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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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Why there is a negative sign in the formula of calculating work done in electrostatics?

$$W = - \int _ { a } ^ { b } \vec { F } \cdot d \vec { r }$$ ( from The Feynman lectures on physics vol. 2, new millennium edition, page 4-4) In this formula why there is a negative sign in the ...
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Question on notation for the inner product of complex vectors

Regarding the wiki: https://en.wikipedia.org/wiki/Sesquilinear_form#Hermitian_form you can see that the wiki states that physics defines the inner product for complex vectors as: $$\langle \, \...
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3answers
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$\Delta K=\Delta U$ vs $\Delta K = -\Delta U$

While doing physics homework, I noticed that in some problems, the change in kinetic energy(KE) is equal to the change in potential energy(PE), even though I learned that conservation of energy shows ...
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1answer
60 views

Exact meaning of 'degree'

I wish to know if there is an exact meaning of degree in physics/math/chemistry. It is used in many cases and it is not clear to me which requirements must have an unit of measurements for carrying ...
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4answers
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Confusion about electron flow and current

So, I learned in class that current is defined as positive charges moving from south to north. However, in all reality, the negative charges are moving, but the convention of positive to negative wors ...
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3answers
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Why is electric field $E^i = E_i$ instead of $E^i = - E_i$?

Let us consider the Minkowski spacetim. Generally, we know that when we lower or raise the index of the convariant or contravairant tensor, we need to use the metric $\eta^{\mu \nu}=\eta_{\mu \nu}=(+,...
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6answers
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Why the work done is positive when bringing 2 opposite charges together?

we know that if the applied force is in the direction of the displacement then work done is positive.But in case of bringing 2 opposite charges from infinite to a certain distance,the work done is ...
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Fourier transform of some discrete, finite, non-uniform signal

Suppose I have some finite signal $x(t)$ of $N$ data points. This signal is produced by some program or some experiment and so is discrete and the difference in time between each data point $\delta t$ ...
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SI redefinition of the kilogram - what is one measuring? [duplicate]

I have been reading about the new SI units and specifically, want to get a better understanding of the definition of a kilogram. It was written that the kilogram will be defined in terms of Planck's ...
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2answers
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Derivation of the Electromagnetic Stress-Energy Tensor in Flat Space-time

I am working on deriving the electromagnetic stress energy tensor using the electromagnetic tensor in the $(-, +, +, +)$ sign convention. However, I have hit a snag and cannot figure out where I have ...
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1answer
53 views

LSZ reduction, momentum diagram, QFT

I was initially confused about which way to choose the sign of the momentum, since it gives rise to different exponential momentum combinations and thus different deltas for momentum conservation. I ...
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0answers
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MKS, why we need 7 base units? [duplicate]

What were the units of temperature in MKS system? Why do we need 7 fundamental units? I think we need only 2: kg, s. All other candela, mol, etc. Should be called derived ones.
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Not understanding about the sign of Fourier transform of power spectrum [migrated]

I don't understand with the below equations the affirmation that $FT(\Delta(\vec{k})$ is the Fourier transform of $\Delta(\vec{r})$ : $$\left\{\begin{array}{l}{\Delta(\vec{r})=\frac{V}{(2 \pi)^{3}} \...
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1answer
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Basic Index Raising and Lowering Question [duplicate]

I am trying to understand the order of the indices when raising or lowering tensors. For example, the electromagnetic tensor: $$F^{\alpha \beta} = \begin{bmatrix} 0 & -\frac{E_{x}}{c} &...
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1answer
47 views

Itzykson & Zuber: Conjugate momentum sign

I can't give myself peace on a confusion about the signs. I'm studying on Claude Itzykson & Jean-Bernard Zuber, Quantum Field Theory, Dover Publications. Metric convention $g_{\mu\nu}=diag(1,-1,-1,...
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2answers
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Electromagnetic Stress-Energy Tensor in curved space-time

I found on Wikipedia that the electromagnetic stress energy tensor in curved space-time with sign convention $(-, +, +, +)$ is $$T_{\mu\nu} = -\frac{1}{\mu_0} \left ( F_{\mu \alpha} g^{\alpha \beta} ...
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1answer
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Path integral calculations $e^{i\omega 0^+}$

