Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
What are the main algorithms the LHC particle detectors use to reconstruct decay pathways? I am just starting to look into how we understand the data from particle collisions.
My question is, what are the algorithms or ways that these detectors interpret the data? Are there standard approaches? Or if not, what are some... | Well, if you have the time... CERN has all the technical design reports for its detectors online at http://cds.cern.ch/. They are excellent reading material.
Start with a search for "ATLAS technical design report" and "CMS technical design report" and work your way trough the references in those documents. Once you un... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/130268",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 3,
"answer_id": 0
} |
The particle content of a given state In Carroll's we read
...The Unruh effect teaches us the most important lesson of Quantum Field Theory (QFT) in curved spacetime, the idea that "vacuum" and "particles" are observer-dependent notions rather than fundamental concepts.
I wonder are we talking about the same observer... | That is exactly the point: if the field is in a vacuum with respect to observer A, and observer B accelerates uniformly with respect to A, then B will observe a field state with nonzero particle content.
It doesn't really matter whether you talk about different observers on different frames of reference, or of a single... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/130439",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
How is photon time of flight/range over sub-millimeter distances measured? I was reading a paper that described how the force a low-thrust torsion pendulum was measured. In it, the paper states a laser is bounced off a mirror and the displacement is "...based upon the beam reflection time." The paper states that the de... | Can you cite the paper, please?
Assuming that it's a modern version of an old experiment, my first guess would be, that the observation uses the fact that the light will induce a constant moment on the torsion pendulum. The response of the pendulum will be an oscillatory motion, for short amounts of time (seconds to mi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/130505",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
How can gravity affect light? I understand that a black hole bends the fabric of space time to a point that no object can escape.
I understand that light travels in a straight line along spacetime unless distorted by gravity. If spacetime is being curved by gravity then light should follow that bend in spacetime.
In N... | It's just a simple concept according to Einstein's relativity when light travels through high gravation field or high mass containing object (i.e. same when a object has high mass means it can attract other objects of lesser mass) the photons present in the light gets attract towards the other object and we see light b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/130552",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
"answer_count": 6,
"answer_id": 5
} |
Finding the cosmological redshift of a galaxy in the expanding Universe Firstly, I understand what the Doppler effect is when it comes to sound or light waves.
From everything that I've read, we are told that the universe is constantly expanding since the all the radiation we observed is red-shifted.
Assuming we are ob... | The answer to this question is that if you can only see one line or feature in the spectrum then the redshift cannot be measured unless you have some other information that leads you to guess what the line or feature in the spectrum is due to (e.g. the 21cm line of hydrogen at radio wavelengths is so strong and ubiquit... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/130607",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Glashow-Weinberg-Salam mass terms At the end of spontaneous symmetry breaking I get these mass terms:
$$W_{\mu}^{\pm}=\frac{1}{\sqrt{2}}\bigl(W_{\mu}^{1} \mp i W_{\mu}^{2} \bigr )$$
$$\mathcal{L}_{mass}=\frac{1}{2} g^2 \frac{v^2}{4} W_{\mu}^{+}{W^{\mu}}^{-} + \frac{1}{2} g^2 \frac{v^2}{4} W_{\mu}^{-}{W^{\mu}}^{+}$$
So ... | Notation $W^{-}, W^{+}$ may confuse in a sense that it may seem that here are two different particles which aren't connected by charge conjugation. But of course, $W^{+}$ is only $(W^{-})^{\dagger}$, so it is an antiparticle to $W^{-}$. So term $( W^{-} \cdot W^{+} )$ is simple $|W|^{2}$ (which is standard for the mass... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/130953",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Wouldn't the presence of dark matter slow the expansion of the universe? If there is a huge element of dark matter in the universe, wouldn't this extra gravity prevent the accelerated expansion of the universe?
| The acceleration of the expansion is currently observed to be happening. This observation is one of the pieces of data we use to infer the amount of dark matter. It tells us that there can't be more than a certain amount of dark matter, because that would be incompatible with the observed acceleration.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131014",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Physical reason for Lorentz Transformation Seeing the mathematical derivation of the Lorentz Transformation for time coordinates of an event for two observers we get the term
$$t'=-\frac{v/c^2}{\sqrt{1-\frac{v^2}{c^2}}}x+\frac1{\sqrt{1-\frac{v^2}{c^2}}}t$$
Now how to make sense physically of the $t-\frac{vx}{c^2}$ fact... | The physical reason IS the constancy of the velocity of light... since I'm writing in a tablet the answer won't be complete, but expect to get you to the mathematical cross-road.
Constancy of velocity of light implies that
\begin{equation}
\frac{d|\vec{x}|}{dt} = c, \quad\Rightarrow\quad d|\vec{x}| = c\,dt.
\end{equa... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131100",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
What is entropy really? On this site, change in entropy is defined as the amount of energy dispersed divided by the absolute temperature. But I want to know: What is the definition of entropy? Here, entropy is defined as average heat capacity averaged over the specific temperature. But I couldn't understand that defini... | A higher entropy equilibrium state can be reached from the lower entropy state by an irreversible but purely adiabatic process. The reverse is not true, a lower entropy state can never be reached adiabatically from a higher entropy state. On a purely phenomenological level the entropy difference between two equilibrium... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131170",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "46",
"answer_count": 14,
"answer_id": 6
} |
Getting first and second time derivative of mean motion of spacecraft - without TLE? The Celestrak website provides information on reading the TLE ( Two-Line Element Set ) format. In Line 1, Column 34-43 & 45-52 give information about First Time Derivative of the Mean Motion and Second Time Derivative of Mean Motion.... | The mean motion $n$ of a satellite is its angular velocity, averaged over one period. In other words, if the satellite rotates around the Earth with period $P$, its mean motion $n$ is
$$
n = \frac{2\pi}{P}
$$
If the Earth were a perfectly spherical symmetric object, and there were no other perturbing agents in the Univ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131216",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Accelerating masses lose energy? If I understand this correctly, accelerating charges lose energy in the form of EM waves because they change the electric and magnetic fields, which "costs" energy. Does that mean that accelerating masses lose energy too, because they change the gravitational field (i.e. curve spacetime... | Yes, it an extremely small effect but it exists in Einsteins general relativity. There is one case of a double star where there rotation around each other seems to lose energy at rate that this phenomena should give according to general relativity
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131293",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Given the spring constant & maximum kinetic energy; length of spring extension? I need to understand the following question before i right my exam tomorrow.
A body attached to a spring with spring constant 100 N/m executes simple harmonic motion. The maximum kinetiv energy of the body is 2J. Calculate the spring exten... | Write down the potential and kinetic energy as a function of position. When the spring is in the middle of the motion, all is kinetic. When it is at the extreme of the range, all is potential. Somewhere between these two extremes, the potential and kinetic energies will be the same; their sum should always be constant ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131527",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What happens when a slow wave reaches lower hybrid resonance? Lower hybrid resonance occurs when $n_{\perp}^2$ goes to infinity, and it occurs only for the slow wave solution, not the fast wave. Since $n_{\perp}$ is proportional to $k_{\perp}$, and $k = \frac{2 \pi}{\lambda}$, it means that the wavelength of the wave ... | Well, I am not sure if your statements are entirely accurate because the fast mode can approach the lower hybrid resonance. In fact, in this regime, it becomes effectively indistinguishable from an electrostatic whistler mode. At low frequency and oblique angles, the fast (or magnetosonic) modes are right-hand polari... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131604",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Amplification of magnetic field can we by any means amplify magnetic signal as we can with electric signal. As both electric and magnetic field can be represented in the form of a wave the analogy seems to be natural.
