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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60 views

Can someone explain, in clear terms (not unmathematical) what short circuit is?

I don't get the idea about what short circuit is. In a few questions I have come across, it appears to be a short circuit but the answers say otherwise. In fact, I don't have a conceptual ...
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In Cosmology, what does it mean to be 'local'?

I'm trying to make a point that there is curvature of spacetime from the metric expansion that contributes to the dynamics of a galaxy. This curvature would be in addition to the curvature caused by ...
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What are these symbols /feynman diagrams physically [closed]

Well the symbols are as follows These are the images from the diagrams, I wanted to know what physical meaning of them, if it has.
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What is a fiducial state in quantum computing? [closed]

I was researching quantum computers and I came across the term "fiducial state". What does it mean in this context?
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What is the difference between two types of velocity?

What is the difference between $v=\frac{s}{\Delta t}$ and $\bar{v} =\frac{\Delta\bar{x}}{\Delta t}$, are they the same?
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1answer
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Why does the time translation operator have a different sign than the time evolution operator?

The time evolution operator is $$\hat U(t)=e^{-i\hat H t}$$ where $\hat H$ is the Hamiltonian, $\hbar=1$, and the state is at time $t=0$. The time translation operator is defined similarly, but with ...
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1answer
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How do we know sound pressure squared is proportional to the power in the sound wave?

I am trying to find the equation that explains why we can say the pressure exerted by a wave (squared) is proportional to the power. I see this claim made a lot but I can't tell where it comes from.
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What kind of tensor is the stress tensor?

I have a lecture on solid mechanics that introduced the stress tensor as a bunch of indices squeezed into a matrix (my point isn't that the indices aren't justified). What I'm trying to understand is ...
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1answer
35 views

Positive binding energy shenanigans

In Introductory Nuclear Physics by Kenneth S. Krane, section 3.3 p.65 is presented the following formula for the binding energy of a nucleon: $$B=\left\{Zm_p+Nm_n-\left[m\left(^{A}X\right)-Zm_e\right]\...
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2answers
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What is meant by “plane of the wire”?

In my textbook, it is given that "The magnetic field produced due to current carrying circular wire at its center is perpendicular to the plane of the wire" What is the meaning of "...
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How is the stress-energy tensor defined?

I know the intuitive explanation of the stress-energy tensor and I have seen equations for stress-energy according to a specific situation, but I have not seen a general mathematical definition. What ...
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1answer
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Physical interpretations of Scattering Matrix $S$, Transition matrix $T$ and amplitude $M$

In QFT we define the scattering matrix from the scattering amplitude as $$ S_{fi} = \lim\limits_{t\rightarrow \infty}\lim\limits_{t_0\rightarrow -\infty }\left\langle f\left|U(t,t_0)\right|i \right\...
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Definitions of Determinants and Permanents in QFT

I have been recently reading a QFT book called: "QFT For the Gifted Amateur". It states in footnote 4 on p. 40 that the determinants and permanents of matrices can be defined as follows: $$ \...
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The definition of generalised momentum in lagrangian mechanics [closed]

Let's assume that no external forces are applied, so the lagrangian is just the kinetic energy. Lagrangian mechanics differentiates it with respect to generalised velocity to get the generalised ...
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2answers
76 views

How is the definition of work motivated?

For most dynamical variables in classical physics, I can understand how one may have decided to introduce them as a result of some "incompleteness" in Newton's laws of motion. For example: ...
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1answer
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How can ideal gases be “elastic” and “ideal” at the same time?

