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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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Is gravitational potential energy of body by $mgh$ negative? [closed]

Consider a 15kg object at 1m from earth ground level, is P.E = 15kg * 9.8m/s^2 * 1m = 147J or P.E = -Gm1m2/r^2 * h = -9.8 * 15kg * 1m= -147J after browsing for a while on debate of potential energy's ...
Neptotech -vishnu's user avatar
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1 answer
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How Can there be a Gravitational Potential when there is NO Gravitational Field? [closed]

How does it make any logic that there exist a potential when there is no net field for example when we have a Hollow Sphere with mass we can find out the the gravitational *potential inside the sphere ...
Aditya Agrawal's user avatar
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1 answer
146 views

What's the difference? $\nabla_\mu e_\nu=\Gamma_{\mu \nu}^\rho e_\rho~\text{ and }~\partial_\mu e_\nu=\Gamma_{\mu \nu}^\rho e_\rho~?$

What's the difference? $$\nabla_\mu e_\nu=\Gamma_{\mu \nu}^\rho e_\rho~\text{ and }~\partial_\mu e_\nu=\Gamma_{\mu \nu}^\rho e_\rho~?$$ In John Dirk Walecka's book 'Introduction to General Relativity',...
Jianbingshao's user avatar
1 vote
4 answers
75 views

Why is work done by force $+mgh$ in the situation of throwing something up?

If there is a particle at point A(at rest) and a force moves it to point B(Above point A vertically)(final velocity = 0 at this point), the work done by gravity is $-mgh$. This I understand as the ...
Gaurav Batra's user avatar
0 votes
3 answers
55 views

Significant figures when product is more by a factor of 10

A rectangular plate has a length of 21.3 cm and a width of 9.8 cm. Calculate the area of the plate. A straightforward question, which you multiply and arrive at 208.74 cm2. Taking significant figures ...
InquisitiveLad's user avatar
2 votes
1 answer
51 views

$CP$-transformation for fermionic bilinears

I am trying to derive the transformation of the fermionic bilinear $\bar{\psi}\psi$ under $CP$ transformation. I know that $P$ acts as: $$\psi(t, \vec{x}) \xrightarrow{P} \gamma^0 \psi(t, -\vec{x})$$ ...
Damiano Scevola's user avatar
3 votes
1 answer
89 views

Equation for real/complex $\phi^4$ theory

On wikipedia (see this link), the Lagrangians of the $\phi^4$ equation for real AND complex scalar fields are given. One may derive the Klein-Gordon equation by inserting into the Euler-Lagrange-...
Octavius's user avatar
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1 answer
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Why did we choose Coulomb's constant $k$ as $9\cdot10^9 \rm Nm^2/C^2$ while define unit of charge instead of any other number?

I understand that while defining charge, Coulomb had to choose any arbitrary value of $k$ to describe unit of charge. But, why did we chose $9\cdot10^9 \rm Nm^2/C^2$ as the value of $k$, but not any ...
Singer Asvj's user avatar
2 votes
3 answers
110 views

Sign conventions for the Lagrangian from the EM Lagrangian density

In Chapter 13.6 of the 3rd edition of Goldstein's Classical Mechanics, Goldstein proposes the Lagrangian density of the electromagnetic field as: $$\mathcal{L} = -\frac{F_{\lambda \rho} F^{\lambda \...
tugboat2's user avatar
2 votes
4 answers
187 views

Is the gravitational potential energy of an object on the ground 0? [duplicate]

In class, we were reviewing kinetic and potential energy and my teacher claimed that on the ground, objects have potential energy. However, as they cannot fall further, isn't their gravitational ...
user386598's user avatar
3 votes
7 answers
849 views

Physical Quantities Sign Convention

I see that almost all physical quantities carry signs. But the confusion I have is what they really mean. Does negative velocity mean decreasing velocity or velocity in the opposite direction? Does ...
Singing Account's user avatar
0 votes
2 answers
82 views

What is the key difference between SI and MKS units?