I want the input and output as magnetic signal.
| Electrical amplification is about using an input signal to modulate a larger amount of power that comes from a separate power supply of some sort.
And yes, there is such a thing as a magnetic amplifier that works on a very similar principle (even though the inputs and outputs are usually electrical). But you can't get ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131661",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 2
} |
Magnetic Force on a Ferromagenetic Material I am currently working on a project involving solenoids, and I needed a force(Newtons, not a measure of magnetic field strength) equation. What I came up with after some digging around on the internet, is the equation:
$$F = (NI)\mu_0\frac{\text{Area}}{2g^2}$$
Where $F$ is f... | The atomic magnetic dipoles in a ferromagnetic material experience a torque that tends to line them up with the crystal axis and another which tends to line them up with any magnetic field. At normal temperatures they can often maintain such an alignment. If the field has a gradient they can also experience a net for... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131885",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 1
} |
How long does it take plasma to radiate its heat? Lets say we have 1 gram of plasma (Argon) at 1 million kelvin confined in a vacuum with electromagnets. If we keep the magnets on but shut down the device that heated the plasma, how long will it take for it to cool off?
| Since your plasma is in a vacuum environment, the only way for it to loose energy is by radiation (conduction transfer through the magnets are neglected). You have thus to consider which bodies are surrounding your plasma and which radiative model is the best ton consider for them. I guess you can consider a black body... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132061",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Electric Potential of Conductors I understand that charges in a conductor reside on the surface of a conductor. So why is it that the neutral inside of a conductor and the charged surface are at the same potential?
| Do you understand that the electric field within a conductor is zero? The charge is mobile, so the internal charge rearranges itself until there is no longer any force to move them: there is no field in the interior. If you understand that, then you will realize that a test particle within the conductor will feel no... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132226",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Photons to Represent a Wave I fear that I have a fundamental misconception about the "wave particle duality" of light, but in a related question, the answerer said, in some sense, that a light wave propagates until it hits something, at which point in time it (can) act(s) like a photon. Which is fine to me, but there a... | The classical electromagnetic field given mathematically by Maxwell's equations can be proven to emerge from a confluence of individual photons, which photons are described by the Quantum Mechanical form of Maxwell's equations. Thus the classical wave is made up by zillions of photons with energy $h\nu$, where $\nu$ is... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132314",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 5,
"answer_id": 1
} |
How can I calculate the force that is applied on a tube by another tube? Let's say there is two tubes(cylinders with no tops or bottoms) with charges $q_1$ and $q_2$, radii $b_1$ and $b_2$, lengths $l_1$ and $l_2$. These tubes are located along the axis of each other's surfaces like in this figure:
If the electric fie... | Without actually providing the mathematical details (which is left for the reader) the basic outline is this:
1.Select a differential segment (a segment of infinitesimal lateral dimensions) on the second cylinder.
2.Write the electric field expression for an infinitesimal charge on the segment.
3.Write the force equati... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132394",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 2
} |
Sound difference between musical instruments I know that the difference between two musical notes is given by the sound frequency, and the difference in volume is given by the amplitude.
What I am wondering is why does the same note sound different on different musical instruments?
What in the wave makes the differenc... | The different tonality of a note in different instruments stems from the different mixes of amplitudes in the harmonic frequencies that the instrument provides.
To be more concrete (and keeping to a slightly simplified view), you play the A note (440 Hz) and then you have the harmonic frequencies 880, 1320, 1760, ... (... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132482",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
Why should any physicist know, to some degree, experimental physics? I've been trying to design a list with reasons why a proper theoretical physicist should understand the methods and the difficulty of doing experimental physics. So far I've only thought of two points:
*
*Know how a theory can or cannot be verified... | Here's a reason that hasn't been touched yet (but is alluded to by your question): to be able to form new theories.
A lot of the most interesting theories in physics comes from someone reading about an experiment and trying to explain the results. We wouldn't have relativity if Einstein didn't read about the Michelson–... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132566",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 6,
"answer_id": 3
} |
Can a third type of electrical charge exist? Upon reading my book on physics, it mentions that there are only two discovered types of electric charges. I wonder if there could be a third type of elusive charge, and what type of effects could it have upon matter or similarly?
| In the Standard Model, electric charge $Q$ is actually part weak hypercharge $Y_W$ and part weak isospin $T_3$
$$Q = T_3 + \frac{Y_W}{2}$$
which can be either positive, zero (electrically neutral), or negative.
In this framework, that's it.
If, in fact, there is another type of electric charge (and its associated anti-... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132654",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "22",
"answer_count": 4,
"answer_id": 0
} |
Poynting vector plane wave I'm calculating the poynting vector for a plain wave and I have some doubt.
$$ \bar S = \frac 1 2 \bar E \times \bar H^* = ... = \frac {| \bar E|^2} {2 \zeta} \hat i_k $$
Now if I consider a cylindrical volume and apply the divergence theorem I get
$$ \int_{s_1} Re \,\, \bar S \,\,\hat i_n ... | Your calculation is right: it is telling you "what goes in, comes out again"!
The plane wave does indeed bear energy. The two nonzero parts of your calculation:
$$ P_1=\int_{s_1} Re \,\, \bar S \,\,\hat i_n dS = - \frac {| \bar E|^2} {2 \zeta} A$$
$$ P_2=\int_{s_2} Re \,\, \bar S \,\,\hat i_n dS = \frac {| \bar E|^2}... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132712",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
How was the formula for kinetic energy found, and who found it? My questions mostly concern the history of physics. Who found the formula for kinetic energy
$E_k =\frac{1}{2}mv^{2}$
and how was this formula actually discovered? I've recently watched Leonard Susskind's lecture where he proves that if you define kinetic... | The author of the law of energy conservation was Hermann von Helmholtz (1821-94). See his classic 1847 paper "Über die Erhaltung der Kraft," translated into English as "On the Conservation of Force." (He called energy force.)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132754",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "31",
"answer_count": 5,
"answer_id": 2
} |
How can metal objects become electrically charged, if current flow means that an equal number of electrons enter/leave the object? I am trying to answer to the question in the title. I am restricting my question to metal objects only.
Here is my logical reasoning:
*
*Current is the flow of charge over time.