The elastic properties of any substance is because of the restoring intermolecular forces operating in it. And I read that the isothermal elasticity of an ideal gas is given by $E_{isothermal} = P$ ...
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Two Definitions of the Weyl Tensor

I'm reading "Textbook in Tensor Calculus and Differential Geometry" by Prasun Kumar Nayak and came across the Weyl tensor/projective curvature tensor $C_{kijl}$. The book states that $$C_{...
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1answer
50 views

Pauli Basis Matrices

When I search for the Pauli basis matrices I find both the following sets but I wonder which one is the right one and why does the first set have an imaginary term which is absent in the second set. ...
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1answer
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Conceptual doubt about representations

I was studying applied group theory to condensed matter, specifically representations. As far as I undersand, we can represent elements of symmetry (rotations, for example) by matrix, being a matrix ...
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What are the differences between FFN, VFN, GMVFN and ZMVFN PDF schemes in QCD?

What are the differences between the fixed flavour number (FFN), variable flavour number (VFN), general-mass variable flavour number (GMVFN) and zero-mass variable flavour number (ZMVFN) schemes for ...
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2answers
58 views

Are mass and force defined simultaneously? [duplicate]

Everything I am talking about here takes place in the Newtonian mechanics, but it is also interesting to discuss everything in the context of special theory of relativity, quantum-mechanics or even ...
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How is spin defined in Quantum Mechanics, exactly?

I know that spin gets a proper and complete definition in Quantum Field Theory, when we account for relativity in our quantum theory. This question is not about this. I am instead interested in ...
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1answer
23 views

What is the meaning of the random surface realization , $\zeta (x,y)$, in optics?

When reading Terahertz Spectroscopy and Imaging, I came across the Kirchhoff Approximation on Rough Surface Scattering Approximations. It is stated that: "The scattered field from the Kirchhoff ...
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3answers
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Electricity vs Current [closed]

BACKGROUND: Electromagnetism has been the hardest concept in physics that I do not understand at all back in my old school. Now, I just started studying robotics which requires the basic understanding ...
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1answer
27 views

What is apparent density?

For any floating body apparent weight and apparent density of body is zero.how.explain also explain what does apparent density meean.
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Brownian Motion (Geometric, Fractional, Drift)

I have been researching Brownian motion for a while and have come across terms/types of Brownian motion such as fractional, geometric, and Brownian motion with drift. I understand the physical meaning ...
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2answers
53 views

What is the significance of negative potential energy

I've learnt this statement as a fact potential energy of bound systems is negative I am interested in knowing the significance or physical meaning of this statement.particulary for electrostatic ...
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1answer
55 views

How do you distinguish between the terms “perfect gas” and “ideal gas”? [closed]

I know that Ideal vs Perfect gas has been discussed somewhere already but obviously it is a pointless discussion and the reason is as follows. If you define Ideal Gas and also if you define Perfect ...
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1answer
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What does it mean that gravity is '“local”? [closed]

What is "local" defined to be? Why don't larger systems affect smaller ones? ie. Don't we need to consider the gravitational pull from all other objects in the universe? Is this "...
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1answer
142 views

Why do self-adjoint operators have to be densely defined?

I have been watching the Schiller lectures on QM and have been going through ‘quantum mechanics and quantum field theory’ by Dimock. Both seem to ensure operators are densely defined, especially if ...
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How we distinguished charges and named them so? [duplicate]

Benjamin Franklin thought of electricity as a fluid, and said when there is abundance of fluid it is positive, stating positively charged fluid flows in electricity. Further, Thomson found that ...
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1answer
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How is a tensor operator defined in terms of commutators?

If $J_i$ represent the angular momentum operators, then a scalar operator $S$ (rank-0 tensor) is defined as an operator which satisfies $$[S,J_i]=0$$ for $i=1,2,3$. $A_i$ is a vector (rank-1 tensor) ...
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3answers
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Weight in Newtonian mechanics

What do we mean by the term weight? Is it an upward force exerted by an object to counteract the downward force of gravity (or) is it the downward force itself? Is the magnitude of the weight always ...
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What are Lagrange points?

We were taught that Lagrange points are where the resultant gravitational intensity is zero. Does that mean the resultant gravitational attraction of an object kept there is zero? If that's the case ...
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2answers
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What does it mean when one says that a vector field is spacelike, timelike, or null separated?

