Use this for questions relating to the proper use of physics terminology or nomenclature.

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240 views

Exact (quantitative) definition of “knee” and/or “cut-in” voltage?

Sort of a 2-part question: 1) I know they are used interchangeably but are the "knee" and "cut-in" voltages of a diode actually the exact same thing? I had thought that the "cut-in" voltage was ...
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2answers
129 views

Confusion with Blackbody Radiation

A blackbody is a theoretical object that perfectly absorbs all the light that falls on it. From what I understand this is an ideal situation and does not actually exist in reality. Certain objects are ...
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0answers
39 views

What does 'vector-like' mean?

What are properties of vector-like field/particle? What's the counterpart of it? Chiral like?
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1answer
1k views

Is there a difference between Electric and Electrostatic Field?

Is there a difference between Electric and Electrostatic Field? All I know is that they both represented with same law suppose we have a Charge placed at the Origin: $$E=\frac{1}{4\pi\epsilon_0}\...
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3answers
789 views

Why are diffraction gratings not called interference gratings? [closed]

It seems to me that diffraction gratings are completely described by the double slit experiment-why then is it called a diffraction grating?
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0answers
162 views

What is a Fermi arc?

What is meant with a Fermi arc in the context of Weyl semimetals? Is this the just a one-dimensional Fermi surface? For example, in electron-doped graphene, the Fermi surface consists of 2 disjoint ...
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2answers
53 views

Sound speed vs Speed of sound

Are 'sound speed' and 'speed of sound' the same thing? If not, what is the difference? If they are, could you clarify how the speed of sound applies in the below description of gaseous clouds? ...
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1answer
33 views

What is the difference between hydraulic radius and geometrical radius in a cylindrical pore?

What is the difference between hydraulic radius and geometrical radius in a cylindrical pore? I was going through the 'stacks' present in an thermoacoustic refrigerators there I came across the ...
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5answers
470 views

Is Parity really violated? (Even though neutrinos are massive)

The weak force couples only to left-chiral fields, which is expressed mathematically by a chiral projection operator $P_L = \frac{1-\gamma_5}{2}$ in the corresponding coupling terms in the Lagrangian. ...
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1answer
169 views

What is a point transformation?

This problem comes from Goldstein. What does $s=e^{\gamma t}q$ mean? Do I just put $q=e^{-\gamma t}s$ into the Lagrangian? But I don't know what that means. I think the point transformation may ...
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1answer
61 views

Is “applying a voltage” the same as “applying a potential” to an electrode?

From what I understand, voltage is the potential difference, but it seems like the terms are used interchangeably. This is confusing me because I am only just learning what these terms mean. I'm also ...
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4answers
15k views

What is the difference between a pulse and a wave?

I wanted to ask what is the difference between a pulse and a wave? According to the definitions of them, they are almost the same. In the websites I looked at, the difference between them was ...
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1answer
49 views

How Does Rotational and Turbulent Motion Contribute to the Pressures of Dynamical Systems?

Rotational and turbulent motion can deepen a gravity well in a galaxy or galaxy cluster. In an analysis of the mass components of a large body, this would appear to be one of the components. Does it ...
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6answers
12k views

Difference between theoretical physics and mathematical physics?

I'm a huge fan of mathematical physics and I know what the formal definitions of those two areas are, I've seen them. But I still get completely baffled when someone asks me to explain it simply. The ...
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1answer
46 views

What does “n-particle reducible” mean?

I am reading Ramond and in page 112 he says "In $\lambda \phi^{4}$ theory, diagrams can be at most three-particle reducible". My question: whether the individual Feynman Diagrams are treated as ...
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4answers
2k views

What is pure energy?

Inside the core of a star thermonuclear fusion reaction fuses hydrogen atom into helium releasing massive heat/light and energy.When a black hole eats up enough stars and gases it devours itself by ...
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0answers
30 views

What is the pure form of energy? [duplicate]

What is the pure form of energy? Einstein in his energy mass equation derivation said that electromagnetic radiation is pure form of energy.
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3answers
160 views

GR: Pseudo Riemannian or Riemannian?

