Law Definition and 999 Threads

Naively there is a conflict between CPT symmetry being at heart of fundamental physics models like QFT, and 2nd law of thermodynamics: saying that entropy grows toward future. Is there really a conflict here - so is physics symmetric or not? How to understand it? Personally I disagree with that...

This is an experiment on a dilute isotopic mixture of helium gas. A spherical vessel of diameter 1 m is first filled with 4He gas to one atmosphere pressure. Then a small amount of 3He gas is introduced through a valve on one side of the vessel. Make a rough estimate of how long one must wait...

I am trying to derive that $$\nabla \times B=\mu_0 J$$ First the derivation starts with the electric field $$dS=rsin\varphi d\theta r d\varphi $$ $$ \iint\limits_S E \cdot dS = \frac{q}{4 \pi \varepsilon_0} \iint\limits_S \frac{r}{|r|^3} \cdot dS $$...

Is third Newton law valid for rotation / Torque? I mean, can we say that for every torque there must be another torque with equal magnitude and opposite direction? This can only be true for contact forces or radial forces, as these forces will create a reaction that will cancel the torque...

If I understand correctly, Gauss’ Law is (roughly) derived as follows: Part A Electric Flux = EA E = q / (∈4πr^2) A of the surface of a sphere is 4πr^2 They cancel out and therefore EA =q/∈ Line 4 seems to only apply to a sphere, as it is based on line 3. Now, Gauss’ Law is applied to...

why it is first law of thermodynamics useful in developing the non-flow energy equation and steady-flow energy equation. First Law Equation (FLE) = 1 1.1 Non-Flow Equation = 2 Steady-Flow Equation = 3 Specific Entrapy of Steady-Flow 4 Mass Flow 5 I understand (i think) but can't explain...

Im trying to apply KCL to the serial c and parallel rl circuit. IZ = = = 330.562 mA but this means incorrect proof. Anyone can help please?

hi guys i was trying to understand the derivation of the child Langmuir law that govern the current flow between the two plates of vacuum tubes and saw this derivation online : and i am stuck at couple of points : when i saw the derivation it seem like a direct application of the Poisson's...

I'm stuck straight out of the gate any help is really appreciated

I need help with the exercises attached in the pictures. Basically, exercise 3 is already solved but I need some help understanding every subpoint (for example, at a) how did we get to 0=H -g* tAO/2 , I know it's from the motion law and vA=0, but why is y(t)=0?). And I tried solving ex 8, but I...

Please see enclosed my calculations for parts 1-3. I am stuck on the final part of of a 4 part question, any help or guidance would be very much appreciated, thanks.

Mary Boas attempts to explain this by pointing out that the situation cannot arise because charges will have to be placed individually, and in an order, and that order would represent the order we sum in. That at any point the unplaced infinite charges would form an infinite divergent series...

Hi PF! I don't understand the sentence: on one side says the energy is preserved, and, at the end, the total energy of the system will change if ##W## or ##Q## is added: ##\Delta{U}=Q+W##. Greetings!

The soultion used polar corrdinates. Acceleration in polar corrdinates have radial and transeverse components.When calculating the acceleration of collar respect to the rod, the solution only calculates the radial component of acceleration. Is it because the collar is on the rod, so the...

Prof. Tong used the Liouville theorem to prove the Poincaré recurrence theorem in his notes, that given an initial point ##P## in phase space, for any neighbourhood ##D_0## of ##P## there exists a point ##P' \in D_0## that will return to ##D_0## in a finite time. To illustrate the theorem, he...

So I found this steam engine on the youtubes: Consider the system not including the candles, only including the can that makes up the boiler shell and the piston, flywheel, etc. The boiler can acts as a hot reservoir; however, the Kelvin-Planck statement of the 2nd law says you can't have an...

A cylinder contains an initial volume V1 = 1m^^3 of a perfect gas at initial pressure p1 = 1 bar, confined by a piston that is held in place by a spring. The gas is heated until its volume is doubled and the final pressure is 5 bar. Assuming that the mass of the piston is negligible and that the...

Let us consider two point charges: one of which moves (in the simplest case) rectilinearly with a constant speed and the other is at rest. The electric field of the first charge is renormalized due to the effects of retardation and generation of a solenoidal electric field (see Am. J. Phys...

