Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Memory fading “striyers” in viscoelastics I was talking to a guy who does polymer moulding, and we were discussing a few industrial issues in melting, mixing and forming shapes, and with a few of my suggestions he rebutted with, "no that would create striyers". So for example, one mixing technique I suggested had the f... | What he is getting at is that, after the grate, the polymer molecules that passed on either side of the grate take time to "knit back together" again (i.e., form entanglements). So the structure of the polymer is temporarily weakened.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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What new features does the Heisenberg Model have compared to the Ising Model? Both the Ising and the Heisenberg Models describe spin lattices with interaction on first neighbors. The Hamiltonian in each case is quite similar, despite the fact of treating de spins as Ising variables (1 or -1) or as quantum operators. In... | One of the main differences is that the Ising model lies on a discrete symmetry (the $Z_2$ symmetry) while the Heisenberg model lies on a continuous one (rotational symmetry). It will affect the phase transitions that these models undergo.
In particular, because of the Mermin-Wagner theorem, there can be no finite-temp... | {
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Is there a way to estimate fan capacity at altitude? Provided you know the capacity of a fan (flow rate) at constant speed and at sea level, is there an analytical way to predict what the flow rate would be at altitude? Or is this specific to the fan's design?
| For a fixed speed, a fan, blower or any turbo-machine in general will deliver the same volumetric flow regardless of the ambient pressure since the machine essentially scoops out a volume of air as each blade of the machine passes the machine's inlet.
$$Q_{SL}=Q_{alt}$$
where $SL$ designates 'Sea Level' as reference a... | {
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Are electric fields produced by static electric charges different from those produced by time-varying magnetic fields? I came across an interesting yet perplexing statement in my physics textbook:
However, we hasten to add that electric fields produced by static electric charges have properties different from those pr... | The only difference is in the structure of the fields:
*
*The field contribution from static charges are non-rotational (curl-free) but divergent.
*The field contributions from time-varying magnetic fields are divergence-less but have non-zero curl.
You can read this right out of the Gauss and Faraday's Laws in the... | {
"language": "en",
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Lagrangian and finding equations of motion I am given the following lagrangian:
$L=-\frac{1}{2}\phi\Box\phi\color{red}{ +} \frac{1}{2}m^2\phi^2-\frac{\lambda}{4!}\phi^4$
and the questions asks:
*
*How many constants c can you find for which $\phi(x)=c$ is a solution to the equations of motion? Which solution has the... | Looks good so far. To find the Hamiltonian you just use that if $L = T - U$ then $H = T + U$ (technically there are some extra assumptions there, but if your case it works out fine). Since $T = 0$ if $\phi$ is constant, you just need to find out which of those values $c$ minimize(s) the potential energy $-1/2 m^2 \ph... | {
"language": "en",
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Neutral points in a system of charges on the vertices of a square There are four positive charges of equal magnitude placed at the four vertices of a square. Is there any point where the electric field vanishes (neutral point) within the square and in its plane, other than its center?
| I believe there is such a point.
Let us assume that unit charges are located in points $[\pm 1,\pm 1]$ of a plane with coordinates $x,y$. Let us consider the field on the abscissa ($y=0$). The $y$-component of the field is zero on the abscissa due to a symmetry. It is obvious that the $x$-component of the field at the... | {
"language": "en",
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Space bends relative to what? We all are aware of gravitational waves, as it bends space and time, black hole squeeze space, but the squeezing, bending, expanding happens reference to what? Since the observable universe is the universe existing within itself, so it bends in reference to whose perspective?
| The easiest way to understand this is to understand the notion of extrinsic versus intrinsic curvature.
Extrinsically curved things follow non-straight lines relative to the space they are contained in.
Intrinsically curved things, however, obey non-Euclidean laws on their own surfaces.
Now, if you fit an intrins... | {
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How to find pseudo-force on a body in a non-inertial reference frame with respect to another non-inertial reference frame? If there is a body in a non-inertial reference frame (say accelerating with acceleration $A_1$) and an observer in another non-inertial reference frame (say accelerating with acceleration $A_2$) ... | The pseudo force depends only on the acceleration of the observer frame (which is why if the acceleration of the observer is zero (in the inertial reference frame case) there is no pseudo force.)
Therefore, given that the acceleration of the observer is $A_2$ the pseudo force acting on any body (which has mass $m$) obs... | {
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Force between proton in a conducting shell and electron outside of shell? There's a proton inside a conducting shell and an electron outside of it. Inside the shell, there is no field due to the electron, but the electron feels the field due to the proton. Therefore the electron should move towards the immobile proton,... | The error that you have made is to apply Newton's third law incorrectly.
On the conducting sphere there are two sets of induced charge.
One set of positive and negative induced charges is due the field of the electron having to be negated so that there is no electric field inside the conducting sphere.
These positive... | {
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Do we need a bounded domain for the Laplace equation to have a non-zero solution $u$? If we have the Laplace equation for electrostatics in free space, that is
$$\Delta u(x) = 0 \quad \quad x \in \mathbb{R}^3,$$
is the only solution $u = 0$? And also, we only get non-zero solutions for $u$ if we instead consider the La... | I'm not sure that I understand what you mean by "bounded domain". This is an answer based on what I understand from the question.
To solve Laplace's equation uniquely, you have to specify either the Dirichlet or Neumann type boundary condition.
EDIT: The boundary of the region of interest can be at finite distances as... | {
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Time dilation for a clock in orbit Suppose that we want to compute the total time dilation for a clock located in an orbiting satellite relative to the clock in our cell phone on the ground.
Consider two different approaches below.
*
*Use special relativity and compute time contraction due to the relative velocity. ... | They are different effects. Special relativity (SR) will do the first part, just calculate from Newton the velocity and you get a slowdown at the clocks in the satellite. With GR you get a faster clock because of the gravitational time dilation. Both apply, subtract one from the other
In GPS orbits the GR effect is abo... | {
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Special Relativity - Non-conservation of Newtonian momentum I'm studying special relativity and I don't quite understand why Newtonian momentum isn't conserved, because everytime I think of the math involved or imagine myself observing some system from various reference frames, it never makes sense to me...
I'll be ver... | Newtonian linear momentum is not conserved because your idea of how to calculate how different reference frames see the same thing is flawed. Usually when people get stuck on this point it's because they have trouble grasping the concept of how the passage of time is reference frame dependent in special relativity (SR)... | {
"language": "en",
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Is measurement of coordinates possible near strong gravity? We know that Schwarzchild metric describes an asymptotically flat spacetime. This means that far away from the event horizon we can safely interpret the $r$ coordinate as distance from the center.
