# Tagged Questions

A convention is a set of agreed, stipulated, or generally accepted norms. It typically helps common efficiency or understanding but is not required, as opposed to a strict standard or protocol.

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### Why does positive work done by internal conservative forces $\implies$ decrease of potential energy?

Potential energy can be thought as the amount of work that the force can potentially do on the point because of its position. $$W=-\Delta U=U_{initial}-U_{final}$$ A positive work done by a force ...
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### How we chose the height while calculating potential energy?

I'm really confused how to chose height when calculating potential energy. If an object is right above a desk, suppose the will we take height from desk? If we take it from a height $x$ from the ...
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### Problem with figuring out sign conventions in QFT

I have a problem with sign conventions in QFT which I have trouble dealing with myself. I self-study and mainly use Weinberg and Peskin. I will present the reasoning following conventions adapted by ...
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### Normalising Generators of a Lie Algebra

Ok, so I'm asking this in physics because I'm currently working through part of Srednicki's text on QFT, even though it's really a maths question. In Srednicki's chapter on non-Abelian gauge theory, ...
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### Definition of Ampere

On Wikipedia it says: This force is used in the formal definition of the ampere, which states that it is "the constant current that will produce an attractive force of $2 × 10–7$ newton per metre ...
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### SI Base Unit definition of mass - obsolete?

According to the formal definition of the SI Base unit of mass, the kilogram, it is stated that "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the ...
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### Why do we still not have an exact definition for a kilogram?

I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It seems this would be fairly important. Edit Today I ...
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### Why the direction of dipole moment is from negative charge to positive charge?

An electric dipole moment is defined as $p = q\times 2d$. How to understand it physically? Why the direction of the electric dipole moment is from negative charge to positive charge?
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### Difference between $\Delta$, $d$ and $\delta$

I have read the thread regarding 'the difference between the operators between $\delta$ and $d$', but it does not answer my question. I am confused about the notation for change in Physics. In ...
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### Can we define tension in a string as the reactive force produced in a string being pulled at both ends?

In my textbook, the definition of tension was given that Tension is the reactive force which exists when string is being stretched at its both end. After it there was a case given that to calculate ...
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### Dealing with negative work

Dumb question, I'm working with vector fields right now, and one question on here tells me to assume an object can take on three paths from a to b. (paths not listed here) for times in [0,1] Now ...
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### Standard convention for $x$ error bars

What is the standard convention regarding the error bars of the independent quantity in a graph? In what situations should I show the $x$ error bars? In case both $x$ and $y$ uncertainties are ...
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### Is optical path length (OPL) usually in units of length, or wavelength?

When I do calculations, I usually define optical path length (OPL) of a path as the integral of index $n$ along a path divided by the vacuum wavelength, so that I can get the phase easily. So for a ...
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### Sign conventions for voltage drop and gain in circuit

In Sears and Zemansky's University Physics book and in many other books in English they define the following sign conventions (used for Kirchhoff loop rule for example): While in many other books (...
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### How do you derive Fleming's left hand rule?

How do you derive Fleming's left hand rule? What is the theoretical explanation for the directions of the magnetic field, current and the force on the current for being oriented in that way relative ...
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### Conventions for work and energy are not going correct in this situation [closed]

Let us consider 2 objects A and B. The initial PE of the objects be U(A)=10J and U(B)=0J. Let F be the force exerted by A on B. Suppose W be the work done by force F. Due to this the PE of both the ...
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### Confused regarding sign convention of silvered plano convex lens

I'm totally confused while finding the sign convention of a silvered plano convex lens. I know that equivalent power of such a system can be found by adding the power of the mirror and twice the power ...
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### What is W.u. as a unit in a nuclear physics context?

In a recent paper (doi/arXiv), of note for causing this recent tussle over its handling by science news media, there's some strange units I can't quite puzzle out. Specifically, the abstract contains ...
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### How can energy be negative in a finite square well?

Say if the potential $V(x) < 0$ in the well but the sides or the scattered states its zero potential, anyways How is that the energy in the well is less than zero? Is it because the potential ...
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### Electric Potential produced by Hollow Sphere

