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A flame war about entropy
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| Pengarang | : | Bruce D. Popp |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 6) |
| Halaman | : | 487-491 |
| Abstrak | : | Following an address by James Swinburne that was published in an extended version in The Electrician in December 1902, John Perry wrote letters to the editor published in two journals in which he objected to an inequality appearing in the definition of entropy; he demanded an equality. He also took the comments as a personal affront, since they could be seen as condemning engineering books he wrote. That began a dispute carried out in British engineering journals about the definition and meaning of entropy in thermodynamics. The extended dispute grew beyond the question of equality vs inequality and exposed uncertainty and confusion among engineers about understanding and using entropy. Several prominent physicists provided contributions either at the request of editors seeking to calm the dispute or on their own initiative. Lord Kelvin provided the terse reply: Clausius had the right definition with an inequality. Oliver Lodge provided a guide for perplexed engineers. Max Planck compared nature to a trader demanding payment in entropy. Poincaré offered a notable and distinctive definition of entropy in terms of changes of state of a system that can only proceed in one direction. |
Daniel Davis, Jr., and his Manual of Magnetism
| Pengarang | : | Thomas B. Greenslade, Jr. |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 6) |
| Halaman | : | 477-486 |
| Abstrak | : | In the preface to his 1842 Manual of Magnetism, the Boston scientist and manufacturer of electrical apparatus, Daniel Davis, Jr., wrote, “Magnetism and Electricity have become related sciences within such a short a period, and their growth has been so rapid, that many important facts which have been observed have not been collected in any scientific treatise, and the amount of unwritten knowledge has been constantly increasing.” His book served as a companion to the apparatus that he manufactured, and it supplied the scientific background to experiments using these instruments. This paper traces the development of some of these instruments and explains their operation, showcasing their ingenuity and inspiring demonstrations for contemporary physics classes. A great deal of his apparatus still exists in museums and the storage closets of schools, college, and universities. I urge you to seek it out and husband it. |
Electromagnetic field momentum in theoretical magnetic monopole-like models
| Pengarang | : | Isaiah Ruz; Douglas Singleton |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 6) |
| Halaman | : | 467-476 |
| Abstrak | : | We show that some standard models of magnetic charge—the Banderet potential, the Dirac string potential, and the Wu–Yang fiber bundle approach—carry an electromagnetic field momentum in the presence of an electrostatic field. This electromagnetic field momentum suggests a violation of the center-of-energy theorem. We show that each of these monopole models also carries a hidden mechanical momentum, which saves the center-of-energy theorem. |
Designing objects that are invisible to electromagnetic waves
| Pengarang | : | Johan Helsing; Shidong Jiang; Anders Karlsson |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 6) |
| Halaman | : | 459-466 |
| Abstrak | : | This article shows that it is, in principle, possible to make a dielectric rod completely invisible to an incident electromagnetic plane wave of a given frequency. Students can derive the conditions that make the rod invisible if they understand the concept of plane waves, the boundary conditions for electric and magnetic fields, and the complex representation of electromagnetic fields. With access to appropriate software, students can determine the bandwidth of the invisibility and investigate whether it is possible to make an invisible rod out of real-world materials. A more advanced project proposed is to use electromagnetic software to find perfectly conducting hollow structures that are invisible to an incident plane wave. |
Enhancing conceptual understanding of Gausss law through asymmetric infinite sheet slab charge examples
| Pengarang | : | Simarjeet S. Saini; Reza Kohandani |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 6) |
| Halaman | : | 454-458 |
| Abstrak | : | Infinite sheets/slabs are one of three geometries, for which the electric field can be calculated using Gauss's law. Frequently, only symmetrical charge distributions are considered, preventing practical applications including p–n junctions and multiple conducting sheets from being introduced in an introductory electromagnetic course. Students also struggle in understanding the requirements for applying Gauss law for calculating electric fields. A common question that confuses students is whether the calculated electric field in a problem corresponds to the enclosed charge within the Gaussian surface or all charge in the distribution. We show that the electric field calculations for infinite sheet/slab charge distributions using the integral form of Gauss's law can be extended in a classroom discussion to asymmetric charges without using superposition, provided the charge is uniform in the two infinite dimensions. These discussions enrich the application of Gauss's law in practical devices and help students develop improved understanding of the law. |
Understanding the oscillatory motion of Saxon bowls: An experimental and theoretical approach
