Confused — 400 discussions

Hey! I have classes of physical chemistry (also had physics in high school) and have some misconceptions about how quantum mechanics work. The question I want to ask is: what's the difference between luminescence (fluorescence/phosphorescence) and (not sure about terminology here) concepts of...

I'm struggling with the concept of net work. If I had a box and was pushing it up an incline would the net work be all the forces parallel to displacement in x direction * displacement in the x direction added up, plus the displacement in the y direction * force gravity parallel to displacement...

Hey everyone I'd be really grateful if anyone could help me out here, been going round in circles for two weeks now and I need to get going soon. Jumping right to the point, here's my problem: We are currently working on a student project and designing a Wind Turbine for future opensource use...

Please explain how or why (I have zero knowledge in physics).. 1. is Voltage the energy required by the elctrostatic forces between 2 points to move a certain quantity of charge between them? 2. Why is it called potential difference? Does this have to do with the difference in potential energy...

I have read that magnesium’s flame is white, and the light emitted includes all the wavelengths of visible light. Calcium, on the other hand, needs less energy to excite its valence electron, and hence, during emission, the radiation given off has a longer wavelength (red light) and its flame is...

Hello. So the last few days there have been articles about how our universe may now contain around 2 trillion galaxies, up from 100-200 billion galaxies that was previously determined. But unfortunately I can't make complete sense of the articles I've read, including the one on NASA's site...

Hello, I am teaching my self real analysis from Pugh's Real Mathematical Analysis, and I came acrosse the construction of reals through Dedekind cuts, but am confused about the following: Dedekind cuts are defined as follows ( Pugh's page 11): A cut in ##Q## is a pair of subsets ##A,B ## of...

So initially S1 and S4 are closed and current flows as seen above. After some time S1 and S4 are open and current flows as seen above. Now I am confused about the polarity of the inductor, what if I said the current flows through D1 and D4 instead of D3 and D2 ?, would that be wrong ?

Homework Statement Let ##f(x,y) = \|x \| - \|y\| - |x| - |y|## and consider the surface defined by the graph of ##z=f(x,y)##. The partial derivative of ##f## at the origin is: ##f_{x}(0,0) = lim_{h \rightarrow 0} \frac{ f(0 + h, 0) - f(0,0)}{h} = lim_{h \rightarrow 0} \frac {\|h\| -|h|}{h} =...

Homework Statement Match the parametric equations with the graphs. In this case, I am stuck on this equation: x = cos t y = sin t z = 1/(1+t^2) Homework EquationsThe Attempt at a Solution So far I have: x^2 + y^2 = cos ^2 t + sin ^2 t = 1 I know this is a circle in the xy-plane, and thus...

Homework Statement A cart that is rolling at constant velocity on a level table fires a ball straight up. When the ball comes back down, will it land in front of the launching tube, behind the launching tube, or directly in the tube? Explain. Homework EquationsThe Attempt at a Solution Why...

Hello! I am a first year student studying Physics. I chose Physics because I really like it. But now that my classes have started, Physics seems a little more demanding than what I previously had in mind. I am also keenly interested in Computer Science. So, I can't decide between the two...

Homework Statement The main question is to derive the Q of the series RLC circuit. This book shows the whole solution but I have a question about a part of their answer. Basically I want to know how ##\frac{1}{2}CV_{Cmax}^2 = \frac{1}{2}C(\frac{i_{max}^2}{wC}^2)## to me it looks like they are...

Homework Statement Find the values of m so that ##y = x^m## is a solution of ##x^2\frac{d^2y}{dx^2} - 3x\frac{dy}{dx} -12y = 0## Homework Equations ##y = x^m## ##y'=mx^{m-1}## ##y''=(m^2-m)x^{m-2}## The Attempt at a Solution So after plugging and chugging we get $$(m+2)(m-6)x^m = 0 $$...

Homework Statement two protons are 1.0m apart. find the force acting on them. q1= 1.602E-19C q2= 1.602E-19c Homework Equations i just don't get how to work it out The Attempt at a Solution ive got this far up to now [(9E+9)(1.602E-19)(1.602E-19)]/(1.0)². don't know where to go from here

Homework Statement I know this is the rule, but I find this very confusing. A negative power, surd, exponent is converted to a positive fraction power. As I said I find this very confusing as a negative is converted to a positive by the addition of a one over it. 2. Relevant equation n -2...