Let there be a hollow sphere (Throughout the question we will ignore the thickness of the sphere) which is positively uniformly charged,q of radius, R. Suppose there to be a test positive charge, q' ...
Fourier transformations: $$\phi(\vec{k}) = \left( \frac{1}{\sqrt{2 \pi}} \right)^3 \int_{r\text{ space}} \psi(\vec{r}) e^{-i \mathbf{k} \cdot \mathbf{r}} d^3r$$ for momentum space and $$\psi(\vec{r}... 1answer 108 views ### What does “Receptor” convention refers to in RC circuits? my question is How can we choose the convention in a circuit and what does it refer to ? Especially for a Capacitor I hear the terms "passive sign convention..." "receptor convention ..." "generator ... 1answer 71 views ### Does the use of \gamma=\left(1-v^{2}/c^{2}\right)^{-1/2} automatically assume a (+ - - - ) metric? [closed] In Special Relativity, does the use of \gamma=\left(1-v^{2}/c^{2}\right)^{-1/2} automatically assume a (+ - - - ) metric convention? For introductory textbooks, the Lorentz factor is is always ... 0answers 23 views ### Sign choice for sigma-matrices I'm trying to figure out the consequences of the sign choice$$ \sigma^\mu = (\mathbf{1},\vec\sigma)\qquad\text{vs.}\qquad \sigma^\mu = (-\mathbf{1},\vec\sigma) \,. $$This choice does not affect the ... 3answers 208 views ### Complex scalar field theory For the complex scalar field theory$$L = -\partial_{\mu}\phi^{*}\partial_{\mu}\phi - m^{2}\phi^{*}\phi + J\phi^{*}+J^{*}\phi,$$Why is there no factor of 1/2 in the lagrangian like in the real ... 0answers 53 views ### Minus sign in analyzing RC with changing emf circuit? I'm learning electromagnetism from Purcell's book. He say that if we have the following circuit:$$-\frac QC + RI = \mathcal E_0\cos\omega t~,\quad I = -\frac{\mathrm dQ}{\mathrm dt} $$The ... 1answer 48 views ### Why Does there Have to be Linearity in Ket and Skew Symmetry? I'm reading Shankar's "Principles of Quantum Mechanics," and on page 8 he states that one axiom in Dirac notation is linearity in ket, and because they are also skew symmetric there is anti-linearity ... 1answer 75 views ### Relation between gravitational field and gravitational potential The gravitational field is the negative differential of the gravitational potential. Now the gravitational potential due to a particle at a distance r is -Gm/r where m is the mass of the ... 3answers 86 views ### What is the Direction of area vector? In case of closed surfaces the area vector is directed outwards the surface. But what is the direction of the area vector in case of an open surface e.g. A thin lamina type of surface. Does it depend ... 2answers 71 views ### Ambiguity in assigning intrinsic parity We know that, fermions can have intrinsic parity either \eta_P=+1 or =-1. How does one then fix the intrinsic parities ofthe elementary particles, uniquely? Again, the intrinsic parity of a baryon ... 1answer 44 views ### Notations used to express direction [closed] We express direction relative to a reference point and call a certain direction positive and it's direct opposite direction negative, by convention. But, what notation should we use to express a ... 0answers 27 views ### Where do the intrinsic parities of particles come from? It is known that some particles have negative intrinsic parity - for example pion \pi. I was wondering if this parity can be understood. I read somewhere that parity of quarks is defined to be ... 1answer 47 views ### Is the net force conventionally shown in a free body diagram? Is it standard convention to display the net force vector on a free body diagram? Internet searches seem to give mixed results. 1answer 105 views ### Bridges between maths and physics: the \tau=2\pi constant [closed] [disclaimer: I am not a math or a CS major, this is probably an easy question for most people on Physics SE.] I just read the tau manifesto explaining - according to its author - the various ... 3answers 42 views ### Need help with signs (+ -) in electric potential work and potential energy problem The problem is from Sears and Zemansky's University Physics, 12th edition in Spanish. The problem says: How much work is needed to assemble an atomic nucleus that contains three protons if it is ... 1answer 67 views ### Use of infinitesimals in physics [duplicate] I want to ask about infinitesimals and non-standard analysis. In physics we always use \mathrm dx,~\mathrm dv,~\mathrm dt etc. as infinitesimal quantities. When we deduce equations in physics, when ... 1answer 68 views ### Number operator in quantum mechanics In quantum mechanics a^{\dagger}a is defined as the number operator, where [a,a^{\dagger}]=1. Why cannot we define aa^{\dagger} as number operator instead of the usual definition? 2answers 254 views ### Different signatures I was working out the christoffel symbols, once where the metric that I am using has (+---) signature and another time where it has (-+++) signature because two books had different signatures and I ... 1answer 27 views ### Does the sign of imaginary part of complex permittivity have any physical meaning? I have noticed some papers having written complex permittivity as e' + je''  and others as e' - je''. The data in literature does not specify the sign. What should I use and does the sign of e'' ... 1answer 47 views ### EFE and Local Minkowski Suppose we view the Einstein Field Equations (EFE) in the context of a boundary value problem with a given stress-energy tensor and boundary conditions. The problem is solved by finding a pseudo-... 1answer 51 views ### Can velocity be negative? [closed] In free fall I use the formula V=g*t and g is negative(-9.8m/s^2). It gives me negative. 2answers 194 views ### The Zero Electric Potential of the “Earth” I know its the potential differences that matter and generally we define the zero of the electric potential according to our convenience. I would like you to look at this standard problem: Charge -... 1answer 95 views ### How does the Einstein summation convention apply to the following equation? This is the equation is in the "mathematical form" section of the following wikipedia article: http://en.wikipedia.org/wiki/Geodesics_in_general_relativity More specifically, the "Full geodesic ... 1answer 97 views ### Staggered Indices (\Lambda^\mu{}_\nu vs. \Lambda_\mu{}^\nu) on Lorentz Transformations I have some open-ended questions on the use of staggered indices in writing Lorentz transformations and their inverses and transposes. What are the respective meanings of \Lambda^\mu{}_\nu as ... 1answer 50 views ### What are the right signs? [closed] This is a question provided in my book: An object of height 2\ \mathrm{cm} is placed at a distance of 2.5f from a concave mirror where f is the focal length. Find the height of the image. ... 2answers 102 views ### What is the zero of potential? A satellite of mass m is travelling at a speed v_0 in a circular orbit of radius r_0 around a fixed mass at O. Taking the zero of potential at r=\infty, show that the total energy of the ... 1answer 54 views ### Should the trace of a product of gamma matrices depend on the convention I use? I am trying to work out$$\text{Tr}[\gamma_5\gamma_\mu\gamma_\nu\gamma_\alpha\gamma_\beta]$$using the same convention as J.J. Sakurai (Advanced Quantum Mechanics), what I get is$$\text{Tr}[\gamma_5\...
It is defined that the contribution of a vertex in a Feynman Graph towards the probability amplitude $M_{fi}$ is proportional to a charge $Q_f$. Yet I seem to cannot find any precise explanation how ...