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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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What is meant by optical density?

I’m trying to research about optical density for a school task, but the definitions online are confusing me. From what I’ve gotten, optical density is: how much the intensity of light is increased or ...
anonymous hehe's user avatar
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1 answer
81 views

GR and Riemann Surfaces -- does the complex plane have anything to do with it?

I have only the vaguest understanding of Riemann Surfaces -- my sense is that Einstein used them in General Relativity because of their shape. But Riemann Surfaces I think are not just deformations of ...
releseabe's user avatar
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1 answer
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By what units is 'degradation to ozone layer' measured?

From Wikipedia: The ozone depletion potential (ODP) of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with trichlorofluoromethane (R-11 or CFC-11) being ...
Sam's user avatar
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1 answer
72 views

Confusing definitions of EMF

I was studying about EMF (electromotive force) recently but had trouble connecting the two definitions of the topic. Here are the two definitions of EMF listed in my book (The book I've referred is: ...
Bhavya Jain's user avatar
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1 answer
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In layman’s language what is the difference between stopping potential and work function?

In layman’s language what is the difference between stopping potential and work function? It feels like both the things mean the same thing, if I am wrong please explain the things to me.
User_5117's user avatar
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1 answer
125 views

What is charge on a fundamental level and what is it caused by?

On bigger bodies, I've learnt so far that charge is caused by a lack or excess of electrons or protons but when you look at an electron or proton, how are they charged? what is the cause of their ...
shriekspeare's user avatar
3 votes
0 answers
94 views

Definition of entropy and microstates (Huang)

The definition of Boltzmann entropy given in Ref. 1 appears to be different from most sources I've seen up to this moment. Let me start from the latter: typically textbooks assume that in a given ...
Mr. Feynman's user avatar
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-1 votes
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How to state that a function has a certain andament in a limit? [migrated]

Assuming we have a function $f(r)$ that has the following limit $$ \lim_{r\to0} f(r) = \frac{5}{3 r^2} \,.$$ What is the correct symbol to express that the denominator goes like $r^2$? Is the ...
Aleph12345's user avatar
2 votes
1 answer
89 views

Confusion on the definition of dielectric permittivity

I don't understand how we're defining the dielectric permittivity, sometimes it's defined as the ability of the material to resist the electric field and sometimes as the ability to permit the ...
Manish Parmar's user avatar
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0 answers
29 views

Meaning of of inertial frames in special relativity [duplicate]

One of the postulates of special relativity is The laws of physics are the same in all inertial frames There is a mention of inertial frames in the postulate. But the predictions of relativity are ...
Niladri Sarkar's user avatar
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How can I call a transformation where only the $z$-axis is modified by a function in each point?

What is called a transformation that maps $z \rightarrow |f(z)|\, z$, while the other axis stays the same? Is it a conformal transformation?
Aleph12345's user avatar
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Intensity of light - questions about the particle standard definition and concerning the units [duplicate]

In the $5.111$ MIT class, lecture $3$, it is written on the last slide "intensity of light = photons per second". Intensity is previously defined in the lecture as the square of the ...
niobium's user avatar
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1 answer
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How to use the definition of a rank-$2$ tensor for this kind of examples?

Suppose that, a rank-$2$ tensor transforms as \begin{align} T'^{ij}=\frac{\partial x'^i}{\partial x^k}\frac{\partial x'^k}{\partial x^l}T^{kl}. \end{align} How to use this criterion to investigate if ...
Perfect Fluid's user avatar
0 votes
2 answers
109 views

What is the connection between moments in probability theory and the moment of inertia?

My question arises as the moment of inertia (MOI) has been described as a second moment. In my understanding if the MOI is indeed a second moment of a distribution of mass, this suggests the MOI could ...
Luke K's user avatar
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1 answer
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Photoelectron and electron in photoelectric effect [closed]

Could you please help me find some answers, as I have been stuck in this for a while and didn't get understandable answers. what is the difference between the photo electron and an electron. The ...
Farheen's user avatar
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2 answers
52 views

What exactly is equivalent resistance?

We all know the basic example of school with series and parallel resistors, so they might be a good place to start. Given two terminals $A$ and $B$ from a circuit, if the current that exists $A$ is ...
hellofriends's user avatar
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4 answers
162 views

What does the $F$ in Newton's second law equation mean?

