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Questions tagged [definition]

The definition tag is used in situations where the question is either about how some term or concept is defined or where the validity of an answer depends on a subtle definition of some term or concept used in the question.

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A question on the strong interaction and charm number

I know that the particle $P_c(4380)^+$, has quark content $\bar{c}cuud$. Furthermore I know that the reaction $K^-+P_c(4380)^+\rightarrow K^-+J/\psi+p$, is strong and the quark content on the left ...
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Why 2nd-order derivatives of $G$ are discontinuous in 2nd-order phase transition?

In most undergraduate books, it is said that for second order phase transition Gibbs free energy and its first order derivatives are continuous at transition temperature. But the second order ...
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2answers
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Precise definition of the Hilbert space in QM?

In QM books (at least those I have read) the definition of the Hilbert space used is somewhat blurred (the "space of square integrable functions" is not enough to define it precisely : which kind of ...
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Inner product axioms and Special Relativity

In Linear Algebra, an inner product is a function $(\cdot,\cdot): V \rightarrow F$ , where $V$ is a vector space over the field $F$. This function satisfies the usual three axioms (Conjugate symmetry, ...
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The Lagrangian and inertial reference frames [duplicate]

From my understanding, my instructor told me that in order to use the Lagrangian, defined as $$L \equiv T - V,$$ to find the equations of motion via the Euler-Lagrange equations, the generalized ...
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1answer
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Luminostiy, Flux, Intensity definitions and meaning

I am taking astrophysics course and we are constantly talking about Luminosity,Brightness,Intensity,Flux etc. I thought that I understand the idea properly but I see now that I dont. Intensity is the ...
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1answer
50 views

Definition of Vector

In a book on General Relativity that I am reading, it defines a vector as an object or array of numbers that transforms like a vector (under rotations). I understand that under rotation $\theta$, a ...
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1answer
1k views

Why is the ratio of two extensive quantities always intensive?

Is this something that we observe that always happens or is there some fundamental reason for two extensive quantities to give an intensive when divided?
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1answer
41 views

What is the difference between linear and plane polarization?

What is the difference between linear and plane polarization?
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How to prove Caratheodory's Concept of temperature

Lenker T D . Caratheodory's concept of temperature[J]. Synthese, 1979 This is a discussion in the article: let $S_i$ be the state space of the thermodynamic system $\sum i = 1, 2, 3$, and assume ...
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3answers
35 views

Defining what it means for a reference frame to move with a velocity $\mathbf{u}$ with respect to another

In describing a Galilean transformation, for example, one might say that if a reference frame $S'$ is moving at a velocity $\mathbf{u}$ with respect to $S$, then an object traveling at a velocity $\...
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3answers
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Ambiguity in the definition of entropy

The entropy $S$ of a system is defined as $$S = k\ln \Omega.$$ What precisely is $\Omega$? It refers to "the number of microstates" of the system, but is this the number of all accessible microstates ...
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What is the formal definition of energy in physics? [duplicate]

What is the formal definition of energy in physics? My question is what is the definition of energy in physics in general ? Moreover is energy a naturally occurring quantity or is a term defined in ...
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2answers
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Field momentum of Klein-Gordon Lagrangian

Given the Lagrangian $L$ of the field $\phi$ the field momentum $\Pi$ reads: $$L_{KG}=-\frac{1}{2}\partial^\mu\phi\partial_\mu\phi-\frac{1}{2}m^2\phi^2$$ $$\Pi=\frac{\partial L}{\partial(\partial_\...
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2answers
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Are $2$ and $1/2$ universal constants? [closed]

For example, if the equation for energy were: $$E = mc^{2.713397972993}$$ clearly $2.713397972993$ would be a universal constant. And in the Einstein field equation: $$R_{\mu \nu} - \tfrac{1}{2}R \...
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1answer
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How do I know that I am observing a quasar?

When I find a faint object on the sky that looks like a star or a far away galaxy how do I know that it really is a quasar? I guess that I could first compare the apparent magnitude (how bright it ...
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5answers
106 views

Difference between voltage, electrical potential and potential difference

I am having hard time to visualize these two concepts in my mind seriously. First of this confusion came from two parallel plates that was connected to a power supply, charged then disconnected from ...
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1answer
63 views

Is a bound state a stationary state?

In Shankar's discussion on the 1D infinite square well in Principles of Quantum Mechanics (2nd edition), he made the following statement: Now $\langle P \rangle = 0$ in any bound state for the ...
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1answer
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Difference between kinematic momentum and conjugated momentum in purely mechanical setup

I don't know much about physics, but I wanted to understand what was the difference between the "kinematic momentum" and the conjugated momentum. As I understand it, kinematic momentum is mass times ...
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1answer
34 views

On the definition of Lagrange's equation

Is it really Newton's third law eq (1.5) ? Isn't the 2nd? I found it on "Abers Ernest Quantum mechanics".
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2answers
94 views

Mathematical formulation of quantum mechanics

I am reading a book on quantum mechanics, but it is difficult to understand. Quantum mechanics is roughly formulated as follows: Physicsl state is a normalized ray $\{e^{i\theta}\psi|\theta \in \...
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0answers
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Principle series, Sharp series , Quantum defect, in Alkali Spectrum

Could anyone help me to understand the relation ship between principle series, Sharp series(https://en.wikipedia.org/wiki/Sharp_series ), electron spin, and atom's inner magnetic field etc. the ...
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2answers
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What is exact definition of potential energy if all forms of energy except kinetic comes under potential energy? [duplicate]

I need to know the exact meaning of potential energy. I AM confused what it actually means.They say all types of energy comes under 2 types kinetic and potential so what is potential energy then? My ...
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1answer
54 views

What is topological material?

