Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Weight distributions If a man is standing on two weighing machines (scales), with one foot on each, Will both machines show equal weight or his weight will be distributed in two machines?
| If the person in concern is standing perfectly, such that his weight distribution is equal over both legs, and if both the weights are calibrated perfectly, then yes, both the weights will show equal readings. (The readings each being half the weight of the person.)
In a realistic case (without 100% perfection), howev... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/183324",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
Does lithium-6 "decay" when hit by a neutron? I am talking about the nuclear reaction
$$
^6\text{Li} + n \rightarrow\ ^4\text{He} +\ ^3\text{H} + 4.78\text{MeV}
$$
A neutron hits a lithium-6 nucleus and together they form an alpha and triton particle. Is it valid to say that the lithium nucleus "decays" when hit by a n... | The process by which the lithium becomes fissile due to neutron capture is called neutron activation. The subsequent decay is simply a fission reaction.
There seems to be a precedent on various sites for such a process to be called a 'neutron capture induced fission reaction', although most of the Google results for th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/183391",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 2
} |
Is true black possible? Black is the absence of light because it absorbs light, but when we create black paint or black objects, light is always reflected, either in all directions in matte or smoothly in shiny black objects, making it never a true black. Would it be possible to use polarization to create an object tha... | Just to add to the above answers, and since to did not limit your question to the visible range - if you define black as absence of light (photons emitted or reflected), then there is no such substance, because according to black body radiation model, everything with a temperature above absolute zero (which is essentia... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/183473",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
"answer_count": 4,
"answer_id": 2
} |
How to calculate the period of the movement from a potential? I have an assignment, where I have an object moving in 1-D with a given mass and energy, and the potential V(x), and I'm supposed to calculate the period of the movement as a function of the energy
$$
V(x)=\begin{cases}\infty &x < -a \\
0... | You can calculate the action integral as a function of energy
$$
J(E) = \oint p_x dx = 2\sqrt {2m} \int_{x_0(E)}^{x_1(E)} \sqrt {E-V(x)}dx
$$
where $x_0$ and $x_1$ are the turning points. (In your case: $x_0 = -a$ and $x_1 = \sqrt{E/\alpha}$). The Period is then given by the derivative
$$
T(E) = dJ/dE
$$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/183645",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Can we quantitatively understand quark and gluon confinement in quantum chromodynamics and the existence of a mass gap? Quantum chromodynamics, or QCD, is the theory describing the strong nuclear force. Carried by gluons, it binds quarks into particles like protons and neutrons. According to the theory, the tiny subpar... | Confinement cannot be rigorously shown in QCD with current techniques, because all analytic results in QCD are perturbative and the perturbative expansion breaks down at low energies where the coupling becomes strong.
QCD has a negative $\beta$-function, i.e. the Yang-Mills coupling grows at lower energies and becomes ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/183748",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Notation of vectors It's very common to see $\text{F} = 30 \text{ N}$ when the problem is unidimensional. Yet, force is a vector. Shouldn't I write $|\overrightarrow{F}| = 30 \text{ N}$? Because if I write $\overrightarrow{F} = 30 \text{ N}$ I'm saying that the vector is equal to an scalar. On the other hand, I rarely ... | Some people use $\mathbf{F}$ instead of $\vec{F}$ or even $\overrightarrow{F}$. I agree that often $F=\| \vec{F} \|$ is a convenient shortcut. So for example
A force $\mathbf{F}=(10 \mbox{ N},0,0)$ has magnitude $\|\mathbf{F}\|=10 \mbox{ N}$.
The components of $\mathbf{F}$ are $F_x = 10\mbox{ N}$, $F_y=0$ and $F_z=0$
... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/183856",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 1
} |
Filming light in slo mo So I was thinking of a cool video I could make when I thought of filming a video of me turning on a light. I thought that if you film yourself turning on a light and slo mo that video enough times (theoretically I you had a camera that could record that many frames per second)will you be able to... | Light moves at about a foot per nanosecond, or a meter every three nanoseconds. In order to capture it propagating across a room over a few frames, you would need to gather something like a billion frames per second. No consumer camera -- indeed no camera on Earth -- is capable of this.
Now there have been people playi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/183907",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Temperature effects on lead against radiation I would like to know if bringing lead to near absolute zero temperatures would have any affects on how resistive it is against gamma radiation. It takes 40 centimeters of lead to reduce gamma radiation effects by a factor of a billion (medium energy levels). Since atoms com... | It is the mass of material more than the thickness that determines the stopping power (which incidentally is a function of energy - so you can't simply state "40 cm reduces gamma flux one billion times" without specifying the energy).
Lead has a positive coefficient of thermal expansion - so the same amount of lead wil... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/184118",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
How electrons move so fast in a electric circuit? Whenever we switch on a bulb......it takes almost no time to glow up.....But we know that the atoms of a solid are tightly packed and there is a very little space between them.