I know there is a serial and parallel circuit I know R1 = 2k, R3 = 1k and i know Vout which is 1V. And i know 5V is coming from supply. I need to calculate the size of resistor R2. I can calculate : 1) IR1 = 5-1V/2000ohms = 2mA 2) UR1 = 0,002A*2000ohms=4V voltage drop on R1 How do i go on...

So my professor says that the implication of 2nd law of thermodynamics is that high quality energy will be degraded into low quality energy. By high quality energy he means something like coal or fuel. By low quality energy he means something like heat entering the cold resevoir in a heat...

I need some help resolving the follow problem. I really don't know where to put the "twice as large as the resultant force on Q3" in order to build an equation. Thank you !

I'm a bit confused on the derivation above. I understand what the goal of the derivation is, as it derives Gauss's Law using the solid angle, but i was wondering if someone could kind of fill in the steps the author skipped and explain the use of the solid angle.

For some context, a Bravais lattice in 3D is described by 3 basis vectors ##\vec{a}_1##, ##\vec{a}_2##, ##\vec{a}_3## s.t. a discrete point in the lattice can be generated with ##\vec{R} = \sum R_i \vec{a}_i##, with ##R_1##, ##R_2##, ##R_3## being integers. A direction is specified by a triplet...

I had already found the Mass of the product (C3H3N) produced by this reaction (theoretical mass at 100% yield) in a previous problem. I did this by finding the Limiting Reagent (C3H6) in the reaction , calculating the number of moles of C3H6 and using the Molar Ratios in the balanced reaction...

F = qE ma = (2*10^-6) * (λ / (2pi*r*ε0) ) ma = (2*10^-6) * (4*10^-6 / (2pi*4*ε0) ) => I am not certain what to put for r ( But I sub in 4 because dist is 4) a = ( (2*10^-6) * (4*10^-6 / (2pi*4*ε0) ) )/ 0.1 a = 0.35950 v^2 = U^2 + 2 a s v = 0 u^2 = -2 a s => Can't sqrt negative so...

From this question, I do not understand why there are three forces exerted at Point C (2 of it being the tension by weight A and the other is the tension by weight B) I understand that there is tension by the two weights but why is there 2 forces exerted by weight A at point C? From the...

After crossing out all the variables which I think equals 0 in the equation, I was left with: ∆PE + ∆U = Qin mg (0.2) + ∆U = 300 10*9.81*0.2 - 300 = - ∆U = -280J This was the answer I derived. However, the correct answer was supposed to be 123.3J. Please can someone explain to me how to get...

Robert Boyle's law states that at constant temperature, the pressure of fixed amount ( i-e number of moles n) of gas varies inversely with its volume. Mathematically, it can be written as $p ∝ \frac1V $(at constant T and n) $\Rightarrow p = k_1 \times \frac1V $ where $k_1$is a proportionality...

Hello, I have attached my attempt at a solution. All is okay until i reach x=5-5e^-x. My assignment suggests that i solve it by substituting x=5-ε into the equation where ε is assumed to be small. Solve for ε and then solved for x (Approximate). Thank you for your help

In this question I can't understand how the loop is opposing the change in flux, I can solve it by simply imagining a bar magnetic with south pole pointing outwards placed inside the plane of image but how do I know that it is the same case as that of bar magnet? What if their is a case in...

How to lose surface integral in derivation of ampere law from biot-savart law if current goes to infinity? How does current that goes to infinity obey Helmholtz theorem for vector fields?

Dear, I am a medical doctor and my physics background is limited to that of secondary school many years ago, but should want to ask for your help. In attachment, you can find a picture of my question. On a scale, we place a jar filled with water. We place in the water a platform which is...

Attached is problem 23.03 from Halliday and Resnick. We have a sphere of uniform negative charge Q = -16e and radius R = 10cm. at the center of the sphere is a positively charged particle with charge q = +5e. We are supposed to use Gauss' law to find the magnitude of the electric field at...