Recently, topological material has been a hot topic in condensed matter physics, but I don't know what is topological material and how to distinguish topological material from band diagram. And how ...
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Positive frequency definition in general spacetime for general fields

In Quantum Field Theory the positive frequency solutions to the classical field equations are quite important since they are the basis of the definition of particles. In Minkowski spacetime we have a ...
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4answers
65 views

Work when there is more than 1 force

I know that for an object with an applied force, the work done is $$W = Fd \cos \theta.$$ I was wondering what would happen when there is another force (e.g. friction)? Is it better to say that the ...
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1answer
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Intuition between this construction of the sympletic form for classical fields

In this paper, Wald presents a quite general construction of a sympletic form for classical fields. If I understood (which I might have not, and in that case corrections are highly appreciated), the ...
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2answers
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Accelerating Charge Radiates, so why can't we make a laser emitter out of it?

I've been taking class where we learnt about different types of laser, it reminded me of an old question I had before. There's been issues with making very short wavelength laser pulse with ...
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1answer
62 views

What is the difference between Non-Conservative and Dissipative?

We often hear these terms. However, they are often confused to be synonyms, but they are not. What are the rigorous definitions of them?
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2answers
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How is mass defined by special relativity?

I am eagerly interested in all kinds of areas of physics. As the question of mass has been around for a pretty long time, I am interested about what modern physics namely special relativity says about ...
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1answer
32 views

What's D lines for hydrogen like atom?

Could anyone tell me what's the definition of D lines for hydrogen like atoms? (Not just those two lines for sodium.)
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3answers
85 views

Rigorous definition of intensive and extensive quantities in thermodynamics

Most books on Thermodynamics give intuitive definitions for intensive and extensive thermodynamic variables. The say, for example that the former is independent of the system's size while the latter ...
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2answers
48 views

What is the difference between velocity and speed? [duplicate]

What is the difference between velocity and speed? could anyone describe it more specific? does velocity have direction?
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1answer
139 views

Confusion about virtual displacement

From Goldstein: A virtual (infinitesimal) displacement of a system refers to a change in the configuration of the system as the result of any arbitrary infinitesimal change of the coordinates $\...
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0answers
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Difference between different approximations in QM and other definition of the integral

I am currently studying the path integral formalism by myself and I am a bit lost within all the different way to solve the integrals we have. I have one big question: It sounds maybe a bit strange ...
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What are phase transiton in different contexts?

I have come across the concept of phase transitions in various contexts. From simple phase transition between different states of matter like water to ice and so on, to phase transition in magnetic ...
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2answers
55 views

Has the kilogram changed?

In late 2018 the kilogram's definition switched from a physical object (the International Prototype Kilogram) to Planck's Constant; a grand move in my opinion. However, I haven't heard of the ...
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4answers
43 views

Difference between Mass Number and atomic mass unit (AMU)

I am struggling to understand the difference between Mass Number and atomic mass unit (AMU). I read several posts on this site but still confused. Mass Number seems like a number (i.e. number of ...
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1answer
40 views

Physics of Neuroscience - Action potential

I am confused about how to understand action potential in the context of physics. Are action potential and potential difference synonymous? If so why is it stated that an action potential 'travels' ...
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1answer
108 views

What is the atomic limit?

I am attempting to grasp topological superconductivity for an assignment and in trying to understand what makes a quantum system topological have came across the following paragraph; "In the case ...
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Non-local operators [duplicate]

As the title suggests I am interested on how to identify that an operator in QFTs is a non-local operator. I have already read similar questions/answers to the topic in stackexchange, but they didn't ...
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0answers
48 views

Determining mass and force separately

I have been thinking.. How can we measure the mass of the body separately from the force? Or should we measure the force and acceleration? But by which means can we gauge our equipment e.g. some ...
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What is the standard definition of quantum spontaneous symmetry breaking?

i found many answers about spontaneous symmetry breaking here but i am not sure to see what is the standard definition of SSB. i am interested in the BCS theory and i would like to know how the ...
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3answers
156 views

What exactly is temperature? [duplicate]

I'm trying to have a really deep understanding of quantum physics and so I went back to what is blackbody radiation until suddenly I realized that my understanding of temperature is that of... well, ...
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3answers
300 views

Why intuitively, do we define symmetries as transformations that map solutions of the equations of motion into other solutions?

Of course, strictly speaking, a symmetry is always a transformation that leaves a given object unchanged. But I'm curious why observable symmetries of physical systems are exactly those ...
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What orbit does a planet have to have to be a planet? [closed]

Other questions have dealt with where you could find a planet-like object, and what a planet has to consist of to count as a planet. But this question is directed as what orbit or path an object would ...
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4answers
129 views

How to interpret this construction of the states in QFT?

Non-Relativistic Quantum Mechanics To make this question clear it might be useful to contrast with non-relativistic quantum mechanics. In any quantum theory, the states of a system are unit rays in ...
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1answer
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What is $\mathbb{Z}_2$ Parity?

While reading about exotic decays of Higgs boson one of the simplest interaction that we come up with which leads to BSM decays is: $$\Delta L = \frac{\zeta}{2}s^{2}|H|^{2}.$$ This is the ...
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1answer
71 views

Dark energy and dark matter difference

From the rotation curve of spiral galaxy it is found that for (>>r ) the total mass is not concentrated at the centre but varies as ~r for which the rotational velocities of the stars far from the ...
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1answer
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What is a precise definition for laminar flow that includes “laminar vortex-shedding”?

I can't find a definition for laminar flow that includes vortex structures. See paper: Numerical calculation of laminar vortex-shedding flow past cylinders and countless others when googled.