# Questions tagged [conventions]

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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### Expression for Potential energy of a hanging mass

If the acceleration due to gravity is $g$ and a mass $m$ is hanging from a fixed support with a thread of length $l$, then it's potential energy ($U$) is given by: $$U = -mgl;$$ This was stated by my ...
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### Contradiction in Thermodynamic Work and Mechanical work [duplicate]

The first law states that $\Delta U = Q - W$ where W is the work the system does on the outside. If there is no heat added, the change in energy is equal to the work done. If the work is negative, i....
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### Help understanding quote on theory and knowledge in Gravitation (Misner, Wheeler, & Thorne, 2017)

I came across an interesting discussion at the start of chapter 3 of MWT Gravitation. It reads: Here and elsewhere in science, as stressed not least by Henri Poincare, that view is out of date which ...
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### Understanding sign convection for entropy change

I recently solved a problem where I had to study the change of entropy for a lake that is freezing. It started off as being plain water at $0$ degrees, that then freezes to some negative temperature. ...
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### Landau/Lifshitz Particle Disintegration

In Landau/Lifshitz "Mechanics", 3e., there is a problem which asks the reader to find the relation between the angles $\theta_1,\theta_2$ in the lab frame when a particle disintegrates into ...
1 vote
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### What does $\overset{\circ}{=}$ mean in the context of thermodynamics? [closed]

Few days ago I started thermodynamics and whilst talking about systems and its type of walls the professor, a part from this one, showed some diagrams including this equal sign. He was talking about ...
1 vote
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### Sign convention for the Lagrangian of a free massive point particle in general relativity

As far as I understand, the Lagrangian of a massive free particle in the context of general relativity is the following: $$L=-mc\sqrt{g_{\mu\nu}\dfrac{dx^\mu}{dt}\dfrac{dx^\nu}{dt}}.$$ But is this the ...
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### The energy-momentum tensor of a scalar field

In Carroll's Introduction to General Relativity: Spacetime and Geometry, I am going to express the components of the energy-momentum tensor of scalar field in three- vector notation, using the ...
1 vote
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### Why can $ϕ(p)$ be Fourier expanded to $ψ(x)$ in quantum mechanics? [closed]

I know the Fourier transform is $$F(\omega)=\int_{-\infty}^{\infty} f(x) e^{-i \omega x} \,d x$$ $$f(x)=\frac{1}{2 \pi} \int_{-\infty}^{\infty} F(\omega) e^{i \omega x} \,d \omega,$$ but in ...
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### Why do we indicate current to be flowing against electrons in a electric circuit ? If it is a convention thenwhy do we draw it along studying it? [duplicate]

I believe that electric current is a kind of force (even when we consider it to be opposite to the flow of electrons). If it is so then what is the nature of this force ? And also tell me if there are ...
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### Does change in metric signature affect the result of loop integrals in momentum space?

I was wondering whether the change in metric signature affect the standard results of loop integrals. Suppose a rank-4 tensor integral's result is known, does changing the metric signature for the ...
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### What is the significance of a reference point in calculating the potential?

The gravitational potential is given as $$U(r)=-\frac{GMm}{R}$$ where $G$ is the universal gravitational constant $M$ is the mass of the earth and $m$ is the mass of an arbitrary object and $R$ is the ...
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### Confusion about the signs we use in Faraday's law for emf

While reading griffiths, I came accross the definition of emf, which is given by $$\epsilon = -\int_a^b \vec{E} \cdot d\vec{l} = \oint \vec{f} \cdot d \vec{l}$$ Here we use $\vec{E} + \vec{f} = 0$ ...
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### Sign of reversible work done

Consider a piston such that it is in mechanical equilibrium with its surroundings. If I pull the piston (reversible work) then the work done by me would be positive. If I push the piston, again work ...
1 vote
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### Why does Dirac bilinear $\bar{\psi}\sigma^{\mu\nu}\psi$ is frequently written with a factor of $i$?

The tensor Dirac bilinear $\bar{\psi}\sigma^{\mu\nu}\psi$ has the matrix tensor $\sigma^{\mu\nu}=\frac{i}{2}\left[\gamma^\mu,\gamma^\nu\right]$. I can understand that the factor of $\frac{1}{2}$ is a ...
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### Lorentz Transformation - Intuitive way to determine when to use Inverse transformation

Any intuitive way to decide when to use Lorentz transformation and when to use its inverse relation? For example: A particle is moving with respect to me with speed of 0.99C. If question is : What ...
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### Correct way to reference to a percentage value's category/label

When a percentage is written as, for example: 20% of sold products were of product A and 80% product B. What is the correct concept / word to use to describe "product A"? Is it units? As ...
1 vote
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### Minus sign for incoming antifermions

In his Diagrammatica, The Path to Feynman Diagrams (Cambridge University Press, 1994; §4.5 "Quantum Electrodynamics", p. 88), M. Veltman reports the following Feynman rule for incoming ...
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