But when close enough to the event horizon the curvature becom... | You seem to have some serious conceptual misunderstandings about the subject. Co-ordinates are not "measured" they are defined on a space time manifold. Proper time on the other hand can be measured. So please think about the concepts correctly. I will recommend reading Einsteins works on the subject.
| {
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Equations of motion for a free particle on a sphere I derived the equations of motion for a particle constrained on the surface of a sphere Parametrizing the trajectory as a function of time through the usual $\theta$ and $\phi$ angles, these equations read:
$$ \ddot{\theta} = \dot{\phi}^2 \sin \theta \cos \theta $$
$$... | Note that you can rewrite your second equation as $$ \frac{\ddot{\phi}}{\dot{\phi}} = -2\cot{(\theta)}\dot{\theta} $$ Each side is an exact differential in one variable, so we can integrate, and Wolfram|Alpha gives $$ \ln{(\dot{\phi})}=-2\ln{(\sin{(\theta)})}+C $$ for some integration constant $C$. We can exponentia... | {
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Concentric Circular Loops We have two concentric circular wire loops. The inner loop has a stable, clockwise current. The outer loop does not have current. If the current of the inner loop increases, what is the direction of the current of the outer loop?
My first thought was "counter-clockwise" using the RHR, increasi... | According to Lenz's Law, the current induced in the outer loop by an increase of the clockwise current in the inner loop has to flow counterclockwise. https://en.wikipedia.org/wiki/Lenz%27s_law.
| {
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Existence of a Hamiltonian system It is well known that a two dimensional system to first order is locally Hamiltonian from Darboux' theorem. For example,
\begin{equation}
\dot x = f(x,y), \qquad \dot y = g (x,y)
\end{equation}
Admits the following Poisson structure,
\begin{equation}
\{x,x\}=\{y,y\} =0 , \qquad \{x,... | There is the well-known condition for a system
$$\dot{x}_i = f_i(x_1,\ldots, x_{2n})\:,\quad i =1,\ldots, 2n \tag{1}$$
to admit a local Hamiltonian re-formulation:
$$\dot{x}_i = \frac{\partial H}{\partial x_i}(x_1,\ldots, x_{2n})\:,\quad \dot{x}_k = -\frac{\partial H}{\partial x_k}(x_1,\ldots, x_{2n}) \quad i =1,\ldot... | {
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Possible Error in Assumption - Griffiths Quantum Mechanics In "Introduction to Quantum Mechanics" by Griffiths, right at the beginning of section 9.1.1 (Time-Dependent Perturbation Theory, The Perturbed System), Griffiths states:
Now suppose we turn on a time-dependent perturbation, $H'(t)$. Since $\psi_a$ and $\psi_b... | A basis is a set of wave functions such that a any wave function can be formed as a linear combination of basis wave functions. Often you choose them to be eigenfunctions of the Hamiltonian. But you don't have to.
If you change the Hamiltonian, you change the egienfunctions, so you change the most common choice for a ... | {
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Dimension of representations of Lorentz Group In my Quantum Field Theory class we were discussing the finite dimensional representations of the Lorentz group. We discussed the vector representation which acts on 4 vectors, and then also the spinor representation that acts on spinors. My issue is that both of these repr... | No, the loss of uniqueness does not have to do with the non-compactness of the Lorentz group. The fact that there is only one irreducible representation of any given dimension is special to the group $SU(2)$ (and by extension, to $SO(3)$). It is not true even for other compact semisimple Lie groups. For example, $SU(3)... | {
"language": "en",
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Why is it that the change in internal energy always uses the formula with Cv in regards to pressure/volume/temperature changes on a gas? Normally I would associate the use of $C_v$ with finding the energy taken into or leaving a system when the volume is kept constant. However, the formula to find $\triangle E_i$ (chan... | If you keep the volume constant then the gas can do no work as $\delta W = P \Delta V = 0$ and so from the first law of thermodynamics the change in internal energy
$\Delta U = \delta Q - \delta W \Rightarrow \Delta U = \delta Q = n c_v \Delta T$
| {
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One straw drinking from many containers of liquid One of my friends brought up a photo:
Which sparked a debate about whether the containers closest to the end of the straw would empty first. I was just wondering if someone could explain if the closest two containers would be empty before the furthest.
| Assuming the straw(s) are full of liquid and you are sucking very slowly (so we are just considering pressure, not fluid dynamics) the fluid levels in all the containers must be equal, otherwise the liquid would flow to equalise them.
| {
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Consequence of approaching infinite mass for near-light speed particles Considering that the inertial mass of an object approaches infinity as the speed of the object approaches $c$, and that inertial mass equals gravitational mass, does this not imply that particles nearing $c$ would have gravitational mass approachin... | Some time before the mass of the particle approaches infinity, surely its gravitational field must make it collapse into a micro-black hole. Or not, the whole thing sounds basically flawed the more you extrapolate its implications.
| {
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Is Dirac's notation really necessary? One thing that's always bothered me in Dirac's notation is that it assumes that the Hilbert space contains a "continuum basis" of vectors $|x\rangle$, which happen to be eigenvectors of an operator $X$ (which has no eigenvalues, only a continuous spectrum that spans the whole space... | The problem, in my view does not lie with the notation as such, but rather with the physical scenario that is being studied. Space-time is continuous infinite. Therefore, one requires a basis that is also continuous and infinite. So, regardless of how one would represent such a basis in terms of a notation, its orthogo... | {
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Intensity of unpolarized light acted upon by a quarter wave plate Why does the intensity of unpolarized light remain unchanged when it pass through a quarter wave plate? A quarter wave plate produces a phase difference between e- ray and o-ray. But the intensity is changed for plane polarized light.
| An ideal quarter wave plate does not absorb any light.
The speed of light through the quarter wave depends on the orientation of the electric field vector relative to the plate. So one orientation is slowed down more than the other and the thickness of the plate is designed so that there is a change of phase of $90^\c... | {
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Why wave theory cannot explain photoelectric effect and provides evidence for particle nature of light? I am able to understand how light can be modeled to have wave characteristics from Young's double slit experiment.
But I am unable to comprehend how we can understand light to have particle characteristics from the p... | Why don't you look a this video an A levels tutorial for the photoelectric effect.
Summary of the observations of the puzzling photoelectric effect:
*
*The electrons were emitted immediately - no time lag!
*Increasing the intensity of the light increased the number of photoelectrons, but not their maximum kinetic ... | {
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Theoretically, could there be different types of protons and electrons? Me and my friend were arguing. I think there could theoretically be different types of protons, but he says not. He says that if you have a different type of proton, it isn't a proton, it's something else. That doesn't make sense to me! There are d... | you are merely arguing semantics. your friend is right because there is no way to distinguish one proton from another. you are right because we might some day find a way to distinguish one proton from another. you two seem to be arguing about what words we would use for the two different types of proton we might some... | {
"language": "en",
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Working principle of inverter I got project on the working of inverter from school. I know this that DC inverter has an alternator switch which constantly changes its direction so that magnetic field is produced in primary coil due to which current is induced in secondary coil and we get output AC. So according to all ... | Low-cost inverters like you see in a car work in this fashion. Ones used in power production, and higher-quality ones used in cars, improve the output by not switching 120 times a second (or 100 if you're in Europe), but many thousands of times. The output is sent into circuits that boost the voltage, sometimes without... | {
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Motion of one body with reference to another I studied that Galileo was punished by the church for teaching that the sun is stationary and the earth moves around it. His opponents held the view that earth is stationary and sun moves around it.
