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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1answer
343 views

What is Quasi-particle weight? [closed]

Hi I just heard this phrase from my classmate yesterday, I am wondering whether someone can explain this real quick for me.
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0answers
17 views

What's the name for the physical attribute that would describe the flexibility of a chain mail?

The individual links of a chain mail are pretty rigid. When deforming a piece of chain mail following its intended use, its flexibility doesn't rely on the elastic or plastic properties of the ...
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2answers
264 views

What is the difference between tangent space and configuration space?

I am doing Lagrangian mechanics and working with Noether's theorem. Please, could you explain the difference between the configuration space and the tangent space?
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2answers
73 views

What does work mean as a concept and not as a mathematical equation? [closed]

I know that work = Force times displacement or the dot product of the force and the displacement. But what does work mean in the physical world? Just like we know that in the physical world, density ...
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1answer
289 views

Technically, what is a spacetime singularity? [duplicate]

In popular science books and articles, one often finds that the BigBang is a singularity of spacetime, and it is expected to be solved by a successful theory of Quantum gravity. Technically what is a ...
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0answers
262 views

Are Heaviside-Lorentz unit and Gaussian unit natural units?

I am studying electromagnetism and have encountered the Heaviside-Lorentz unit and Gaussian unit. In these unit systems, the constant $c$ keeps occurring. For example, the Maxwell equations are of ...
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2answers
3k views

What is the difference between vapour and steam?

What is the difference between vapour and steam? These two terms seemingly are similar but have different meanings. trying to define these two gets us a similar definition which is not actually true I ...
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2answers
342 views

What is a pseudotensor, really, and how to tell one?

I have some trouble with the concept of a "pseudotensor". Wikipedia distinguishes between that and a tensor density (e.g., here, where both concepts are used simultaneously) while e.g. in Eric ...
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3answers
2k views

Is work equal to energy? [duplicate]

In my textbook, there is written that: Done work = spent energy Suppose I am riding a motorcycle. I start my journey from a place and then going to various places I come back to the exact point(I ...
4
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1answer
67 views

Conservative force definition

Classical Mechanics, by John Taylor defines a conservative force F as a force that satisfies: F depends only on the particle's position and no other variables. Work done by F is the same for ...
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2answers
3k views

Distance and Displacement from a velocity vs. time graph

My teacher is saying that the distance covered will be equal to the area of the trapezium in the graph, but the displacement will be equal to the area of the triangle (with purple hypotenuse). I ...
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1answer
564 views

KMS States and Thermal States

In the algebraic approach to QM and specially to QFT one defines KMS states, for example, as Wald does in his lecture notes: Let $\mathscr{A}$ be one $\ast$-algebra describing a system and $\...
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1answer
130 views

Effective mass definition

Given an energy spectrum $\epsilon_k$, there are two common definitions of the effective mass $m^*$: $$\frac{1}{m^*}=\frac{\partial^2\epsilon_k}{\partial k^2},$$ $$\frac{1}{m^*}=\frac{v_F}{k_F},$$ ...
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2answers
95 views

What is the definition of a Collisional Fluid?

I am unsure whether this means the particles in the fluid must physically collide or does 'collisional' also apply to particles interacting in general, eg via gravity? I cannot find a good definition ...
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1answer
61 views

Definition of second and QM

Wikipedia says that the (newly proposed, more rigorous) definition of a second is: The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium ...
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1answer
168 views

Perfectly inelastic collision

Why do bodies move with the common velocity after collision in a perfectly inelastic collision? What can be the possible explanation for it? Why don't they just go in any arbitrary direction rather ...
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0answers
59 views

Pseudotensor in different dimensions

In this topic What is pseudo-tensor? one answer was the next: The action of parity on a tensor or pseudotensor depends on the number of indices it has (i.e. its tensor rank): The action of parity on ...
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1answer
40 views

Definition of a calorie - confusion

Wikipedia defines a calorie as the (approximate) amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius at a pressure of 1 atmosphere. Is it the case then that, ...
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1answer
264 views

Whats the difference between a linear and non-linear sigma model?