*In a ... | If you have a complete circuit, every piece of metal will gain and lose the same number of electrons and will not have a net charge. If you connect two plates, one to each end of a battery, the battery will take charges from the plate connected to the positive terminal and send charges to the plate connected to the ne... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132846",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Direction of current in concentric cylinders Example 7.2 in David Griffiths E & M book (3rd edition) has a side view of 2 concentric cylinders, with smaller radius $a$ and larger radius $b$. The region in between $a$ and $b$ has conductivity $\sigma$. "If they are maintained at a potential difference $\textit{V}$, wh... | The Lorentz force on a charge in an electromagnetic field is
$$F=q(E+v \times B) \ \ .$$
For an electron between the cylinders, $q$ is negative, and $E$ is defined as pointing outward, so the electron will experience a force radially inward. But due to the unfortunate sign convention used for currents, electrons flow... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132917",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Who does work while walking? While walking, the work done by friction is zero.
But who does the work, actually? How someone is getting displaced?
This situation also arises when someone climbs without slipping or is climbing a ladder.
| When you are walking you are doing work against gravity and friction.
Consider this - when you walk on a flat surface, you shift your body weight on to say right leg. Lift the left leg and move it by a step. For the next step, you shift the weight on the left leg, lift the right leg and move forward. Thus you move.
Wha... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/132978",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 10,
"answer_id": 8
} |
Deviation from 2D trajectory I need to find out how far a ball is from the predicted trajectory in 2 dimensional space and I know the start and end position of the ball in both dimensions. Along with that I know the initial velocity and the angle at which the ball was launched.
Every single variable is known in this ... | It is worth noting that the force due to air resistance is usually modeled as
$$\vec{F} \propto -\vec{v} \ \ \ \text{or} \ \ \ \vec{F} \propto -|v|^2 \hat{v} $$
Intuitively this makes sense: the faster you go, the more drag you should experience. The minus sign means that the force acts in opposition to the direction y... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/133150",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Potential difference between point on surface and point on axis of uniformly charged cylinder Question:
Charge is uniformly distributed with charge density $ρ$ inside a very long cylinder of radius $R$.
Find the potential difference between the surface and the axis of the cylinder.
Express your answer in terms of the v... | Like Eternal Code said, using a cylinder inside the original problem cylinder is the right approach. If you use Gauss' Law, you should find that the electric field inside the infinitely long, uniformly charged cylinder is
$$E=\frac{ρr}{(2ε_0)} $$
Now, to calculate the potential difference between the surface and axis ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/133322",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 2
} |
Mass of small fluctuation around vacuum For a potential $V$, how do we define the mass of a small fluctuation around its vacuum? For example I have the potential
$$ V_\mathrm{eff}(\phi) = \frac{1}{2} \left(\frac{\rho}{M^2} - \mu^2\right) \phi^2 + \frac{1}{4} \lambda \phi^4. $$
What is the definition of the mass of smal... | The mass of a small fluctuation is usually defined as $$ \pm m^2= \frac{d^2V}{d\phi^2}\biggr|_\text{VEV}$$
The sign depends on your conventions. This makes sense in analogy with the canonical free field potential $$V_\text{free}=\pm \frac{1}{2}m^2\phi^2$$
for which the above formula is clearly right. More generally, w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/133565",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Question about Metropolis Monte Carlo in the case of equal energies If configuration A is equal to configuration B in a Metropolis Monte Carlo method, do you still do the attempted update?
| Was initially posted as a comment. (Comment removed now)
The micro-states are changing nonetheless, a different point in the phase space of your system, so the system is evolving, even though the two states are part of the same macro-state. Finally remember that the Metropolis probability criterion for accepting moves ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/133647",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Is the Higgs Boson a Force Carrier? I am told there are four fundamental forces, and each of these forces has a boson that acts as its carrier.
Reading this http://www.fnal.gov/pub/science/inquiring/questions/higgs_boson.html I find that the Higgs Field is not a force field
The Higgs field is not considered a force. ... | The difference between the Higgs boson and the bosons of the three/four fundamental (depending whether you include gravity as a quantized theory or not) actions is that the latter are associated with gauge symmetries, while the Higgs plays a role in spontaneous symmetry breaking. Photons, W- and Z-bosons, gluons and g... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/133713",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 1,
"answer_id": 0
} |
Advantages/Disadvantages of "hanging off" a motorcycle when leaning The closest question I could find with regards to this subject was this one:
Countersteering a motorcycle
However, it does not address the specific physics of what I would like to know.
There are 3 ways to lean when turning a motorcycle:
*
*Upper bo... | All the force to accelerate or turn a motorcycle come from the wheel/ground interface. So to turn a bike quickly means a larger force on the wheel (from friction on the ground).
If the bike were upright when a strong sideways force were applied, it would quickly rotate the bike around its center of mass and dump the r... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/133766",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
How can there be a voltage when there is no current? I'm told at school that the Electromotive Force (e.m.f) of a battery equals the potential difference between the terminals of the battery when there is no current.
How is that possible? How can there be a potential difference with no charge flowing?
| If you insert a dielectric in a circuit, you will not see any current but obviously there is a potential difference across the dielectric. To have a potential difference, you just need an electric field inside the material. This electric field might drive a current if the charges are mobile.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/133862",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 2
} |
A Conceptual Problem With the Field Equations of General Relativity I have two questions:
*
*Suppose that we have an amount of energy in the form of a perfect fluid in the right hand side of Einstein field equations (energy momentum tensor), this will lead to a gravitational field, the gravitational field itself has... | This is the famous back-reaction problem in perturbative gravity. To avoid it, we typically only work to a few orders in a perturbative series (though the PPN people will go farther than seems sane when doing numerical work, but you can't blame them considering that radiation only shows up at 2.5 PPN). It is unclear ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/133919",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 1
} |
A light so strong it has a shadow I have recently taken an interest in shadows. I know that in order for a shadow to exist that you must have a solid in the way of the light. My hypothesis is that there can be a light so strong, like a laser beam, that it acts like a solid in the sense where it doesn't let light pass t... | Your hypothesis is basically correct, in theory, even in a vacuum.
Light consists of electromagnetic radiation. According to classical electrodynamics, electromagnetic fields in a vacuum are linear, which means that one light beam will pass right through another, completely unaffected. But according to quantum electr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134000",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 0
} |
If everything is relative to each other in this universe, why do we keep the Sun to be the reference point? and study the solar system and universe relative to it and why not relative to the Earth?
| A reference frame at rest with the Sun is, with a good approximation, an inertial system (much better than one at rest with our planet or other bodies in the Solar system, essentially in view of the hugely larger mass of the Sun). Physics in inertial reference frames has the simplest form. For instance the motion of pl... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134071",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 5,
"answer_id": 1
} |
How to prove the Levi-Civita contraction? I want to prove the following relation
\begin{align}
\epsilon_{ijk}\epsilon^{pqk}
=
\delta_{i}^{p}\delta_{j}^{q}-\delta_{i}^{q}\delta_{j}^{p}
\end{align}
I tried expanding the sum
\begin{align}
\epsilon_{ijk}\epsilon^{pqk}
&=&
\epsilon_{ij1}\epsilon^{pq1}
+
\epsilon_{ij2}\eps... | You need to use the fact that the only isotropic (invariant under rotations) tensors are $\epsilon_{ijk}$ and $\delta_{ij}$. Any other isotropic tensor must be expressible in terms of these two tensors. Clearly, $T_{ijlm}=\epsilon_{kij}\epsilon_{klm}$ is isotropic. It is a tensor that is (i) antisymmetric under the exc... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134156",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 3,
"answer_id": 1
} |
Is the charge of an ion evenly distributed? This question relates to:
Gauss' law and ions?