This is a super simple question: does $F$ represent the net force exerted on an object or the force it exerts on another object as a result of momentum? Say a ball is rolling. In this specific ...
user386598's user avatar
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4 answers
87 views

What are internal and external forces?

what is internal and external force? How do I decide what forces are internal and external? in work-energy theorem and conservation of mechanical energy, do we consider internal, external, or both ...
android's user avatar
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0 answers
49 views

Why is warming food in the microwave work and not heat? [duplicate]

In Schroeder's An Introduction to Thermal Physics, heat is defined as the "spontaneous flow of energy from one object to another" and work is defined thermodynamically as any other transfer ...
PineappleThursday's user avatar
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0 answers
57 views

Second derivative of energy with $k$

In solid-state physics, we often encounter the velocity operator (e.g. this post) which the eigenvalues can be written as $$ V^\mu(\vec{k}) = \frac{1}{\hbar} \frac{\partial E}{\partial k^\mu} \biggr|_{...
Bio's user avatar
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Meaning of $n$-critical point

My lecture notes about field theory refer to a tricritical point as a point in which a continuous phase transition line meets a discontinuous phase transition line. In the following it refers to a ...
Dirac's delta's user avatar
5 votes
3 answers
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Is the definition of geodesics different if the electromagnetism is added to GR?

In plain GR, geodesic are defined by: $$ \nabla_{u} u^{\,\mu} = 0 $$ where $u^{\,\mu}$ is the four-velocity of the particle. Now, I don't understand if this holds true also in the electromagnetic case,...
Aleph12345's user avatar
1 vote
1 answer
97 views

What is the relationship between Clausius Inequality and 2nd Law?

I am confused about the application of the 2nd Law for reversible and irreversible processes and cycles. I want to know how the Clausius principle, the Kelvin-Planck statement, and the Clausius ...
Skaeler's user avatar
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4 votes
1 answer
474 views

Clarifying the definition of pressure in statistical physics

I am studying David Tong's lecture note on statistical physics, and I have a question regarding the precise definition of pressure. I checked other postings in this community, but was unable to get ...
Sangchul Lee's user avatar
8 votes
1 answer
896 views

Two values of Rydberg Constant

In my textbook I came across two values of Rydberg Constant: \begin{align} R&= 109\,677\ \mathrm{cm}^{-1} \\ R_h&= 2.18 × 10^{-18}\ \mathrm J \end{align} when we are calculating energy of the ...
Curious Guy's user avatar
0 votes
2 answers
65 views

What is $r$ in a metric signature in general relativity? If $v$ and $p$ are the time and spatial coordinates?

The Wikipedia article on metric signatures says that the signature of a metric can be written $(v,p,r)$, where $v$ is the number of positive eigenvalues, $p$ is the number of negative eigenvalues, and ...
Kurt Hikes's user avatar
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-1 votes
2 answers
77 views

Rest Mass Energy

What exactly is the Rest mass energy? I know the usual goes Like it is total energy stored in mass $m$ that is in Rest. so for someone moving relative to the frame that the mass $m$ is in (a rest ...
Hello's user avatar
  • 73
7 votes
1 answer
314 views

What is the difference between Born approximation and tree-level processes?

The answer to this question says that Born approximation is essentially equivalent to the tree-level. This can be seen from the Feynman-diagrammatic version of Born series discussed in many NRQM ...
Sanjana's user avatar
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0 answers
22 views

What exactly are cohesive and adhesive forces?

Are cohesive and adhesive forces defined only when at least one of the interacting materials is a fluid?
Anvi Mahajan's user avatar
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0 answers
49 views

On the definition of topological order

I am currently reading the "Quantum Information Meets Quantum Matter -- From Quantum Entanglement to Topological Phase in Many-Body Systems". I am trying to understand the proper definition ...
Truth and Beauty and Hatred's user avatar
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0 answers
26 views

What is the so-called momentum density? [duplicate]

What is the so-called momentum density? I am reading the paper by Pitaevskii, in which he stated that it is well-known. He studied the nonlinear Schroedinger equation, but it seems that the concepts ...
poisson's user avatar
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0 votes
0 answers
40 views

Is displacement vector always the shortest path?

I read that the displacement vector of a particle is the shortest path between its initial and final positions since it's a straight line joining the two points, this holds true for me till a 2D ...
Manish's user avatar
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0 votes
0 answers
40 views

Is there a practical distinction between functions of state and functionals in thermodynamics?

In thermodynamics, and more precisely when talking about continuous systems, some sources [1, 2] introduce functionals of state: $$F[s(x), \dots]:=\int_VdV(x)f(s(x),\dots,x)$$ In order to derive ...
GvPStack's user avatar
0 votes
1 answer
52 views

What does Density really mean? [duplicate]

We say that density is mass per unit volume and if an object has high density that means it has more amount of matter contained per unit volume but as we know mass is not the amount of matter ...
SHASHWAT RAI's user avatar
1 vote
1 answer
55 views

Definition of “quasi-locality” in Wilsonian RG scheme

I’m studying about the holographic RG with this paper. In that paper they say Wilsonian action expects quasi locality, but I’m not sure what “quasi-locality" exactly means. If quasi-locality ...
Positron3873's user avatar
1 vote
2 answers
96 views

How were angles measured in ancient times?

