# 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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### Sign conundrum while deriving electrostatic potential

Consider a fixed, positive Point charge $q1$, kept at the origin. Another (positive) charge, $q2$, is being brought from $\infty$ to the point $(r,0)$, by an external agent slowly. We wish to ...
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### Problem with the formula for electric intensity due to a charged sheet

The formula for the electric field due to a charged sheet is $\LARGE\frac{\sigma}{2\epsilon_0}$. Now, if I want to find the electric intensity of a point between two equally and oppositely charged ...
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### How is the reference point of potential energy defined in electric fields?

Consider the diagram below of a positive charge and three points marked in the field X, Y and Z. I understand that for a gravitational field, we define the potential energy as being always negative, ...
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### The exact nature of the spacetime interval

The book I will reference here is (specifically page XXXV of) The Geometry of Physics by Frankel. I asked this question a long time ago when I was a complete newcomer to general relativity. There was ...
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### Why use two spacetime indices to label Lorentz generators?

I've seen (e.g. in Srednicki) the following notation for the connection between a Lorentz transformation $\Lambda$ and the Lorentz generators $M^{\mu\nu}$: \begin{equation} {\Lambda^\mu}_\nu = {\left(...
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### Showing that the Wess-Zumino Lagrangian is invariant under a SUSY transformation

I want to show that the free Wess-Zumino Lagrangian is invariant under a SUSY transformation, e.g. following this reference (section 3.1). However, I have a hard time understanding the daggers and ...
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### In relativity, why does spacetime diagram have position on $x$-axis and time on $y$-axis?

In regular Newtonian physics, we use $x(t) - t$ graph to under position of a particle but why is it that in special relativity that we switch up the axis notation? I was seeing the lecture by Leonard ...
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### How to calibrate potential function?

The original question: Given sphere with radius $R$ and charge $Q$, and an infinite wire that goes through the center of the sphere with charge density $\lambda$. Find an expression for the ...
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### Direction of Integration of Electrostatic force over a circular arc, direction of the resultant force changing with interchange in limits?

Problem: A uniformly charged circular arc with linear charge density $m$ subtends angle $\theta$, find the net force acting on a charge placed at its center if total charge of the arc is $Q$ and the ...
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### A confusion about the direction of the current density

I'm contradicting my premise again and again and I have no idea what I'm doing wrong. Say there is a given current density in a charged infinite cylinder, $J$ in the $\hat{z}$ direction. It follows ...
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### How to decide which way are the limits of an integral?

So, I'm trying to calculate the electric field at a point $r$ distance away on the perpendicular bisector of a finite line charge having uniform charge density $\lambda$ I arrive at the following ...
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### Why isn't the work done by gravity positive in this situation?

I want to find the work done by the force of gravity to move an object of mass $m$ from infinity to a point $P$ at distance $r_p$ from a body of mass $M$ (that I assume fixed). The formula should be \...
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### The form of gauge invariance

When imposing local gauge invariance for a simple lagrangian describing a free Dirac fermion field: $$\mathcal{L}=\bar{\psi}\left(i\gamma^{\mu}\partial_{\mu}-m\right)\psi$$ Is there a particular ...
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### Addition of a constant potential in the Schrodinger equation

A really elementary question: Solutions of the single particle, non-relativistic, time-dependent Schrödinger equation for a spinless particle of mass $m$ alone in space, with no forces acting on it, ...
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### Changing the convention of positive and negative charge holders in an atom

Would there be any specific change in the physical/chemical laws if one day we changed the convention that electrons are negatively charged to the one that electrons are positively charged and vice ...
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### What is the sign of Weinberg angle?

One has $\cos{\theta_W}=m_W/m_Z$, and $\sin^2{\theta_W}=1-(m_W/m_Z)^2$ Is the sign of the Weinberg angle defined or an important quantity? If yes, are there constraints?
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### Can we set an arbitrary reference level of elastic potential energy

$$ΔE_e=\frac{1}{2}k(x_f^2-x_i^2)$$ Where $k$ is the spring constant and $x$ is the displacement from equilibrium position. Are we allowed to select an arbitrary reference level of elastic potential ...
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### Why does taking sign convention in Ray optics generalize the formulas?

I have read a lot about why sign conventions are used in ray optics. It is because the formulas are different for different lens and object. My teacher also said that sign conventions are like the ...
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### Is the sign convention necessary in solving kinematics?

I have started with kinematics and I have a really basic question of following sign convention i.e taking up and right positive and down and left negative. Is such a convention necessary when I can ...
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### Why is the annihilation operator associated with the positive frequency solutions?

I am looking for a qualitative explanation, if possible, of the following passage from Peskin & Schroeder, bottom of page 26, concerning the quantised real Klein-Gordon field: A positive ...