Questions tagged [conventions]
A convention is a set of agreed, stipulated, or generally accepted norms. It typically helps common efficiency or understanding but is not required, as opposed to a strict standard or protocol.
1,269
questions
0
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1answer
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Do we put negative sign while calculating inward flux by Gauss Divergence theorem?
Suppose we have a vector field $\vec A=ax\hat i + by\hat j+cz\hat k$ and we want to calculate its inward flux $\int \vec A\cdot\vec {dS}$ over the spherical surface $x^2+y^2+z^2=1$(with area vector ...
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0answers
32 views
What does negative value for potential energy mean?
What does negative value for potential energy mean and which is more stable in potential energy for example -3000 or -4000? I read that it doesn't mean anything and the values of the energies are ...
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0answers
66 views
Relativistic Electromagnetism (Undergrad Level) book with metric $\eta_{\nu\mu} = (-1, +1, +1, +1)$
I have looked several special relativity books but in each of them the metric is defined as $\eta_{\nu\mu} = (+1, -1, -1, -1)$.
Is there a book where the metric is defined as $\eta_{\nu\mu} = (-1, +1, ...
0
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1answer
48 views
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.
...
2
votes
1answer
71 views
Why is the $\frac{1}{2}$ significant in $\frac{1}{2}\rho v^2 C_d A$? [duplicate]
The formula for drag force is $F_D=\frac{1}{2}\rho v^2 C_d A$. Why is the $\frac{1}{2}$ significant here? I think that the drag coefficient $C_d$ already serves the purpose.
Is it for a historical ...
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0answers
60 views
Direction of $r$ in $-dU/dr$?
While stating $F(conservative)= -dU/dr$; what is/are the condition(s) on $r$ of which $U$ is a function of. Does it need to be along (either parallel or antiparallel to) $F$?
This can be critical when ...
2
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2answers
66 views
Symmetry factor of Feynman diagram in $\phi^4$ theory at second order
I am trying to figure out the symmetry factor of the above diagram in the $\phi^4$ theory. The correct answer is $S=4$ (in that the Feynman diagram gets a multiplicative factor of $\frac{1}{4}$) but I ...
0
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1answer
58 views
Is Earth's North pole actualy a South pole?
As far as I am concerned, the compass is facing along the magnetic field lines on the Earth's surface which in turn points towards the North pole by its design. But for a magnet, the field lines go ...
-1
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1answer
30 views
Potential energy in the gravitational field - Why is $r_2$ striving against infinity?
why is $r_2$ striving against infinity in the formula $š = šŗšš(\frac{1}{š_1}ā\frac{1}{š_2})$, so its often simplified to $š = \frac{šŗšš}{r}$ ?
I know that in the final formula, r is the ...
0
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2answers
69 views
Is the direction of current wrong in this circuit? Why? [closed]
I know how to measure the equivalent resistance of this circuit, we say R2, R3 and R4 are in parallel combination and I know the reason of this too. But if we consider the direction of electricity of ...
2
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2answers
63 views
Why not make GPE positive?
I was teaching a student about negative GPE with the formula:
$$U=-\frac{GMm}{r}$$
He wasn't very happy with why it was negative and came up with this formula instead:
$$U'=-\frac{GMm}{r}+Bm$$
where $...
0
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0answers
26 views
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 ...
4
votes
2answers
97 views
What happens if we expand out fields in terms of different functions?
When we "expand" our classical fields, for example the Dirac field, in the standard way which we later go on to "quantise":
$$\psi(x,t)=\int d^3\tilde k \sum_{a=1,2}\left(b_a(k)u^a(...
2
votes
2answers
151 views
Why is there a specific negative sign in front of the $m_{12}$ term of the 2HDM Higgs potential?
Why is there a specific negative sign in front of the $m_{12}$ term of the 2HDM Higgs potential?
(but not for the $m_{11}$ and $m_{22}$)
See for example: https://arxiv.org/abs/1106.0034
Eq. (2) Page 6:...
2
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1answer
72 views
Why is the neutral component of Higgs doublets in 2HDM theory different according to the document of reference?
According to a reference on Higgs (anatomy of Higgs, tome 2, by Djouadi)
arXiv:hep-ph/0503173
On formula 1.55 page 30, the neutral component is on the top component of the first Higgs doublet, and in ...
1
vote
2answers
76 views
Why did Feynman use a negative sign while defining the potential energy function?
Why is it ($-U$) and not ($+U$)?
And how do we know it's going to help us if we take it to be negative beforehand?
0
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1answer
26 views
What sign conventions are used here? Geometrical optics
I'm reading Theory and Problems in Optics By Eugene Hecht (Schaum's outline series).
I'm not able to understand, What sort of sign convention are they using?
It says,
Table 4-1 summarizes the sign ...
1
vote
2answers
51 views
What is the height in the potential energy formulation?
