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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96
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25k views

Why was carbon-12 chosen for the atomic mass unit?

The atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Was there any physical reason for such a definition? Were they trying to include electrons in the atomic mass unit? Why not ...
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Why is it “bad taste” to have a dimensional quantity in the argument of a logarithm or exponential function?

I've been told it is never seen in physics, and "bad taste" to have it in cases of being the argument of a logarithmic function or the function raised to $e$. I can't seem to understand why, although ...
81
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6answers
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Why does public mains power use 50-60 Hz and 100-240 V?

Is there a physical reason behind the frequency and voltage in the mains electricity? I do not want to know why exactly a certain value was chosen; I am rather interested to know why that range/order ...
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10answers
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Should zero be followed by units? [duplicate]

Today at a teachers' seminar, one of the teachers asked for fun whether zero should be followed by units (e.g. 0 metres/second or 0 metre or 0 moles). This question became a hot topic, and some ...
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11answers
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Why is the charge naming convention wrong?

I recently came to know about the Conventional Current vs. Electron Flow issue. Doing some search I found that the reason for this is that Benjamin Franklin made a mistake when naming positive and ...
65
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What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?

The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the ...
49
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2answers
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Difference between $\Delta$, $d$ and $\delta$

I have read the thread regarding 'the difference between the operators $\delta$ and $d$', but it does not answer my question. I am confused about the notation for change in Physics. In Mathematics, $\...
42
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1answer
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Why is the partition function called ''partition function''?

The partition function plays a central role in statistical mechanics. But why is it called ''partition function''?
42
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Identification of particles and anti-particles

The identification of an electron as a particle and the positron as an antiparticle is a matter of convention. We see lots of electrons around us so they become the normal particle and the rare and ...
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4answers
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Can units be plural? [duplicate]

I was in a conversation with my senior engineer where he kept on insisting that we can use plural when we write down any unit. I argued that it is not the 'common' practice or even throughout my whole ...
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Square bracket notation for dimensions and units: usage and conventions

One of the most useful tools in dimensional analysis is the use of square brackets around some physical quantity $q$ to denote its dimension as $$[q].$$ However, the precise meaning of this symbol ...
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2answers
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Symbols of derivatives

What is the exact use of the symbols $\partial$, $\delta$ and $\mathrm{d}$ in derivatives in physics? How are they different and when are they used? It would be nice to get that settled once and for ...
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7answers
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Why is gravitational potential energy negative, and what does that mean?

I usually think of gravitational potential energy as representing just what it sounds like: the energy that we could potentially gain, using gravity. However, the equation for it (derived by ...
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5answers
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Why do we use the electron volt?

Why do we use the electron volt? Why did it come to be the electron volt and not, say, just a prefix of the joule, like the nanojoule? Does the electron volt represent anything particular as far as ...
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4answers
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Why is the potential energy equal to the negative integral of a force?

Why is the potential energy equals to the negative integral of a force? I am really confused with this negative sign. For example, why there is a negative sign in the gravitational potential energy ...
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2answers
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Why is the cut off mass for massive stars 8 solar masses? Why can't it be 10-11 solar masses or so?

I know that stars having a mass greater than or equal to 8 solar masses are termed "massive stars". But why is the cut-off 8 solar masses?
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4answers
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Why are grams usually only expressed as milligrams, grams or kilograms?

I'm a physics (and electronics and astronomy, etc.) enthusiast. As I learn and research topics, I notice that many SI units are often expressed using a variety of prefixes, such as in electronics ...
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12answers
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Pulley system. Why do I need to put a minus sign?

I am a mathematics undergraduate who's struggling to understand elementary physics. The following exercise looks particularly obscure to me. A $3.70\,\text{kg}$ mass (let's call it $m_1$) is ...
17
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3answers
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If electrons were positive and protons were negative, would life be different? [duplicate]

This was a question on a worksheet during my first week in a class on Electromagnetism. The answer is essentially: No. Life would be no different if electrons were positively charged and protons ...
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4answers
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Why is the mol a fundamental physical quantity? [duplicate]

I am starting to study physics in detail and as I read about physical quantities, I was puzzled why mol (amount of substance) is taken as a physical quantity. A physical quantity is any quantity ...
17
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2answers
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Why add a minus sign in the formula for gravity?

