Russell K. Hobbie

Single-pool exponential decomposition models: potential pitfalls in their use in ecological studies

The importance of litter decomposition to carbon and nutrient cycling has motivated substantial research. Commonly, researchers fit a single-pool negative exponential model to data to estimate a decomposition rate (k). We review recent decomposition research, use data simulations, and analyze real data to show that this practice has several potential pitfalls. Specifically, two common decisions regarding model form (how to model initial mass) and data transformation (log-transformed vs. untransformed data) can lead to erroneous estimates of k. Allowing initial mass to differ from its true, measured value resulted in substantial over- or underestimation of k. Log-transforming data to estimate k using linear regression led to inaccurate estimates unless errors were lognormally distributed, while nonlinear regression of untransformed data accurately estimated k regardless of error structure. Therefore, we recommend fixing initial mass at the measured value and estimating k with nonlinear regression (untransformed data) unless errors are demonstrably lognormal. If data are log-transformed for linear regression, zero values should be treated as missing data; replacing zero values with an arbitrarily small value yielded poor k estimates. These recommendations will lead to more accurate k estimates and allow cross-study comparison of k values, increasing understanding of this important ecosystem process.

Contributions to the impedance cardiogram waveform

We have developed a cylindrically symmetric model with which to study what physiologic variables might contribute to the impedance cardiogram signal. We find the major contributions in this model to be due to dilation of the aorta and carotid arteries, changes in conductivity of blood in these same vessels due to red cell reorientation during flow, changes in the conductivity of the lungs, and changes in heart volume. The calculations suggest that the popular equation used to determine stroke volume from thoracic impedance data is not accurate under all conditions of the circulatory system.

The Electrocardiogram as an Example of Electrostatics

The relationship of the electrocardiogram (EKG) to the charge distribution in the heart provides an interesting example of electrostatics for a general physics course. The qualitative features of the EKG can be explained by a simple electrostatic model which ignores the electrical conconductivity of the body. This model is developed and used to explain some EKG patterns.

Atoms and Light

This chapter describes the biological properties of infrared, visible, and ultraviolet light. Photons are emitted or absorbed when atoms or molecules change energy levels, leading to the techniques of infrared spectroscopy and Raman scattering. Scattering and absorption are described by the cross section. When scattering dominates, light is governed by the diffusion equation. Photons can be absorbed and emitted in a continuous range of frequencies, leading to thermal radiation. Blue and ultraviolet light are used for therapy, and can also be harmful to the skin and eyes. Lasers heat tissue, a process that is modeled by the bioheat equation. The chapter closes with a discussion of vision.

Resource Letter MP-2: Medical Physics

This Resource Letter provides a guide to the literature on the uses of physics for the diagnosis and treatment of disease. It does not include molecular biophysics but does include biomedical engineering.

Osmotic Pressure in the Physics Course for Students of the Life Sciences

Ideal gas models for equilibrium osmotic pressure and for nonequilibrium flow of solvent through an ideal semi-permeable membrane are presented. These models can easily be introduced in general physics courses for students of the life sciences. The models are justified in Appendices, which include a summary of the irreversible thermodynamics of flow through membranes. Some physiological examples are presented, including the Nernst equation and Gibbs-Donnan equilibrium.

THIN LITHIUM TARGETS SEALED IN NICKEL FOR LOW OXYGEN CONTAMINATION.

A technique is described for making pure lithium targets in the range of 100 μg/cm2 by encapsulation in a sputtered nickel film. The oxygen impurity is about 2%.

Physics Useful to a Medical Student.

The author recently audited all of the courses taken by first and second year medical students at the University of Minnesota. On the basis of that experience, he recommends physics topics which those students will find useful, such as the Boltzmann factor, diffusion and osmosis, Thevenin’s theorem, nerve conduction, a detailed discussion of the electrocardiogram, relaxation oscillators, feedback, Fourier series, correlation functions, and power spectra. In addition, examples are given of applications of physical principles from each of the major areas of physics.

Teaching Exponential Growth and Decay: Examples from Medicine.

A treatment of exponential growth and decay is sketched which does not require a previous course in calculus. Rate of change is introduced by considering compound interest and is determined graphically by using semilog paper. Examples of exponential growth and decay are given which will interest premedical students. These include bacterial growth, sterilization, survival in certain chronic diseases, clearance, and drug absorption.

A Simplified Treatment of the Quantum-Mechanical Scattering Problem Using Wave Packets

This paper presents a treatment of quantum-mechanical scattering suitable for a senior level course in modern physics. The entire treatment is based on the use of wave packets instead of unrealistic plane waves. The asymptotic behavior of the scattered wave packet is derived from physical considerations, and the Born approximation is then obtained for a wave packet in a manner which displays the details of the scattering process.