# Tagged Questions

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.

259 views

### The standard definition of current

The book says current is the rate of flow of charge per unit time, but I don't understand whether it is rate of flow of charge through a single cross-sectional area per unit time or the entire amount ...
215 views

### Gravitational acceleration

'What is difference between free fall acceleration g and gravitational acceleration a?***a is with subscript g.In my textbook it is written that "free fall acceleration = gravitational acceleration - ...
74 views

### What exactly is conservative vector field?

I'm studying calculus, but since the example involved a physical concept. I will ask here: This is how it goes: This means that in a conservative force field, the amount of work required to ...
72 views

### How are these two Riemann tensor equations equivalent?

Poisson in A Relativist's Toolkit defines the Riemann tensor as$$A_{\,;\alpha\beta}^{\mu}-A_{\,;\beta\alpha}^{\mu}=-R_{\phantom{\mu}\nu\alpha\beta}^{\mu}A^{\nu}.$$ Foster and Nightingale's A Short ...
132 views

### Why can't we precisely define physics? [closed]

While reading a textbook, I came across this statement: "A precise definition of physics is neither possible nor necessary." I was curious why it is not possible but the textbook never ...
186 views

### What is the definition of 'relative population' in context of partition function?

In statistical mechanics, what is the definition (or mathematical definition) when authors refer to relative population in the case of a classical particle system?
102 views

### A car is moving in a circular trajectory with radius R=20m. The equation of motion is : x(t) = 15 + 8t – t^2 [closed]

We have to find the distance the car has traveled after 3s. Actually this question came up in an important exam, and the answer was: L=x(3)-x(0) But I think this is the answer for displacement, ...
478 views

### The definition of mutual capacitance

I am not sure I completely understand the definition of mutual capacitance. Let's say we have two conductors, $A$ and $B$, so that the following holds: Both conductors are isolated. $A$ is isolated ...
417 views

### What are global and local gauge invariance defined as they are?

I'm sorry for the triviality of my questions. Why is $\bar{\psi} = e^{i \theta}\bar{\psi}$, where $\theta$ is a real number, used as the global gauge transformation? Why $e^{i \theta}$; what's the ...
185 views

### Need help on electric potential definition

I'm having trouble understanding electric potential. In my book it says "an electric force acts on a charge situated in an electric field." I understand this part. Then it goes on to say "If a charge ...
175 views

### Good layman definition of the critical point(phases) and supercriticality

I've heard of this point among others, but never really got what it meant. Wikipedia makes one's head spin. The only thing I picked up is that it occurs between liquid and gas, and displays ...
14k views

### Inertia Vs Momentum

At my recent lesson on kinematics, my teacher taught about inertia and momentum. This is what she said. Inertia: a characteristic of an object that resists changes to its state of motion. Momentum: ...
66 views

### How can I physically demonstrate potential difference in a circuit to a 14 year old?

Children of this age have a fair idea about current, resistance, and batteries. Potential difference is a thing that cannot be felt or physically visualized. A teenager asked me if he can touch '...
1k views

### Why do we use capacitors when batteries can very well store charges?

Can batteries be used instead of capacitors? I am trying to figure out a basic, superficial and any obvious difference between the two.
75 views

Say we have a four component spinor $\psi$: $$\psi=\begin{pmatrix}\psi_L\\\psi_R\end{pmatrix}$$ Is the Hermitian adjoint of this: $$\psi^\dagger =\begin{pmatrix}\psi_L^\dagger \psi_R^\dagger\end{... 2answers 988 views ### Susceptibilities and response functions It is often confusing whether a susceptibility is the same as a response function, specially that often they are used interchangeably, in the context of statistical mechanics and thermodynamics. Very ... 0answers 161 views ### How is Infinitesimal coordinate transformation related to Lie derivatives? I am reading the book "Gravitaion and Cosmology" by S. Weinberg. In section 10.9, while discussing Lie derivatives of tensors of different ranks, he makes a general comment: The effect of an ... 3answers 322 views ### Can we rigorously define force? I'm looking to get rigorous definitions on which to base the important quantities in classical mechanics. To me a "rigorous" physical definition is an operational definition -- that is one in which ... 3answers 348 views ### What is actually a conservation law? Though in his lectures, Feynman didn't define conservation law, he did use it while explaining divergence theorem: [...] heat is conserved. That is, no heat is generated inside the material and ... 3answers 379 views ### Definition of non-degenerate metric tensor We know that a metric has a property which is called non-degeneracy. I was searching for what does that mean and saw it associated with the fact that det(g_{\mu\nu})\neq0. How does this relate to ... 2answers 105 views ### When momentum coincides with impulse If an object is pushed for some time by some force from a resting state, can I say that at the exact moment the force is removed, impulse equals momentum? 2answers 119 views ### How this formula for work follows from the definition? If a particle moves along a path \gamma : I\subset \mathbb{R}\to \mathbb{R}^3 then the work done by a force \mathbf{F} is defined by$$W = \int_{\gamma} \mathbf{F} = \int_{I}\mathbf{F}(\gamma(t))\...
286 views

What is quantum hadrodynamics? Can anybody give a proper explanation? What are the standard books and sources of information that can be found on the internet for better understanding?
117 views

