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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19
votes
6answers
123k views

What is the difference between stress and pressure?

What is the difference between stress and pressure? Are there any intuitive examples that explain the difference between the two? How about an example of when pressure and stress are not equal?
14
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5answers
49k 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?
17
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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 ...
5
votes
3answers
4k views

Newton's first law: is his concept of (force of ) inertia still useful and used?

The force of inertia is the property common to all bodies that remain in their state, either at rest or in motion, unless some external cause is introduced to make them alter this state. That is ...
17
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5answers
3k views

What is the difference between a functional and an operator?

What is the difference between a functional and an operator? When we define an operator in physics, e.g. the momentum operator as $\hat{p} = i \frac{d}{dx}$, it is said this operator acts on the wave ...
15
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1answer
1k views

What is a general definition of the spin of a particle?

In quantum field theory, one defines a particle as a unitary irreducible representations of the Poincaré group. The study of these representations allows to define the mass and the spin of the ...
7
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7answers
19k views

Simple explanation of quantum mechanics [closed]

Can you please describe quantum mechanics in simple words? When ever I read this word (quantum computers, quantum mechanics, quantum physics, quantum gravity etc) I feel like fantasy, myth and ...
4
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2answers
35k views

How is focal length defined for a two-lens system, separated by a distance $d$?

I have found the formula for the effective focal length $f$ of two thin lenses with focal lengths $f_1$ and $f_2$ separated by distance $d$ to be $$ \frac 1f=\frac 1{f_1}+\frac 1{f_2}-\frac d{f_1f_2}....
13
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1answer
1k views

What's the most fundamental definition of temperature?

What's the most fundamental definition of temperature? Is it the definition concern about average energy, number of micro states, or what? By "fundamental", I mean "to be applied" in such general ...
8
votes
2answers
688 views

How come the universe is made of matter and not antimatter?

Antimatter is like matter on opposite day: it has the same properties as the stuff that makes up planets, stars and galaxies, but one vital piece is different—its charge. The universe supposedly ...
7
votes
3answers
4k views

What is the general statistical definition of temperature?

Temperature in an isolated system is defined as: $$\frac{1}{T} = -\frac{\partial{S(E,V,N)}}{\partial{E}} $$ But I wonder how one can generalize this to a random system. Or for instance to a point in ...
7
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4answers
7k views

The elusive difference between force and impulse

Impulse is defined as the product of a force $F$ acting for a (short) time $t$, $J = F*t$, and that is very clear. What I find difficult to understand is how a force can exist that doesn't act for a ...
6
votes
1answer
822 views

Equivalent definitions of vectors

Equivalent definitions of vectors. In maths a vector is an object that obeys some axioms of a vector space. But in physics a vector can be thought as an object which is invariant under rotations of ...
4
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3answers
4k views

Why is the absolute zero -273.15ºC?

I can't find an answer of why the lowest temperature is -273.15ºC. Is it deduced theoretically or is it experimental? An explanation is that when any gas volume tends to zero, the temperature will be ...
2
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2answers
11k views

Definition of Significant Figures

In my textbooks, significant figures are defined as: “Significant figures by definition are the reliable digits in a number that are known with certainty.” “A significant figure is the one ...
3
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1answer
3k views

What is the meaning of “moment”?

What is the meaning of moment? I'm little confused about the word as there are some terms like moment of momentum, moment of mass, moment of force, etc. I want to know what exactly is meant by the ...
6
votes
3answers
9k views

What is displacement? Position relative to a reference point or change of position

What is the "official" or most useful definition of displacement in the context of kinematics? There are two common ones: Displacement is the length and direction of a line from a fixed reference ...
2
votes
2answers
117 views

What is a basic physics general definition of a 'potential'?

From the Wikipedia: In physics, a potential may refer to the scalar potential or to the vector potential. In either case, it is a field defined in space, from which many important physical ...
2
votes
2answers
566 views

Is the Reynolds Number well defined for a given system?

I have been looking for definitions of the Reynold number, and by far the most common definition is that the it is given by: $$ Re\equiv \frac{LU}{\nu}$$ Where $L$ is the characteristic length scale, $...
91
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3answers
19k views

Why was carbon-12 chosen for the atomic mass unit?

The atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Was there any physical reason for such a definition? Were they trying to include electrons in the atomic mass unit? Why not ...
8
votes
3answers
19k views

What exactly is the “coherence” between waves?

I know, by definition, that coherence means that a pair of waves have constant phase difference. What does this mean? Does it mean they always have a 360 degrees, or 0 degrees phase difference? Or ...
4
votes
1answer
359 views

How a reference frame relates to observers and charts?

Recently I've been watching the General Relativity lectures from the "International Winter School on Gravity and Light" by Frederic Schuller. In those lectures he made the following two definitions: ...
3
votes
1answer
13k views

Definition respective derivation of angular momentum formula

I am reading An Introduction to Mechanics by Kleppner and Kolenkow (2014). On page 241 is the definition of the angular momentum: Here is the formal definition of the angular momentum $\vec{L}$ of ...
0
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4answers
645 views

What is the rigorous quantitative definition of the concept of “Energy”? [closed]

First of all I acknowledge you that I posted this Question on many other forums and Q&A Websites. So don't be surprised if you found my question somewhere else. I bet when the experts saw the ...
14
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1answer
3k views

Unitary quantum field theory

What do physicists mean when they refer to a quantum field theory being unitary? Does this mean that all the symmetry groups of the theory act via unitary representations? I would appreciate if one ...
6
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4answers
3k views

Is the Lagrangian density a functional or a function?