Wikipedia says In physics, a sigma model is a physical system that is described by a Lagrangian density of the form: $L(\phi_1,...,\phi_n)=g_{ij} d\phi_i \wedge d\phi_j$ With Einsteins ...
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2answers
495 views

Definition of granular material

I am doing a project on some properties of granular materials. I might have to face people who ask the definition of granular material. How can we define granular materials? One answer is "it is sand-...
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1answer
63 views

What exactly is equilibrium?

What exactly is equilibrium? I'm riding my scooter and I get this doubt. The maximum power the engine of my scooter can deliver is constant. As I pull full throttle, the scooter accelerates, the ...
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2answers
539 views

The actual definition of entropy

According to the inequality of Clausius, $$S\ge \dfrac {q_{rev}}T$$ , where $S$ is the entropy of the system, $q$ the heat absorbed by the system during a reversible process and $T$ is the ...
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2answers
251 views

Electric current at a point

When learning about electric current in circuits, I have learned that at each point in the circuit there is a current associated with it which is defined as the charge per unit time passing through ...
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2answers
388 views

Electromotive force is a potential difference or it is an energy?

Electromotive force is a potential difference or it is an energy?
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1answer
37 views

Confusion regarding EMF definition

The emf of the battery source can be defined as the potential difference between two electrodes when no current is flowing. Now how I can apply this definition of the emf which is produces by change ...
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1answer
78 views

Mathematical form of the magnetic force - a vector field or not?

Is the magnetic force a vector or a vector field? The magnetic force is written without arguments: $$ \mathbf F=q \mathbf u \times \mathbf B \tag{1} $$ Does it mean that $\mathbf r=(x,y,z)\in\mathbb ...
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1answer
4k views

What is the definition of parity operator in quantum mechanics?

In classical mechanics, the position vector changes under parity as $\textbf{x}\to -\textbf{x}$. This is the definition of parity in classical mechanics. Operators (such as $\mathcal{O}$) change ...
0
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1answer
96 views

Lower critical Reynolds number [closed]

The definition given in my text is : “is the value at which the flow remains laminar however agitated the tank water is “ I found that unclear. Also , What other factors affect the critical value of ...
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3answers
4k views

What is considered a wavelength?

What is considered a wavelength? I am getting confused here. I keep seeing one wavelength is the distance when the wave repeats itself. So at the two highest points that's the wavelength. Now I'm ...
2
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3answers
326 views

Physical Interpretation of the Diffusion Constant $k$

I have read technical explanations of the interpretation: ''Diffusion coefficient is the proportionality factor D in Fick's law (see Diffusion) by which the mass of a substance dM diffusing in ...
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3answers
181 views

Why is the moment of inertia the integral of the “second moment”? What is the derivation for the formula? [duplicate]

In my dynamics textbook, the introduction to the moment of inertia reads: "Moment of inertia is the integral of the "second moment" about an axis of all the elements of mass dm which compose the ...
2
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2answers
861 views

What is the radial quantum number $n_r$?

As we know, the principal quantum number $ n=1,2,3,... $ is related to the radial quantum number $ n_r=0,1,2,... $ by $$ n=n_r+\ell+1 .$$ What is the physical (or chemical) definition of the ...
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2answers
2k views

What is the Wilsonian definition of renormalizability?

In chapter 23.6, Schwartz's quantum field theory book defines renormalizability as follows, paraphrasing a bit for brevity: Consider a given subset $S$ of the operators and its complement $\bar{S}$....
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1answer
130 views

Number of free indices on a tensor, how's it called?

Suppose we're working with a spin-$s$ field, represented by a totally symmetric tensor of order s (or rank, as we confusingly call it in physics), i.e. $$\varphi_{\mu_1 \cdots \mu_s}$$ Assume that, ...
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1answer
94 views

What is the definition of $\overleftrightarrow{\partial}$ in Dirac Lagrangian?

