Examples from Kinematics(2009) Frank, Brian Wallace; Scherr, Rachel E; Hammer, David; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Physics education research has long emphasized the need for physics instruction to address students' existing intuitions about the physical world as an integral part of learning physics. Researchers, however, have not reached a consensus-view concerning the nature of this intuitive knowledge or the specific role that it does (or might) play in physics learning. While many early characterizations of student misconceptions cast students' intuitive thinking as largely static, unitary in structure, and counter-productive for the purpose of learning correct physics, much of contemporary research supports a conceptualization of intuitive thought as dynamic, manifold in structure, and generative in the development of expertise. This dissertation contributes to ongoing inquiry into the nature of students' intuitive thought and its role in learning physics through the pursuit of dynamic systems characterizations of student reasoning, with a particular focus on how students settle into and shift among multiple patterns of reasoning about motion. In one thread of this research, simple experimental designs are used to demonstrate how individual students can be predictably biased toward and away from different ways of thinking about the same physical situation when specific parameters of questions posed to students are varied. I qualitatively model students' thinking in terms of the activations and interactions among fine-grained intuitive knowledge and static features of the context. In a second thread of this research, case studies of more dynamic shifts in students' conceptual reasoning are developed from videos of student discussions during collaborative classroom activities. These show multiple local stabilities of students' thinking as well, with evidence of group-level dynamics shifting on the time scale of minutes. This work contributes to existing research paradigms that aim to characterize student thinking in physics education in two important ways: (1) through the use of methods that allow for forms of empirical accountability that connect descriptive models of student thinking to experimental data, and (2) through the theoretical development of explanatory mechanisms that account for patterns in students' reasoning at multiple levels of analysis.Item Nonlinear Dyanmics in Biological Systems: Actin Networks and Gene Networks(2009) Pomerance, Andrew; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two problems in biological systems are studied: (i) experiments in microscale deformations of actin networks and (ii) a theoretical treatment of the stability of discrete state network models of genetic control. In the experiments on actin networks, we use laser tweezers to locally deform actin networks at the micron scale as a model of the action of molecular motors and other cellular components, and we image the network during deformation using confocal microscopy. Using these tools, we observe two nonlinear effects. The first observation is that there are two time scales of relaxation in the network: the stress induced by deformation relaxes rather quickly, however, the strain relaxes at a different rate. Additionally, upon removing the deforming force, the initial rate at which the strain relaxes seems to be independent of the amount of stress still in the network. The second observation is that large deformations are irreversible, and imaging the network implies that a large-scale snapping event seems to accompany this irreversibility. In the theoretical treatment of gene networks, we focus on the stability of their dynamics in response to small perturbations. Previous approaches to stability have assumed uncorrelated random network structure. Real gene networks typically have nontrivial topology significantly different from the random network paradigm. In order to address such situations, we present a general method for determining the stability of large Boolean networks of any specified network topology and predicting their steady-state behavior in response to small perturbations. Additionally, we generalize to the case where individual genes have a distribution of `expression biases,' and we consider non-synchronous update, as well as extension of our method to non-Boolean models in which there are more than two possible gene states. We find that stability is governed by the maximum eigenvalue of a modified adjacency matrix (&lambdaQ<\sub>), and we test this result by comparison with numerical simulations. We also discuss the possible application of our work to experimentally inferred gene networks and present approximations to &lambdaQ<\sub> in several cases.Item A Search for Muon Neutrinos from Gamma-Ray Bursts wih the IceCube 22-String Detector(2009) Roth, A Philip; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two searches are conducted for muon neutrinos from Gamma-Ray Bursts (GRBs) using the IceCube detector. Gamma-Ray Bursts are brief and transient emissions of keV/MeV radiation occuring with a rate of a few per day uniformly in the sky. Swift and other satellites of the Third Interplanetary Network (IPN3) detect these GRBs and send notices out via the GRB Coordinate Network (GCN). The fireball model describing the physics of GRBs predicts the emission of muon neutrinos from these bursts. IceCube is a cubic kilometer neutrino detector buried in the deep antarctic ice at the South Pole that can be used to find these