Weinberg at page 300 of The Quantum Theory of Fields - Volume I says: $L$ itself should be a space integral of an ordinary scalar function of $\Psi(x)$ and $\partial \Psi(x)/\partial x^\mu \,$, ...
3
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2answers
2k views

What is pseudo-tensor?

What is the pseudo-tensor in relativity? How do we transform tensor and pseudo-tensor under parity?
0
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2answers
153 views

Definition of magnetic field

Can we define magnetic field at a point as: Force on a point magnetic north pole at that point divided by its pole strength. Anything wrong in this definition? (The concept of point magnetic pole ...
5
votes
4answers
5k views

What is the physical meaning of electric potential, potential difference, and voltage?

When resembling the electricity flow through a wire to people walking through a street: electrons are people, current is the number of people, resistance is the barriers on the way. But what is the ...
4
votes
1answer
2k 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?
6
votes
8answers
13k views

Is there a definition of “direction” in physics?

Is there an actual definition of "direction" (that is, spatial direction) in physics, or is it just one of those terms that's left undefined? In physics textbooks it's always just taken for granted ...
5
votes
2answers
2k views

Understanding relationship between work and energy

I've read over 10 books about work and energy, and I just simply can't understand it. First of all, they go ahead and randomly define that work is force times distance: $$W=F X \cos\theta$$ Okay, ...
5
votes
4answers
1k views

How to define heat and work?

In textbooks, heat is usually defined as the energy transfer due to temperature difference. However, we don't know what temperature is in the first place. I think it's better to define heat first and ...
4
votes
1answer
3k views

Will centre of gravity coincide with centre of mass if density of object is non-uniform?

I read that for bodies of very large dimensions, but having non-uniform density, the centre of gravity does not coincide with centre of mass. I can understand that with large dimensions the strength ...
3
votes
1answer
7k views

Definition of “intensive” and “extensive” properties

Today I was asked what does it mean for a physical property of a system to be intensive. My first answer, loosely speaking, was: "It is a property that is local." I was specifically thinking ...
3
votes
2answers
21k views

Why is a degree Celsius exactly the same as a Kelvin?

How on earth is it possible that the difference between two temperatures in Celsius and Kelvin is exactly the same. Given the historical definition of Celsius, I find it hard to believe that this is ...
2
votes
2answers
319 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, ...
2
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1answer
117 views

What is Work? What does the quantity suggest intuitively? [duplicate]

The mathematical formula for work says that work is force into displacement, but what is the philosophy behind it? I mean what does the quantity suggest?
2
votes
1answer
3k views

What is the difference between a tensor, vector, and a matrix? [duplicate]

I'm currently going through notes on a physics course and I'm having trouble understanding the difference between a tensor, a vector, and a matrix. I know that a vector is a kind of tensor and that a ...
1
vote
2answers
8k 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, ...
0
votes
2answers
277 views

How has the definition of gravitational potential energy been derived?

The definition of gravitational potential energy is - The gravitational potential energy of an object at a point above the ground is defined as the work done is raising it from the ground to that ...
-1
votes
2answers
6k views

Work done by ideal gas

Our textbooks state that work done by the gas is, $$dW = P_{external}\space dV$$ But why can't it be $$dW = P_{gas} \space dV$$ Why do we consider the external pressure instead of the pressure of ...
3
votes
0answers
664 views

What's the difference between hypothesis, theory and law? [closed]

What's the difference between hypothesis, theory and law? I think I'm confused now because I exactly learned the misconception one as explained below in high school. According to this university's ...
2
votes
1answer
323 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 ...
1
vote
1answer
430 views

Defining four-vectors in General Relativity?

Nearly every general relativity textbook I look in has a section on four-vectors. However, this section is usually titled something like 'Four vectors in Special Relativity', 'Special Relativity (...
0
votes
2answers
304 views

What is the degree of freedom?

In here, https://en.wikipedia.org/wiki/Degrees_of_freedom_(mechanics), the degree of freedom is defined as "the number of independent parameters that define its configuration." So, if $N$ particles ...
0
votes
2answers
680 views

Can I define the term energy in terms of work?

Recently, I'm doing my personal task which is to formalize every definition and concept in physics, by means of formal language and of course with intuitional notes. Because I found myself that the ...
-2
votes
1answer
63 views

Clarification of the definition of potential at a point

My textbook defines the potential at a point to be equal to the work done in bringing a unit positive charge from infinity to that point, and then explains that the point contains another unit ...
17
votes
2answers
7k views

What is a non linear $\sigma$ model?

What exactly is a non linear $\sigma$ model? In many books one can view many different types of non linear $\sigma$ models but I don't understand what is the link between all of them and why it is ...
23
votes
3answers
12k views

What is a mode?

The word mode pops up in many fields of physics, yet I can't remember ever encountering a simple but precise definition. After having searched fruitlessly on this site as well, I feel that even ...