Is General Relativityy described by Pseudo-Riemannian manifold or Riemannian manifold? I cannot understand the vast difference between the two manifolds. In books, General Relativity is looked as a ...
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0answers
23 views

What does “at%” refer to in sol-gel method?

I'm reading on the preparation of ZnO thin films for UV stimulated emission and I found a method called Sol-Gel. During this I found a paragraph saying, " the ratio Ag/Zn in the sol is 3 at%......". ...
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0answers
12 views

What is the difference between a planet and a dwarf planet? [duplicate]

What is the difference between a planet and a dwarf planet? And is there a way to determine a dwarf planet from a normal planet in a unknown solar system? (If we discover a unknown solar system)
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2answers
710 views

What is the difference between a hollow (sphere/cylinder) and a shell (cylinder/sphere)? [closed]

First of all, it was hard to "tag" this question ! Anyways, I see some contradictions in what a shell or a hollow cylinder is in some sources in the internet based on some figures they show. I ...
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1answer
106 views

What is the difference between a quasar and an Active Galactic Nucleus (AGN)?

In answering another Phys.SE question about quasars - Why no new quasars? - an issue arose about which object is the nearest quasar. That got me puzzled. To what is the label "quasar" attached, as ...
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2answers
68 views

What is $\mathcal{L}_M$?

Usually we derive the Einstein field equation in vacuum starting from E-H action $$S= \int{\sqrt{-g}d^4x(\frac{c^4}{16\pi G})R}.$$ But in case we wanted to get $$R_{\mu\nu}-\frac{1}{2}g_{\mu\nu}R=\...
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4answers
66k views

What is the difference between phase difference and path difference?

I have learnt that path difference is the difference between the distance travelled by two waves meeting at a point. If that is path difference,then how will one know what is phase difference and how ...
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1answer
55 views

What does dimensionless quantity 'number of $g$' mean?

I am doing data analysis in which I found a quantity named "no. of $g$". I don't know what it means or what is its usage. Look at the image below. I want to know the meaning and usage of "no. of $...
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1answer
59 views

Naming anti-electrons and anti-protons [closed]

I think I understand why an anti-electron is called a positron (since it symbolizes positive charge) But why then is anti-proton called an anti-proton and not a negatron?
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1answer
304 views

Which transformations are canonical?

Which transformations are canonical? Why do canonical transformations preserve the measure of integration in phase space?
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1answer
1k views

Where did the Word 'Physics' Come From? [closed]

As the title suggests, where did the name for the discipline 'Physics' come from? - does it mean something in Greek?
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1answer
50 views

What is an awkward constant and is there a relation to ad-hoc hypotheses?

[I'm not entirely sure whether this is the right board, since it is not a technical but a soft question]. I'm reading the textbook "Spin Dynamics - Basics of Nuclear Magnetic Resonance", 2nd ed., (...
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3answers
2k views

What is a mode?

Admittedly, this seems like a very simple question. The word mode pops up in every field of physics, yet I can't remember ever having read what I felt was a precise and sensible definition. After ...
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3answers
933 views

What is meant by a “c-number”?

In Chapter 2 of David Tong's QFT notes, he uses the term "c-number" without ever defining it. Here is the first place. However, it's easy to check by direct substitution that the left-hand side ...
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2answers
1k views

What do physicists mean when they say “speed of light”?

Does it make sense to say, "The speed of light varies?" Some may say right off the bat "Yes, it changes as a wave passes through a different medium." However, I'd like to say no, because when I hear ...
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0answers
95 views

Is the phrase “coupling constant” interchangable with “ strength of interactions”?

Can I use the terms coupling constant and strength of interactions, interchangeably, or are there more subtleties to the term coupling constant that I am not aware of? Coupling Constants from ...
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1answer
65 views

Is there a special name for thermodynamic process during which no work is performed?