Why do we add a minus sign in our formula for gravity, when we might as well choose the unit vector $r_{21}$, instead of $r_{12}$? I'm just wondering why we choose this convention. Is it because it'...
17
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3answers
4k views

Why is ampere still a base unit? [duplicate]

The ampere is still a base unit, according to the SI brochure. However, in my perception the recent redefinition of units effectively defines the Coulomb as e/(1.602 176 634 × 10^−19), and the ampere ...
16
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3answers
379 views

Does swapping “$i$” with “$-i$” change a physical theory?

Mathematically speaking, "$i$" and "$-i$" are the two roots of the equation $x^2+1=0$ and it seems to me at least that there is no obvious way of distinguishing between them. Thus, ...
15
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6answers
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Why the electric potential of Earth is zero?

For a localized charge distribution the potential is set to zero far away from the charge distribution (at infinity) Now, when grounding a conductor, i.e. connecting it to Earth, it is said that we ...
15
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4answers
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Why is an electron negatively charged, and what is the difference between negative and positive charges?

Nobody has yet defined the actual meaning of a charge, or why a negative charge is different from a positive charge. Everybody knows that positive charge is due to protons and negative charge is due ...
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3answers
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Why is Fourier space called as momentum space?

If I take a periodic wavefunction $\psi\left(\vec{r}\right)$ and then take the Fourier space dispersion of the wave function as defined below $$ \psi(\vec{k})=\iiint_{-\infty}^{+\infty}\psi\left(\vec{...
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Near Earth vs Newtonian gravitational potential

Newton's Law of Universal Gravitation tells us that the potential energy of object in a gravitational field is $$U ~=~ -\frac{GMm}{r}.\tag{1}$$ The experimentally verified near-Earth gravitational ...
14
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4answers
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Are insulators and conductors arbitrary categories?

I have seen charts showing the transition from insulator to semi-conductor is at $10^{-8}~\frac{\text{S}}{\text{cm}}$ and between semi-conductor and conductor is $10^{3}~\frac{\text{S}}{\text{cm}}$. ...
14
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1answer
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What is a base unit in the new SI, and why is the ampere one of them?

One question that comes up pretty much always in introductory electromagnetism courses is Why the base unit of electrical measurements is the ampere and not the coulomb, and the usual answer is that ...
13
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2answers
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Why is the Newtonian expression for kinetic energy called the “first order” approximation of the relativistic expression?

In many texts, the non-relativistic (Newtonian) kinetic energy formula $$\text{KE}_\text{Newton} =\frac{1}{2}mv^2$$ is referred to as a first order approximation of the relativistic kinetic energy $$\...
13
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3answers
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Thermodynamics - Sign convention

I use the sign convention: Heat absorbed by the system = $q+$ (positive) Heat evolved by the system = $q-$ (negative) Work done on the system = $w +$ (positive) Work done by the system = $w -$ (...
13
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3answers
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Why is a conservative force defined as the negative gradient of a potential?

I'm learning about work in my dynamics class right now. We have defined the work on a particle due to the force field from point A to point B as the curve Integral over the force field from point A to ...
13
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4answers
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Why does the direction of a dipole moment go from negative to positive charge?

An electric dipole moment is defined as $p = q\times d$ (for two point charges $\pm q$ separated by a distance $d$). What is the physical meaning of this quantity? Why does the direction of the ...
13
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4answers
870 views

Do bras and kets have dimensions?

I'm trying to understand more intuitively what bras and kets are, but some aspects of them remain a mystery to me. We usually think of $\psi (x)$ as having dimension of $[1/\sqrt{L}]$ so that ...
12
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3answers
4k views

Earthing a conductor

This may be a basic question, but I have never understood it completely: why is an earthed conductor always at zero potential? I would say it is because theoretically one can suck up charge from the ...
12
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2answers
1k views

What is precisely the energy scale of a process?