prediceted but still unobserved neutrinos. It is sensitive to them by detecting Cherenkov light from secondary muons produced when the neutrinos interact in or near the instrumented volume. The construction of IceCube has been underway since the austral summer of 2004-2005 and will continue until 2011. The growing IceCube detector will soon be sensitivite to the high energy neutrino emission from GRBs that is predicted by the fireball model. A blind and triggered search of the 22-string IceCube data for this neutrino emission was conducted. The principal background to the observation of neutrinos in IceCube is muons generated in cosmic-ray air-showers in the atmosphere above the detector. Atmospheric neutrinos make up a separate irreducible background to the detection of extraterrestrial neutrinos. A binned stacked search of 41 bursts occuring in the northern hemisphere greatly reduces the muon background by looking for tracks moving up through the detector. The atmospheric neutrino background is greatly reduced by the temporal constraints of the search, making it effectively background free. 40 individual unbinned searches of bursts occuring in the southern hemisphere extend IceCube's sensitivity to the higher background regions above the horizon. No significant excesses over background expectations are found in either search. A 90% confidence upper limit on the neutrino fluence from northern hemisphere bursts is set at 6.52 x 10-3 erg cm-2 with 90% of the expected signal between 87.9 TeV and 10.4 PeV.Item Transport in Poygonal Billiard Systems(2009) Reames, Matthew Lee; Dorfman, J. R.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The aim of this work is to explore the connections between chaos and diffusion by examining the properties of particle motion in non-chaotic systems. To this end, particle transport and diffusion are studied for point particles moving in systems with fixed polygonal scatterers of four types: (i) a periodic lattice containing many-sided polygonal scatterers; (ii) a periodic lattice containing few-sided polygonal scatterers; (iii) a periodic lattice containing randomly oriented polygonal scatterers; and (iv) a periodic lattice containing polygonal scatterers with irrational angles. The motion of a point particle in each of these system is non-chaotic, with Lyapunov exponents strictly equal to zero.Item Control, Dynamics, and Epidemic Spreading in Complex Systems(2009) Nagy, Viktor; Ott, Edward; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we investigate three problems involving the control and dynamics of complex systems. (a) We first address the problem of controlling spatiotemporally chaotic systems using a forecast-based feedback control technique. As an example, we suppress turbulent spikes in simulations of the two-dimensional complex Ginzburg-Landau equation in the limit of small dissipation. (b) In our second problem we examine the dynamical evolution of the one-dimensional self-organized forest fire model, when the system is far from its statistically steady-state. In particular, we investigate situations in which conditions change on a time-scale that is faster than, or of the order of the typical system relaxation time. (c) Finally, we provide a mean field theory for a discrete time-step model of epidemic spreading on uncorrelated networks. The effect of degree distribution, time delays, and infection rate on the stability of oscillating and fixed point solutions is examined through analysis of discrete time mean-field equations.Item An Epistemic Framing Analysis of Upper Level Physics Students' Use of Mathematics(2008-07-11) Bing, Thomas Joseph; Redish, Edward F.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mathematics is central to a professional physicist's work and, by extension, to a physics student's studies. It provides a language for abstraction, definition, computation, and connection to physical reality. This power of mathematics in physics is also the source of many of the difficulties it presents students. Simply put, many different activities could all be described as "using math in physics". Expertise entails a complicated coordination of these various activities. This work examines the many different kinds of thinking that are all facets of the use of mathematics in physics. It uses an epistemological lens, one that looks at the type of explanation a student presently sees as appropriate, to analyze the mathematical thinking of upper level physics undergraduates. Sometimes a student will turn to a detailed calculation to produce or justify an answer. Other times a physical argument is explicitly connected to the mathematics at hand. Still other times quoting a definition is seen as sufficient, and so on. Local coherencies evolve in students' thought around these various types of mathematical justifications. We use the cognitive process of framing to model students' navigation of these various facets of math use in physics. We first demonstrate several common framings observed in our students' mathematical thought and give several examples of each. Armed with this analysis tool, we then give several examples of how this framing analysis can be used to address a research question. We consider what effects, if any, a powerful symbolic calculator has on students' thinking. We also consider how to characterize growing expertise among physics students. Framing offers a lens for analysis that is a natural fit for these sample research questions. To active physics education researchers, the framing