Let $W$ denote the work done on a system during a thermodynamic process. Is there a commonly-accepted, dedicated term for a process during which $W=0$?
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1answer
34 views

5UDH Tandem electrostatic accelerator

What does UDH stands for in the name of an electrostatic accelerator 5UDH-2 Tandem accelerator? Does it is related to its working or just a trade mark?
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2answers
120 views

What is meant by the velocity of a star?

I recently read somewhere that among other things like size, radius, distance from earth, luminosity, age, etc of a star, velocity was another variable. What is exactly meant by the velocity of a star?...
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2answers
213 views

The role of anharmonic oscillator(s) in Heisenberg's 1925 paper

I am talking about the most famous paper of Heisenberg, which I know from the translation of van der Waerden (Sources in Quantum mechanics, North Holland, 1967). After introducing matrix mechanics ...
3
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1answer
490 views

What is meant by a “stiff” or “soft” equation of state (wrt neutron stars)?

I am currently trying to understand the history of the development of the equations of state and structure of neutron stars. In my textbook, I frequently encounter phrases such as "The ...
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4answers
3k views

What's the difference between an entangled state, a superposed state and a cat state?

1) Can a state be entangled without also being a superposition? (Please give an example.) 2) Can a state be a superposition without being entangled? (Again, an example please.) 3) And what about ...
2
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0answers
65 views

QFT: What does “finiteness” mean?

As above: what is the definition of a QFT to be "finite"? That all UV corrections are finite and there are no divergences at all? That there are divergences, but these divergences can be absorbed ...
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1answer
142 views

Quantum mechanics successive measurements (incompatible observables) [closed]

I started a "principles of quantum mechanics" course this year, and already have a problem with successive measurements (using Dirac notation). The question is about incompatible observables A and B. ...
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1answer
294 views

What is meant by 'probability of transition per unit time'?

Today I came across a term used by Feynman in his thirteenth lecture: 'probability per unit time' to go from $| 1\rangle$ to $|2\rangle$ while initially being at $|1\rangle$. This is the excerpt fom ...
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3answers
1k views

Quantum field theory variants

Wikipedia describes many variants of quantum field theory: conformal quantum field theory topological quantum field theory axiomatic/constructive quantum field theory algebraic quantum field theory ...
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1answer
69 views

Understanding “Propagation in a Crystal Lattice”: What is the difference between 'amplitude leakage' & 'probability leakage'?

I started reading, today, Chap 13: Propagation in a Crystal Lattice of Feynman's Lectures III. But, I couldn't understand some of his writings as: If you have a harmonic oscillator which is ...
2
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2answers
631 views

What is meant by thermal penetration depth?

What is meant by thermal penetration depth? I am doing a project on Thermoacoustics. while researching I came across about thermal penetration depth.I searched over the net but i didn't get a clear ...
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1answer
134 views

Definition of the “support” of the reduced density matrix

Some of the papers in condensed matter physics use the word "support" (space). For example, the following papers use the support especially for the reduced density matrix. http://journals.aps.org/prb/...
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0answers
40 views

Meaning of 'nuclear surface vibrations'?

Would like to know the meaning of nuclear surface vibrations as mentioned by Bohr in his paper titled 'The coupling of nuclear surface vibrations to the motion of individual nucleons'. The paper talks ...
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1answer
43 views

Is “approximative reduction” general knowledge to physicists?

I came across this concept called "approximative reduction", about which there are some papers, e.g. in this collection called Structure and Approximation in Physical Theories. Very briefly, it ...
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0answers
36 views

Nomenclature of nuclear excited states

I read in an online portal about $^{112}$Sn nucleus making a transition from $0_{g.s}^{+} \rightarrow 2_{1}^{+}$ state. Also, some higher excited states were named as $0_{2}^{+}$, $3_{1}^{-}$, etc. ...
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1answer
124 views

What did Feynman mean by 'energy shift' here?

I was reading Feynman's Lectures III's Chapter 10: Other Two-State Systems. There he discussed about hydrogen molecular ion having two base states: The amplitude of the molecule to go from $|1\...