Is the charge distribution in an ion spherically symmetric due to quantum mechanical effects or do we assume it when using Gauss's law, as in the linked question, to make the calculation easier?
I think it is the former but I a... | It will help you understand the quantum mechanical picture if you read up on atomic orbitals. These are the loci around the nucleus where the electrons have a probability to be found. You will see that the orbitals have a shape, which depends on the angular momentum of the state. The electrons carry the charge and th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134217",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Riemann curvature tensor notation in Wald This question is entirely on tensorial notation in Wald's General Relativity. When specifying the properties of the Riemann tensor on pg39, he states:
$R_{[abc]}^{\quad \ \ \ d} = 0$
and
For the derivative operator $\nabla_a$ naturally associated with the metric, $\nabla_a g_{... | *
*The square brackets mean antisymmetrization. That is:
$$ X_{[a_1a_2\dots a_n]} = \frac{1}{n!}\sum_{P\in S(n)} \text{Sign}(P) X_{a_{P(1)}a_{P(2)}\dots a_{P(n}} $$
where $S(n)$ is the set of permutations of $n$ elements, and $\text{Sign}(P)$ is the sign of the permutation $P$, that is, $\text{Sign}(P)=-1$ if you need... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134363",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Converting two component product to four component notation Consider the product of two left Weyl spinors in the notation commonly found in supersymmetry,
\begin{equation}
\chi ^\alpha\eta_\alpha = \chi ^\alpha \epsilon _{ \alpha \beta } \eta ^\beta
\end{equation}
This is equal to,
\begin{equation}
\left( \begin{... | Following this ref, one sees that, in some basis where the current is diagonal ($3.2.16$), then a term like $\chi \eta$ is just a part of the mass term ($3.2.17$).
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134433",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Difference between heat capacity and entropy? Heat capacity $C$ of an object is the proportionality constant between the heat $Q$ that the object absorbs or loses & the resulting temperature change $\delta T$ of the object. Entropy change is the amount of energy dispersed reversibly at a specific temperature. But they ... | https://en.wikipedia.org/wiki/Standard_molar_entropy?wprov=sfti1
$$dQ = T \ dS \tag1$$
$$dQ = C \ dT \tag2$$
Interesting, right? In $(1)$, the whole $T$ multiplies the infinitesimal $\frac{\text{J}}{\text{K}}$. In $(2)$ it's the opposite: the whole $\frac{\text{J}}{\text{K}}$ multiplies the infinitesimal $T$.
But you h... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134496",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 6,
"answer_id": 3
} |
Is there any evidence that matter and antimatter continuously appear and disappear on the edge of a black hole? I heard Stephen Hawking got a Nobel prize for this, someone said there was no evidence for it which I find quite strange since he got an award for it.
| Direct experimental evidence of Hawking radiation is going to be exceedingly difficult to obtain. The radiation from stellar mass black holes is so small as to be undetectable, and we haven't (yet) worked out how to small black holes in the lab. At the moment there is no direct experimental evidence, and we have to acc... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134655",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 0
} |
Difference between Veneziano amplitude and Virasoro shapiro amplitude I have been study about Veneziano amplitude and Virasoro Shapiro amplitude.
I want to summarize this two amplitude in the following way, please check that i am understand them properly.
Veneziano amplitude : they are open string amplitude for disk. ... | Well, there is the Kawai-Lewellen-Tye (KLT) relations, which says that a closed string amplitude is roughly speaking a product of two open string amplitudes. See e.g. Ref. 1.
References:
*
*Z. Bern, Perturbative Quantum Gravity and its Relation to Gauge Theory, Living Rev. Relativity 5 (2002) 5; Section 3.1.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134706",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
How is force exerted on a wall equal to derivative of hamiltonian with respect to wall position? I'm trying to understand a solution of a problem in Landau, Lifshitz "Quantum mechanis. Non-relativistic theory" in $\S22$ "The potential well":
Determine the pressure exerted on the walls of a rectangular "potential box" ... | I Assume that the Hamiltonian only depends on $a$ through a potential term $V(a)$. This is the case more often than not. Then
$$\frac{\partial H}{\partial a}=\frac{\partial V}{\partial a}$$
Now, invoke Newton's second law:
$$ \vec{F}=-\frac{\partial V}{\partial \vec{x} }$$
and the result follows quite simply.
Alterna... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/134861",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Hydrogen Bomb Mass to Energy? How much mass is converted to energy when a hydrogen bomb explodes? I remember an eighth grade chemistry class where, by going through the nuclear processes, my teacher estimated that roughly 2g of matter was converted in a fission bomb.This is a surprisingly small amount of mass! I have... | Of course all other replies do not consider the fact that in a hydrogen bomb only 30% of yield comes from fusion, ie. hydrogen. The rest comes from fission as in the known sequence:
fission (plutonium) -> fusion (hydrogen) -> fission (depleted uranium in the case)
So we have to correct all of the calculations by multip... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135013",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 3
} |
Work done against gravity The work done against gravity is $mgh$, well at least that's what my textbook says.
I have a question:
I can apply a force say 50N, so total work done = $mgh + mah$. Where $ma$ = Force.
But the truth is irrespective of the force applied, the work done against gravity is always $mgh$. Why?
For ... | You might want to change your question title to "Work done by gravity," because that is what is implied by the variables mgh. Of course, you can add a greater force than that of gravity, which would cause whatever object to which the force is applied to accelerate (since the forces are not in balance). No matter how ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135175",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Is there a classical analog to quantum mechanical tunneling? In comments to a Phys.SE question, it has been written:
'Tunneling' is perfectly real, even in classical physics. [...] For sufficiently large temperatures this can put the system above a hump in its potential energy.
and
the only difference between the ... | Frustrated total internal reflection is an optical phenomenon. It's such a close analogue to quantum tunneling that I sometimes even explain it to people as "quantum tunneling for photons". But you can calculate everything about it using classical Maxwell's equations.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135225",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "17",
"answer_count": 7,
"answer_id": 4
} |
The purpose of auxiliary lens in microscope Can someone explain me the purpose of auxiliary lens in microscope.
The specification says:
Eyepiece: Extra wide field 10x Eyepiece w Spectacle Correction 30mm Ocular
Objective 0.7-45x , Auxiliary 2x
And the manufacturer claims that it is having magnification of 90X. Is it... | Auxiliary lens normally serve to cover different zoom ranges.
for example here: http://www.2spi.com/catalog/ltmic/ZTX-3E-Microscope.shtml they say:
Objective lens:
Zoom type objective lens that has a range of 1x to 4x comes "standard"
with the Series 3 "package". We also have available optional auxiliary
lens t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135305",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Acceleration: A ball sliding down the inside of a semicircular bowl Edit: To clarify, this is a homework question. But I couldn't care less about getting the answer to this specific question as it's not even assigned yet. It looks interesting and I want to understand the concepts behind it. Not understanding these thin... | do it with conservation of energy
first of all find out the potential energyat the top of the semi circular bowl
keep it equal with the kinetic energy at lowest point
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135371",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Reflection - reaction force direction Let's say an object hits a wall. When the object is reflected does the direction of the reaction force caused on the wall look like the red arrow? Does that direction depend on how "strong" object is reflected?
| Since you are saying an object i am considering it to be a rigid particle.Now,since the particle strikes the surface as in your figure.it gives a downward force on the surface and hence the reaction is obviously upward.