The question i want to ask is if the absolute motion has no meaning,are the ... | Because the sun has more mass and a higher inertia, so the gravitational attraction between the two will impact the Earth's momentum more than the Sun's.
If we look at the two bodies in isolation and have no prior knowledge of the system the only thing that breaks the symmetry is mass. If gravity exerts an equal force ... | {
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Measuring very small temperature differences Can one use a thermometer with $\pm$5 mK accuracy to measure a temperature difference of 2 mK (the measurement is near 100 mK temperature on a sample on an ADR)? Using the same thermometer, I am thinking to measure temperature of the sample, heat the sample slightly, measure... | Yes, of course you can do it. The 'accuracy' is from a calibration, after all, and your temperature-difference determination establishes a short-term new calibration. If the apparatus were to have hysteresis (the meter pointer is sticky), or if there were interfering signals (the power supply ripple dominating
an el... | {
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Why is velocity inversely related to pressure in a flow? I've seen the equations that give this relationship, and I understand the math and have seen it worked out in problems. But I don't have a qualitative, conceptual grasp on the relationship.
Is the pressure that which is exerted by a small element of the flow, or ... | The "pressure is inversely related to velocity" formula is applied when fluid is in dynamic motion and $P =f/a$ as area is inversely to velocity. Instead, in $P = fv$ pressure is directly proportional to velocity and it is applied only on static fluid
$P〆v$ in static fluid
$P〆1/v$ in dynamic fluid
| {
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Why is direct current needed to produce an electromagnetic field using a solenoid? I am performing an experiment for school investigating the magnetic force of a solenoid. While doing this experiment I realized that I needed to connect the solenoid to the DC output of the power supply instead of the AC.
I am perplexed ... | To be fair, you don't actually need continuous DC current, but you do need a waveform that is always in the positive (or always in the negative) voltage range. An AC source with a simple half-wave or full-wave rectifier (4 diode) will work fine without the need for a transformer or DC converter. It is still DC, it's ju... | {
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Models of gravitational waves: Do we need to modify our understanding? So, now we got the first observation of gravitational waves, and that's indeed an amazing revolution in the field of astronomy and cosmology. So one can start discussing whether the previous model(s) were completely to the point or not, isn't it ? I... | I think the answer is clearly no, we don't. The detections have agreed extremely well with numerical models based on GR so far. So this is another test that GR has passed with flying colours.
It may well be that we will learn new things -- and with luck surprising things -- about the statistics of objects and this mi... | {
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Moment of Inertia of Annular Quadrant I am measuring the moments of inertia for various numbers of annular quadrants placed on a torsional oscillator. I know $\displaystyle{I=\frac{1}{2}M(R^2+r^2)}$ for a whole annulus. If I want the moment of inertia of only an annular quadrant, would I divide the formula above by fou... | It sounds like you are doing the following TeachSpin experiment.
If I understand correctly, brass quadrants are placed symmetrically on a rotation table, at the same distance from the axis as for the annulus.
When the axis is the same, the distribution of mass about the axis is the same for the quadrant as for the whol... | {
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What is the prevailing opinion in scientific community about Hans C. Ohanian's description of spin? In the paper What is spin?, Am. J. Phys. 54 (1986) 500, by Hans C. Ohanian, spin is described as a circulating flow of energy in the wave-field of a particle. Is this the generally agreed upon explanation of intrinsic a... | I read the abstract.
The basic reason it is not referred to or used is that main stream physics has elementary particles as point particles in the standard model and any wave nature attributed to the particle is on the probability distribution of its location in space and time.
it can be shown that the spin may be re... | {
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Why aren't there simulateous spin z and x eigenstates? Consider the spin $1/2$ case and suppose I have an eigenstate of $S_z$. This has $\sigma_z=0$. The uncertainty principle states that $\sigma_z \sigma_x \geq \frac{\hbar}{2} |\langle S_y\rangle|$. In this case $|\langle S_y\rangle|=0$, so the uncertainty principle i... | The simple answer why $\S_x$ and $\S_z$ can not be simultaneously specified is of course since they do not commute, i.e. $[S_x,S_z]=S_y$. This leads to your uncertainly relation $$\sigma_x\sigma_y\geq\frac\hbar2|\left<S_y\right>|$$
Since in your special case $\left<S_y\right>=0$ you can choose $\sigma_z=0$ without viol... | {
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What actually oscillates in quantum harmonic oscillator with given frequency? Quantum harmonic oscillator is said to be describing motion of microscopic stuff (like atoms in molecules). But unless one keeps on measuring the position on the atom, it doesn't exist at all. It's in superposition of several possible positio... | Hamiltonian eigenstates viz. $\hat{H}\left|n\right\rangle =E_n\left|n\right\rangle$ span the Hilbert space. At time $t$ the state vector may be written as $\left|\psi\left( t\right)\right\rangle =\sum_n \psi_n\left( 0\right)e^{-i\omega_n t}\left|n\right\rangle$ with $\omega_n=\frac{E_n}{\hbar},\,\psi_n\left( 0\right)=\... | {
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Clebsch-Gordan coefficients for all the normalized $S_\textrm{tot}^2$ eigenstates of a three spin system with spin $s_\textrm{tot} = 1/2$? The total spin operator is defined as $\vec{S}_\textrm{tot} = \vec{S}_1 + \vec{S}_2 + \vec{S}_3$ with $\vec{S}_1 = S\otimes\mathbb{I}\otimes \mathbb{I}$, $\vec{S}_2 = \mathbb{I}\o... | Yes. Combine spins 1 and 2 to obtain $j_{12}=0$ and $j_{12}=1$. Then combine each with the remaining spin 3 to get the total spin -$1/2$ states.
Note that there are two sets of spin-$1/2$ states, "coming from" $j_{12}=0$ and $1$ respectively. The states are distinct (as you will see) even if the total spin are the ... | {
"language": "en",
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What is the difference between conventional current and electronic current? what is the difference between conventional current and electronic current?
How are they linked to one another?
| • Electric current can be either positive or negative, but conventional current is always positive.
• The conventional current for an electron flow is positive, whereas the electrical current is .
• For a flow of positive charges, both the electric current and the conventional current are the same.
• Since almost every... | {
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How to detect femtosecond pulses with slow PMT's? In pump-probe measurements, we can use femtosecond lasers to study ultrafast processes, such as molecular vibrations. PMT's are very slow compared to the pulses, so how can we use them to detect such short pulses?