Hi, I'm learning the basics of EE, and I never fully understood voltage and current. So, in a very simple circuit, if we increase voltage, the rate of elétrons will increase due to the increase of the force that "moves them" right? Then in the end, voltage's only purpous is to manipulate...

Suppose two rigid circular coins are lying on top of each other. A horizontal force on the top coin is applied. So, I think there are two possible scenarios: 1. Both move together with common acceleration (if friction between them is strong) 2. Only the top coin slips over the bottom (if...

I am currently confused about anti-symmetrization of wavefunctions. In a thread "Still confused about super position and mixed state", kith responded that anti-symmetrization was not done thus the none of the bras and kets shows any properties of a possible state of a molecule I have mentioned...

Hi, I am just going through a textbook on electronics and have come across an explanation that uses potential dividers to work out potential values for resistors. I was wondering if anyone is able to explain how the author got the bit after "which can be written as": R1=(4.9/10.1)*R2 How did...

I am confused about pure state or in mixed states. I've seen several threads on this forum, but I still can't get the grasp of it. I only have very little quantum chemistry to know what these means. So instead, I want to know the answer for specific examples so that I can get an idea. So I...

I get easily confused when asked questions about electric field. One question that is bothering me is... Is it possible to have 0 electric field at a point, but at the same point have electric potential? I've read that the answer is yes, there can be a potential without a electric field, but...

Hi. I have come across the following equation < x | p | ψ > = - iħ d/dx < x | ψ > where p is the momentum operator. I'm confused as to how p which acts to the right on | ψ >can then be taken outside the inner product so it now also acts on the bra < x | ? Thanks

Hi everyone, first time poster here, I'm studying for a Materials exam, one of my weakest subjects and this question is causing me a lot of pain: Beryllium (Be) is a HCP metal with an atomic weight of 9 g mol-1 , an atomic radius of 0.112nm and a density of 1850 kg m-3 . Calculate its c/a ratio...

I know it may sound a bit cocky but it's true. I was good in academics since school time and I loved physics, so i pursued it. Got my BS and MS in physics. During my course of studies, I got interested in writing and SEO (Search Engine Optimization). I also like music and gave drumming a shot...

Homework Statement Homework Equations W_g = mgdcos(phi) The Attempt at a Solution So the angle between the force doing work (x component of F_g) and the displacement (down the hill) is 0 degrees. aka phi is 0 degrees. This is true for (a), (b) and (c). so since cos(0) = 1, the work done by...

Hi, I am working on the laminar flow and during my calculation at the outlet of my nozzle my Re was even greater than 120,000 however my other calculations seemed legit to me. And someone told me that I am calculating the Reynolds number of an open channel and for that flow can remain in laminar...

Good evening. I've got stuck in this question from my book of Mechanics of Materials by Beer and Johnston about torsion. The allowable shearing stress is 15 ksi in the 1.5-in.-diameter steel rod AB and 8 ksi in the 1.8-in.-diameter brass rod BC. Neglecting the effect of stress concentrations...

I understand the basics of the stress-energy tensor (I think) but I still have a couple questions about it. But first, I'd like to give a quick run down of what I do understand, and I would appreciate if one of you could correct me where I am wrong and also answer my questions afterward. So...

I'm not quite sure how to describe this phenomena but in my town we have a large boardwalk structure and it is closed off almost entirely underneath by a sea wall. this leaves slits on top with the boards and slits near the top of the sea wall where a concrete beam will pass through. at certain...

Suppose we have a differential equation with initial conditions ##y_{0}=y^{\prime}_{0}=0## and we need to solve it using a Green Function. Then we set up our differential equation with the right side "forcing function" as ##\delta(t^{\prime}-t)## (or with ##t^{\prime}## and ##t## switched I'm a...

Hi. I have come across the following equation eA eB = eA+B e [A , B]/2 where A and B are operators. Is this the B-C-H formula or is it a version that only applies under specific conditions on the operators ? Does this equation imply that eA+B ≠ eB+A ? Thanks

Fairly simple question I'd like someone to help with please. My teacher has me confused so I figured I'd turn to the forums for help. 1. Alright, so in an exam we had a ray diagram to draw. "Complete the ray diagram shown in Figure 6" they said (I sketched a replica * thumbs up*...

