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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53 views

Difference between left- and right-handed, helicity and chirality

What is the difference? I know there is the (almost) same question What's the difference between helicity and chirality? but when a particle is given as left-handed. Is it helicity or chirality?
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1answer
241 views

Definition of one-particle irreducible diagrams

Text books often defines one-Particle Irreducible diagram (1PI diagram) as a connected diagram which does not fall into two pieces if you cut one internal line. Is this internal line the full ...
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0answers
25 views

What do the terms “Frame specific work” and “Frame dependent work” mean? [on hold]

I was reading a paper published in A.J.Phy. American Journal of Physics 60, 356 (1992)[All About Work] by A. John Mallinckrodt and Harvey S. Leiff There I found this two terms but when tried to ...
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1answer
308 views

Mathematical definition of wavefront in case of non-harmonic waves

What is the general mathematical definition of wavefront? Wavefront is the surface where, at fixed time, the phase is constant But for non-harmonic waves we cannot talk about phase as the function ...
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0answers
18 views

What is Wedge Constraint? (Definition)

What is Wedge Constraint? How can we apply it in the following problem? The block of mass $m$ slides on a wedge of mass $m$ which is free to move on the horizontal ground. Find the accelerations ...
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0answers
52 views

What motivated Gibbs's definition of Gibbs entropy? [on hold]

What motivated Gibbs's definition of Gibbs entropy? I have read and I think that I agree with the idea that if we have to choose probability distribution for an unknown system then it is a good idea ...
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1answer
142 views

Proper definition of Internal energy

In Internal energy of a gas ($U$), we have to include ALL types of energies possible like $KE$, bond energies, nuclear interaction energies, etc.., However we have derived that $U = (f/2)nRT$ from ...
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1answer
106 views

The invariance for time-reversal: some clarifications

On the Physics.SE I haven't found a simple definition of the invariance for time-reversal. Is it possible to have a simple explanation (also using differential equations) of the invariance for time-...
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2answers
5k views

Understanding terms Twist and Wrench

In kinematics, physics and especially robotics, we often encounter the terms Twist and Wrench. Twist is (LinearVelocity, AngularVelocity) and Wrench is (Force, Torque). The reason I'm confused is I ...
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1answer
65 views

What's the difference between canonical quantization and second quantization?

I am wondering the difference between the canonical quantization and the second quantization in quantum field theory. For example, a harmonic chain, one can write down its lagrangian density $\...
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2answers
61 views

What does the term $\mathcal O(\epsilon^2)$ mean?

In the highest upvoted answer to Where does the $i$ come from in the Schrödinger equation? the author writes the following equation: $$ U^\dagger U=(\mathbb I+\epsilon^* A^\dagger)(\mathbb I+\...
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1answer
414 views

What is capacitance, in general?

In circuit analysis software capacitance can be measured between any two nodes of a circuit or of a multiterminal device. In practical terms we take $C_{ij}$, the capacitance between $i$ and $j$ as ...
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3answers
10k views

Differentiating Propagator, Green's function, Correlation function, etc

For the following quantities respectively, could someone write down the common definitions, their meaning, the field of study in which one would typically find these under their actual name, and most ...
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3answers
53 views

Is a rotating object moving or stationary?

Because it rotates in situ, its center of mass does not move, so it is static, but it is rotating, so it is not static, then is it static or moving?
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3answers
2k views

Definition of non-degenerate metric tensor

We know that a metric has a property which is called non-degeneracy. I was searching for what does that mean and saw it associated with the fact that $det(g_{\mu\nu})\neq0$. How does this relate to ...
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7answers
29k views

Are matrices and second rank tensors the same thing?

Tensors are mathematical objects that are needed in physics to define certain quantities. I have a couple of questions regarding them that need to be clarified: Are matrices and second rank tensors ...
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4answers
27k views

Why is the potential energy equal to the negative integral of a force?