I know that the pulses themselves are used for pulse ch... | You can still do pump probe spectroscopy with a femtosecond laser. As long as the detector's gate is wide enough to capture the pulse, you should be fine. As for getting the vibrational information, the pulse needs to be passed through a spectrometer to get your vibrational information. (We did a similar thing for femt... | {
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Gravity between two Photons (I searched for an answer online already but I couldn't quite find what I was looking for...)
I thought about this for a long time now. If two Photons fly in the same direction, one behind the other one, for my understanding the one behind the other one should be pulled towards the photon in... | They do not interact because their rest masses are zero.
You can not attach to the photon neither an inertial nor a gravitational "mass" as
$$ m \neq\frac{\hbar \omega}{c^2}$$
See for example on this topic https://arxiv.org/abs/physics/9907017.
This part is pure kinetic part and is related to the momentum of a photon... | {
"language": "en",
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Flow rate of a liquid Can one use the following formula for calculating the flow energy rate of a fluid?
$Q = Cp_{fluid} T \dfrac{dm}{dt}$
where dm/dt is the mass flow, T the temperature and Cp the specific heat.
And how would it be related to the flow energy rate $\dfrac{dm}{dt}Pv$?
Thanks!
| only if the flow is isothermal. You derive this from the state equation (ideal gas law) but when you take the derivative wrt time each side you have to use the chain rule. And if temperature can change, then you need to include its derivative.
| {
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How attitude indicator (gyro horizon) adjusts to the Earth's curvature? Image a plane is flying from North to South and is crossing equator. How gyro horizon would maintain correct pitch angle? (Or East-West?)
I assume that pitch angle is correct at takeoff, so the further plane flights, the more difference would be be... | I've been crushing flat earth nonsense with solid facts and mathematics for about two years now. Their understanding of gyro physics (much like any of their other scientific understanding) is flawed, and they base most of their claims on these flawed understandings.
Gravity is an acting force on the attitude indicator... | {
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Where am I going wrong in finding the center of mass of this sphere? A solid sphere of density $ρ$ and radius $R$ is centered at the origin. It has a spherical cavity in it that is of radius $R/4$ and which is centered at $(R/2, 0, 0)$, i.e. a small sphere of material has been removed from the large sphere. What is the... | You dont have to complicate using such integrals. Just do this. Assume the larger sphere to be complete and find its centre of mass (which is its geometrical centre of course). Now for the cavity, assume it is made of negative mass(no such thing of course, just eases the calculation).The mass of the smaller sphere woul... | {
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Should zero be followed by units? Today at a teachers' seminar, one of the teachers asked for fun whether zero should be followed by units (e.g. 0 metres/second or 0 metre or 0 moles). This question became a hot topic, and some teachers were saying that, yes, it should be while others were saying that it shouldn't be u... | First, as long as the quantity under consideration possesses a unit, yes, because of the importance of the Consistency of Units or Dimensional analysis. Second, in a minor key: in experimental physics, a pure zero is not likely to happen in practice. Therefore, whenever a zero is very close 0.0000000000257, it is impor... | {
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Electromagnetic induction with light bulbs
I chose C as the voltage, which was initially split between the two lamps, will only be "used" by lamp 1, thus increasing the brightness. However, the answer is D.
| Assume that the copper wire a very much smaller resistance than that if the bulbs and that the resistance of the bulbs $R$ is independent of the current passing through them.
Let the induced emf in the loop be $2\mathcal E$.
The current in the circuit which produces a certain equal brightness for the two bulbs is $I =... | {
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What happens to a photon as it experiences an increasing amount of force opposite to its momentum? Could it be frozen? Image that a photon is emitted from a sphere, moving orthogonally away from the sphere's surface. As the weight of this sphere increased, how would the increasing gravitational force affect the motion ... | Photon momentum does not correspond to its velocity, but its frequency, so if you applied a force opposite to its momentum, it would not slow down, but redshift.
As photons carry no charge, the only force applicable is gravity, and its general relativistic description is more kinematical than dynamical (the photon gets... | {
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Operator functions: why is $f(A)$ uniquely defined? In Nielsen and Chuang, they write: Let $A = \sum_a a|a\rangle \langle a|$ be the spectral decomposition of $A$. Define $f(A) = \sum_a f(a) |a \rangle \langle a|$. Apparently this is uniquely defined. I'm having trouble seeing why this is.
If we used some other orthono... | This stems from a misformulation of the spectral theorem. In a proper mathematical text one never sees it stated using a basis precisely because that is not unique. Physicists often assume "for the sake of simplicity" a nondegenerate spectrum, where every eigenvalue has an algebraic multiplicity of 1, and consider nont... | {
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How many photons can exist in a cubic box of unit volume, simultaneously? Suppose that we have a cubic box of unit volume. Simultaneously, how many photons can exist in such box? Is there any limit?
| You have to keep in mind that if we are not at absolute zero the atoms in the walls of the container will have thermal energy and therefore they will vibrate and emit/absorb photons. In an equilibrium situation, the electromegnetic radiation in the box will be blackbody radiation (assuming we are dealing with an isolat... | {
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Momentum and acceleration of an atom after emitting of a photon? I'm a second year physics student and we've been talking about light and the fact that it carries momentum. I've been thinking about a situation where there is an excited atom that has an electron at a high energy level. When the electron jumps back down ... | The momentum depends on the wave length (energy). The time from the wave starts until it reaches its top phase will be the acceleration time. The acceleration amount depends on the mass of the atom.
| {
"language": "en",
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Why are planets not crushed by gravity? Stars can be crushed by gravity and create black holes or neutron stars. Why doesn't the same happen with any planet if it is in the same space time?
Please explain it in simple way. Note: I am not a physicist but have some interest in physics.
| The particles which make up atoms are electrically charged, and they repel each other when they get too close to each other. Gravitational forces only attract one particle to another, and never repel, but they're extremely weak compared to the electrical force. To create a black hole, the gravitational force needs to o... | {
"language": "en",
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Charge leakage from two suspended charged spheres Recently I was going through "Problems in General physics" by I E Irodov. In Electromagnetics chapter, there is a question how much is the charge leakage from two spheres suspended by a silk thread (3.3).
Two small equally charged spheres, each of mass $m$, are suspen... | To your questions:
(1) Charge leakage $dq/dt$ is the charge loss current of the spheres due to the conductivity of the air.
(2) If the spheres are losing charge, the repulsive Coulomb force decreases and thus also the distance x decreases. If the spheres maintain a constant distance x, while they are kept apart by the ... | {
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Two Cases Of Harmonic Motion Caused By Gravity I'm a highschool student and we learned not so long ago Simple Harmonic Motion, and I'm trying to analyse "similar" cases which I thought of.
Here we have a body (with mass $m$) being affected by the gravity of a body whose mass is $M$, yet it doesn't collide with it (it ... | Harmonic motion $(x=A\sin(\omega t))$ depends on having a restoring force which is proportional to the displacement from the equilibrium position.