So essentially you are supposed to find the mass percentage of N2H4 in the Hydrogen/Nitrogen mixtures. The way I understood it, the only way to find the mass percentage of the gasses is to find the mole percentage. Which I got as 25%, based on the fact that the ratio of product gasses to...

hi guys i saw this problem online about using the MATLAB ode45 to solve the nonlinear Hoock's law and its specifically stated that the nonlinear hoock's law is given by $$F = k\;u + \epsilon\;u^{3}$$ , but when expanding the potential function in a Taylor series where you obtain the force...

My attempt: Realize we can work in whatever coordinate system we want, therefore we might as well work in the rest frame of the fluid. In this case ##u^a=(c,\vec{0})##. The conservation law reads ##\nabla^a T_{ab}=0##. Let us pick the Levi-Civita connection so that we don't have to worry about...

The first law of thermodynamics states that the change in the internal energy of a system equals the net heat transfer into the system minus the net work done by the system. In equation form, the first law of thermodynamics is ΔU = Q − W. in the mathematical expression of the first law of...

1. speed of light = 3*10^8 ms^-1 length of an Earth year=365 days*24 hours*60minutes*60seconds=3.15*1 0^7 seconds distance=speed*time 1 light year= 3*10^8 *3.15*10^7=9.46*10^15m 4.2 light years = 4.2*9.46*10^15m=3.9732*10^16m ~ 3.97*10^16m (to 3sf) 2. Intensity=power/area Power=1.4MW=1,400,000...

From Maxwell's equations \partial_\nu F^{\mu\nu}=J^{\mu}, one can derive charge conservation. The derivation is 0\equiv \partial_\mu \partial_\nu F^{\mu\nu}= \partial_\mu J^{\mu} { \Rightarrow}\partial_\mu J^{\mu}=0. However, a circular reasoning exists in it. For the sake of better...

Answer : Using Pascal's law, this is my answer : ##\color{blue}{\boxed{\vec F_a = \vec F_c < \vec F_b}}##. Reasoning : Forces ##F_a## and ##F_c## are equal because the pressures required at the two cylinders for case (c) is the same as that required in (a). It doesn't matter how many of those...

Hello, I am new and I have a question about the lenz law. How I am suppose to find DA/dt and o in this problem? A= 9 but D(9)/dt is equal to 0 ? And I don't really know what is the angle o means in this formula. If I found the voltage i can find the current with I= eps/Req .

Good afternoon, I have a question regarding this derivation that I'm covering in Thornton & Marion's "Classical Dynamics of Particles and Systems". In it, we're covering the most basic equation of motion for a particle falling in a medium. I understand the process of starting with Newton's 2nd...

The following is from the 2018 AQA AS Further Maths/Mechanics Paper 2 exam: 'Two smooth spheres A and B of equal radius are free to move on a smooth horizontal surface. The masses of A and B are m and 4m respectively. The coefficient of restitution between the spheres is e. The spheres are...

The next chapter in my personal studies is Law of Sines and Law of Cosines. I am not there yet. I know there is a Law of Tangent but it is rarely used and never taught in a typical trig course anywhere. Why is the Law of Tangent ignored?

My question relates to whether the 2nd law of thermodynamics is a an empirical or mathematical law. If we can reason purely from the laws of statistics that entropy can only increase, then the 2nd law of thermodynamics cannot be falsifiable and therefore shouldn't be considered a scientific...

The three laws are basic principles in Newtonian Mechanics. The principle of superposition of forces is part of the second law.But before we make the assumption of superposition of forces,we have to make another assumption which is the principal of superposition of motion.why are...

The so called -1st (Minus First) law of thermodynamics, I believe states that information cannot be destroyed, or something along those lines. Does anyone know where I can read more about it? I think I've heard Dr. Leonard Susskind talk about it but it's been hard to find the video or any other...

Well, I guess that is pretty much my question.

If you take a bar magnet and place a wire with current a short distance from the end, Lorentz's law can be used to accurately predict the location and magnitude of the resulting forces. The same is true if you use a large volume uniform magnetic field to create an induced field in a bar ferrite...

I´m currently studying Newtons 2nd law and I don't get how can force only depend on mass and acceleration and not on velocity.. I mean, if I punch something with my fist going at constant velocity ( acceleration=0) it stills has a force right?