Most examples I've seen describe the (gravitational) potential energy e.g. with an example of a ball thrown upwards, and explain how the sum of the ball's kinetic and potential energy is constant at ...
0
votes
1answer
39 views
Why don't we take $-e$ in equations involving electrons? [closed]
$$dQ = -enAv_ddt$$
is not the given equation but
$$dQ = enAv_ddt$$
is where from $Q$ = charge, $n$ = electrons per unit volume, $A$ = area of cross section, $v_d$ = drift velocity and $t$ = time.
...
2
votes
1answer
72 views
Palatini identity sign
I was deducing the Palatini identity using the variation of the Christoffel symbols and got to the following result:
$$ \delta R_{\mu\nu}g^{\mu\nu}=(\nabla^\mu \nabla^\nu-g^{\mu\nu} \square) \delta g_{...
0
votes
1answer
38 views
Why is there a negative sign in the time evolution operator when defining in/out states (Peskin/Schroeder)
This relates to Peskin & Schroeder's QFT book, equation 4.70 on page 104.
To define in and out states we take our initial state and evolve it far into the past, and do the same for our final state....
0
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3answers
120 views
Why is SI-unit of density $\frac{\rm kg}{\rm m^3}$? [closed]
The CGS unit of Density is: $$\frac{\rm g}{\rm cm^3}$$
Then how can the SI or MKS unit of density be:
$$\frac{\rm kg}{\rm m^3}$$
Shouldn't it be:
$$\frac{\rm kg}{\rm dm^3}$$
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0answers
26 views
Origin in the discrepancy between engineers' and physicists' notation of waves? [duplicate]
my question is very simple. Physicists use this notation in order to write a (for example) plane wave:
$$
\xi(z) = \xi^+ \mathrm{e}^{+\mathrm{i}kz} + \xi^- \mathrm{e}^{-\mathrm{i}kz},
$$
where $\xi^+$ ...
0
votes
1answer
32 views
Why (physically not mathematically) is the induced EMF of a rotating coil the negative derivative of the graph for flux?
The graphs for a rotating coil of the flux and then the induced EMF looks like this.
So I understand mathematically that the emf will be at a maximum when the gradient of the flux is at its biggest. ...
10
votes
6answers
3k views
Why is the standard uncertainty defined with a level of confidence of only 68%?
The widely used and accepted form of representing the uncertainty of a measurement is described in the GUM (Guide to the Expression of Uncertainty in Measurement).
There it is recommended to use the ...
1
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0answers
54 views
Feynman rules for derivative couplings with identical particles
Consider the interaction Lagrangians given by
\begin{align*}
\mathcal{L}_1 = A_\mu \ \pi^{0} \ \partial^{\mu} \pi^{0}\\
\mathcal{L}_2 = B^{\mu\nu} \ \partial_\mu \pi^{0} \partial_{\nu} \pi^{0}\\
\...
1
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1answer
45 views
Can we write $3.9\ \mathrm{M\Omega} \pm 5\%$ as $3.9 \pm 5\%\ \mathrm{M\Omega}$?
I have a doubt regarding units. Can we write $3.9\ \mathrm{M\Omega} \pm 5\%$ as $3.9 \pm 5\% \mathrm{M\Omega}$?
1
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3answers
109 views
In vector algebra, why does the cross product of unit vectors like ${\bf i}\times{\bf j}={\bf k}$ become negative when the order is reversed?
Why is it ${\bf i} \times {\bf j} ={\bf k}$, but if the order is interchanged it becomes ${\bf j} \times {\bf i} =-{\bf k}$? I did not understand this concept and I would love to know where I am ...
0
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0answers
30 views
Notation problem in Yukawa-like interaction - from SUSY?
I am struggling to understand the meaning of a term in a Lagrangian of a paper I am reading. I think it is a notation issue. The term is:
$$\lambda_e \bar{\nu}_L^c l_L \Sigma^{\dagger}_e$$
where $\...
0
votes
1answer
62 views
How to choose the sign of the differential?
I know this is a very simple question, and I have searched it too. How to avoid incorrect symbols in calculation results.I donāt understand how to choose the sign of $ds$.
An object moves from a to b,...
3
votes
1answer
115 views
Totally antisymmetric part of a rank 3 tensor
In Hobson's General Relativity: An Introduction for Physicists book, pg. 95, the totally antisymmetric part of a rank 3 tensor $t_{abc}$ is defined as
$$t_{[abc]} = \frac{1}{6}(t_{abc}-t_{acb}+t_{cab}-...
0
votes
1answer
51 views
Why is the positive terminal of an ammeter connected to the positive terminal of a battery?
I always thought it's because, if we consider a circuit ABCDE with only a battery and an ammeter, if we connect the negative terminal to the positive terminal the sum of the voltages across loops ...