As you must be knowing normal reaction is perpendicular to the surface.So break the black arrow in c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135503",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
$\rm Lux$ and $W/m^2$ relationship? I am reading a bit about solar energy, and for my own curiosity, I would really like to know the insolation on my balcony. That could tell me how much a solar panel could produce.
Now, I don't have any equipment, but I do have a smartphone, and an app called Light Meter, which tells... | Lux is a unit of "illuminance" and is based on the eye's response to light and each wavelength is weighted based on the percentage the eye is capable of perceiving. The curve is loosely a bell curve so at deep purple the eye may only "see" 5% of the W/m2 available while in the fat green part of the visible curve it ma... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135618",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 4,
"answer_id": 2
} |
Why does this condition ensure that the residue of the propagator is 1? The corrected propagator is given by $$\Delta'(q)=\frac{1}{q^2+m^2-\Pi^*(q^2)-i\epsilon}$$
($\Pi^*$ is the sum of all irreducible one-particle amplitudes) I get that the residue of the original propagator around the pole $q^2=-m^2$ is $$\frac{1}{2\... | The original propagator has a pole at $q^2=-m^2$, the mass shell. For $m=\sqrt{-q^2}$ be the true mass of the particle, we have $\Pi^*(-m^2)=0$ and require that the residue of the modified propagator is unity around $q^2=-m^2$. Recall that for a meromorphic function $f(z)$ we have
$$\oint f(z)dz=2\pi i\sum_k\operatorna... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135702",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 1,
"answer_id": 0
} |
What technology can result from such expensive experiment as undertaken in CERN? I wonder what technology can be obtained from such very expensive experiments/institutes as e.g. undertaken in CERN?
I understand that e.g. the discovery of the Higgs Boson confirms our understanding matter. However, what can result form t... | In practice very little new technology results from experiments like those at CERN. While they are pushing the envelope on some things like the design of resonators, power klystrons and particle detector technology, the immediate technological return on those things is relatively small, even though one can argue that m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135764",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "49",
"answer_count": 8,
"answer_id": 4
} |
Tensor product notation convention? For two particle state, the Dirac ket is writren as
$$\lvert\textbf{r}_1\rangle \otimes \lvert\textbf{r}_2 \rangle. $$
Then how do we write its bra vector,
$$\langle\textbf{r}_1\rvert \otimes \langle\textbf{r}_2\rvert ~~\text{or}~~\langle\textbf{r}_2\rvert \otimes \langle\textbf{r}_1... | It is a matter of definition of whether you want to revert the order of vector spaces on tensor products or not when going to the complex conjugate vector space, i.e. in physics jargon: from ket-spaces to bra spaces. Different authors use different conventions.
In particular, in the case of super vector spaces with Gr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135914",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 3,
"answer_id": 1
} |
How many wavefunctions are in a minimal basis set for benzene? I am reading Modern Quantum Chemistry by Szabo and Ostlund and on page 62 he says "A minimal basis set for benzene consists of 72 spin orbitals." I tried to understand this number but failed.
Previously he illustrates his concept of a minimal basis set wit... | You have one 1s atomic orbital for each H atom, and one 1s, 2s and three 2p for each C. This makes a total of 36 atomic orbital in the whole molecule, and so you have 72 spin states.
The 2s and 2p's orbitals are going to hibridate giving three 2sp$^2$ orbitals and one 2p orbital, so it has its characteristic $\pi$-delo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135992",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Specular or Diffuse Reflection - Overhead Projection Question:
What type of reflection is exhibited as light waves from an overhead projector reflect off a white projection screen and are scattered throughout a room?
I said specular reflection, but my friend says diffuse because the light rays are expanding outward. ... | Your friend is correct.
It's not specular, because that would mean it's a mirror, and you'd only see the image if you were at the angle of reflection. Further, a mirror does not act as an image plane, so you might have difficulty ( :-( ) perceiving the image. Basically you'd need another lens to re-image the source.... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136035",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Why is the phase space a symplectic manifold rather than a manifold with a metric? Why does phase space require a symplectic geometry rather than a metric? Is there some scenario where a metric is unable to describe the notion of length in phase space, specifically in relation to the uncertainty principle?
| *
*A metric structure $g$ and
*a symplectic structure $\omega$
are two very different structures, although sometimes they can co-exist in a compatible way.
Unlike a symplectic structure, there are no Jacobi-like identity and no Darboux-like theorem for a metric structure.
There exists a unique torsionfree metric c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136182",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 2,
"answer_id": 0
} |
DC Generator with magnet as rotor DC generators convert the AC current in them by split ring commutators right and the graph of the current will be like this
but the question is how would be the graph if the magnet is the rotor and not the armature? Me and a part of my friends are on the opinion that the the graph wil... | When the magnet is rotating and the coils are stationary, you have an AC generator unless you play tricks with brushes. This is because the net flux change in the coil after a complete revolution is necessarily zero - and thus the net voltage must be zero too.
The sinusoidal waveform in your second sketch is therefore... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136256",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Why can I see my breath after I pop my ears? I have this weird but reproducible thing that I can see my breath for less than a second even in hot/humid weather. The key to do that is to pop my ears - I have to do that often, it's like I'm on a flight, not sure why my ear is that way... Anyway even if I just try to forc... | The relative humidity of air is pressure dependent. Your method of popping your ears involves increasing the pressure of the air in your mouth. And if you sufficiently compress a volume of air that has a high relative humidity, you can increase the air's relative humidity beyond it's saturation limit, which causes th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136338",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
Why does Energy-Momentum have a special case? I was reading Energy-momentum, and I came across this simplified equation:
$$E^2 = (mc^2)^2 + (pc)^2$$
where $m$ is the mass and $p$ is momentum of the object. That said, the equation is pretty fundamental and nothing is wrong when looked upon, I similarly also believed thi... | Special equations of the kind you mention are useful as they elucidate limiting behaviors.
Your example is no different from a statement like: the hypotenuse $c$ of a right triangle with legs $a$ and $b$ with $b \ll a$ is given by $c = a + \frac12 b^2/a$. The Pythagorean relation $c^2 = a^2 + b^2$ is valid for any rig... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136407",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 10,
"answer_id": 3
} |
What is zero impedance in AC circuit? If a capacitor is connected with an inductor, then because $$Z=\frac{1}{j\omega C}+j\omega L,$$ the Z may be zero. Does that mean when I apply a voltage, the current will be infinite large?
What's more, in transmission line theory, the characteristic impedance could be $\sqrt{L/C}$... |
Does that mean when I apply a voltage, the current will be infinite
large?