In Goldstein Classical Mechanics Chapter 7 (3rd edition, page 287), the authors classify vector fields as follows: Name Time Portion Space Portion (Magnitude$)^2$ Type Coordinate $ct$ $\mathbf{r}$ $...
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1answer
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What is the difference between rate of cooling and rate of heat loss?

I had a perception that rate of cooling and rate of heat loss are synonymous because the rate at which it looses heat indicates the rate at which it is cooling but this question changed my view There ...
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How is the term 'current' defined in QFT?

Reading papers and books about QFT, the term current is often mentioned with examples like the quark current in QCD or the electromagnetic current in QED. I was wondering, if there is a precise ...
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3answers
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What's considered an interaction in Quantum Mechanics?

Clearly I'm not an expert of QM but recently I came across the fact that particles do not have intrinsically a specific location until they interact (very ambigous term to me) with something else and ...
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1answer
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Difference In $\vec{i}$ , $\vec{j}$, $\vec{k}$ vector and $\hat{i}$ , $\hat{j}$, $\hat{k}$ cap? [closed]

There are actually two ways in which unit vector is shown in my textbook. The first one is: $\hat i, \hat j, \hat k $. The second one is: $\vec{i}, \vec{j}, \vec{k}$. What is the difference between ...
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1answer
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A general question about precession

Does precession (of a top for example) occur only whenever the angular velocity vector is not parallel to a principal axis? I have yet found any clear definition of precession and when it is defined ...
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Confusion on different propagators of QFT

I have a very naive confusion regarding the propagators of QFT. I have come across the terms: (i) Retarded propagator, (ii) Advanced propagator, (iii) Feynman propagator ... I can comprehend the ...
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1answer
80 views

PMNS matrix versus CKM matrix — how precise is the analogy?

PMNS matrix is said to be the matrix for the neutrinos as the CKM matrix for the quarks. See https://en.wikipedia.org/wiki/Pontecorvo–Maki–Nakagawa–Sakata_matrix#The_PMNS_matrix However, I am ...
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2answers
94 views

Contravariant/covariant vectors or components?

Related: Covariant vs contravariant vectors Note: by vector I try to refer to the physics entity, not to the list of components. From wikipedia page and others: $$ \mathbf{v} = q_i \mathbf{e^i} = q^i \...
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What are the Lorentz Transformations between polar coordinates? Or can Lorentz Transformations be Non-Linear?

This question rises from the comments on @G Smith's answer's to this question https://physics.stackexchange.com/a/603032/113699 Precisely I was trying to understand the Lorentz Transformations between ...
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What is the height in the potential energy formulation?

Most examples I've seen describe the (gravitational) potential energy e.g. with an example of a ball thrown upwards, and explain how the sum of the ball's kinetic and potential energy is constant at ...
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2answers
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What is the meaning of resistance?

I am a 10th grade student and I have a little trouble in knowing what is resistance? I know this might be a silly question for others but My thinking of resistance is that it opposes the flow of ...
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4answers
768 views

What is meant by coordinate time? Isn't it time measured on a clock? If not then what does it measure?

In this question- Is proper time equal to the Invariant Interval or the time elapsed in the Rest Frame? @Dale in the comments says- no $dt$ is never physical time. It is always coordinate time. The ...
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Various Definitions of 'Ghosts'

I have seen various different definitions of a ghost field in the literature. For example, one can find many examples where ghosts are simply defined as any field with a negative sign in the kinetic ...
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3answers
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If Heisenberg uncertainty principle involves the standard deviation of quantities then why do we use it in a different way as here?

Heisenberg uncertainty principle is mathematically given as $$\sigma_x \cdot \sigma_p \ge {{\hbar} \over {2}}$$ The two terms on the left being the standard deviations of position and momentum. But on ...
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Why only reversible adiabatic processes are called isentropic and not irreversible ones?

For both reversible and irreversible adiabatic processes, $dQ =0$,and by the definition of entropy $dS=dQ/T$, it should imply that entropy is constant for both. Why it is not so?

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