# 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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### What is the correct form of Dirac equation?

Usually the Dirac equation in curved space is written as $$i\Gamma^{\mu}D _{\mu}\Psi-m\Psi=0,$$ where $\Gamma_{\mu}$ are curved space gamma matrices and $D_{\mu}$ is covariante derivative. This ...
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### How is the Time Derivative of the the Electric Field Equal to the Current Density in Gaussian Units?

The microscopic form of Ampere's law with the Maxwell addition in Gaussian units states, \begin{equation} \nabla \times \vec{B} = \frac{1}{c} \left ( 4 \pi \vec{J} + \frac{\partial \vec{E}}{\partial ...
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### Sign convention for spherical mirrors

What is the sign convention for this formulation of the spherical mirror equation? $$1/i + 1/o = 1/f$$ f is negative for convex mirrors. i is image distance. o is object distance. (How can the ...
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### If I were to change the definition of refractive index [closed]

We know that the Refractive Index is just a number that gives an idea of how much velocity of light decreases in comparison to vacuum. So, it makes sense to define a variable, n as: n=c/v where v ...
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### Why $1 \ \mathrm{fm}$ is often used in modern physics?

In the International System, we all know that a one femto (or one Fermi) is equal to $$1 \ \mathrm{fm}=10^{-15} \ \mathrm{m}$$ What is the historical reason why this unit of measure was adopted as ...
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### Formulas with complete sign conventions for spherical mirrors and thin lenses

I'm looking for two things: Something like the third equation in the chosen answer in the link, but for spherical mirrors, and Something that covers conventions for the radii of curvature and focal ...
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### Why electric potential energy at a point is the same as amount of work done to bring the charge to that point from infinity?

Let's consider a positive charge at a point in the space. Now, if we want to bring a another positive charge to a point within the electric field of the first charge, then we have to do some work ...
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### Cross product and handeness

I'm having some difficulties understanding the cross product in a left-handed coordinate system. I want to compute $\hat{i} \times \hat{j}$ for both systems in the picture (the first one is right-...
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### Velocity in circular motion, $v = r × \omega$ or $v = \omega × r$?

I know it might sound silly to ask, but is the relation between linear velocity and angular velocity of an object undergoing circular motion $v = r × \omega$ or $v = \omega × r$? I didn't notice it ...
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### Transformation of metric by diffeomorphism: pushforward or pullback?

Let $(M,g)$ be a smooth manifold with a metric tensor of signature $(p,q)$. The signature isn't really important for this question so we leave it general. If $\Phi : M\to M$ is a diffeomorphism we ...
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### Potential - When and when not to use infinity as a reference point?

I'm reading Griffiths' Electrodynamics and having a bit of a problem to understand when we can use infinity as a reference point and when it cannot be used. One question asked to calculate the ...
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### Orientation and sign convention in 2D electrodynamics using differential forms

I've been following this paper for a treatment of electrodynamics using differential forms. In particular, they demonstrate that Maxwell's equations expressed using differential forms are form-...
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### What is the fundamental reason behind electric potential decreasing in the direction of increasing electric field? Is there any exception to this?

Does this increase and decrease necessarily have to be linear in nature? I have seen this statement being thrown around which states that in direction of increasing electric field the potential ...
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### Symbolizing frequency as $v$

In my school textbook $v$ is symbolized to the frequency of the wave. Is that correct? I also saw this convention used in Chemistry: The Central Science By Theodore L. Brown, H. Eugene LeMay Jr
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### Mars longitude reference?

Platforms like NASA's JPL HORIZONS web-interface for solar system dynamics often have obtainable parameters like sub-solar or sub-terrestrial longitude for a given time and celestial object. (example: ...
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### Confusion on negative charges, current, and direction

My text book seems to have two contradictory statements about the same subject. Under the electrical potential and capacitance chapter, it says that negative charges move from lower to higher ...
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### Angle $\phi$ in astronomical coordinate transformations

I was reading about the transformation between horizontal coordinates (Alt-Az) and equatorial coordinates (RA-Dec) from Fundamental Astronomy by H.Karttunen. In the image, one can see the angle $\phi$...
### How does one obtain $\hbar$ as $\frac{h}{2\pi}$?
I'm reading Dirac's Principles of Quantum Mechanics. He defines $\hbar$ to be the real number satisfying the following relation $$uv - vu = i\hbar[u,v]$$ where $u$ and $v$ are dynamical variables, ...