Teaching Faraday’s law of electromagnetic induction in introductory physics courses is challenging. We discuss some inaccuracies in describing a moving conductor in the context of electromagnetic induction. Among them is the use of the ambiguous term “area change” and the unclear relation between Faraday’s law and Maxwell’s equation for the electric field circulation. We advocate the use of an expression for Faraday’s law that shows explicitly the contribution of the time variation of the magnetic field and the action of the Lorentz force, which are usually taught separately. This expression may help students’ understanding of Faraday’s law and lead to improved problem solving skills.

An approach emphasizing the complementarity of electric and magnetic fields within a weak relativistic approximation is suggested as a basis for presenting electromagnetism in an introductory university physics course. Within the electromagnetic unification approach, ''magnetic force'' and ''electromagnetic induction'' are each taught in a manner consistent with mechanics from a qualitative relativistic point of view. The Lorentz force and the magnetic flux rule are treated similarly, linking electrical and magnetic phenomena and improving the integrity and self-consistency of the course. The status of Faraday's integral law is discussed and is shown to be of limited validity in this context.

This study analyzes the operational knowledge of the weight concept of high school students after two educational levels: Introductory and advanced physics courses. The results show that most of the students at both levels construct alternative understanding of weight, which can be represented in operational schemes. The study shows that upparent and true weight concepts are poorly assimilated by most of the advanced placement students. Students' confusion is interpreted as reflecting the misfit of the weight concept imuge by students and the weight concept definition currently adopted by most of U.S. physics textbooks and identifying weight with the gravitational force. The origins of the shortcomings of the alternative knowledge about weight are listed and discussed. An alternative, operational definition of weight, separating it from the gravitational action at a distance, might be preferable in the educational context. Students' intuitive weight considerations have close historical parallels, which could elucidate the epistemological roots of their alternative knowledge. 0 1996 John Wiley & Sons, Inc. I NTRO D U CTlONLearning and teaching are related multidimensional processes of an extremely complicated nature, both of which are far from being fully explored (Niedderer & Schecker, 1992). The constructivist approach to teaching science recognizes this complexity and elaborates on students' active roles in these interactive processes (von Glaserfeld, 1989(von Glaserfeld, , 1992. Common in recently discussed learning theories is the recognition of the interaction between initial and newly acquired knowledge schemata, causing complex structural transformations of the learner's knowledge.

This paper addresses the effects of nonlinear LOG compression on the amplitude of the backscattered signals. The changes in the statistical characteristics of signals were examined and cornelated with the compression parameters. We develop an analytical formulation for the probability density function (PDF) of logcompressed amplitude signals. To obtain the theoretical F'DF, a Rayleigh distributed signal is subjected to a scaled log-compression of the form nlln(x)+n2. Such transformation is common in medical ulnasound image formation as it allows independent control over the dynamic range and gain of the displayed image. The resulting Fisher-Tippett F' DF and its statistical parameters are derived and compared with the empirically measured statistics of u limages of scattering phantoms. The comparisons reveal a great degree of similarity between theoretical PDF and the histogram of the image, even though the goodness-of-fit tests indicate statistical mismatch from theoretical model due to factors such as non-ideal log compression transfer, noise, envelope smoothing, etc..

Although textbooks usually present the conservation laws of energy and momentum only in the laboratory frame of reference, it is important that an introductory physics course show that they are valid in any inertial frame. It can be useful to apply the conservation laws in more than one frame of reference so as to highlight that they are precise only in closed systems. This is of particular note if one of the interacting partners is of near-infinite mass, whose share of the redistributed energy cannot always be neglected. In this sense, the notion of ''infinitely large mass'' is frame dependent. Balancing the energy and momentum in more than one frame of reference can help resolve some common difficulties with regard to energy conservation. We show also that the energy-work theorem holds in noninertial frames where inertial forces are treated as external. © 1997 American Association of Physics Teachers.