In MKS as well SI system, the length, mass and time are measured in the units meter, kilogram and second, respectively. Then how the SI units are different from MKS units? What are the key ...
San26's user avatar
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1 vote
2 answers
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Does the exponential representation of Dirac delta function depend on choice of Fourier convention?

Is it always true that $$\delta(\omega) = \frac{1}{2\pi}\int_{-\infty}^{+\infty} e^{i \omega t} dt , $$ regardless of your Fourier convention? For example, if I choose to use the Fourier convention ...
photonica's user avatar
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2 votes
1 answer
265 views

Are zeros to the right really significant? [duplicate]

In one of YouTube lectures about significant digits I saw this: I was completely shocked. Both scientific notation numbers when converted to integer would represent same quantity of $1200 \text{kg}$ ...
Agnius Vasiliauskas's user avatar
3 votes
5 answers
1k views

Can a positive charge have a negative potential?

I'm studying about electrostatic potential due to a point charge and was wondering - can a positively charged point have negative electrostatic potential? V=Kq/r for a positive point charge. Assume B ...
Mel's user avatar
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How to add and subtract significant figures?

There are actually two ways given to add or subtract significant figures in different books .In books like concepts of Physics by HC Verma it is written that round of the numbers given first and then ...
ARNAV CHADHA's user avatar
1 vote
1 answer
58 views

Why does current lag behind voltage by 90° in an inductive circuit?

We can easily prove this with KVL, a little bit calculus, and trigonometry. We get voltage as a sine function and current as a cosine function. Then we apply 'Add sugar to coffee' and get that current ...
Harjot Dhillon's user avatar
1 vote
0 answers
38 views

Electrostatics: Electric Potential of Earth [duplicate]

I have a very basic question which is: Why we consider the potential of Earth to be zero? Generally in physics we consider the electric potential at infinity to be zero. But while reading about ...
Aritra's user avatar
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1 vote
2 answers
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Why is the overall phase of a wave traveling backward notated as $-kx - \omega t$ instead of $kx + \omega t$?

in textbook i read the equation for the wave equation that travel forward in the $+x$ direction is $$ y(x,t) = Ae^{i(kx -ωt)} $$ and for wave equation that travel backward in the $-x$ direction is $$...
Tulip Lhospita's user avatar
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2 answers
162 views

Are there any non-SI base units? [closed]

Are there any base units that are not in the SI system? If so why aren't they? Edit: By this I don’t mean other systems that you can convert to (ie. Feet, Ounces, Hours, etc) I mean new base units ...
Gabriel Tellez's user avatar
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0 answers
65 views

Hamiltonian flows and Poisson Brackets confusion

I have been using results from this paper in calculations. In sections 2.4 and 3.4 they perform a canonical transformation into new coordinates consisting of constants of motion. My question is ...
Geigercounter's user avatar
2 votes
1 answer
79 views

If helicity of photons is +1 then the light is right- or left-circularly polarized?

In particle physics, we say: a particle has +1 helicity (right-handed) if its momentum and spin are parallel, or it has -1 helicity (left-handed) if its momentum and spin are antiparellel. Now, if we ...
Atom63's user avatar
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3 votes
1 answer
83 views

Why is the term 'Accuracy of 1 part in $x$' used?

My question stems from how we measure the accuracy of Caesium clocks. Most Caesium clocks are said to have an accuracy of '$1$ part in $10^{14}$.' I understand that the terminology means to convey ...
Smarika Singh's user avatar
0 votes
1 answer
66 views

Name of metric used by Friedmann

In his original paper, Friedmann used the following dynamic and symmetrical metric: $$\mathrm{d}s^2=a(t)^2\left(\mathrm{d}\chi^2+\sin (\chi)^2\left(\mathrm{d}\theta^2+\sin (\theta)^2 \mathrm{d}\phi^2\...
Vincent ISOZ's user avatar
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0 answers
40 views

What convention, if any, exists regarding the algebraic signs of constants and variables such as in this exercise?

