Confusion Definition and 1000 Threads

For the Gordon identity $$2m \bar{u}_{s'}(\textbf{p}')\gamma^{\mu}u_{s}(\textbf{p}) = \bar{u}_{s'}(\textbf{p}')[(p'+p)^{\mu} -2iS^{\mu\nu} (p'-p)_{\nu}]u_{s}(\textbf{p}) $$ If I plug in $\mu$=5, what exactly does the corresponding $(p'+p)^{5}$ represent? 4 vectors can only have 4 components so...

Perhaps not an appropriate place to ask this. I've completed a question which is phrased as the following: Explain, on thermodynamical grounds, why the minimisation of the energy E(S, V, N) yields the equilibrium state of a system with fixed entropy S, volume V and number of particles N. I...

As a layman, I have come to understand basic expansion (meaning without dark energy) as objects which are not locally gravitationally bound are moving away from each other due to some inertia created from an initial period of inflation. This rate of recession is the Hubble constant and it...

Since the particles aren’t rotating, but just traveling in a straight line, there should be no angular momentum whatsoever. Since the magnetic moment of magnetic dipoles can be theoretically represented as coming from a loop of electric current, or particle orbiting, thus magnetic moments are...

When we deal with an infinite-dimensional basis,the normalization condition of this basis becomes <x|x'>=δ(x-x')(here for example the position basis).Same thing for momentum eigenstates <p|p'>=δ(p-p'). Lets look now on the eigenvalue problem of the momentum operator: \hat p | p \rangle =p |...

In the derivation of the Clausius inequality, T is the temperature of the reservoir at that point in the cycle, but in the definition of entropy it becomes the temperature of the system. This seems to work for a Carnot cycle, where the two are the same, but for other processes, such as an object...

Suppose that I have a set of axioms in first-order logic. And suppose that I have several inequivalent models for this set of axioms. And suppose that I want to choose one specific model. To choose it, I need to make some additional claims which specify my model uniquely. My question is the...

Homework Statement I'm reading the book Relativity, Gravitation, Cosmology by Ta-Pei Cheng. I'm in the part where he derived the gravitational time dilation formula for static gravitational field, τ1=[1+(Φ1-Φ2)/c2]τ2. This implies that clocks at a higher gravitational potential will run...

I know several math formulas, like which I will write below. \int_{x_0}^{x_1} f(x) dx \frac{\int_{x_0}^{x_1} f(x) dx}{x_1-x_0} \frac{\int_{x_0}^{x_1} f(x) dx}{2} f(x_1) - f(x_0) \frac{f(x_1) - f(x_0)}{x_1-x_0} \frac{f(x_1) - f(x_0)}{2} \frac{f(x_1) + f(x_0)}{2} And I know too that...

I've been reading a bit about the very intriguing summation \displaystyle \sum_{n=0}^{\infty} {n} and it seems \frac{-1}{12} is the result but apparently with a lot of subtleties and caveats. It is those that I am trying to understand now. At first reading it appeared totally incongruous to...

Dipole moment is the measure of strength of electric dipole and is equal to product of either charge and distance between two charges. P=ql but as we know when charges are close the force b/w them is strong n when they are far away force decrease. But in above equation strength increases with...

While I am reviewing my undergrad physics for qualifying exam, I became confused about the numerous selection rules. (1) We have the selection rules for spontaneous emission in a hydrogen atom: Δl=±1 ,Δml=0,±1. This rule is the easiest to understand by evaluating <n'l'm'|z|nlm> (2) We have the...

I'm just trying to build Doppler shift equation for Moving observer and stationary source. I have an problem in a step. I'm attaching a screen-shot please check it and tell me where to this step ( 1 - v/u)^-1 comes from ? Thanks in advance.

Hi, I have a revision book that has an example in it for the tension on a rope if a 20 Newton force is hanging on the centre of a rope suspended between two points. The rope dips 25 Degrees either side. So effectively an upside down pair of right angled triangles is formed. The Hypoteneuse...

When designing the fuel plumbing for an efi system, do pressure drops in the fuel lines need to be considered when there is a pressure regulator? If so why? Been reading up a lot on plumbing design lately about how the goal is to minimize restrictions in the lines which ultimately cause...