So how the electrons travel through them irrespective of so much blockages in the conductor??... | Electrons can sneak pass all the atoms because of their wave function. They behave like waves not like particles. In short, because of quantum mechanics. In a periodic assembly of atoms like metallic solid they should not feel any resistance when moving through but because it is not perfectly periodic they feel aperiod... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/184317",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 4,
"answer_id": 2
} |
Entanglement of Tripartite States Is there any simple algorithm to determine the entanglement of a tri-partite state?
In particular, what is the proof for entanglement of $ |GHZ\rangle $ and $ |W\rangle $ states?
$ |GHZ\rangle =\frac{1}{\sqrt{2}}(|000\rangle+|111\rangle) $
$ |W\rangle =\frac{1}{\sqrt{3}}(|001\rangle+|0... | $\newcommand{\ket}[1]{\left| #1 \right>}$A state $ \ket \psi \in H_1 \otimes H_2 \otimes H_3$ is said to be entangled if there exist no coefficients $a_i,b_i,c_i$ such that:
$$\ket \psi = \sum_{ijk} d_{ijk} \ket{e^1_i} \otimes \ket{e^2_j} \otimes \ket{e^3_j} = \sum_i a_i \ket{e^1_i} \otimes \sum_j b_j \ket{e^2_j} \s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/184426",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Polarization and Reflection The polarization is a property of waves that can oscillate with more than one orientation. Given this, when the light is reflected from a surface, does the reflection change the property/orientation of waves?
| Yes it does. Since the direction of the light beam changes with reflection also the direction of polarization. This is mostly because the observer is in a fixed coordinate system and the light beam changes its local coordinate system during reflection.
For an idealized reflector and an observer which moves along with t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/184527",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why is the electric field inside a charged conductor zero in the electrostatic case? I am trying to understand the idea (or the fact) that most books introduce which is about the electric field inside a charged solid conductor.
Books tell that the field has to be zero everywhere inside solid conductor, otherwise charge... | We assume that the the electric field is uniform for a charged solid sphere. It follows that the electric charge of the sphere is equal to
$$ Q = \rho V$$
Where $\rho$ is the charge density and $V$ is the volume. Therefore,
$$ Q = \rho V = \frac{4}{3}\rho R^3$$
We create a Gaussian surface in the form of a sphere of r... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/184606",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Inductance of two parallel wires I've been asked to calculate the inductance per unit length for two wires or radius $a$ separated by $2d$ where $2d>>a$.
Starting from $\int_{s} B.dS = LI$ Im not sure what surface to take?
For each wire the field at a distance r away is given by $\int_{l} B.dl = \mu_0 I$ and by superp... |
Answer if anyone is interested. In the end the areas outside the inner edges of the wire cancelled by symmetry and so the surface i was looking for was the area enclosed.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/184728",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Is it possible to write the fermionic quantum harmonic oscillator using $P$ and $X$? The Hamiltonian of the quantum harmonic oscillator is
$$\mathcal{H}=\frac{P^2}{2m}+\frac{1}{2}m\omega^2X^2$$
and we can define creation and annihilation operators
$$b=\sqrt{\frac{m\omega}{2\hbar}}(X+\frac{i}{\omega}P)\qquad{}b^{\dagger... | Let's start from
$$H = \hbar \omega \left(f^\dagger f - \frac{1}{2}\right),$$
with $\{f, f^\dagger\}=1$, $\{f, f\} = 0$ and define fermionic position and momentum coordinates by
$$ \psi_1 = \sqrt{\frac{\hbar}{2}} \left(f + f^\dagger\right) \\
\psi_2 = i\sqrt{\frac{\hbar}{2}} \left(f - f^\dagger\right) $$
with the fol... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/184815",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 3,
"answer_id": 0
} |
Difference between fusion plasma and fluorescent lamp plasmas How is the plasma in a compact fluorescent lamp (CFL) different from a plasma in say ITER or the sun? Why does ITER need 100MK and a CFL can work at practically room temperature (apart from the filament)?
Or could ITER also create a plasma by charging the ga... | ITER needs very high ion temperatures (100M K) so the deuterons and tritium nuclei are fast enough to overcome electrostatic repulsion and undergo thermonuclear fusion. A CFL only needs to have a conductive plasma in order to have an electron current exciting atoms in the gas.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/184899",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
Is Chern-number for free fermion system always limited by total band number, i.e. number of orbits with a unit cell? If so, how to see that?
Also I think it has been proven that the total Chern-number for free fermion system is 0?
If you know how to prove it, please make some comment or hopefully a sketch of proof.
I... | https://arxiv.org/abs/1205.5792 The first example in the paper is $C=3$ on a triangular lattice with two orbitals per site. It is essentially three-layers of Haldane's honeycomb lattice model, but stacked together in a clever way so the translation symmetry is restored.
UPDATE: In fact two-band free fermion Hamiltonian... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/184986",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Does Time change over temperature? I am not a physicist, I am just an engineer.
But I dared to ask whether the temperature changes the perception of time.
Let's consider a particle that "stops" at absolute zero. I was thinking as a hypothesis, that our perception of time changes and the particle actually does not stop... | Your idea does not seem to work if you have two particles at different temperatures. Assume you "stop" one of them but not the other. Then does the time slows down for only one particle and not the other? or how would you explain that?