No, not even in the context of ideal circuit theory. It's a bit subtle since we're using phasor voltages and currents and that requires a couple of assumptions to hold in order to be valid.
When those assumptions don't hold, we have to see ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136553",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 0
} |
How do photons "decide"? I was reading that when horizontally polarized light hits a vertical Polaroid all the light is blocked out. But when the Polaroid is off the vertical, some but not all photons "decide" to jump into the new plane of polarization. Could this be a "road less traveled" kind of effect?
If a run of t... | There is no such mechanism. The probability for a photon to pass through a polarizer at an angle $\theta$ is $\cos^2(\theta)$, regardless of what has happened before, and regardless of how many photons "at once" try to pass through it. As Bell's theorem tells us, the quantum world is really random (or non-local).
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136633",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Complex Quantum Wave Can the complex nature of quantum wave arise from the fact that particle is represented as wave packet in spatial frequency and particle's total energy is represented as wave packet in time frequency?
Those wave packets are connected since $E=p^2/2m+V$, but since wave-like particle posses definite ... | No, this is not a valid explanation. Pardon my possible simplifications, but I understand the reasoning in the following way:
*
*$k$ bears independent information
*$\omega$ bears independent information
*We can "store" only one piece of information in the real line, so we need "two folders", which is provided by ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136751",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What allows us to distinguish between two different voices even if they're singing the same note? I love Freddie Mercury. I love Robert plant. I once heard Robert Plant sing a Freddie Mercury song at a tribute concert. Plant sang the song beautifully, but it was obvious that the song was different from the original -- ... | No voice sings in a "pure tone", i.e., while the voice is in tune, the sound signal is composed of various harmonic frequencies. This gives you the "color" of the voice, and that makes the two voices distinct.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136922",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Are there other places to look for the gravitational wave, other than space? The recent BICEP2 announcements about the existence of gravity waves made the news, and made a big pitch for cosmic inflation, in the process they also claimed to detect the existence of gravitational waves. Have the results of the experiment ... | Since the original BICEP2 publication there has been increasing suspicions that what they had seen was actually just a signal from interstellar dust. We've been waiting for data from the Planck experiment on the dust signal, and that data has just been released. Sadly it looks as if BICEP2 did indeed just see dust and ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136988",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Alternative liquid for Galileo thermometer So a friend of mine broke my Galileo thermometer recently. The glass tube and the liquid inside were lost, but the bulbs survived. I've cleaned out an old tall glass candle, and tried filling it with water. Even when the water is steaming hot the bulbs still float, so the liqu... | You would be wise to somehow determine the exact fluid used by the original manufacturer.
Consider that each of the floats has a fixed density, and has a temperature marked on its hanging tag. So you need a liquid which will have the correct, different density at each temperature marked on a tag. In short, the liquid... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/137149",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 0
} |
Will 5 pizzas in the same Hot Bag stay warmer than 5 pizzas in 5 separate Hot Bags? For example, say I am delivering 5 pepperoni pizzas to 5 different addresses. In one scenario, I Keep all 5 in the same insulated Hot Bag, I carry that bag to the door, and I quickly remove one of the pizzas from the bag to give to the ... | I don't think there will be any difference. Provided the bags are ideal insulators and the pizzas are at the same temperature, there will be no exchange of heat. Even with all of them in the same bag, there will be no exchange of heat as no temperature gradient exists!
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/137252",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 4,
"answer_id": 1
} |
What causes the random movement of particles inside a conductor? I'm reading about currents in electricity right now, and it was mentioned that even if there's no electric field inside a conductor, charged particles inside are still undergoing random movement.
I wanted to know what forces cause this random movement to ... | In the quantum mechanical description of a conductor all energy levels of the conductor are filled up to some specific energy level, called the Fermi level. This is because of the Pauli exclusion principle, which says that electrons with the same spin cannot occupy the same energy level and thus causes higher energy le... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/137425",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Why Earth's atmosphere behaves like an ideal gas? In every book I found this sentence like an assumption, without explananions, somebody can help me understand it better?
| Look at the definition of ideal gas .
An ideal gas is a theoretical gas composed of many randomly moving point particles that do not interact except when they collide elastically. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is amenable to analysis under stati... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/137491",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Where does the $\partial \vec{E}/\partial t$ term from Maxwell's equation go in Ampere's Law? One of Maxwell's Equations (ME) is:
$$\nabla\times\vec B = \mu_0\vec J+\epsilon_0\mu_0 \frac{\partial \vec E}{\partial t}.$$
While Ampere's Law (AL) is:
$$\nabla\times\vec B = \mu_0\vec J.$$
Griffiths E&M book derives that for... | I believe that Ampere's Law is wrong in some situations. When Maxwell looked into it, he discovered that Ampere's Law is not always true, so he modified it to get the Ampere-Maxwell Law.
According to my understanding, this can be shown if you have a wire with a current flow that is charging a capacitor. If you calculat... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/137625",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 4,
"answer_id": 2
} |
What is the difference between diffraction and interference of light? I know these two phenomena but I want to know a little deep explanation. What type of fringes are obtained in these phenomena?
| Diffraction occurs when a wave encounters an obstacle or a slit these characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength, whereas Interference is the phenomenon where waves meet each other and combine additively or substractively to form comp... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/137860",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "34",
"answer_count": 7,
"answer_id": 2
} |
Classical toy models of particles with intrinsic spin Related to my question here (spacetime torsion, the spin tensor, and intrinsic spin in einstein cartan theory), I'd like to be able to put test particles on a manifold with non-zero torsion and see how this affects the motion.
The action for a free particle is usual... | Note that in classical systems, spin is not quantized but just a parameter, so the question of ''higher spin'' is not really meaningful. There remains only the question of statistics.
Lagrangian principles for classical Fermions were first discussed in:
J.L. Martin,
Generalized classical dynamics, and the ‘classical... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138014",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
$ε_0$ affects electric field intensity, but $μ_0$ doesn't affect magnetic field intensity? I'll be honest: this question is actually a homework problem. I've spent the past hour going through Google and several textbooks trying to answer the question "Why does $ϵ_0$ affect electric field intensity but $μ_0$ does not af... | The only thing I can see them going for is the fact that only two of $\epsilon_0$, $\mu_0$ and $c$ are independent, and typically, a modern view will fold $\epsilon_{0}$ into the definition of charge, and declare $c$ to be the fundamental constant used to transform space into time in special relativity, making $\mu_{0}... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138077",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
What would happen if an accelerated particle collided with a person? What would happen if an accelerated particle (like they create in the LHC) hit a person standing in its path?
Would the person die? Would the particle rip a hole? Would the particle leave such a tiny wound that it would heal right away? Something else... | While a single LHC particle wouldn't be doing much harm, being hit by the LHC beam would be certainly deadly and it would damage the machine badly. Any dense matter that comes into the LHC beam will instantly act as a beam dump. We have a very good idea about what happens in the LHC beam dump, see e.g. http://iopscienc... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138151",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "36",
"answer_count": 4,
"answer_id": 0
} |
Why does electron move in an elliptical path? According to Sommerfeld's atomic model, an electron moving around a central positively charged nucleus is influenced by the nuclear charge. As a result of which, the electron moves in an elliptical path with the nucleus situated at one of the foci. But how the path of elect... | Considering the way matter waves are associated with all moving particles, it seems inconceivable to me that electrons cannot move in other than elliptical orbits.
