# 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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### 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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### 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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### 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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### 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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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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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 ...