analysis presented in this dissertation can provide a useful tool for addressing other research questions. To physics teachers, we present this analysis so that it may make them more explicitly aware of the various types of reasoning, and the dynamics among them, that students employ in our physics classes. This awareness will help us better hear students' arguments and respond appropriately.Item Problems in Spatiotemporal Chaos(2007-11-26) Cornick, Matthew Tyler; Ott, Edward; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we consider two problem areas involving spatiotemporally chaotic systems. In Part I we investigate data assimilation techniques applicable to large systems. Data assimilation refers to the process of estimating a system's state from a time series of measurements (which may be noisy or incomplete) in conjunction with a model for the system's time evolution. However, for practical reasons, the high dimensionality of large spatiotemporally chaotic systems prevents the use of classical data assimilation techniques such as the Kalman filter. Here, a recently developed data assimilation method, the local ensemble transform Kalman Filter (LETKF), designed to circumvent this difficulty is applied to \RaBen convection, a prototypical spatiotemporally chaotic laboratory system. Using this technique we are able to extract the full temperature and velocity fields from a time series of shadowgraphs from a Rayleigh-Benard convection experiment. The process of estimating fluid parameters is also investigated. The presented results suggest the potential usefulness of the LETKF technique to a broad class of laboratory experiments in which there is spatiotemporally chaotic behavior. In Part II we study magnetic dynamo action in rotating electrically conducting fluids. In particular, we study how rotation effects the process of magnetic field growth (the dynamo effect) for a externally forced turbulent fluid. We solve the kinematic magnetohydrodynamic (MHD) equations with the addition of a Coriolis force in a periodic domain. Our results suggest that rotation is desirable for producing dynamo flows.Item Submillimeter Test of the Gravitational Inverse-Square Law Using a Superconducting Differential Accelerometer(2007-11-21) Prieto, Violeta A; Paik, Ho Jung; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The inverse-square law of gravitation is tested at submillimeter distances. To minimize Newtonian errors, the experiment employs a near null source, a circular disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, are suspended on the two sides of the source mass at a nominal distance of 180 micrometers. The source mass amplitude of motion is 16.1 micrometers. The signal is detected by a superconducting differential accelerometer. Careful matching and alignment makes the detector highly immune to platform vibrations. To reduce the thermal Brownian motion noise as well as the temperature noise of the instrument, the experiment is cooled to 1.7 K by pumping on liquid helium. In this dissertation, I discuss the assembly, design, and design improvements of the inverse square law experiment. I perform a comprehensive analysis of the errors, identify the problems with the apparatus, and show ways to improve the design of the experiment. With the improved design, it will be possible to achieve a sensitivity of |alpha| = 2 x 10^-3 at lambda = 150 micrometers, which will improve the current experimental limits by one order of magnitude at 150 micrometers and by over two orders of magnitude at shorter distances.Item Efficient Surface Conversion for Neutral Atom Detection(2007-11-16) Hughes, Patrick; Coplan, Michael A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Neutral atom detection is a useful way of studying astrophysical plasma structures such as the heliosphere and planetary magnetospheres. When plasma ions undergo charge exchange with the neutral background gas, energetic neutral atoms (ENAs) are generated. These neutral atoms travel in straight lines from the point of charge exchange because they are not subject to deflection by the electric and magnetic fields in space. As a result ENAs can be used to image the plasma structures from which they originate. ENAs in the energy range from a few eV to a few keV are particularly worth studying and are best detected by conversion to negative ions at a surface, a method that has been successfully used by ENA imagers on the Imager for Magnetosphere-to-Aurora Global Exploration (IMAGE) spacecraft. The function and construction of the imager is dependent upon the efficiency of the conversion surface used. A surface with a high conversion efficiency would allow the imager to be smaller and still collect a measurable signal compared to an imager using a surface with low conversion efficiency. The previously used conversion surface had an efficiency of about 1%. In order to find a more efficient conversion surface, detailed as well as comparative measurements of conversion efficiencies were taken at two facilities. The surfaces studied are polished tungsten, highly ordered pyrolytic graphite, diamond-like carbon, a secondary electron emitting leaded glass, gold, silver and platinum. The work function and smoothness of some of the sample surfaces were measured. These measurements have been compared with measured conversion efficiencies to identify those surface properties that are critical for conversion efficiency. For many surfaces, adsorbates and roughness appear to play an important role in conversion efficiency.