This problem is from the original exercise set to accompany The Feynman Lecture on Physics. I am absolutely not asking how to find the solution. I am asking if there is some convention regarding the ...
Steven Thomas Hatton's user avatar
0 votes
1 answer
35 views

Question about meaning of "bar"-ing in the context of Dirac fields

Following chapter 38 of Srednicki, "bar"-ing means (based on eq. 38.15) $$\bar{A} = \beta A^\dagger\beta$$ One can show for instance that $$\bar{\gamma^\mu} = \gamma^\mu$$ My question is, ...
JohnA.'s user avatar
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3 votes
4 answers
364 views

Regarding the signature of special relativity

in special relativity we add time as a dimension and replace euclidean space $ \mathbb{R}^4 $ with a pseudo-euclidean space $ \mathbb{R}^{1,3} $ of signature $ (1,3) $ by defining a quadratic form $\...
Tomás's user avatar
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1 answer
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Clarification regarding the meaning of Universal Time UT1

I've been reading the book "From Sundials to Atomic Clocks: Understanding Time and Frequency" by James Jespersen and Jane Fitz-Randolph which is available at https://www.nist.gov/system/...
russell.price's user avatar
11 votes
8 answers
4k views

Why do we associate negative sign to electric charges when they are a scalar quantity?

My confusion arose when going through the additive property of charges where charges with opposite signs subtract each other. My knowledge of electrostatics is limited, but I am sure that signs are ...
Shaan's user avatar
  • 121
-1 votes
2 answers
126 views

Why do we multiply the Euler-Lagrange equations by negative one?

As I've learned classical mechanics from different sources, I've seen both $$\frac{d}{dt} \left( \frac{\partial L}{\partial \dot{q}_k} \right) - \frac{\partial L}{\partial q_k} = 0,$$ and $$\frac{\...
wlancer's user avatar
  • 183
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0 answers
62 views

Why does Newtonian gravity necessarily imply that gravitational potential energy is negative? [duplicate]

Why can't the zero value of gravitational potential energy be set at a different point instead of infinite distance within Newtonian gravity? I am not asking why is gravitational potential energy ...
Manuel's user avatar
  • 476
0 votes
2 answers
96 views

The sign of Work in Turbomachines point of view

As I know from Thermodynamics work done by the system is negative and equals to $U=Q-W$. In this picture it clearly describes the process in which when we take some pebbles from the piston the system ...
Ulvi Rahimov's user avatar
-1 votes
3 answers
271 views

Why something connected to the ground has 0 potential?

I'm studying the concept of grounding of neutral wire in AC. I have done an extensive research on the internet to answer the question "Why the neutral wire has 0 potential ?". The answer is ...
InTheSearchForKnowledge's user avatar
1 vote
0 answers
39 views

Generator normalisation in the covariant derivative

A common convention for the definition of the covariant derivative in the SM is $$ D_\mu = \partial_\mu - i g_s \frac{\lambda^a}{2}G^a_\mu - \cdots $$ where $\lambda^a$ are the Gell-Mann matrices. In ...
infernophys's user avatar
1 vote
1 answer
66 views

Fierz idendity (supersymmetry)

So basically I have two Fierz identities involving spinors: $$\psi^a \psi^b = -\frac{1}{2} \epsilon^{ab} \psi \psi$$ And $$\overline{\psi}^{\dot{a}} \overline{\psi}^{\dot{b}} = \frac{1}{2} \epsilon^{\...
LSS's user avatar
  • 980
0 votes
0 answers
52 views

Matter vs antimatter asymmetry per particle [duplicate]

What is called matter and what is called antimatter is just a convention, isn't it? For example, suppose we call the bottom, the charm and the down quark antimatter and we call the strange, top, and ...
anoniem's user avatar
  • 147
0 votes
2 answers
93 views

What is actually electric current? [closed]

Electric current is the rate of flow of charges (electrons) or the rate of flow of positive charge. Okay, I get it. But here's my question. The electron flows in the wire and then the current flows in ...
Moksh Singh Dangi's user avatar
1 vote
2 answers
103 views