Why is the potential energy equals to the negative integral of a force? I am really confused with this negative sign. For example, why there is a negative sign in the gravitational potential energy ...
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1answer
3k views

What is the definition of how to count degrees of freedom?

This question resulted, rather as by-product, the discussion on how to count degrees of freedom (DOF). I extend that question here: Are necessary1 derivatives such as velocities counted as individual ...
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0answers
35 views

Rigorously define degrees of freedom

I want to understand if there is truly a rigorous definition for the degrees of freedom in a system. Say all of a system's physical states are contained in some set $S$. A seemingly acceptable (and I ...
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2answers
2k views

What does “degrees of freedom ” mean in classical mechanics?

The definition I come up with is $3M - N$ ...where $N$ is the number of constraints. I assume $M$ is the number of distinct points. In what context is it used? According to Wiki it says "an ...
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0answers
44 views

Mathematical formulation of the concept of temperature

We were taught the following. Consider three systems $A$, $B$ and $C$, which have exactly two independent variables each, $(X,Y)$, $(X’,Y’)$ and $(X’’,Y’’)$. Thermal equilibrium for $A$ and $B$ is ...
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1answer
23 views

Sensitivity to initial conditions and predictability

Are there examples of chaotic systems that are predictable and at the same time sensible to initial conditions? or would that violate the notion of sensibility to initial conditions? Lets imaginge A ...
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4answers
144 views

Does energy exist, or is it just a relationship between other fundamental things?

Could anyone tell me what energy really is? I searched for it, and some people said that energy doesn't exist physically and it is not fundamental, but it is a relationship between other fundamental ...
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0answers
67 views

General definition of symmetry in physics?

I've looked at a number of questions on what symmetries are in physics, such as this one, this one and this one. However, I found the questions and answers to be not completely satisfying because they ...
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8answers
40k views

What Is Energy? Where did it come from?

The simplistic undergrad explanation aside, I've never really understood what energy really is. I've been told that it's something when converted from one kind of something to another kind, or does ...
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1answer
69 views

What is the most general conception/formalization of “energy”? [duplicate]

(disclaimer: I am not a physicist). I have been taught about the concept of energy in a fairly ad-hoc way. i.e. I was simply "told" that kinetic energy equals $\frac 1 2 m v^2$, and similarly I was ...
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4answers
9k views

What's the real fundamental definition of energy?

Some physical quantities like position, velocity, momentum and force, have precise definition even on basic textbooks, however energy is a little confusing for me. My point here is: using our ...
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2answers
257 views

Equivalency of $Q$ Factor Definitions

The Q factor is defined (seemingly) as $$Q=2\pi\frac{\mathrm{energy \, \, stored}}{\mathrm{energy \, \,dissipated \, \, per \, \, cycle}}$$ however on Wikipedia is says that the Q factor can be ...
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1answer
38 views

A doubt regarding Modelling physical phenomena and position uncertainty

For example, in velocity, when we say $v=\frac{dx}{dt}$, there is no proof for it. Its almost like an axiom. Something taken to be true, without a proof. How do I know that for every $x=f(t)$, $v=f'(t)...
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0answers
40 views

What is Rectangular Simple Harmonic Motion (SHM)?

While studying Simple Harmonic Motion and Lissajous figure, I found a term called Rectangular SHM. But what is it actually? Is it something like square wave? What is the difference between Linear and ...
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2answers
52 views

Soft X-rays and Hard X-rays

How to know how much of kilovolts are in soft X-rays, and in hard X-rays? (Ex: 80kV is in soft X-rays or hard X-rays?
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1answer
20 views

Specific total enthalpy VS Specific enthalpy

What is the difference between specific enthalpy and specific total enthalpy in the context of fluid flow?
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5answers
31k views

What is sound and how is it produced?

I've been using the term "sound" all my life, but I really have no clue as to what sound exactly is or how it is created. What is sound? How is it produced? Can it be measured?
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2answers
48 views

What is a mass moment?