Close examination of the harmonics & resonance effects of phase waves & matter waves, it becomes apparent that they need to move in elliptical orbits in ord... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138311",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 3,
"answer_id": 2
} |
Simple estimation of the critical temperature of water I'm trying to develop fermi estimation skills and I came up with a question for which I don't even know where to start from. Here goes:
Is it possible to estimate the critical temperature (say in Kelvin degrees) of water in a simple way using fermi estimation?
By c... | I guess you can use the Van der Waals equation and some estimates of the molecule volume and intermolecular attractive forces. The parameters of the critical point depend on these characteristics, so you need to assess them.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138403",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 3,
"answer_id": 2
} |
Why rubber is incompressible material? Why rubber is incompressible material? I know its Poisson's ratio is nearing to 0.5. So I don't understand physically, what it means by 0.5 Poisson's ratio and incompressibility. When I tried searching it, I found that rubber (or similar polymers) conserve volume after deformation... | Assuming a cube of side l with normal stress in one direction, its final volume after deformation would be:
$$V=(l+d1)*(l-d2)^2=l(1+d1/l)*l^2(1-d2/l)^2$$
$$ d1/l=\frac {\sigma}{E}=\epsilon $$
$$ d2/l=\frac {\nu \sigma}{E} $$
$$\nu =0.5$$ $$d2/l=\frac { \sigma}{2E}$$
$$V=l^3(1+\epsilon)(1-\epsilon /2)^2$$
$$V=... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138562",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 6,
"answer_id": 4
} |
Complex theory in physics I'm a physics graduate and usually encounter with complex numbers in physics.
For example, in electrical engineering,
Why do we express capacitive reactance as an imaginary number..?
Can't it be expressed as real, as we generally do?
Forgive me, if my question is logically incorrect, But I do ... | For your engineering problems, the complex numbers are usually introduced to simplify the problem solving. Just take the complex reactance as an example. If you trace back to the place where you introduce the complex numbers, you will find that it is when you solve the "forced oscillation" equations$$\frac{d^{2}q}{dt^2... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138643",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Origin of quark masses Does all the mass of the quarks in the standard model come from the Higgs sector or is there also a contribution to quark masses due to QCD chiral symmetry breaking?
| 99.9% of the mass of a hadron or a meson comes from confinement in QCD. Confinement is a special feature of QCD due to its non abelian symmetry which leads to a negative beta function. It is confinement that also leads to a breaking of the chiral symmetry at about 200 MeV or the radius of a hadron (about 1 femto meter)... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138704",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Why is the Moon not redder at moonrise/moonset? Okay we all know about Raleigh Scattering, which makes the sky blue. And by the same token, sunsets appear red because sunlight traveling through more atmosphere will "lose more blueness" as it's scattered away.
But what about the Moon? The Moon is just reflected sunlight... | This is just an opinion, but the moon on the horizon is simply less visible than the sun is. I suspect that color changes it makes are more subtle and less easily noticed. However full moons are often noticeably orange. Here is a page with a wonderful time lapse view.
http://www.pikespeakphoto.com/moon-rising.html
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138795",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Gibbs free energy and maximum work I'm getting confused between two important results.
The Gibbs free energy is $G = H-TS$
where $H$ is the enthalpy and $S$ is the entropy.
When the temperature and pressure are constant the change in the Gibbs energy represents maximum net work available from the given change in system... | More of a layman's answer because @higgss and @By_Symmetry answered with very good support.
I think a lot of the times that the 'work definition' of Gibbs Free Energy comes up during the discussion of hypothetical thermodynamic processes, "what if the volume decreased?", or "what if this chemical decomposed into these ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138955",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 2
} |
If light is electromagnetic then can light produce electricity or attract metals? what I mean to say is that if light is electromagnetic in nature then shouldn't it show electric or magnetic properties on matters? Like if light falls on a metal it should produce current due to its electric nature but it doesn't. Seco... | An example of a charged, magnetic particle that can be manipulated by light is the electron. With light it can be excited into a higher atomic or molecular orbital. When it falls back it emits light. Light or better the closely related microwaves can flip its magnetic moment in EPR.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/139034",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 5,
"answer_id": 4
} |
What are the "other" Hadamard matrices? The Pauli matrices
$$ X = \begin{pmatrix}0&1\\1&0\end{pmatrix}, Y=\begin{pmatrix}0&-i\\i &0\end{pmatrix},\,\text{and}\, Z=\begin{pmatrix}1&0\\0&-1\end{pmatrix} $$
can be used to construct the Hadamard gate
$$ H=\frac{1}{\sqrt{2}}(X+Z)=\frac{1}{\sqrt{2}}\begin{pmatrix}1&1\\1&-1\en... | Realize that an arbitrary rotation around the axis $\mathbf{n}$ is given by $R_\mathbf{n}=\cos(\alpha/2)I-i\sin(\alpha/2)\hat{\mathbf{n}}\cdot\mathbf{\sigma}$ and an arbitrary unitary operator can be written as $U=\exp{(i\gamma)}R_\mathbf{n}$ with $\gamma$ some phase factor. Thus, in general, any operations on the qubi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/139114",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How detectors in particle colliders can differentiate neutrons from antineutrons? Their mass is the same. None of them interacts with EM fields. And their decay (around 1000s) is far too slow to see their decay products yet in the detector.
How is it then possible to differentiate them?
| The experiment which has measured the most stringent limit on neutron to anti-neutron oscillations (i.e. produce neutrons, let them fly for some time and then look if you find anti-neutrons) has used a 130 micrometer thick and 110 cm diameter carbon foil. This target had a probability greater than 99% for anti-neutrons... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/139540",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 0
} |
Moment of inertia of rods Ok so I'm extremely comfortable with calculating moment of inertia of continuous bodies but how do we do it for a system not continuous.
For example if 3 rods of mass $m$ and length $l$ are joined together to form an equilateral triangle what will be the moment of inertia about an axis passing... | The moment of inertia for a system of $n$ point masses, $m_i$, at distances $r_i$ from the pivot is simply:
$$ I = \sum m_i r_i^2 \tag{1} $$
We normally calculate $I$ by integration, i.e. we take each point mass to be an infinitesimal element of our continuous object and integrate to add up the moments of inertia of al... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/139625",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 0
} |
Are critical exponents below and above the critical point always same? The scaling relations don't distinguish the the critical exponents below and above the critical value. In the mean field level, I understand these critical exponents are same whatever one approaches the phase transition from the order phase or disor... | For what it's worth, it is claimed that the critical exponents differ above and below the critical point for some exactly solvable 2-dimensional model: http://www.ujp.bitp.kiev.ua/files/journals/49/11/491114p.pdf (Ukr. J. Phys., v.49, #11, p.1122 (2004)).