Defintion of gravitational potential

I am not much clear regarding the defintion of "gravitational potential": Is the work done for bringing the unit mass from infinity to that point by, gravitaional force or external force? (...
Cerebral cortex 's user avatar
0 votes
3 answers
57 views

A question regarding the concept of potential difference between two points in an electric field, as stated in my 12th grade book

My 12th grade physics book on electrostatics says: Potential difference between two points in electric field can be defined as work done in displacing a unit positive charge from one point to another ...
Hufaiza Hufaiza's user avatar
0 votes
1 answer
65 views

Significant Figures when dealing with bearings

Significant figures are used to ensure that the value is precise, and fall in within error in the positive and negative direction. 327 degrees true can also be written as N33degreesW. As such, would ...
Jay Chen's user avatar
0 votes
1 answer
90 views

The Klein-Gordon equation and the sign of the mass term

A derivation of the Klein-Gordon equation starts with the following lagrangian for a scalar field ϕ: $$ L=\frac{1}{2}g^{ab}(∇_a\phi)(∇_b\phi)-V(\phi) $$ If we plug this lagrangian in the Euler-...
DrD's user avatar
  • 141
0 votes
2 answers
99 views

Problem with understanding the definition of electric potential [closed]

Here is the definition I know: $V(\vec r)-V(\vec r_0)=\int_{\vec r_0}^{\vec r} -\vec E \cdot\vec dr$ I have lots of problems with this topic. How can I choose the starting point? and does it matter ...
Dor's user avatar
  • 11
2 votes
1 answer
120 views

Columns, rows, dotted, undotted, $SL(2, \mathbb{C})$ reps, and building Dirac spinors from Weyl spinors

I'm looking through Introduction to Supersymmetry by Muller-Kirsten and Wiedemann, along with any other resource I can find. I'm specifically trying to understand the concepts and notations for ...
Gleeson's user avatar
  • 213
0 votes
1 answer
88 views

Sign choice for line element while finding potential due to point charge [closed]

The canonical derivation for the $1/r$ potential due to a point charge is as follows: We consider an electric field of the form $$\mathbf{E}=\frac{q}{4\pi\epsilon_0 r^2}\hat{\mathbf{r}},$$ and ...
user1394273's user avatar
0 votes
1 answer
28 views

Signs terms including acceleration

Am I right in saying that accelerations can only be given a sign if a coordinate system is defined in relation to which they are described? Is this idea applied to any vector quantity? Is there a ...
Sylvia's user avatar
  • 123
0 votes
0 answers
42 views

Time direction ambiguity in stationary observer's 4-velocity

Working in the mostly + convention, for a timelike geodesic in Minkowski spacetime we have the requirement that (c=1): $$u^\alpha u_\alpha=-1$$ and that it of course obeys the geodesic equation. If we ...
Mario's user avatar
  • 49
2 votes
1 answer
119 views

Time evolution vs. Time translation in QFT

There is a certain sign mismatch between the time translation operator and time evolution operator in quantum field theory which I hope someone can illuminate. From my understanding, a Poincaré ...
Mishary Al Rashed's user avatar
0 votes
0 answers
29 views

Langevin dynamics with position dependent random force

Suppose I want to solve for some Langevin dynamics. Let us consider a single particle: $$m\ddot {\mathbf r}=-\eta\dot{\mathbf r}+\mathbf F$$ where $\mathbf r$ is position, $\eta$ is viscosity force ...
Mauricio's user avatar
  • 5,568
1 vote
0 answers
20 views

Rescaling the null coordinates

Given a $4$-dimensional spacetime described by four coordinates $(t,r,\theta,\phi)$, we usually define the null coordinates by, \begin{equation} u = \frac{t-r}{2}, \quad v = \frac{t+r}{2} \end{...
mathemania's user avatar
0 votes
1 answer
74 views

Why minus sign in definition of angular momentum field density tensor?

The following generalization of angular momentum is given on page 571 of the third edition of Goldstein's Classical Mechanics:$$\mathcal M^{ij}=-(x^iT^{j0}-x^jT^{i0}).\tag{13.44}$$The metric used in ...
Albertus Magnus's user avatar

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