This is not true for your 1st case. Although there will be an oscillation, it is not harmonic. You can avoid the infinite force at $x=0$ by giving mass $M$ a finite radius ... | {
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The Schrodinger equation for quantisation of light? Why not with the Dirac equation? Light is a born relativistic. However, when we quantise the electromagnetic wave, we start with the time-independent Schrodinger equation, which is a non-relativistic equation. Why is this fine?
Wouldn't it more make sense if we use t... | Light obeys Maxwell's equations. So when we quantize light, the typical starting point is the Maxwell Lagrangian:
$$\mathscr{L} = \frac{1}{4}F^{ab}F_{ab}$$
where
$$F_{ab} = \partial_{a}A_{b} - \partial_{b}A_{a}$$
where $A_{a}$ is the 4-dimensional vector potential $(\phi, {\vec A})$ such that ${\vec E} = -{\vec \nabl... | {
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Why is nuclear force spin dependent? Why nucleons with parallel spins have greater nuclear force than the ones with anti-parallel spins?
I just want a clear and easy explanation. Thank you!
| Spin dependence of nuclear force is highly non trivial. As an example, exceptional stability of some particular nucleus with nucleon number (also known as magic number) 2, 8, 20, 28, 50, 82, and 126
can be explained by considering the spin-orbit interactions between nucleons.
To see the role of spin dependence, the deu... | {
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Simple Electric Field Problem Solve the Electric Field distance z above a circular loop of radius r. The charge/length = $\lambda$
The arc-length is 2$\pi$r. So the smallest portion of the circle is 2$\pi r \delta \theta$ and charge is therefore
\begin{align}
q&=2\pi r \delta \theta*\lambda
\\
R&=\sqrt{r^2+z^2}= \tex... | The length element should be $r d\theta$ not $2\pi r d\theta$. So the charge element is
$$dq=\lambda r d\theta$$
but not
$$dq=\lambda 2\pi r d\theta.$$
| {
"language": "en",
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What does "point of application of force" mean in the given context? I faced a particular conceptual doubt while solving a textbook problem. I will initially write the complete question in my textbook and then try to boil it down to a single conceptual doubt so that it complies with the rules of Physics Stack Exchange.... | Think about gravity for example. It acts on every point of an object, and one finds the center of gravity by doing a "weighted" sum of the point locations.
$$ x_{\rm CM} = \frac{ \sum x \Delta m}{\sum \Delta m} $$
If gravity varied by location as $g(x)$ then the above would be
$$ x_{\rm CM} = \frac{ \sum x \Delta m\, g... | {
"language": "en",
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acceleration and potential energy due to conservative force how can we find acceleration due to a conservative force as a function of time when potential energy due to the conservative force is given as function of position(e.g. U(x)=x^2)
| For a conservative field, the force $F$ can be calculated from the potential energy gradient $dU/dx$
$$
F = -\frac{dU}{dx} = -2x
$$
where $U(x) = x^2$ in your example.
If the mass, $m$, is known, Newton's second law gives
$$
\ddot{x} = \frac{F}{m} = -\frac{2}{m} x
$$
which is the differential equation for an undampe... | {
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Does the top plate of a capacitor hold half of the capacitor's charge or all of it? I am a little confused conceptually about the charge of a capacitor held by the top plate. Is it equal in magnitude to $q$, or is it half of $q$? It makes more sense to me for it to be half of $q$, with the other half existent on the ot... | When we say that the charge of a capacitor is $Q$, we mean that one plate has charge $Q$ and the other plate has charge $-Q$.
That is to say, we don't mean that the capacitor has a net charge of $Q$ (as I suspect you might be thinking) since, in fact, the net charge of a charged capacitor is zero.
By charged, we mean t... | {
"language": "en",
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Example of a transformation that is not canonical Can someone please give me an example of a transformation that is not a canonical transformation?
| Consider the transformation $x \mapsto x, p \mapsto v = \frac 1 m [p - qA(x)]$ where $A$ is a vector potential. Then $\{v,v\} = \frac q {m^2} v\times B$ where $\{\cdot,\cdot\}$ is Poisson bracket and $B = dA$ is the magnetic field, so this transformation is not canonical.
More concretely:
With the Hamiltonian $H = (p-... | {
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Discrete Langevin Equation We have the Langevin equation, that describes the motion of a particle in a viscous medium, given by
\begin{equation}\label{Langevin}
\frac{dv}{dt} = -\gamma v + \zeta(t)
\end{equation}
With the conditions that
\begin{equation}
\langle \zeta(t) \rangle = 0
\end{equation}
\begin{equation}
\la... | The short answer to your question is $\zeta(t) \rightarrow \sqrt{\frac{\Gamma}\tau}\xi_n$. The easiest reason to give for the square root is dimensional analysis. $\zeta$ is dimensionful, but $\xi$ is dimensionless, so using dimensional analysis in the variance equations will give you the square root.
In order to deduc... | {
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Where does gravitational energy come from? We've all heard mass tells space how to curve and curved space tells matter how to move. But where does the energy to curve space come from? Likewise where does the energy that curved space uses to push planets around come from? I mean if I tell my son to clean his room, an... | Mass is the same as energy by $E = mc^2$ So the energy that curves space-time is the mass that curves space-time. Then you could ask why does mass curve space? As far as I know that is equivalent to asking where do Einstein's field equations come from. For that one needs a complete and consistent theory quantum gravity... | {
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Frozen lake ice formation Yesterday I went walking on the Lagorai mountain, in northern Italy. Passing near the Lake Erdemolo, I noticed a small part of frozen water from the lake, separated from the rest (the lake was not frozen at all). I found a quite strange ice formation, which I cannot fully comprehend.
At the bo... | You might be seeing evidence of 'frost heave'. Moisture in the soil will,
in cold conditions, both freeze at the surface (adjacent to cold air), and
wick up the existing ice crystal from its (relatively warm) base near the
soil, to the cooler upper tip (where the newly arrived water freezes).
Thus, ice crystals can b... | {
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Understanding rocket problem intuitively
A rocket is trying to land on a planet. The mass of the rocket is $1\,\rm kg$, and the gravitational acceleration of the planet is $1\,\rm m/s^2$. The rocket starts the free fall at $20\,\rm m$ above the surface of the planet (initial velocity is $0$), and can use the thrust fo... | To minimise the impact speed, there are two ways to think about this:
*
*You want to maximise the work done on the rocket:
$$\mathrm{Work = Force \times Distance}$$
*You need to minimise the time spent falling.
| {
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How much information can you obtain from a pulsar-black hole system? Imagine that we have detected an interesting source in the sky that we believe is generated by a pulsar orbiting a black hole.
