Concept Definition and 1000 Threads

The concept's main theorem goes as follows: Suppose that w = F(x,y) satisfies the following criteria at point A = (a,b). \begin{cases}\exists\delta > 0\colon F\ is\ continuous \forall (x,y)\in B(A,\delta)\\ \exists \frac{\partial}{\partial y}F , continuous \\ F(A) = 0, F_y \neq 0\end{cases}...

I was wondering if the fermi level lies in the centre of the forbidden energy gap on pure semiconductors, then how does it (by definition) have a probability 1/2 of being filled? Shouldnt the fact that the fermi level lies in the forbidden, forbid the fermi level from having any probability of...

I had a physics lab that is introducing Potential and Kinetic energy into are unit on projectiles. The first question was solve for the impact speed of a projectile launched horizontally at a 13m/s, this impact speed is -14.36m/s. The next question was find the impact speed of the same...

Homework Statement I understand how to solve for stationary points, and then take the second derivative and input the values of x to determine the nature of the stationary points, if x > 0 then the stationary point is a minimum and if x < 0 then the stationary point is a maximum. What I am...

So I'm no physics expert, by no means. But I'm working on a project that involves some weird physics elements, and I'd like it to be scientifically accurate; (if somewhat pseudo-scientific or liberally applied for flavor) and I'd like some feedback and help. The goal, in other words, isn't to be...

Hi there! I would like to understand the basic concepts of how mathematics is builded up from sets of elements to anything else (i.e: an equation) For example here is a formula: 2-1 So are there different sets of elements like A=(2) B=(-) C=(1) ? And maybe there is an operation with sets...

I'm looking at lecture notes on General Relativity by Sean M. Carroll, and after defining the Riemannanian tensor in the usual theorem, the extent to which the partial derivatives of a vector field fail to commute, it says ' having defined curvature tensor as something which characterizes the...

As we can not meaningfully compare a vector at 2 points acted upon by this operator , because it does not take into account the change due to the coordinate system constantly changing, I conclude that the elementary differential operator must describe a change with respect to space-time, How do...

Hi I'm currently reading up on magnetism and its relation to currents but there's one thing that frustrates me and the textbook doesn't seem to elaborate on it. I understand that a current produces its own magnetic field (and we use the right-hand rule to determine this), it also makes sense...

This is SO much more forward thinking than just "hey, we're going to build a car that can drive and park itself" http://www.businessinsider.com/r-daimlers-self-driving-concept-car-turns-into-mobile-living-room-2015-1 Very cool.

This is Newton's law of universal gravitation. $$F=G\frac{m_1.m_2}{r^2}$$ Gravitational field $$g$$ is derived from this formula $$g=G\frac{m_1}{r^2}$$ This is named gravitational "field" strength. If Newton knew nothing about "field concept" and formulated his formula in the form of "action...

Why momentum of phonons called quasimomentum? Is it due to the fact that wavenumber of vibrations confined in a first Brillouin zone, and therefore can not have an arbitrary value as "usual" momentum?

Usually when we've done free-body diagrams with cars, the direction of friction was reasonably opposite the direction of the motion. But here it's different... 1. Homework Statement All I want to know is why the direction of friction is forward. My only guesses are that the wheels are...

Could someone please explain to me the concept and theories of 'Quantum Physics', as I am quite perplexed at this topic, but simultaneously intrigued at the thought of it.

And it seems like it's so straightforward for everyone else, which is incredibly frustrating. It's about the relationship between pressure and flow rate of a fluid moving in a pipe or closed system. Often what I've heard is, they have an inverse relationship...but then there are cases where...

What is entropy ?, And how it effect the system?

Disclaimer: I'm just an amateur enthusiast who is a programmer during the day I suppose everyone who takes an interest in physics who doesn't have the mathematical tools to further their enquiries end up writing posts like mine, and that at the very least it's nice to see where the ideas reside...

Homework Statement Hi, I've been having problems visualizing and interpreting a situation where there is zero potential in a point, equidistant, between two opposite charges. What is the significance of this? Here's a sample problem:Consider two point charges. One has a charge of +1 μC and the...

I was imagining how one could design a newer, more efficient city then the cities we have right now and I came up with a fairly basic design concept that I later Googled and found I was not the first. That concept was that of a snowflake. But why design a new city? Currently modern cities are...

Could somebody disprove the following concept of a Maxwell's demon: Let's say we have a very small container, the size and a form of a nanotube, similar to ellipse, and only two gas molecules randomly flying inside. According to probability we will quite often find molecules in the different...

I was reading about mass inflation inside BH in wiki few years ago (and it did not make any sense to me for multiple reasons) Recently I tried to google this term again - with almost no hits - so this subject (mass inflation) has disappeared from wiki and from science?

I don't understand the concept for polar and non-polar semiconductors, generally speaking about propagation of existence of phonons (acustic or optics). Thanks. I don't find any thread about this concepts.

Hello, My question is, there is a concept of contour integration. Which is choosing a circular contour space sort of, and integrating along that. How do you do contour integration? Secondly, there is something going around called integrating along a square. I tried searching only, a lot...

Suppose $\int f d\mu < \infty.$ Let $$h(\omega)=\begin{cases}f(\omega) \ \ \ \text{if} \ \ f(\omega)\in \mathbb{R} \\ \\ 0 \ \ \ \text{if} \ \ \ f(\omega)=\infty\end{cases}$$ How to show $h$ is measurable and $\int f d\mu = \int h d\mu?$ **Attempt:** It is known that the product of two...