In my course, the teacher wrote the Dirac Lagrangian as : $$ \mathcal{L}=\frac{i}{2} \bar{\psi}\gamma^{\mu}\overleftrightarrow{\partial_\mu} \psi -m \bar{\psi} \psi $$ I just would like to ...
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0answers
63 views

Definitions in physics [duplicate]

In physics, I have come across some definitions of physical quantities (one such definition is that of the physical quantity Force, $\vec{F} \equiv m\vec{a} $) that do not tell us what physical ...
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0answers
328 views

What is an orthogonal point transformation?

It is regarding classical mechanics. I know that a point transformation is the transformation of generalized coordinates. But what is meant by "orthogonal" point transformations?
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2answers
326 views

Operational definitions in Newtonian Physics

Operational definitions are constructed from the observations we make in nature. Experiments show us that two objects $m_1$ and $m_2$ in a local inertial frame, isolated from the rest of the universe, ...
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4answers
524 views

Are bosons matter?

The title explains the question. Are bosons matter? As I have seen, there are three answers to this question: No, only fermions are matter. Yes, but only those with mass. Yes, all bosons are matter. ...
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2answers
323 views

How is fluid pressure defined?

In general, the pressure $p$ due to force acting on a surface of area $A$ perpendicular to it is defined as $$p=F/A$$ assuming that the force is distributed uniformly over $A$. Otherwise, we can ...
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2answers
430 views

Isometry group of Minkowski Space

from the book "Quantum Field theory and The Standard Model - Schwartz M.D": "The group of translations and Lorentz transformations is called the Poincare group $ISO(1,3)$ (the isometry group of ...
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4answers
63 views

How are these two definitions of angular momentum related? [duplicate]

I've seen angular momentum defined as: $$\ L=I \omega\ $$ In dynamics, the notation is different and states: $$\ L_o = r × (mv) \ $$ How are these definitions related, if they are describing ...
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2answers
504 views

What does it mean for an action to be invariant under $x \to x'$, $\phi \to \phi'$?

I'm suddenly getting very confused about a basic question. Suppose somebody tells you that the action is invariant under the transformation $$x \to x', \quad \phi(x) \to \phi'(x').$$ I realize this ...
5
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1answer
163 views

Understanding conservative forces

I'm trying to better understand conservative forces. I have a decent intuitive idea of what they are, but I've recently learned the mathematical rigor behind it which has made me have some questions. ...
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2answers
92 views

What is work definition exactly? [duplicate]

i learn about work and most book define force x distance. The definition given not very informative and the book don't describe much about it. All they do is give above definition and then a lot of ...
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1answer
48 views

Potential definition

I am visiting a course for Event Engineering and I want to ask you, if following sentence is right. I have a large disagreement about it. A measured Voltage, which is based on a point e.g. 0V, is ...
4
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1answer
196 views

Definition of $i^+,i^-,i^0, \mathscr{I}^+,\mathscr{I}^-$ in a general spacetime

I've seem sometimes a construction being carried out specificaly in Minkowski spacetime: One picks the standard metric tensor $$g = -dt^2 + dr^2 + r^2 d\Omega^2$$ an introduces two new coordinate ...
2
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1answer
330 views

What is the correct definition of the Jarlskog invariant?

In this lecture on neutrino physics, Prof. Feruglio defines the Jarlskog invariant as $$J=\text{Im}(U_{\alpha i}^{*} U_{\beta i} U_{\alpha j} U_{\beta j}^{*})\tag{1}$$ where $U$ is the neutrino mixing ...
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3answers
1k views

Phase space in classical mechanics

I am new in classical physics and I frequently come across the terms phase space and phase trajectory. Can anyone please explain to me what they are in a simple language?
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0answers
152 views

Quantum (spin/thermal) Hall v.s. (Spin/Thermal) Quantum Hall effects

Are the following concepts defined correctly, as I understand: Quantum Hall effect is a quantum-mechanical version of the Hall effect, observed in two-dimensional electron systems subjected to low ...