The challenge here is the following:
What physically relevant information could you extract from the observation of this sy... | The system you describe could be considered analagous to the famous Hulse-Taylor binary system, which consists of a pulsar and a "quiet" neutron star companion in a close orbit.
A paper by Weisberg et al. (2010) describes how a detailed timing analysis of the pulsar alone will yield the masses of both components (to 5 ... | {
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Change in enthalpy equal to heat transferred
*
*Is the relation $ \Delta H = Q _P $ valid for both reversible and irreversible processes or only for reversible processes?
$Q_P$ is the heat exchanged at constant pressure.
*Specifically, is it valid for this case: saturated water (mass fraction = 1) is enclosed in... | I don't know if it is not true for every irreversible process, but it is certainly not true for the process you described in item 2. And it is not true for the case of a so-called constant pressure irreversible expansion or compression of a gas, where, during the deformation, the external pressure is held constant at ... | {
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Thermal equilibrium of universe I am just an interested layman in the field of cosmology. In the usual account of the development of the universe, it is stated that about 380.000 years after the big bang the electromagnetic radiation was in thermal equilibrium with matter.
Does this mean that the universe was (and mayb... | Yes. You are completely right on all accounts. At one point you suggest the universe is still in thermodynamic equilibrium --- this is not the case. As you later point out, after recombination, photons are baryons do decouple. Once average densities become low enough that the diffusion time (the time it takes for t... | {
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Is an electron attracted to one of the magnetic poles ...? I noted the question above had been posted. And I wanted to comment, but nay, it was locked out. However, what of the old 'CRT' tubes, in which magnetic fields are used to steer the electron stream ? Now I know the 'Electron's' do not get attracted, or repul... | Thanks for all the input. I think I just answered my own question. (The site suggested refining question, but can't get there easily). Aurora Borealis. I hope that answers the question I answered. (Grin).
Charged particles hit the earths magnetic fields and follow those lines to the poles. I want to thank one of t... | {
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"url": "https://physics.stackexchange.com/questions/291667",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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"answer_id": 2
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Calculating height of a ball formula based on bounce I'm trying to figure out a formula for the new height of a ball, up until it stops bouncing.
The time iteration between each could be for example one second; and gravity of course would be 9.8
I've tried doing research at different websites such as physics @ illinois... | R.H=h/4 [t2÷t1] ²
t1=Time of free fall
t2=Time of flight
h=Free falling height
We can calculate the first Rebouce by this equation.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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How do one show that the Pauli Matrices together with the Unit matrix form a basis in the space of complex 2 x 2 matrices? In other words, show that a complex 2 x 2 Matrix can in a unique way be written as
$$
M = \lambda _ 0 I+\lambda _1 \sigma _ x + \lambda _2 \sigma _y + \lambda _ 3 \sigma_z
$$
If$$M = \Big(\begin{... | Let $M_2(\mathbb{C})$ denote the set of all $2\times2$ complex matrices. We also note that dim$(M_2(\mathbb{C}))=4$, because if $M\in M_2(\mathbb{C})$ and
$M=\left(
\begin{array}{cc}
z_{11} & z_{12}\\
z_{21} & z_{22} \\
\end{array}
\right)$, where $z_{ij}\in \mathbb{C}$,
then
$M=\left(
... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
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General method of deriving the mean field theory of a microscopic theory What's the most general way of obtaining the mean field theory of a microscopic Hamiltonian/action ? Is the Hubbard-Stratonovich transformation the only systematic method? If the answer is yes then what does necessitate our mean field parameter to... | In second quantization, the mean field approximation consists in approximating some combination of operators $A$ by a $c$-number $\langle A\rangle$. For example,
for Bose-Einstein condenstate $A=b_0$, for Cooper pairing $A= a_{\mathbf{p}\uparrow}a_{-\mathbf{p}\downarrow}$, in the Hartree-Fock approximation $A=a_{\math... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why do some chemicals take electrons from other chemicals? How can some chemicals, if they have an equilimbrium of electrons, take away electrons from other chemicals?
One example I believe is placing a small amount of gallium on top some alluminum and watching the alluminum melt.
Why does the gallium, if it is at equ... |
One example I believe is placing a small amount of gallium on top some alluminum and watching the alluminum melt. Why does the gallium, if it is at equilibrium state, need more electrons?
No, gallium dissolving aluminium is not an example of electron exchange but of formation of an alloy.
Exchange of electrons betwee... | {
"language": "en",
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Is there a low-bugdet method to measure micronewtons? I will have to measure very small EM forces (on the order of 10^-6...10^-5 Newtons) acting on a small copper sphere or plate (of about 0.05 kg).
I could hang the copper object with a long thread and use a micro-Newton sensor but I can't find any vendor that gives p... | Rewritten in response to comments.
The torsion balance is often used to measure gravitational attraction of small objects.
You could make a torsion balance with your copper sphere. Add a metal sphere B as shown in the diagram. Calculate the gravitational attraction. Measure the deflection. Use this to calibration your... | {
"language": "en",
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Does a trumpet operate using an open air column or a closed air column Just as the title states. I could not find a coherent answer online.
Many thanks in advance
| The trumpet is considered a closed-ended pipe, but this is not a straight pipe. The flared end (at the bell) modifies the overtone structure so that all the integer multiples (rather than just the odd multiples) of the fundamental are present. The fundamental is also modified by the flare. See Thomas Rossing's The Scie... | {
"language": "en",
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Young's Double Slit Experiment-Problem Can it be proved that there would be no change in the "fringe width" when the main illuminated slit(s) is shifted to a position, which makes an angle of $\Theta$ with the original position of the source slit?
My try - I first found out the fringe width in a normal double slit wher... |
Hope you understand by the above explanation . I had a sample question also.
| {
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What do the commutators of the Hamiltonian with the spin operators mean precession-wise? I proved that $[H, S_z] = 0$, while $H$ and $S_x,S_y$ do not commute.
I showed this using matrix representations, now I am to comment on my results with respect to spin precession and I need help for that - how exactly do commutato... | The commutator of an observable like $S_x$ or $P_y$ (the spin in the x direction and the y component of the momentum respectively) with the hamiltonian will tell you about the time evolution of that observable. It tells you how that observable changes with time.
This is given through the heisenberg equation:
$\frac{dA}... | {
"language": "en",
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Why are only some portions of the EM spectrum able to pass through a metal dog bowl?
In this photo you can see part of the bowl, which I turned upside down and placed on top of various electromagnetic sources to determine whether the waves could pass through it. The graph in the background illustrates my results.
Fo... | Long-wavelength and short-wavelength radiation passes for different reasons: long-wavelength does not actually pass, but bends around the bowl due to diffraction (or is reflected from the walls), as the wavelength is greater or of the order of the dimensions of the bowl, and short-wavelength radiation passes because it... | {
"language": "en",
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Why can the $1$-point correlation function be made to vanish? The $1$-point correlation function in any theory, free or interacting, can be made to vanish by a suitable rescaling of the field $\phi$.