Coupling constants run with the energy scale $\mu$. But what is exactly this energy scale. My question is, if I have a physical process, how do I compute $\mu$?
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2answers
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Variation of the metric with respect to the metric

For a variation of the metric $g^{\mu\nu}$ with respect to $g^{\alpha\beta}$ you might expect the result (at least I did): \begin{equation} \frac{\delta g^{\mu\nu}}{\delta g^{\alpha\beta}}= \delta^\...
11
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1answer
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Who (and Why) started the “electrons are negative, protons are positive” convention? [duplicate]

For some reason everyone labels electrons using a minus sign and protons using a positive sign, even though the opposite seems more intuitive: Who started the convention that electrons should be "...
11
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4answers
5k views

How to indicate that a unit is dimensionless [duplicate]

For my dissertation I am preparing a list of symbols used in the text, which basically is a table that consists of the symbol, a short explanation and the dimension it has as indicated below: ...
11
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3answers
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Advantages of using different metric signatures in relativity and QFT

I am studying General Relativity and some basic QFTs. It bothers me a lot that different books use different metric signatures, i.e. $(-+++)$ and $(+---).$ Can anyone tell me the advantages of using ...
11
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2answers
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Historical reason behind using $ν$ instead of $f$ to stand for frequency in the equation $E=hν$?

Normally, we use the letter $f$ to stand for frequency in equations. $$T = 1/f$$ $$v = \lambda f$$ $$Φ +E_k = h f$$ So I'm curious as why the letter $ν$ (nu) is used to represent frequency in the ...
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3answers
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Performing Wick Rotation to get Euclidean action of a scalar field $\Psi$

I'm working with the signature $(+,-,-,-)$ and with a Minkowski space-time Lagrangian $$ \mathcal{L}_M ~=~ \Psi^\dagger\left(i\partial_0 + \frac{\nabla^2}{2m}\right)\Psi. $$ The Minkowski action is $$...
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7answers
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Why does it make any physical sense for a body to have negative potential energy?

Sorry to ask the long way, but trying to make the question clear so that I can get a clear answer. Why does it make any physical sense for a body to have negative potential energy? If a body A is ...
10
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6answers
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Why is the standard uncertainty defined with a level of confidence of only 68%?

The widely used and accepted form of representing the uncertainty of a measurement is described in the GUM (Guide to the Expression of Uncertainty in Measurement). There it is recommended to use the ...
10
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2answers
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Normalising Generators of a Lie Algebra

Ok, so I'm asking this in physics because I'm currently working through part of Srednicki's text on QFT, even though it's really a maths question. In Srednicki's chapter on non-Abelian gauge theory, ...
10
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3answers
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What is the meaning of the double complex integral notation used in physics?

In Altland and Simons' condensed matter book, complex Gaussian integrals are introduced. Defining $z = x + i y$ and $\bar{z} = x - i y$, the complex integral over $z$ is $$\int d(\bar{z}, z) = \int_{-\...
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1answer
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Is 4-volume element a scalar or a pseudoscalar in special relativity?

In general relativity 4-volume element $\mathrm{d}^4 x = \mathrm{d} x^0\mathrm{d} x^1 \mathrm{d} x^2\mathrm{d} x^3$ is clearly a pseudoscalar (or scalar density) of weight 1 since it transforms as $\...
10
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3answers
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Staggered Indices ($\Lambda^\mu{}_\nu$ vs. $\Lambda_\mu{}^\nu$) on Lorentz Transformations

I have some open-ended questions on the use of staggered indices in writing Lorentz transformations and their inverses and transposes. What are the respective meanings of $\Lambda^\mu{}_\nu$ as ...
9
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2answers
4k views

Why is the work done on a charge calculated from infinity?

Why is the work done on a charge calculated from infinity to a point? Why not from one particular point to other?
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2answers
2k views

Why do we still not have an exact (constants-based) definition for a kilogram?

I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It would seem this should be fairly important. Edit: ...

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