Hi everyone, I am a little confused conceptually about electrostatic voltage and DC voltage. I did my bachelors in biochemistry (where I’m working today), I’m learning/relearning physics as a hobby. I did well in high school physics and first year physics but I’m afraid I’ve forgotten most of...

Homework Statement the answer that i get is different with the answer provided , is my answer wrong ? i got ( cos(2n -1) / 2n-1 )instead of ( cos(n+1) / n+1 ) Homework EquationsThe Attempt at a Solution [/B]

we all know the equation for KE 1/2mv^2 you do this simple math and you get an number that is in KE units , Joules. so, why is the KE unit equation, 1 Joule = Kg x (m^2/s^2) ?? 1Kg traveling at 1m/s would have a KE of 1Joule, but using the KE equation, it would be 1/2 a Joule. why is...

Hello I am confused with the energy conservation equation in thermodynamics. Is it U2 - U1 = Q - W ? This is what most books say. Or is it H2 - H1 = Q - W ? Which should I use to solve problems? For example we have a gas with U1 and P1V1 (which makes H1). After a process, we have Q...

The part I'm confused about - in order to have two distinct observations of a system you want to check for symmetry isn't something asymmetrical is required to distinguish them... as distinct observations. If we say a system is completely closed and are restricted to refer only to that system...

Hi. For a diffeomorphism between 2 manifolds Φ : M → N with a tangent vector v in M I have the following equation for the push-forward of v ( Φ*v)f = v( Φ*f) where Φ* is the pull-back. I understand this equation but i have also come across the following equation for the push-forward...

I am certain this has been asked before many times before since the cat in the box first did not come out, and I am just overlooking it. And I am using the cat in the box for the exact purpose it was orginally created as I understood, to simplify the concept to its very base pieces to allow...

Why are basis vectors represented with subscripts instead of superscripts? Aren’t they vectors too? Isn’t a vector a linear combination of basis vectors (and not basis co-vectors?) In David McMahon’s Relativity Demystified, he says, “We will often label basis vectors with the notation e_a...

n^2 - 1 / (n^3 + 6n) If I use the nth divergence test, I plug ∞ in (limit as n -> ∞) for n and since the degree on the bottom is larger I get 0, which means it converges. However, if I use the limit comparison test and compare it to: n^2/n^3, which = 1/n, which diverges -> n^2 - 1 / (n^3 + 6n)...

My textbook says: "if ## V = W_1 \oplus W_2 ##,, then a linear operator ## T ## on ##V ## is the projection on ##W_1## along ##W_2## if, whenever ## x = x_1 + x_2##, with ##x_1 \in W_1## and ##x_2 \in W_2##, we have ##T(x) = x_1##" It then goes on to say that "##T## is a projection if and only...

Hi all, I have a question about Slater Determinant for excited states. Let's say we want to construct approximate (2 level) wavefunction of He in some certain state. Since we have two electrons in two level system with spin in consideration, we can construct total of 4 different wavefunctions...

Im having trouble with my thought process for spin states of a system of two electrons Using Total Spin 'S' and Total spin mag quantum numbers 'MS' as state ket |S MS> My textbook states... " Three Symmetric Spin states Triplet spin stats for twin identical spin -1/2 particles | Up Up> = |S...

Problem is in the picture below. The problem is already solved, but I am confused as to why C(dv/dt) is positive and not negative, since the current is coming out of the capacitor and towards the point v(t), which is not away from v(t) like the solution shows. But if I solve the problem with...

I'm feeling pretty silly not being able to understand this. Let's say that you have a piston (red) sitting inside of a housing (black) like this Let's say that something like this was lying on its side on a flat table. By my understanding, if the contact between the piston and the housing...

In the giancoli physics textbook for engineers and scientists, it says "E=F/q where E is the force on a small positive test charge at rest..." Then about one paragraph later it proceeds to say "If q is positive, F and E point in the same direction. If q is negative, F and E point in opposite...

Homework Statement I am trying to solve a problem from a popular quantum mechanics text. I am learning on my own. I am trying to calculate the variance, which is <x^2>-<x>^2 = variance in x. I posted a photo of the problem as a picture that is linked below as well as the solution, I simply...

Homework Statement "An automobile travels on a straight road for 40-km for 30 km/h. It then continues in the same direction for another 40-km at 60 km/h. (a) What is the average velocity of the car during the full 80 km trip? (Assume that it moves in the positive x direction.) (b) What is the...