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/139690",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 1
} |
Is not force required to start each and every motion,uniform or non-uniform however they might be?
A body which is at rest will remain at rest; and a body which is in motion will continue motion in a straight line as long as no unbalanced force acts on it.
This is Newton's 1st law of motion,everyone knows. But what N... | I think that a force will be definitely required to accelerate a body from rest to a velocity, however, once it has achieved that velocity you should be able to cease applying the force and it will continue that at that velocity in a frictionless ideal world. After you cease applying the force it should be undergoing u... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/139778",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Do transmitters create magnetic fields and radiation? In the company I work for (I am a software engineer) we develop a system which uses a transmitter (antenna) that creates a magnetic field. I also know that radio station transmitters create radio waves, so I am somewhat confused.
Do the coils built into the transmi... | Every change in a magnetic field automatically creates an electric field and vice versa. For technical purposes, however, magnetic antennas create a stronger magnetic near field (i.e the field that can be measured less than a wavelength away from the antenna), while "electrical" antennas create a stronger electric near... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/139826",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Is there a difference between the speed of light and that of a photon? As in the title I am curious whether there is a difference between the speed of photon and the speed of light, and if there is what is the cause of such a difference?
| Comment to the question (v4):
It seems relevant to mention that there in principle could be a difference between the universal speed limit constant $c$ (which is usually casually referred to as the speed of light in vacuum), and the actual speed of light in vacuum if the photon has a rest mass, at least from an experim... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/140923",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 0
} |
How the polarization of electromagnetic wave is determined? What help us determine the polarization of electromagnetic wave . Does perpendicular electric and magnetic field determine it or does the direction of propagation ?
| You can uniquely define the polarisation of a plane wave from any of the following:
*
*The electric field vector as a function of time $\vec{E}(t)$ and the magnetic field (or induction) $\vec{H}(t)$ (or $\vec{B}(t)$;
*The wavevector $\vec{k}$ and two scalar functions of time, the latter being the transverse compone... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141050",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Derivation of temperature-volume relationship for a reversible adiabatic expansion of an ideal gas We start with $\delta q = 0$ and $dU = C_{V}(T)dT = \delta w$. Why can we take the heat capacity at constant volume, when this process is an expansion so volume increases?
| This is valid for ideal gas whose molar number is constant $n$. Why?
When a fluid changes volume, the equation
$$
dU =dQ - pd V
$$
is obeyed. Formally dividing by $dT$ we obtain
$$
\frac{dU}{d T} = \frac{d Q}{dT} - p\frac{d V}{d T}.
$$
If we now consider only processes where $V$ remains constant, the relation
$$
\frac{... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141130",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Eigenvalues being physical observables I think I'm comfortable with the PDE solutions to the Schrodinger equation. But as soon as we start putting these values in a matrix (in dirac notation), I lose my understanding and everything becomes plug-and-chug math magic.
I'm wondering if anyone has an understanding as to w... | A measurement is not a primitive in physics. Rather, a measurement is a physical process that takes place according to the same laws of physics as any other physical process. Those same laws apply to the measurement apparatus, to the person doing the measurement and to the records he makes of the measurement. What dist... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141227",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
How to make a rotating linearly polarized (not circular polarization) beam from a single beam? One way to make a linearly polarized beam rotating at frequency $\Delta f\approx10\mbox{MHz}$ is by combining two circularly polarized beams, one left-handed and one right-handed, and where one beam is at a frequency $f$ and ... | Faraday Effect. The plane of polarization in a medium is rotated when exposed to a magnetic field. Solenoid, glass rod.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141322",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Normalization of a wavefunction that's superposition of two unknown energy eigenfunctions Question:$$\psi(x)=A(3u_1(x)+4u_2(x))$$where $u_1(x)$ and $u_2(x)$ are energy eigenfunctions.
How to normalize function $\psi(x)$?
My intuitive solution:
I got $$\int^{\infty}_{-\infty}|\psi(x)|^2dx=A^2\int^{\infty}_{-\infty}(9u_... | We are told that $\left|1\right>$ and $\left|2\right>$ are energy eigenstates, meaning they are eigenstates of the Hamiltonian $\hat{H}$, a Hermitian operator: ${\hat H}^\dagger = H$. Eigenstates of a Hermitian operator with different eigenvalues are orthogonal (see this): $\left<m|n\right> = \delta_{mn}$ (using Kronec... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141413",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What happens to waves when they hit smaller apertures than their wavelenghts? I was wondering this for quite a long time now. Let's say you have a water wave (like ripples, not the ones you see during tsunamis) with wavelength 10 m. Imagine you put a boundary with an opening of 1 m. Will diffraction be observed? Accord... | http://www.nature.com/nature/journal/v445/n7123/box/nature05350_BX1.html
You can find a complete answer in above link.The theory which is introduced can be valid for mechanical waves such acoustic and water waves.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141556",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 6,
"answer_id": 5
} |
Baryon in terms of quark fields – spinor index structure What is the most general way to write down a current describing a baryon made from quarks $\psi_i^\alpha$? Let's say we suppress flavour indices but want to write colour $(i,j)$ as well as spinor $(\alpha,\beta)$ indices. Then I suppose the colour structure would... | I found an explicit form for a baryon current in a paper by B.L. Ioffe. I still have problems understanding a certain aspect, which I thought warranted a new question here: Charge conjugation matrix in baryon current
Anyways, to be complete, a current describing the isobar $\Delta^{++}$ can be found in equation (13) th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141647",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 1,
"answer_id": 0
} |
Power loss due to dipole antenna position mismatch If we have two dipole antennas, it is well known that in order to transfer maximal power, two dipole antennas should be parallel and on the same height, which means that line that connects their middle points is perpendicular to both antennas. I wonder now, if two dipo... | You'd need to consider the angle between the two antennas and the distance between them. Dipole antennas do not radiate uniformly into $4\pi$. If you're looking up or down at the poles of the antenna, you will see no radiation (ideally). Looking at a direction transverse to this, the radiation is at a maximum. The ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141805",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Kinetic energy vs. momentum? As simple as this question might seem, I failed to intuitively answer it.
Let's assume there is a $10,000$ $kg$ truck moving at $1$ $m/s$, so its momentum and KE are: $p=10,000$ $kg.m/s$ and $KE=5,000$ $J$.
Now if we want to stop this truck, we can simply send another truck with the same ma... | Regarding scenario 1:
we can simply send another truck with the same mass and velocity in
the opposite direction to collide with the first one, and both of them
will stop because of the conservation of momentum.
If one assumes a totally inelastic collision then, by conservation of momentum, it is true that both t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141891",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 5,
"answer_id": 1
} |
Why the EMF of a battery doesn't depend on distance between the two electrodes? I have read that resistance of a conductor is directly proportional to its length. So, the EMF of a conductor (electrolyte of battery) should increase with increase in length/distance between the electrodes..is it not so?
| If you are talking about the open circuit voltage of the battery, then internal resistance and resistance of any conductors connected to the battery are irrelevant. The voltage across a resistor is proportional to the current thru it, see Ohm's law. When there is no current, there is no voltage drop across a resistor... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/141925",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.