When computing correlation functions using the path integral formulation, I often need to compute integrals such as $$ \int_{-\infty}^\infty \frac{d\omega}{2\pi} \frac{1}{i\omega -\epsilon} $$ ...
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1answer
52 views

Sign of Lagrange multiplier

Hello I have a short question. Say I would consider a pendulum and define the Lagrangian as usual being \begin{align} L = \frac{1}{2} m(\dot{x}^2 + \dot{y}^2) - mgy \color{red}{-} \lambda (x^2 + y^2 ...
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0answers
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Direction of the current and the electrons [duplicate]

If the electric current is defined as Coulomb/second, why is the direction of the current is just the opposite of the direction of the flow of electrons?
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4answers
102 views

Direction of $d\mathbf{l}$

A solid sphere has charge $q$ and radius $R$. Find the potential at a point a distance $r$ from the center of the sphere where $r>R$, using infinity as the reference point. My attempt: From Gauss'...
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2answers
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Sign in $\Delta U= -\int\vec{F}\cdot d\vec{l}$

So I have been trying to understand why there is a negative sign in the following$\Delta U= -\int\vec{F}\cdot d\vec{l}$ I wanted to try to understand it by trying to derive the gravitational ...
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2answers
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The Electromagetic Tensor and Minkowski Metric Sign Convention

I am trying to figure out how to switch between Minkowski metric tensor sign conventions of (+, -, -, -) to (-, +, +, +) for the electromagnetic tensor $F^{\alpha \beta}$. For the convention of (+, -,...
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1answer
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$\delta^{(2)}$ convention

In this note: https://arxiv.org/abs/hep-th/0410165 at page 12 there is a delta-function constraint written as: \begin{align} \delta \left( ^ { U } M \right) = \prod _ { i < j } \delta ^ { ( 2 ) } \...
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1answer
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Arrow convention for neutral particles in Feynman diagram

In a Feynman diagram, the arrows on the charged particles are along the direction of negative charge flow. As a result, the arrow is not always directed from the vertex of creation and not always ...
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Origin of 2π normalization factor in chemical $J$-coupling compared to the Heisenberg model

In chemistry, particularly the field of NMR spectroscopy, the interaction between two (nucleic) spins (or so I guess?) is governed by the Hamiltonian: \begin{align} \mathcal{H}=2\pi\cdot J_{ij}{\vec {...
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2answers
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Computation in QFT

I'm always a mess with the upstairs and downstairs notation. To be specific, say I want to calculate the Euler-Lagrange equations of \begin{equation} \mathcal{L} = \frac{1}{2}\partial^\mu\phi \...
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4answers
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Deriving the formula of potential difference duo to a solid conducting sphere

The doctor in university was deriving a formula and I can't understand how it works A sphere with charge Q The Sphere's radius is R, and we are trying to derive a formula for potential difference at ...
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1answer
35 views

Sum over real photon polarizations. The minus sign

Ok for real photons there is the formula when summing over the polarizations: $$ \sum_{\lambda=\pm}\epsilon^{*\mu}_\lambda\epsilon^\nu_\lambda = -\eta^{\mu\nu}$$ But if I have a matrix element of ...
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2answers
54 views

Why is there no negative sign in the formula of work against electric field?

Why is there no negative sign in the first formula of work done against electric field where as second formula below has it?
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1answer
64 views

What exactly makes negative energy negative?

Antimatter is the opposite of matter since it has opposite electric charge (e.g. proton -> positive whilst anti-proton -> negative). So what exactly makes negative energy negative? What property does ...
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1answer
39 views

Coordinate axes, convention, and the isotropy of space

Say something has cylindrical symmetry so I align it on an axis to take advantage of that symmetry (this will make things like calculating the volume of a cylinder much easier if the $z$ axis pierces ...
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3answers
102 views

Why the electric potential inside a conductor doesn't equal zero?

Assume that a negative charge in the space near of it a small positive charge, Then for sure that the negative charge will attract the positive charge naturally because it has an electric field around ...
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2answers
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If an object in free-fall is moving upward, the object's velocity is decreasing. Does this mean that the acceleration is -9.8 m/s/s?