I don't see how the Copenhagen interpretation and the second law of thermodynamics can be compatible. In the Copenhagen interpretation, upon losing coherence the system chooses a single definite state and all other possible states are eradicated. This seems to be losing entropy to me, as it's...

Suppose I have electric field of the form ##\mathbf{E} = 3x\mathbf{i} + 3y\mathbf{j}##. Calculating the charge density gives me ##\rho = \epsilon_0 \nabla\cdot\mathbf{E} = 6\epsilon_0##. But now if I turn one of the components of the field in the opposite direction, for example ##\mathbf{E} =...

Homework Statement A rotational axis is directed perpendicular to the plane of a square and is located as shown in the drawing. Two forces, F1 and F2, are applied to diagonally opposite corners, and act along the sides of the square, first as shown in part a and then as shown in part b of the...

Homework Statement A 500-N person stands 2.5 m from a wall against which a horizontal beam is attached. The beam is 6 m long and weighs 200 N (see diagram below). A cable attached to the free end of the beam makes an angle of 45 degrees to the horizontal and is attached to the wall. a) draw a...

My Question is very interesting. I want an exact answer after correct assessments. Let there is a box or rocket or something in which light starts from point A to point B. When Light is generated from point A, at the exact point that thing box or rocket or frame moves from point X and it moves...

I have found a book from my father epoch of undergraduate student in Physics. It was apperently Physics 101, and was written with a typewriter and the formulae with pen. I have gone straight to Special Relativity section and, to my dismay, and even if all formulae were indeed correct, the...

The Cauchy Ratio test says: If $ \lim_{{n}\to{\infty}}\frac{a_{n+1}}{a_n} < 1 $ then the series converges. OK. Now I read that for a power series (of functions of x), the same test also provides the interval of convergence, i.e. If the series converges, then $...

Homework Statement Consider the following in cylindrical coordinates \rho,\varphi,z. An electric current flows in an infinitely long straight cylindrical wire with the radius R. The magnetic field \mathbf{B} outside of the thread is...

A blackbody is a theoretical object that perfectly absorbs all the light that falls on it. From what I understand this is an ideal situation and does not actually exist in reality. Certain objects are close to being a blackbody but they do not absorb 100% of the light that hits it (i.e. some...

Hi all, According to quantum mechanics, the graviton is the measure of the smallest amount of curvature possible in space-time. I read that the mass which would be required to create this curvature is Planck Mass which is close to the value 10^-5g. But the elementary particles, like electrons...

Hey all! I have a silly doubt In acids and bases As you guys know The concentration of H+ ions For a weak acid Which does not undergo Complete dissociation Is given by mod H+=square root of Ka times C Where C is the initial concentration Of the acid And k is the ionization constant In the same...

Background: I have limited knowledge on QM and only know that magnetic fields split the energy levels of an atom It seems counter-intuitive to me that as the atoms slow down along the tube the magnetic field strength decreases http://es1.ph.man.ac.uk/AJM2/Atomtrapping/Zeeman-B-field.jpg as...

Hello everyone, I'm talking about retaining urine in the bladder,not renal failure, so I can't think of uraemia as a course. So what is the reason behind this. Thanks :)

For question about the falling object we will always take T=mg+ma so we take mg and ma positive for question about the object rolling down the inclined plane we will say that mg sin theta - ma=0 although mg sin theta and ma is still on the same direction, we take mg sin theta positive but we...

Mentor note: moved the thread to the homework forum So I was given a few problems to take home, to complete by the professor. My question is not regarding how to solve the problems but rather, if I understand the instructions. We are given the following instructions. Find the general...

According to equivalence principle, gravity can be treated like acceleration "locally". Based on this principle we can treat a non-inertial frame at rest and explain the fictitious forces (of Newton's Laws) as gravity. From this we can prove that time elapses at different rates at different...

Homework Statement Suppose that 70% of the statisticians are shy, whereas 30% of the economist are shy. Suppose also that 80% of the people at a large gathering are economists and the remaining 20% are statisticians. If you randomly meet a person at the gathering and the person is shy, what is...

Hello, I am a first year science teacher doing my best with teaching physics for the first time (my degree is chemistry but I am in a very small school). I am teaching projectile motion. I was creating a worksheet and trying to solve a problem I made up when I realized something wasn't working...

