electric field and potential of an infinite flat surface charge
another derivation question from griffith's intro to electrodynamics. this is the equation for which i'm trying to follow the derivation for: $$\frac{∂V}{∂n}=\vec{\nabla}V\cdot \hat{n}$$ electric field...
View ArticleLP Modes
Hello experts! We know that each LP mode is derived from the other modes. For example, [itex]LP_{01}[/itex] mode is derived from the [itex]HE_{11}[/itex] mode. Right? But what are other modes for...
View ArticleA question on Electromagnetic theory
Hi, In EM theory to derive the 'wave equation for electric field in a conducting medium' we make ρfree to be zero, but we still use σ to be nonzero. My question is why we are doing this ? σ denotes the...
View ArticleCircular Waveguide equation help
Hello experts! I am looking for the proof of the following equation:...
View ArticleHow prove angular momentum is conserved from the Lagrangian.
Hey, It's a simple question (hope so). How do you know (analitically) wether angular momentum is conserved based solely on the Lagrangian? Let me elaborate, for example to prove that the linear...
View ArticleResistance of a discharging capacitor
I am trying to calculate the maximum current drawn from a capacitor that is connected to a resistor of a very low value (a few mΩ). Because the resistor does not have a large resistance as I've seen...
View ArticleWhy doesn't light cancel itself?
In a beam of unpolarised light, there are thousands of light waves of all polarisations. At a given point along the beam, each of these elements represents an electric and/or magnetic field in some...
View Articlecorrelation time of Brownian rotational diffusion
Hi everyone, I have a tough question. I could not solve it for a long time. could someone help me? Thanks! Question: If I know the orientation (θx, θy and θz) of an object over time, how may I...
View ArticleCoils that compress with an inclination.
Hi, I have an intersting problem to solve. Imagine a Sprint/Coil with Stiffness K. The same has been kept upright on a flat surface. Upon this coil, is hooked a wooden ledge of mas m. Due, to its...
View ArticleCalculating actual power (wattage) created by a simple generator
Hi All, I am thinking about building a simple electrical generator, and I am trying to make sure I clearly understand the theory and expected results before starting the project. I have two basic...
View ArticleDeriving motion of a displaced piston on a gas cylinder
First, I'm a second year high school physics student so my thinking may be half-baked. I was wondering what motion a piston would exhibit if it was the frictionless lid of an ideal gas cylinder with a...
View ArticleWork done on a rigid body in a collision
Sorry if this question has already been considered in this Forum (I'm sure it is but I can't find it now). A rigid ball slide without friction in an horizontal plane and collide elastically with a...
View ArticleUniqueness given specified surface charges and voltages
Suppose we have a collection of conductors for which the voltage is specified on some conductors and the surface charge is specified on others. Is there a coherent way to specify this as a boundary...
View ArticleMagnetic Field Lines around a Magnet
Hi, please could someone explain why the magnetic field is stronger at the poles of a magnet. I know that the field line density is strongest at the poles and more spaced out in the middle. I get that....
View ArticleCalorimetry of a non-isolated system
I had a bottle half full of ice and filled it with water and placed it in my fridge. I noticed that the ice was taking a significant time to melt. In fact after half a day it looked like it hadn’t...
View Article'Constant' vector field is equivalent to some scalar field
To every scalar field s(x,y) there corresponds a 'constant' vector field x = A s(x,y) and y = B s(x,y), where A,B are direction cosines. The vector field is only partially constant since only the...
View ArticleSemi-Classical/Classical derivation of ideal gas partition function
In the semi-classical treatment of the ideal gas, we write the partition function for the system as $$Z = \frac{Z(1)^N}{N!}$$ where ##Z(1)## is the single particle partition function and ##N## is the...
View ArticleWant to understand ke formula without having to accept any assumptions
To prepare for a future physics course, I googled Newtons Laws and then found about the Work Energy Theorem. I can see how Newtons Laws makes sense but I dont get why the kinetic energy formula is...
View ArticleFirst variation of convolution of two nonlinear functions
A new variational principle is presented in this paper: http://arxiv.org/ftp/arxiv/papers/1112/1112.2286.pdf When trying to derive something like the equation of motion of a Duffing oscillator, I take...
View ArticleDirection of Frictional Force
When a body is held or rests in equilibrium in contact with a surface (e.g. a slope, the edge of a block etc.) how do we work out which direction the friction acts in? I'm used to the friction acting...
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