Is this mathematical concept EVER used in real life or "higher" levels of math? I just find it to be a practically useless thing. It's like some guys sat around and invented this math concept just for the sake of it. Or, am I wrong?

Homework Statement The 87 kg painter in the figure below is using a large stepladder. The base of the ladder is 3 m wide, and the ladder is 7 m high. The painter is on a step that is 2.0 m above the floor. The horizontal bar of the ladder has a mass of 12 kg, and the entire ladder has a mass of...

I'm having some trouble reconciling different concepts when it comes to pressure. I know that: 1) Pressure in a container or in any fluid in a closed container depends on depth by P=Patm + pgd. 2) Pressure is felt on all sides of an object. So if you have a cube in tank of water, then the...

I know that there exists a force of gravity between person A and person B that are action reaction pairs. My question is, is this gravitational field inherent in matter(i think higgs boson) look like an electrostatic field between matter? I ask this because, it seems that the moon and the...

Hi everyone. I ran into a problem while attempting my Fourier Series tutorial. I don't really understand the "L" in the general formula for a Fourier Series (integration form). I shall post my question and doubts as images. Thank you for any assistance rendered. <I am solving Q3 in the image.>

I am kind of curious what topics to read to understand this concept more. Suppose I want to find a function f: A \rightarrow B, where if you look at the pre-image of any point b \in B, the size of the pre-image of b will be quite large. Essentially, I want to find functions that map from very...

I am reading An Introduction to Rings and Modules With K-Theory in View by A.J. Berrick and M.E. Keating (B&K). In Chapter2: Direct Sums and Short Exact Sequences in Sections 2.1.14 and 2.1.15 B&K deal with ordered index sets in the context of direct sums and products of modules. Although it...

Ut+6UUx+Uxxx=0 [kdv eq] Why to solve this do you need U(x,t=0)? Why is it a initial value problem? This should probably be really obvious. I think I've forgotten some basic background stuff, just starting my course in solitons... Thanks for your help.

Hi, I have newly encountered with a subject called LaTeX. I try to learn it. I know it is a document preparation program. Can I install on my PC and start to use it like Matlab? For example where can I use this code? http://en.wikipedia.org/wiki/AMS-LaTeX Thank you.

Homework Statement Total enthalpy is defined such that: h+(V^2)/2 = constant ... (1) h0 is the stagnation enthalpy. Stagnation means V=0. Plug this V into the above, get h=h0. Hence h+(V^2)/2 = h0 ... (2) My question is we already defined V=0, so how could there still be a V^2/2 term in...

What do you all have to say about the concept of multiple universe?

Hello. I am an undergraduate physics major and am struggling to identify a senior project research question that I can pursue over the next 2 semesters. I already opted to do my project on heliophysics, but that is obviously a rather broad subject. While doing general reading, I came up with...

I was working on a problem out of Griffiths, and have become a bit confused. The problem is regarding to Earnshaw's theorem, which states that a particle cannot be held in stable equilibrium by electrostatic forces. (3.2 for anyone with the text). He suggests a cube with a positive charge on...

So I've been trying to understand a concept in gyroscopes and I hope the guys and girls here can help me with it. I first read it here, the part about mounting 2 gyroscopes with their axes at right angles to each other. It said that the platform will remain totally rigid. I tried to...

How does an inductor works? I know that it stores energy in form of magnectic fields, and all the effects it does to a circuit, but i don't know how it really, physically works. The electrons go through the coils and induce a magnectic field. Why it is stored? Why doesn't the magnectic field...

Immediately after two separated charged particles are released from rest, both increase in speed. What happens to the charge of the particles?

So what actually is the cause of electrical Resistance?

A spacecraft on its way from Earth to the Moon is pulled equally by Earth and Moon when it is closer to the Earth's surface or closer to moon's surface. How to calculate the effects of moon and Earth on spacecraft ?

The mutual induction of magnetic and electric fields can produce light. Can anyone explain this concept in detail? Hoe does this happens?

So this is my first post to PF, but I've been a long time reader. I graduated with a civil engineering degree a year ago and since just before graduation, I've had an engineering concept that has stayed in my mind very persistently for the entire time. I will warn you ahead of time; this will...

I'm having trouble getting my head around entropy. In an isolated system, entropy can only remain the same or increase. Is this the same for a thermodynamics cycle? What I mean is, if I drew a cycle on a PV diagram, would the entropy keep increasing? I can't see how that would work, that would...

OK so let me apologies if I don't seem to fully understand the concept of free fall and am making a far fetched connection. I've been recently reviewing some of my old course material such as vector calculations, Newtonian mechanics,etc, and while reading up on basic vector calculation the...

The definition of species is "a group of organisms that can interbreed with each other". So, how is interbreeding of two dogs of different species, say a german shepherd and a pitbull, possible? And the concept of a "pure" and "impure" breed - how will you explain it? As according to the...

Let A(i) be the event that the first ith person have different birthdays for i=1,2,3...,n. We note that A(i+1) is a subset of Ai such that Ai(A(i+1))= A(i+1) I wonder why A(i+1) is a subset of Ai. If the first 3 people have no birthdays in common, shouldn't that also mean the first 2 people...

1. Hey everyone! I am currently studying for my final exam and I am trying to review work to start out. When I was reading, it gave me an example of where a person is holding a textbook and the person moves the textbook in the horizontal direction. Under the picture they gave an explanation...

Homework Statement Question 1) a mass m=200g or an unknown metal is heated to a temperature T1=200°C and then dropped into m=50g of water at a temperature T2=20°C in an insulated container. The water temperature rises within a few seconds to T3=39.7°C and then changes no further. Specific heat...