I have a problem (that I am not asking you to solve) where it takes $4s$ for an object to travel upward to its maximum height, and we have to determine how far the object travelled. If the ...
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0answers
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Confusion in sign convention of magnification?

A concave mirror forms a real image three times larger than the object on a screen. Object and screen are moved until the image becomes twice the size of object. If the shift of object is 6 cm. Find ...
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1answer
49 views

Confusion on Centigrade/Celsius Scale

There came the centigrade scale. The issue was that many solid/liquid thermometric substances didn't respond to temperature linearly so different thermometers produced different results. This was ...
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1answer
45 views

Why was this convention for Significant Figures set up?

In my textbook $3$ rules have been stated for rounding off digits for maintaining the same level of precision as in the provided experimental values, or simply for maintaining the number of ...
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2answers
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Is the voltage between the two points $A$ and $B$ denoted as $U_{AB}$ or $U_{BA}$? And why?

Consider the following circuit Is the voltage between the two points $A$ and $B$ denoted as $U_{AB}$ or $U_{BA}$? And why?
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1answer
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Doubt in a derivation for electric potential at a point

My physcis teacher in school has given me this derivation for electric potential at a point. What bothered me the most was why the small work was negative but the large work positive.Could anyone ...
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4answers
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Why do physicists take the convention that a force field is the negative gradient of a scalar field?

A conservative force is naturally the vector gradient of a given scalar field . I don't know why the convention to put the negative sign in front of the gradient operator. Or is this just a ...
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1answer
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Direction of dipole moment is different in different case [closed]

Why is it that the direction of dipole moment is different in Physics and Chemistry? I mean how can the same quantity have different directions in different subjects. A simple answer is definitely ...
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3answers
82 views

Potential energy of an oscillating pendulum

The following question always confuses me. for an oscillating pendulum why the potential energy is given by: $$V = mgL(1-\cos\theta)$$ Why not $$V = mgL\cos\theta$$ Is this a convention or there ...
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2answers
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Work kinetic energy theorem

I don't understand the wordings of this theorem. Can someone please help me in understanding this? Secondly on what basis are the sign conventions in this theorem applied? I get confused in plus and ...
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3answers
51 views

Inserting an arbitrary phase in the equation for driven damped oscillations

In Classical Mechanics by Taylor, we find the solution to the differential equation of a damped oscillator with a sinusoidal driving force: $$\ddot{x} + 2\beta\dot{x} + \omega_0^2x = f_0\cos\left(\...
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2answers
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Why have a conversion factor of $10 \:\rm mA / 10 \: mV$ instead of $1\:\rm A/1\:V$?

I’m working with a device that measures current by measuring voltage and then giving current with a conversion factor of $10 \:\rm mA / 10 \:mV$. Why write this instead of $1\:\rm A/1\:V$?
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2answers
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Why is the ground state of a particle in a box called $n=1$?

For a quantum harmonic oscillator, the ground state in most sources is referred to as $n=0,$ and this state has zero nodes. For a particle in a box, the ground state in most sources is called $n = 1.$...
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Name of real-valued representation of density matrix?

This is a specialization of my question https://math.stackexchange.com/q/3157300/ on math.SE. There are many ways to write the density matrix $\hat \rho$ as vector $\vec \rho$. In the Liouville space,...
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1answer
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General equation of a traveling wave [duplicate]

I have seen the equation of a wave written as  $y = A \sin (kx – \omega t)$. However sometimes I see it written as $y=A\sin(\omega t-kx).$ Please notify the correct one and provide sufficient reasons.
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Electrical systems with multiple connections to earth - is earth modelled as one region of equal potential here?

In the picture we can see a realization of a TT-System, an electrical system with multiple connections to earth at different localiations. I allready know (from this question) that the coulomb-...
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0answers
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Question about Pauli Matrices

I found the following identities about Pauli matrices from the lecture notes of Supersymmetry. $$((\sigma^{\mu})^{\alpha\dot{\alpha}})^{\ast}=(\bar{\sigma}^{\mu})^{\dot{\alpha}\alpha}$$ $$((\sigma_{\...
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1answer
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Virial radius of a collection of point masses

I am experimenting with multi body simulations in astronomy ($N$-body algorithms). It is customary to give the results in dimensionless units. One of them is a length unit, and the virial radius is ...