I would like to understand this statement.
With the above goal in mind, consider the following theory:
$$\mathcal{L} = ... | The 1-point function is constant in spacetime because of translation invariance, i.e. $\langle \phi(x)\rangle = \phi_0\in\mathbb{R}$ for all $x\in\mathbb{R}^4$. Obviously, the 1-point function of $\phi'(x) := \phi(x) - \phi_0$ is zero since the expectation value is linear. So $\phi\mapsto \phi' = \phi + \phi_0$ gets ri... | {
"language": "en",
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What is the role of pillars in bridges?
As I can see in the picture, there are so many pillars which are holding the bridge. This picture gave a question to me that what are these pillars doing below the bridge?? An appripriate answer could be "these are providing support to bridge".
I tried to get the answer as foll... | In addition to the other answers, I think it's important to understand that you can't just make an arbitrarily strong pillar. Or rather, you can't make the ground the pillar stands on arbitrarily strong. So, depending on the geology that the bridge is standing on, you may need more or less pillars.
Of course you can ... | {
"language": "en",
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If an electron is in ground state, why can't it lose any more energy? As far as I know, an electron can't go below what is known as the ground state, which has an energy of -13.6 eV, but why can't it lose any more energy? is there a deeper explanation or is this supposed to be accepted the way it is?
| You might as well have asked: "Why can't the string on my guitar produce a lower sound?" The short answer is: the lowest sound depends on the quality of the string, on the tension of the string and on the restricted length of the string. So it's just a matter of configuration! It's the same with an electron in a hydrog... | {
"language": "en",
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What does this symbol $\odot$ mean? While reading through a physics textbook, I came across the use of sub-scripted ☉s.
Here's the context:
Stars between 0.5M☉ and 10M☉ will evolve into red giants...
I'm assuming it's to do with the life-span of a star; however, I don't know exactly how.
I searched google/wikipedia how... | The symbol in question, $\odot$, usually denotes the Sun. The solar mass, $M_\odot$, is often used as a unit of mass in astronomical/astrophysical texts. Another example is the solar luminosity, $L_\odot$.
| {
"language": "en",
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Phonons and heat conduction What is the physical picture of heat conduction in a poor conductor? In particular, I'm curious about the role of phonons in conduction in poor conductors. I know that phonons (within the harmonic approximation) move without being scattered and would lead to infinite thermal conductivity. Th... | Thermal conductivity is defined as a ratio of energy flux and temperature gradient (up to a factor). If phonons move freely, arbitrary energy flux can exist without temperature gradient (so the finite phonon velocity does not make the thermal conductivity final). See details on the role of scattering, say, at http://ww... | {
"language": "en",
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Small nucleus emission from a larger nucleus Like alpha decay, is there the possibility of a small (n,z) nucleus coming out of a large (N,Z) nucleus? Why lithium and beryllium don't decay out of big nuclei as helium does ?
| The decays you are considering are determined completely by the energetics. The alpha decay occurs so readily because the alpha nucleus is so tightly bound. Lithium and Beryllium are much less tightly bound so the energetics are less favorable. You can explore the possibility of such decays by looking at the masses of ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/294125",
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What's happening when a force is applied but there's no displacement? I've thought of an example that can explain my doubt properly.
Let's take an electric motor which is connected with an object through a rope so that when the motor starts to rotate, it brings the object towards itself. However, the mass of the object... | I do apologize for not simply commenting, as I am not able to do so, but I found this answer to a different question that explains why a motor releases heat energy when held stationary. Again, apologise for not really giving any original answer, but thought this would help.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Partition function for classical indistinguishable particles and Bose particles We have two particles that can be in either level $E_0 = 0$ or in level $E_1$.
If we treat them as Bose particles, then the partition function will be:
$$ Z = 1 + e^{-\beta E_1} + e^{-2\beta E_1}, $$
whereas if we treat them as classical in... | In quantum statistics there there is never any over-counting. In this system containing two particles (= a quantum many-body system) the only three possible many-body states are
$$\vert \psi_A \rangle = \vert E_0 \rangle \otimes \vert E_0 \rangle \qquad \text{with energy} \quad E = 0$$ (with which I denote particle one... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/294540",
"timestamp": "2023-03-29T00:00:00",
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Strange quark decay into two down quarks and an anti-down quark I saw that a $\Sigma^+$ can decay into $n+\pi^+$, which means that the $s$-quark must decay into $dd\bar{d}$. However, is there a Feynman diagram to represent this? I cannot find one for either the $\Sigma^+$ decay or the $s$-quark decay. I have only just ... | This is a leading order Feynman diagram (which may be referred to as a 'gluonic penguin') for a $s \to dd\bar{d}$ transition. It also holds for any other kind of $q \to q'q'\bar{q'}$ flavour-changing neutral current transition (i.e. when $q$ and $q'$ have the same charge). FCNC decays are pretty interesting because the... | {
"language": "en",
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"source": "stackexchange",
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Confusion between two different definitions of work? I'm doing physics at high school for the first time this year. My teacher asked us this question: if a box is slowly raised from the ground to 1m, how much work was done? (the system is only the box)
Using the standard definition, $W = Fd\cos(\theta)$, the work shou... | F is not the sum of the forces on the block, it is the force which is doing the work. It is either the force provided by the person (if you want to find the work done on the block by the person) or the force of gravity (if you want to find the work done on the block by gravity). You choose.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/295245",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
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Entanglement and wormholes: Are they the same? Some "recent" studies (e.g., http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.111.211603) popularized in the news (e.g., http://news.mit.edu/2013/you-cant-get-entangled-without-a-wormhole-1205 or https://www.ias.edu/ideas/2013/maldacena-entanglement) argue that qua... | Non-locality of entanglement is not experimentally yet fully proven beyond doubts/loopholes. It must have been proven mathematically though. Non-locality of Entanglement correlations has not been sufficiently scrutinized yet. All efforts are geared towards proving non-locality via Bell's inequality. Bell's inequality i... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Free and bound current- and charge density in Maxwell equations The first and fourth Maxwell equations are often denotet in vaccum:
$$
\nabla \cdot \mathbf{E} = \frac{\rho}{\epsilon_0}
$$
$$
\nabla \times \mathbf{B} = \mu_0\left( \mathbf{j}+\epsilon_0 \frac{\partial{\mathbf{E}}}{\partial{t}}\right)
$$
and in matter:
$$... | The electric field $E$ is the field we apply, what we express with the first Maxwell equation is that its sources must come from the total density charge $\rho$. In a material, there will be some fixed charges, so the presence of $E$ will induce some dipoles, and this will make Polarization $P$ appear. Then polarizatio... | {
"language": "en",
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Mock equation of state I am student of astrophysic i need to meaning of "mock equation of state".
And is there any specific way to generate mock equation of state.
| I guess that what is meant is a relationship between the pressure of the gas and it's density, temperature and composition.