### What are the “generations of matter”?

After a series of clicks on New Scientist and Wikipedia, I ended up on the Wiki article for "generations of matter", and I didn't quite understand it. I believe (and this may be wrong) that different ...
236 views

### What is the theoretical instantaneous temperature of a gas?

When we measure the temperature of a gas we typically integrate the molecular collisions and wind up with an 'average' temperature due to the sensor comprising a relatively large thermal mass. And ...
48 views

### Integral limits when calculating the work

If I integrate $$dW= \vec{ F} \cdot d\vec{\ell}$$ which are the limits? In $$\int\limits_{W_{inf}}^{W_{sup}}dW= \int\limits_{\vec{\ell}_{1}}^{\vec{\ell}_{2}} \vec{ F} \cdot d\vec{\ell}$$ it is ...
153 views

### Transpose of (1,1) tensor

When we transpose a (1,1) tensor, shall we simply switch the two indices while keeping their upper/lower positions or switch them and also switch their upper/lower positions? In general, would the ...
230 views

### Degrees of freedom in double Atwood machine?

Why the degree of freedom in double Atwood machine (one block on one side and a pulley with one block in its each side on other side) is 2 and not 1? According to the formula $s=3*n-m$; where $n=$...
82 views

### Difference between Higgs mechanism and Higgs decay

What is the difference between Higgs Mechanism and Higgs decay? I know that Higgs mechanism is a process which provide the mass to gauge boson. In general, many literature explain Higgs mechanism ...
2k views

### When is an object at rest?

When is an object considered to be at rest?
12k views

### What is the exact difference between diffusion, convection and advection?

I have tried to explore the information but still not very clear on the exact difference between diffusion, convection and advection. Can anyone help me out to clear my concept?
1k views

### Can we define tension in a string as the reactive force produced in a string being pulled at both ends?

In my textbook, the definition of tension was given that Tension is the reactive force which exists when string is being stretched at its both end. After it there was a case given that to calculate ...
466 views

### What is a photon? [duplicate]

I'm trying to get a definitive and clear answer to the question of what a photon actually is. Textbooks seem to give quite vague explanations, all starting with Einstein's idea that a quanta is a form ...
66 views

### Who is said to do Work, me or the body?

If I subject my force to a body and it is displaced then the work is said to be done. What is that work done by? Is it said to be done by me or that body?
865 views

### Conventional definition of ideal fluid

According to Landau&Lifshitz, an ideal fluid is one with zero viscosity and a negligible thermal conductivity. This is also the FR.wikipedia version: En mécanique des fluides, un fluide est ...
254 views

### What is the difference between the diffusion equation and the heat equation?

I know that the diffusion equation is a more general version of the heat equation. But what is the exact difference informally?
2k views

### What's the difference between microscopy and spectroscopy?

Both methods collect particles or electromagnetic waves, and in both methods it's possible to reconstruct a 2D image, which may represent morphology (AFM, LEED for example), electronic structure (STM, ...
1k views

### Definition of a normal mode?

What is the formal definition of a normal mode for a string? And how does this relate to the definition from e.g. wiki that seem to be applied to discrete systmes of particles only? Also on a string ...
80 views

### What's the difference between “spectromicroscopy” and “microspectroscopy”? [duplicate]

Both definitions that I found are rather vague. (Related question: What's the difference between microscopy and spectroscopy?)
182 views

### What is thermodynamic equilibrium?

The state of thermodynamic equilibrium is typically defined referring to the system's behaviour in the future. Can the definition be formulated in terms of measurable quantities?
17k views

### What is basically the difference between static pressure and dynamic pressure?

What is basically the difference between static pressure and dynamic pressure? While studying Bernoulli's theorem, I came before these terms. The law says: When the fluid flows through a small ...
486 views

### Extensive variables in thermodynamics

Extensive variables in thermodynamics are those which scale linearly with the system size. It is known that a ratio of two extensive variables is an intensive variable. Now, the number of particles (...
160 views

### Bounded operator - definition?

As mentioned also in Bounded and Unbounded Operator, an operator $A$ is said to be bounded, if $$\|Af\|\leq k \|f\|,$$ where the constant $k$ does not depend on the choice of $f$ (let us consider a ...
99 views

### What are the Fermi and Debye temperature constants?

What are the Fermi temperature and Debye temperature constants? We were discussing these in class and I don't fully understand what these constants are or why we have them. Can anyone explain?
60 views

### Cosmology: what is a quantity that is called “$h$” in regard to angular size of a galaxy?

I am trying to solve a Cosmology problem, but a certain quantity $h$ appears in it, of which I do not know the definition (I have never seen it mentioned anywhere before). So I thought maybe someone ...
75 views

### Is entalphy a presence of energy or a change in energy?

I see that the words enthalpy and change in enthalpy are often used interchangibly. Do they mean the same thing? Are change in enthalpy and enthalpy different? What is the true definition of ...
55 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 ...
126 views

### Definition of “nonlinear” in the context of perturbation of gravity

What exactly is the definition of a nonlinear perturbation when applied to a background spacetime metric? I have seen so called "linear perturbations" which look like ds^2 = -(1+2\Phi)dt^2 +a^2(1+...