What is one degree (angle measurement)? I know that we calculate one second using atomic clock, one kilogram using Planck's constant, one meter using speed of light but how do we define one degree? If ...
Payal Payal's user avatar
0 votes
0 answers
49 views

What is the definition of a conformal symmetry? [duplicate]

I have been very confused by this after some recent reading. So as far as I know, a conformal transformation (according to the definition in di Francesco et. al.'s book on CFT) is an active coordinate ...
QFTheorist's user avatar
0 votes
0 answers
91 views

What are regular event horizons?

I am studying uniqueness theorems of Black Holes and I often see the word "regular event horizon" instead only event horizon. Many textbooks and literature do not define this term. Please ...
Talha Ahmed's user avatar
4 votes
5 answers
193 views

Where does the four-vector potential $A^\mu$ originate?

Now the default answer to this may be, "It has no origin because it's a definition", but let me just try to justify my concern here. On page 842, equation (22.91) of "Modern ...
Electra's user avatar
  • 295
0 votes
0 answers
45 views

What exactly is a brane? [duplicate]

what exactly is a brane? i know that in order to get the equations of motion of a relativistic string by varying the Nambu-Goto action one has to impose a Neumann boundary condition and a Dirichlet ...
Tomás's user avatar
  • 309
0 votes
1 answer
31 views

Pressure sensors vs. piezoelectric sensors

In simple terms, what is the difference between pressure sensors and piezoelectric sensors? I did some google search online and could not come to a good conclusion. Say you make a 'sensor' in the lab ...
Acrobatic-Ad-168's user avatar
1 vote
1 answer
59 views

What stationary charges exactly mean?

When learning about magnetic fields, it is said that stationary charges do not have any effects due to magnetic fields. So when explanations are given, is the spin of the charges not considered? And ...
MADHUMATHI S's user avatar
0 votes
0 answers
19 views

Precise Definition of Degrees of Freedom [duplicate]

I am taking Analytical Mechanics and while reading Goldstein's and LL something bothered me: can I say that a degree of freedom is an independent (generalized) coordinate? What bothers me is that we ...
user avatar
1 vote
2 answers
66 views

Defining Nuclear Reaction and Beta Decay

I use a school textbook that defines nuclear reaction in the following way. I will go through what does make sense to me, and then point out the thorns. "When an atom changes into a different ...
HBP's user avatar
  • 11
0 votes
3 answers
119 views

Why is entropy defined in the way it is?

In thermodynamics course we were taught that in a reversible adiabatic process of an ideal gas the entropy change is zero. Now what I didn't get intuitively is that if the system is compressed ...
Ankit's user avatar
  • 8,220
11 votes
5 answers
3k views

Is there a formalization of the butterfly effect?

According to my understanding, the butterfly effect says, very informally, that even a tiny perturbation in a physical system can lead to significant alterations in future states of the physical ...
user107952's user avatar
  • 1,252
1 vote
3 answers
137 views

What are relativistic particles? [duplicate]

What are relativistic particles? I got it in a question of mechanics. So, what is it about a particle that makes it "relativistic"?
Nishkarsh Singh's user avatar
2 votes
2 answers
201 views

Can the composition law of a group be defined only when considering a representation or realisation of the Group?

When we talk about, lets say, the Lorentz group, we define the action of the Lorentz transformation $\varLambda$ on \begin{alignat}{1} x^{\mu} & \in\mathbb{R}^{1,3},\\ x^{\mu} & \rightarrow x'^...
HypnoticZebra's user avatar
3 votes
2 answers
83 views

In what sense is $\int (u \cdot \nabla) u \cdot u dx$ an energy flux?

Due to the nature of this question I have have cross-listed it on mathSE. Let $u$ be either a solution to either the Euler equations or Navier-Stokes equations over a domain $\Omega$. In fluid ...
CBBAM's user avatar
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0 votes
0 answers
58 views

What is the name of the transformation from one harmonic oscillator basis to another centered elsewhere?

If I have a harmonic oscillator basis centered at $x=2$, how do I rewrite it in terms of the harmonic oscillator basis centered at $x=0$? To be more specific: If $|\Psi_n\rangle$ is the $n$th ...
Idieh's user avatar
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