No, there is no specific way to make something that you have defined in a completely general way, without describing any context - is this a star, a compact object, the interior of a star forming ... | {
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What is the origin for the name "partition function" given to $Z = \sum e^{-\beta E_i}$? Does anyone know why is the function $Z = \sum e^{-\beta E_i}$ called "partition function"?
For example, does it have a connection to the mathematical term "partition of $A$" which is a representation of the set $A$ as a disjoint... | It is appropriate to call $Z$,
$$Z=\sum_{i \in \, \mathrm{states}}\exp \left( -\beta E_i\right)$$
the partition function as it describes how probabilities are distributed amongst all the states with energies $E_0, E_1$, and so forth. To see this, note that the expected value of a property $Q$ is,
$$\langle Q \rangle = ... | {
"language": "en",
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How can one derive Schwarzian derivative action as low energy effective field theory invariant under global $SL(2,\mathbb{R})$? In a recent paper (page 47, below eq (4.173)) they make a passing claim that the Schwarzian derivative action can be derived using effective low energy field theory reasoning. I imagine they m... | The answer to this question is rather simple. Let us assume that our field is $f(x)$ and we want to find an effective low energy action invariant under the $SL(2,\mathbb R)$ transformations,$$f(x)\to\frac{a\, f(x)+b}{c\, f(x)+d}~,\tag{1}$$with $a,b,c,d\in\mathbb R$ and $ad-bc=1$. Let us look at these transformations on... | {
"language": "en",
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"source": "stackexchange",
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Osmotic Pressure: Comparing Solutes and Ideal Gases The equation describing osmotic pressure is
$\Pi=\frac{n}{V} RT $
which is just like the ideal gas equation law
$PV=nRT$
So how much of an analogy is there between ideal gases and solutes? Is there a extended version of the equation for osmotic pressure that matche... | The van t'Hoff equation for the osmotic pressure can be expanded by a virial expansion similarly to the extension of ideal gas equation. In the gas case, the coefficients of the virial expansion are related to the van der Waals constants a and b. Thus you can probably also obtain a similar equation of state for the osm... | {
"language": "en",
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Why is bench pressing your bodyweight harder than doing a pushup? Why does bench pressing your own bodyweight feel so much harder than doing a push-up?
I have my own theories about the weight being distributed over multiple points (like in a push-up) but would just like to get a definite answer.
| When you're doing press-ups, your arms have a mechanical advantage over the mass of your body. The force that they exert does not pass through your COM and, instead, exerts a torque on your mass with your feet being the fulcrum of the lever.
When you benchpress, your arms are exerting a force that passes straight throu... | {
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Electric field associated with moving charge I have recently started to learn about the electric field generated by a moving charge. I know that the electric field has two components; a velocity term and an acceeleration term. The following image is of the electric field generated by a charge that was moving at a cons... | When the charge is moving at constant velocity, it emits a non-uniform electric field that can be calculated from the Lienard-Wiechert potentials. This field has a velocity component but no acceleration component, as the charge is not accelerating.
When the charge is not moving, it emits a spherically symmetric electri... | {
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"source": "stackexchange",
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Using a Ring Imaging Cherenkov (RICH) detector I'm not clear on how RICH (Ring Imaging Cherenkov) detectors identify particles, as much as I've tried to read up on it. So they measure the angle Cherenkov light is emitted at, which is related to the refractive index of the material, speed of light and the speed of the p... | A RICH alone doesn't ID a particle. The only thing it measures is velocity.
But by combining that information with either momentum (from a spectrometer) or energy (from a calorimeter) you can often deduce the species or at least constrain it to some range of possibilities.
As for why they are used to separate pions and... | {
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Lorentz transformation, problem with derivation I have a question about the Lorentz transformation:
In the derivation it's said that two systems S and S' should at $t=0$ and $x=0$, overlap. We get the following transformation rules:
$t'=\gamma_0(t-v_0x/c^2)$
$x'=\gamma_0(x-v_0t)$
$y'=y$
$z'=z$
My question is: What ha... | There are three types of transformations that preserve the spacetime interval.
*
*Boosts: these are transformations like the one you gave. They transform between systems moving relative to each other, which have the same origin. If we write spacetime coordinates as column vectors like
$$ X = \left( \begin{matrix}
t... | {
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Why faraday rotation is not effective in case of circular polariation? As in case of linear polarization the plane of polarization gets rotated. Why it's not happening with circular polarization?
| The Faraday effect is the rotation of the polarization plane (proportional to the length $l$ and magnetic field $B$ of linearly polarized light when propagating along a magnetic field $B$ applied to a (transparent) material. Linearly polarized light can be decomposed into left and right circularly polarized light. The ... | {
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How to distinguish "system" and "environment" in quantum decoherence Quantum decoherence distinguishes the whole big system into "system" and environment, and shows how system, when density matrix is traced over environment, comes to be decoupled from environment.
But this requires distinguishing environment from syste... | The fact that the whole system is a quantum system is not relevant to this issue and neither does special relativity prohibits this separation as it is not physical.
The separation is made on the basis of what is the system that is the subject of an experiment. The environment is then everything else, including you.
De... | {
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Sensation of atmospheric pressure Pressure is force divided by area, and force is mass times acceleration.
Now, the newton [N] is the force needed to accelerate 1 kg by 1 m/s, and the kilogram-force [kgf] is the force needed to accelerate 1 kg by g m/s, where g is the standard gravity.
The standard atmospheric pressure... | Solids and liquids are what we call incompressable. They do not change their volume a great deal when a pressure is applied to them. A comparable metaphor for you is that you can park the wheel of a car on top of a phone book. That phone book is under thousands of pounds of force, but it doesn't implode because soli... | {
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Is the Earth gaining or losing mass over time? The earth presumably loses mass because molecules of the atmosphere disassociate and fly off into space where the solar wind carries them away.
On the other hand the earth gains mass because particles of dust and meteorites strike the earth and accumulate onto it.
Which fo... | We really don't know the figures of mass loss, or mass gain through cosmic dust accumulation.
Mass loss through Hydrogen and Helium
According to some calculations, the Earth is losing 50,000 tonnes of mass every single year, even though an extra 40,000 tonnes of space dust converge onto the Earth’s gravity well, it’s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/297622",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 2,
"answer_id": 0
} |
Is the Photovoltaic Effect observed only in P-N junctions? So far, all the sources I've found explain the photovoltaic effect in relation to p-n junctions when talking about solar cells (since that's what they're made out of), but I was wondering if one could explain the photovoltaic effect in general without having to... | Another example of a system that exhibits photovoltaic action is a metal-semiconductor junction, or a Schottky junction.
In general photovoltaic action will occur at an interface involving a semiconductor. At this interface, say the interface between an n and p doped semiconudctor, there is an inherent asymmetry that ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/297706",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
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