2
votes
2answers
69 views
The value of the volume form on an orthonormal frame
In a Riemannian space, what is the value of the canonical volume form on a frame? In particular, say, an
orthonormal frame. One does not usually need to know anything about the
value of a ...
0
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0answers
52 views
How is the Minkowski space oriented?
This might seem like a weird question, but I am trying to prove the following result:
\begin{equation}
\mathrm{d}*F=\frac{4\pi}{c}J\Leftrightarrow\begin{cases}
\nabla\cdot E=4\pi\rho\\
\nabla\times B=\...
0
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0answers
20 views
Battery/cell circuit symbols
I'm teaching some younger kids physics, and we are looking at circuits.
When recapping circuit symbols, I remembered that there were different ones for a cell and a battery. A battery symbol looks ...
0
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2answers
113 views
What does the symbol $Ī¼$ represent?
I know that Ī¼ represents the prefix micro.
However,
I read in few books (reliable, at least in my country) and other resources, that the symbol Ī¼ represents micron(micrometre) too.
Now, if the ...
2
votes
2answers
57 views
Writing Numbers with Uncertainties
I am having a little trouble determining the right way to write a number with an uncertainty:
I did a calculation in which the uncertainty was 150, while the value with physics meaning was about to ...
1
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1answer
42 views
Integrating cross products
I know that while integrating dot product to two vector quantities along a line integral, the limits of the integration implicitly takes care of the direction in which we integrate from here and here.
...
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3answers
102 views
There is something I don't understand in the answer of the following question [duplicate]
Q. The gravitational field due to a mass distribution is given by $E=k/(x^3)$ in $x$-direction. Taking the
gravitational potential to be zero at infinity, find its value at a distance $x$.
for the ...
0
votes
2answers
91 views
How to figure out if the vector product of force and displacement is $-\mathbf fd\mathbf x$ or $\mathbf fd\mathbf x$?
Consider the force due to a point charged particle at the origin and the particle which we move to also be charged with the same charge.
When we move a particle from point $\mathbf{x}$(position vector)...
6
votes
4answers
1k views
Why is gravity considered a negative vector in a pendulum question?
In many places and tutorials, gravity is often considered as a negative vector.
I am confused as to why is that? I though I was missing something from trigonometry but it was just negative in first ...
-1
votes
2answers
109 views
Verifying $W = \int \vec{F} \cdot d\vec{x}$
Verifying formula for Work;
$$W = \int \vec{F} \cdot d\vec{x} \quad(i)$$
Let us consider a very simple scenario; I will solve first by simple Maths and then by calculus.
Scenario 1: (Force vector is ...
0
votes
1answer
46 views
Can I draw a variable resistor with an arrow looking to the left? What does the side the arrow look at signifiy?
Like this:
Instead of this:
And what does the direction that the arrow look at signify?
5
votes
1answer
174 views
Quantization of Klein-Gordon field (what is creation operator there and what annihilation)
Recently in my class we studied quantization of fields and I'm brooding over
an argument/ motivation on the construction
of the quantization of the Klein-Gordon field. Recall the "classical"
...
2
votes
2answers
86 views
4-Vector Gradient and Contravariant Derivative
I am self-studying General Relativity, and the course of study I am following has started to introduce me to index notation. The texts I am using (Carroll, Schutz) begin with a geometric slant on ...
6
votes
2answers
104 views
$A^{\alpha}=(\phi,\vec{A})$ or $A_{\alpha}=(\phi,\vec{A})$?
I have seen both equations, $A^{\alpha}=(\phi,\vec{A})$ is from Wikipedia and $A_{\alpha}=(\phi,\vec{A})$ is from my lecture. Which one is right?
My thoughts: As far as I know, $A$ is a 1-form, so $A(...
1
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0answers
38 views
Algebra of Noether's charges and algebra of symmetry transformations
I'm trying to understand the connection of algebra of transformations under a commutator and algebra of Noether's charges under Poisson bracket.
I have a problem that results I infer from theoretical ...
1
vote
1answer
33 views
Gravitational potential energy sign
Following is a small derivation just so I can explain my question. The gravitational potential energy is:
$$(*)U_g = -\frac{GMm}{r}$$
And:
$$ \Delta U =-GMm(\frac{1}{r_{final}} - \frac{1}{r_{initial}})...
1
vote
2answers
47 views
What do primed coordinates refer to in Griffith's section on Helmholtz Theorem?
In his section on Helmholtz theorem Griffith uses primed coordinates in the integrals that define $U$ and $\mathbf{W}$. Now, I understand that primed coordinates are used for sources (charges and ...
2
votes
2answers
77 views
Sign Convention in Field Theory
I am working with some field theory and I am at a point where I would like to change my sign convention from $(-+++)$ to $(+---)$. I am worried at some point I will just equations with the wrong sign ...