I am currently reading through a document Finding Moments of Inertia from MIT, page 4, and I am a little confused as to one of the concepts that they use. In this document, there is mention of a mass ...
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0answers
95 views

A concise definition of a frame of reference in Newtonian mechanics?

I've read Wikipedia's entry on frame of reference and also followed all of the references cited in the text (Salençon, Brillouin, Norton, etc) but I'm struggling to find any concise definition in all ...
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1answer
37 views

Definition of non-conservative force [duplicate]

In defining conservative force, we say that "The potential energy difference is path independent." However, as far as I understand, potential energy only exists when there is a force field. ...
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0answers
65 views

Physicist path integral and cylinder set measures

Path integral via discretization So let me start with what seems to be the point of view of physicists (corrections are highly appreciated since this is what I understood!). Let a quantum system with ...
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3answers
274 views

Equivalent definitions of total angular momentum

Consider the equality \begin{equation}\exp\left(-\frac{i}{\hbar}\boldsymbol{\phi J}\right)\left|x\right>=\left|R(\phi)x\right>,\end{equation} where $\left|x\right>$ denotes a position ...
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2answers
77 views

Definition of “specific gravity”

I've learnt that a specific quantity is an extensive quantity divided by the mass. How does the definition of specific gravity fit into this scheme?
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6answers
4k views

Is the Big Bang defined as before or after Inflation?

Is the Big Bang defined as before or after Inflation? Seems like a simple enough question to answer right? And if just yesterday I were to encounter this, I'd have given a definite answer. But I've ...
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3answers
86 views

WHY did physicist defined velocity as displacement divided by time, why not displacement * time? [closed]

V=S/T. As per my knowledge i think ratio as division and it don't give any meaning like this much displacement in this much time. So i think physicists only used division as notion for velocity. But ...
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2answers
478 views

What is difference between variations of the work and virtual work?

I really want to know whether or not both equations are the same mathematically. I think that they are the same, I just want to be sure. (Reference: this website.)
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0answers
42 views

Off-shell vs half off-shell vs fully off-shell $T$-matrix

I know what are on-shell particles, but I want to know what are off-shell, and half off-shell, and fully off-shell states? and how we decide to consider one of these states in evaluating $T$-Matrix?
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0answers
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Physics Equivalent of IUPAC Gold Book

I wanted to look up a few definitions and found them to vary from source to source so I wondered if there was a book such as IUPAC Gold Book in Chemistry which formally lists and defines almost all ...
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0answers
44 views

Why definition of potential energy and law of conservation of mechanical energy is misleading several times?

I regularly see 1 or 2 questions on this website about the definition or application of potential energy.The users fundamentally ask the same thing in every question. What I have learned till now is:-...
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1answer
89 views

Is four velocity always given by $U^{\mu} = d x^{\mu}/d\tau$?

I was taught that four-velocity is defined as $${\bf U} = \frac{d \bf x}{d\tau}$$ and that it has the components $$U^{\mu} = \frac{d x^{\mu}}{d\tau}$$ where $d\bf x$ is the four displacement and $\...
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3answers
105 views

Is there a better definition of magnetic field than this?

It may seem a trivial question but the definition of the magnetic field in everyday books is misleading. "It is the region or area around a magnetic material in which its magnetic force can be felt." ...
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3answers
20k views

273 + degree Celsius = Kelvin. Why 273?

Temperature conversion: $$273 + \text{degree Celsius} = \text{Kelvin}$$ Actually why is that $273$? How does one come up with this? My teacher mentioned Gann's law (not sure if this is the one) but ...
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2answers
57 views

What is irrotational flow? How to judge?

For example, when the wing moves horizontally, the direction of fluid flow changes first to upward at the leading edge of the wing and then to downward at the trailing edge. Does it rotate? If the ...
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1answer
36 views

What is a pseudopure state?

In the paper titled "Experimental Implementation of the Quantum Baker’s Map" by Weinstein et al. (Phys. Rev. Let. 89 (2002)), the author says something like [...] the pseudopure state corresponding ...