Hello PF. Sometime ago I posted a question about the light clock train thought experiment. I didn't understand the answer, so if I may I'll ask it again. In the light clock on the train thought experiment, the clock is calibrated to each time the light strikes the mirrors, which are 1 metre...

I'm new to bra-ket notation and am slightly confused; given an infinite square well with eigenvectors: \phi = \sqrt{2/a}\sin( (n\pi x)/a) And we assume the form: H |φ> = E_n |φ> How would you then represent φ in terms of a column matrix, because that what I thought |φ> represents. Given...

Consider a system of 2 identical fermions. $$\psi_{k_1,k_2}(x_1,x_2,m_1, m_2) = \langle x_1\,x_2\,m_1\,m_2\mid \psi \rangle$$ According to what I have read we can construct a state with the right antisymmetry properties by $$\psi_{k_1,k_2}(x_1,x_2,m_1, m_2) =...

I have been trying to understand some facts about the Laniakea supercluster. I found information at three sites which when compared with each other creates some confusion. a. http://www.ifa.hawaii.edu/info/press-releases/Laniakea/ b. https://en.wikipedia.org/wiki/Laniakea_Supercluster c...

I looked in forum and i couldn't find a useful thread (useful for me). I am currently using C# and PHP and I am going to learn C and java for university soon For general uses i mostly use windows (Photoshop , ...) . I currently have ubuntu on my laptop (wanted to install kali too). I want to...

I am trying to understand the concept of interatomic spacing. Below is a link to three pictures, and my questions is: why are they different? I thought atoms were packed together touching each other like in the first picture but the second and third imply otherwise. Any explanation or help is...

Homework Statement I have a question that involves comparing the energy output of a laser to the energy required to make tea. I've already calculated the energy to make tea and I know what to do to find how long it takes to make the tea but I'm struggling with the energy output part. It says...

I have a few general questions about how wave packets relate to particle creation and motion. 1) In the following video, Steve presents us with a simple case of a free particle at rest, where we have zero momentum and thus know the momentum exactly, but as a consequence we have no idea what...

I've come across two different approaches to quantifying what l is in the equation for hydraulic gradient Δh/L. In this first picture L is the parallel distance along the datum across the reference plane But in this second picture L is the length along the pipe Why are the two L's...

Homework Statement Consider a point charge Q placed at point O.Consider any point p in the field of the Q.Charge q is placed at point p.Charge q is then moved from point p to s the distance i=between p and s is dr.Calculate work done. y Homework Equations ##dW=##-##F^→.dr^→## The Attempt at a...

Homework Statement Look I have an exercise about trusses in vector mechanics. It's the following: https://scontent-lax3-1.xx.fbcdn.net/hphotos-xfp1/v/t1.0-9/12003339_1645511085733429_8210043018491702007_n.jpg?oh=4955534403fb6f3f5f9a40c9c0b8ecbc&oe=569761ED I have to determine the forces in...

The text says: "For the values shown, Vin < +10 volts puts the op-amp output at positive saturation, and VOut= Vin, When Vin exceeds +10 volts the diode closes the feedback loop, clamping the output at 10 volts. " My inference: When Vin<+10V the opamp increases its output in positive direction...

Homework Statement "Julie drives 100 mi to her Grandmother's house. On the way, she drives half the distance at 40 mph and half the distance at 60 mph. On her return trip, she drives half the time at 40 mph and half the time at 60 mph." Homework Equations a. "What is Julies average speed on...

I am confused as to what a quasi particle is. I know that it only occurs when a microscopic system acts in an unorthodox way, but are they truly a particle? Are they fundamental? Have they been proven? If the answer to all of these questions is yes, why are they excluded from the standard model?

Principle of least action states that the particle choose the path which optimizes its action.Where the action is defined by S=t1∫t2(L)dt and L is the Lagrangian of the system.This leads to δS=0 and it is a condition to optimize S. I will summarize what confuses me about this principle in a...

rude man submitted a new PF Insights post Modulation vs. Beating Confusion Continue reading the Original PF Insights Post.

I know that when we have to identify the nodes for solving a circuit with nodal analysis we always have to designate a reference node (which will be the ground node) as a reference for measuring the others' voltages. However I was practicing this and I found this example on...