The conventions tag has no wiki summary.
2
votes
4answers
101 views
Potential energy sign conventions
Almost every book on physics that I read have some weird and non-clear explanations regarding the potential energy. Ok, I do understand that if we integrate a force over some path, we'll get a ...
2
votes
1answer
75 views
Plane waves in QFT
Suppose we work in the metric $(-1,+1)$.
How do we describe an incoming particle with a plane wave; $\exp(-\mathrm ikx)$ or $\exp(+\mathrm ikx)$?
What's the difference?
Does it change if we work in ...
2
votes
1answer
63 views
Sign crazyness on the stress energy tensor?
I would like to know on what depends the sign of the stress energy tensor in the following formula :
$T_{\mu\nu}=\pm(\rho c^2+P)u_{\mu}u_{\nu} \pm P g_{\mu\nu}$
In my case the metric is equal to ...
1
vote
2answers
105 views
What is the most natural value of Heaviside step function at zero argument?
In many physical applications, the Heaviside step fuction is defined as $$H(x) = \left\{\begin{eqnarray}
1, \quad x>0 \\
0, \quad x<0
\end{eqnarray}\right.$$
The value $H(0)$ is left undefined. ...
2
votes
1answer
180 views
Why the direction of dipole moment is from negative charge to positive charge?
An electric dipole moment is defined as $p = q\times 2d$. How to understand it physically? Why the direction of the electric dipole moment is from negative charge to positive charge?
2
votes
2answers
142 views
Inner Product Spaces
I am trying to reconcile the definition of Inner Product Spaces that I encountered in Mathematics with the one I recently came across in Physics. In particular, if $(,)$ denotes an inner product in ...
0
votes
1answer
92 views
Sign and Four Acceleration Special Relativity
If $ use $(+,-,-,-)$ sign convention then four position, four velocity become positive but four acceleration becomes negative!
$x_{\mu}x^{\mu}=\tau^2c^2,$
$U_{\mu}U^{\mu}=c^2,$
...
1
vote
1answer
59 views
When are leap seconds added in various time zones?
I understand that technically when a leap second is added, it is added after midnight UTC, but I'm unclear how the addition is handled in other timezones. For precise reckoning of course (e.g. ...
2
votes
1answer
375 views
Levi Civita Symbol and contravariance vs covariance
I have a question regarding the Levi-Civita symbol and contravariance vs covariance. Some of this was asked in a previous post, but I think I need more clarification.
Consider the magnetic field:
...
3
votes
0answers
104 views
Dirac action and conventions
I have a (possibly) fundamental question, which is driving me crazy.
Notation
When considering the Dirac action (say reading Peskin's book), one have
$\int ...
1
vote
1answer
80 views
What's the common consensus on the meaning of “physical change”?
I'm doing a textbook problem that shows a "molecular level" view of some matter, little colored balls, before and after, and there are, among the four questions, two that say: 1) Did a physical change ...
0
votes
1answer
71 views
Is this phase right?
Hello at physics lectures we wrote a phase of a sine wave like this:
$$\phi = kx - \omega t$$
Is this right? As I recall the phase of a wave should be written like this:
$$\phi = \omega t - kx$$
...
11
votes
1answer
343 views
Why is the partition function called ''partition function''?
The partition function plays a central role in statistical mechanics.
But why is it called ''partition function''?
1
vote
1answer
281 views
Levi-Civita symbol in Euclidean space
Suppose a component of tensor field is described by $B^k=\varepsilon^{kij} \phi_{ij}$. If we define $B^k$ in an Euclidean space then does the rising or lowering of the indices of the Levi-Civita ...
0
votes
1answer
62 views
What does the latin letter mean in Bayer name of a star such as “$\Upsilon$ Andromedae $b$”?
What does the latin letter mean in Bayer name of a star such as the "$b$" in "$\Upsilon$ Andromedae $b$"? I tried in vain to look it up in Wikipedia or elsewhere on the Web.
4
votes
1answer
284 views
Is 4-volume element a scalar or a pseudoscalar in special relativity?
In general relativity 4-volume element $\mathrm{d}^4 x = \mathrm{d} x^0\mathrm{d} x^1 \mathrm{d} x^2\mathrm{d} x^3$ is clearly a pseudoscalar (or scalar density) of weight 1 since it transforms as ...
4
votes
3answers
326 views
Why do we still not have an exact definition for a kilogram?
I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It seems this would be fairly important.
Edit
Today I ...
1
vote
1answer
90 views
Conversion of the units BeV (US) and GeV (UN)
What is the difference in the definition of a billion electron volts in United states (US) and United Nations (UN)?
When the US people say billion, do they mean $10^{12}$ or $10^9$?
0
votes
3answers
162 views
Why is 'the period' marked as letter T?
I'm not a native English speaker and I was wondering, why 'the period' got the letter $T$.
I've asked myself the question when I was thinking about stuff related to the frequency. I.e.:
$f$ - ...
2
votes
1answer
135 views
What is the origin of the naming convention for position functions?
In physics, position as a function of time is generally called d(t) or s(t). Using "d" is pretty intuitive, however I haven't ...
2
votes
3answers
455 views
How is Planck's law defined?
Now, I found three different definitions of Planck's law:
$$
P_1(\nu,T) = \frac{8 \pi}{c}\frac{h \nu^{3}}{c^2} \frac{1}{e^{h\nu/kT}-1}
$$
$$
P_2(\nu,T) = 2\frac{h \nu^{3}}{c^2} ...
0
votes
1answer
101 views
Is reflection just a particular case of scattering?
I was talking to a colleague about optical scattering from a metallic nanoparticle, and we had a very simple doubt. If you have a particle that's small compared to the illuminated area, you can use ...