| Pengarang | : | Hui Xiang Sim; Bernard Ricardo |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 6) |
| Halaman | : | 447-453 |
| Abstrak | : | The Saxon bowl, a bowl with a hole at its base, has been used by ancient people as a timing device. Although the apparatus used is simple, the device is surprisingly accurate and robust to small deviations in initial conditions. This paper investigates the effect of oscillation on the submersion time of a Saxon bowl with factors such as the initial release conditions and drag coefficient. The results of numerical simulations are consistent with the robustness of this timing device. Additionally, an analytical model is presented that is reasonably consistent with the experimentally observed dependence of the sinking time on the hole radius and bowl mass. |
The right way to introduce complex numbers in damped harmonic oscillators
| Pengarang | : | Jason Tran; Leanne Doughty; James K. Freericks |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 5) |
| Halaman | : | 437-440 |
| Abstrak | : | The introduction of complex numbers to solve the damped harmonic oscillator in classical mechanics can seem mysterious to students. We show how an approach based on Born and Jordan's 1930 quantum mechanics textbook both demystifies the use of complex numbers in classical mechanics and makes a strong connection to quantum mechanics. In that work, they converted the harmonic oscillator equations of motion into two uncoupled first-order equations. In classical mechanics, this mapping explicitly introduces complex numbers into the motion of the oscillator and directly shows how to solve for the position and momentum observables. Here, we explain how this mapping works and show how to demystify the use of complex numbers in damped-driven harmonic oscillators. As an added bonus, this approach also shows how to determine the energy as a function of time and makes a strong connection to quantum mechanics. |
Natural relationship between classical orbits and quantum Hamiltonians for the Kepler problem
| Pengarang | : | Jason Tran; Leanne Doughty; James K. Freericks |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 5) |
| Halaman | : | 434-436 |
| Abstrak | : | Central force problems involve coupled linear differential equations of motion. By decoupling the differential equations, we show how to efficiently solve for the orbits of the Kepler problem without any complicated integrals. Introducing quantization provides a deep connection between the solution for these classical orbits and the quantum Hamiltonian of hydrogen for states that have definite total angular momentum. |
Low-cost optical heterodyne detection including interference between two independent He–Ne lasers
| Pengarang | : | Michael Daam; Antje Bergmann; Carsten Rockstuhl; Ronny Nawrodt |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 5) |
| Halaman | : | 430-433 |
| Abstrak | : | Several publications describe experiments for university laboratory courses using and demonstrating optical heterodyning, and these experiments are included in many advanced instructional laboratory courses. A key aspect of these experiments is the measurement of beat notes using fast photodetectors and rf spectrum analyzers for frequencies up to at least 1 GHz. These spectrum analyzers alone typically cost on the order of US $1000 or more. Here, we show that beat notes can be measured with a photodiode and a spectrum analyzer based on a software defined radio for a total of about US $50. The spectrum analyzer presented also provides a waterfall plot to easily follow the time evolution of the spectrum, and FM demodulation to enable FM signal transmission with an open-cavity He–Ne laser. |
From light emission to solar power: Experiment on LED's photovoltaic behavior
| Pengarang | : | Tiago V. Fernandes; Luís P. Gonçalves; Rodrigo Rito |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 93 (No. 5) |
| Halaman | : | 422-429 |
| Abstrak | : | An experimental study to investigate the fundamental similarities between light-emitting diodes (LEDs) and solar cells (SCs) for educational purposes is here presented. Through a hands-on experiment using inexpensive, commercially available LEDs and simple laboratory equipment, students are introduced to characterization techniques, such as electroluminescence (EL) and current–voltage (I–V) measurements, as well as to optoelectronic devices, photovoltaic physics, spectroscopy, light-to-matter interactions, and solid-state physics concepts. Four LEDs of different colors—red, amber, green, and blue—are used in this study. The measured EL spectra allowed an estimation of the bandgap energy of the material constituting each LED's optical active layer, establishing a relationship with the LED color. Photovoltaic mode I–V curves are measured under outdoor sunlight, demonstrating that all LEDs exhibited photovoltaic behavior, generating electrical energy from incident light. The red LED produces sufficient electrical energy to power a small calculator, while the green LED exhibited the poorest photovoltaic performance. Shockley–Queisser (SQ) theory is employed to comparatively investigate the “photovoltaic” quantum efficiency (QE) of the LEDs, enabling students to relate the results to the well-known green gap problem in LED “emission” QE. This work effectively illustrates the similarities between LEDs and SCs, bridging the gap between theoretical knowledge and practical application, providing students with a valuable learning experience in solid-state physics, semiconductor-related topics, and photovoltaic technology. |