电磁力及其对MIG焊接熔池流场的影响

MIG焊接熔池具有不规则的表面边界,温度较高且分布极不均匀,在电磁力、浮力、表面张力等的作用下发生剧烈运动.基于熔池表面变形较大时电弧电流密度的双峰分布模型,建立了电磁力的计算模型.采用数值模拟技术研究了熔池中的流体力学行为,揭示了焊接工艺参数对熔池流场的影响规律.实验表明,计算与实测结果符合良好.     

Are Field Derivatives Needed in Linear Constitutive Relations?

General linear constitutive relations need not contain any dependence on field derivatives, and the standard boundary conditions need not be modified. We show that the answer to the question asked as the title of this communication is therefore in the negative.     

TheB (3) field as a link between gravitation and electromagnetism in the vacuum

The emergence of theB ⁽³⁾ field in vacuo has shown that electromagnetism is non-Abelian and similar in structure to gravitation. In this paper the Christoffel symbol used in general relativity is developed for electromagnetism in curvilinear coordinates: The former becomes describable as the antisymmetric part of the gravitational Ricci tensor. Therefore gravitation and electromagnetism are respectively the symmetric and antisymmetric parts of thesame Ricci tensor within a proportionality factor. Both fields are obtained from the Riemann curvature tensor, both are expressions of curvature in spacetime.     

Is evans' longitudinal ghost fieldB (3) unknowable?

M.W. Evans has hypothesized every photon to have a longitudinal "ghost" magnetic field and commented extensively on this field, most recently in this journal. It is pointed out that Evans' field—being time-independent and spatially uniform—is unknowable, and thus lies outside the pale of physics.     

Learning Electricity with NIELS: Thinking with Electrons and Thinking in Levels

Electricity is regarded as one of the most challenging topics for students of all ages. Several researchers have suggested that na?ve misconceptions about electricity stem from a deep incommensurability (Slotta and Chi 2006; Chi 2005) or incompatibility (Chi et al. 1994) between na?ve and expert knowledge structures. In this paper we argue that adopting an emergent levels-based perspective as proposed by Wilensky and Resnick (1999), allows us to reconceive commonly noted misconceptions in electricity as behavioral evidences of “slippage between levels,” i.e., these misconceptions appear when otherwise productive knowledge elements are sometimes activated inappropriately due to certain macro-level phenomenological cues only. We then introduce NIELS (NetLogo Investigations In Electromagnetism), a curriculum of emergent multi-agent-based computational models. NIELS models represent phenomena such as electric current and resistance as emergent from simple, body-syntonic interactions between electrons and other charges in a circuit. We discuss results from a pilot implementation of NIELS in an undergraduate physics course, that highlight the ability of an emergent levels-based approach to provide students with a deep, expert-like understanding of the relevant phenomena by bootstrapping, rather than discarding their existing repertoire of intuitive knowledge.

The photomagneton B(3) and longitudinal ghost field B(3) of electromagnetism

The concepts are introduced of the longitudinal ghost fieldB ⁽³⁾ and photomagnetonB ⁽³⁾) of electromagnetism:B ⁽³⁾ = B⁽³⁾ =B⁽⁰⁾/, whereB ⁽⁰⁾ is the magnetic flux density amplitude and the angular momentum operator of a photon beam. The major implication is that the individual photon hasthree degrees of polarization, the longitudinal one being accompanied by the ghost fieldB ⁽³⁾ which has no energy or linear momentum, and is generated from the angular momentum of the photon.     

电磁学课程思政的探索与实践

为了落实"立德树人"的根本任务,将知识传授、能力培养、价值引领融为一体,在电磁学课程教学中开展了课程思政的探索.首先研究了基于课程思政理念的电磁学教学大纲与教学目标的修订、电磁学课程思政类型、准确提炼课程思政映射点的方法.其次探索了电磁学课程思政的几种方法和途径、电磁学教学内容及其课程思政映射点.实践表明电磁学课程思政...     

Quasi-topological electromagnetism:Dark energy,dyonic black holes,stable photon spheres and hidden electromagnetic duality

We introduce the quasi-topological electromagnetism which is defined to be the squared norm of the topological 4-form F ∧ F. A salient property is that its energy-momentum tensor is of the isotropic perfect fluid with the pressure being precisely the opposite to its energy density. It can thus provide a model for dark energy. We study its application in both black hole physics and cosmology.The quasi-topological term has no effect on the purely electric or magnetic Reissner-Nordstr o¨m black holes, the dyonic solution is however completely modified. We find that the dyonic black holes can have four real horizons. For suitable parameters, the black hole can admit as many as three photon spheres, with one being stable. Another intriguing property is that although the quasitopological term breaks the electromagnetic duality, the symmetry emerges in the on-shell action in the Wheeler-De Witt patch. In cosmology, we demonstrate that the quasi-topological term alone is equivalent to a cosmological constant, but the model provides a mechanism for the dark energy to couple with other types of matter. We present a concrete example of the quasi-topological electromagnetism coupled to a scalar field that admits the standard FLRW cosmological solutions.     

On linear electromagnetic constitutive laws that define almost‐complex structures

It is shown that not all linear electromagnetic constitutive laws will define almost‐complex structures on the bundle of 2‐forms on the spacetime manifold when composed with the Poincaré duality isomorphism, but only a restricted class of them that includes linear spatially isotropic and some bi‐isotropic constitutive laws. Although this result does not trivialize the formulation of the basic equations of pre‐metric electromagnetism, it does affect their reduction to metric electromagnetism by its effect on the types of media that are reducible, and possibly its effect on the way that such media support the propagation of electromagnetic waves.     

Wesson’s Induced Matter Theory with a Weylian Bulk

The foundations of Wesson’s induced matter theory are analyzed. It is shown that the empty—without matter—5-dimensional bulk must be regarded as a Weylian space rather than as a Riemannian one. Revising the geometry of the bulk, we have assumed that a Weylian connection vector and a gauge function exist in addition to the metric tensor. The framework of a Weyl–Dirac version of Wesson’s theory is elaborated and discussed. In the 4-dimensional hypersurface (brane), one obtains equations describing both fields, the gravitational and the electromagnetic. The result is a geometrically based unified theory of gravitation and electromagnetism with mass and current induced by the bulk. In special cases on obtains on the brane the equations of Einstein–Maxwell, or these of the original induced matter theory.