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/185094",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Phase added on reflection at a beam splitter? If we have light of a particular phase that is incident on a beam splitter, I assume the transmitted beam undergoes no phase change. But I thought that the reflected beam would undergo a phase change of $\pi$. I have, however, read that it undergoes a phase change of $\pi/2... | It actually depends on what kind of beam spitter you have.
I'll give a general treatment and shows that the conclusions of both Emilio Pisanty and Steven Sagona are basically correct, corresponding to different specific beam splitters, which are all common in the laboratory. For simplicity we don't consider loss in thi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/185155",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 5,
"answer_id": 0
} |
Crane Balancing, Center of mass I am working on the ICPC 2014 Problem C "Crane Balancing"
The initial idea was to calculate the center of mass of the polygon, which I did via this equation:
Where the Area A:
Now, the solution is to binary search over the mass and look for th e maximum mass M where the crane is still... | Assume that your crane is lying on the side set L on the axis c, where L can be just a point, can be just a line section [Ai, Bi], or can be unity of points and line sections.
You don't need to know how the mass affects the crane, you just need to now the minimal and maximal mass, for which the x coordinate of the cent... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/185232",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
What would be the rate of acceleration from gravity in a hollow sphere? Lets say the Earth is hollow and you are in the center of it (Same mass except all of it is on the outside like a beach ball) If you move slightly to one side now your distance is closer to that side therefore a stronger gravitational force however... | If the mass/charge is symmetrically distributed on your sphere, there is no force acting on you, anywhere within the sphere. This is because every force originating from some part of the sphere will be canceled by another part.
Like you said, if you move towards on side, the gravitational pull of that side will become ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/185298",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 2,
"answer_id": 0
} |
Particle Physics Decay Question - Eta Prime Decay Parity/Angular Momentum Conservation I was hoping someone could clarify why the following decay does not occur:
$ \eta ^{'0} \rightarrow \pi ^{0} + \rho ^{0}$
The quark compositions and spin parity are as followed:
$ \eta ^{'0} : (u\bar{u}+d\bar{d}+s\bar{s}) / \sqrt{3} ... | This decay (occurring via the strong interaction) violates the charge conjugation since $J^{PC}(\pi^0) = 0^{-+}, J^{PC}(\rho^0) = 1^{--}, J^{PC}(\eta'^0) = 0^{-+}$.
The charge conjugation transforms a particle in its anti-particle. In the case of the 3 particles involved in this decay, they are all their own anti-parti... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/185394",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Density of states of 3D harmonic oscillator Consider the following passage, via this image:
5.3.1 Density of states
Almost all of the spin-polarized fermionic atoms that have been cooled to ultralow temperatures have been trapped by magnetic fields or focused laser beams. The confining potentials are generally 3D harm... | Absorbing the irrelevant ħω constants into the normalization of the suitable quantities, for the 3D isotropic oscillator, $\epsilon=n+3/2$, while for each n the degeneracy is $(n+1)(n+2)/2$; (see SE ). Scoping the power behavior of a large quasi-continuous n, leads you to the answer.
The number of states then goes l... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/185501",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
Why is the shadow of a wind turbine a bit slow at first then very quickly, etc. and not equivalent to the wings? When I stand at a wind turbine and look at the shadow on the ground, the shadow is a bit slow at first then very quickly, etc. Very strange.
| Depending on the angle of the sun, the shadow becomes elongated so that it traces out an ellipse rather than a circle. This means that the shadows of the blades must traverse a different distance across the ground but within the same time period, thus giving rise to the periodic variation in velocity.
If the sun is at ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/185703",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
Is the Planck length the smallest length that exists in the universe or is it the smallest length that can be observed? I have heard both that Planck length is the smallest length that there is in the universe (whatever this means) and that it is the smallest thing that can be observed because if we wanted to observe s... | There is a tiny bit more going on than the otherwise excellent answer by zeldrege suggests. Imagine that you wish to probe an unspecified object to examine its structure. If we use light to look at the structure of an object, we need to have its wavelength smaller than the size of the details we wish to look at. Prob... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/185939",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "83",
"answer_count": 4,
"answer_id": 1
} |
Meniscus attached to an inclined plate To be more specific, suppose a hydrophilic infinite plate is stuck into a semi-infinite region of water, above the water is a semi-infinite region of air, when the plate is stuck into the water vertically, the contact angle is $\alpha$, as shown in the figure below:
Needless to s... | Section 2 of the paper below uses geometric & variational techniques to show that the Young-Dupre relation still holds for a meniscus formed by a solid of rotation that makes an arbitrary angle with the vertical:
http://www.unisanet.unisa.edu.au/staffpages/stanmiklavcic/cm_anziam2.pdf
I can't imagine that the result wo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186034",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 3,
"answer_id": 1
} |
Number of Nodes in energy eigenstates I have a question from the very basics of Quantum Mechanics. Given this theorem:
For the discrete bound-state spectrum of a one-dimensional potential let the allowed energies be $E_1<E_2< E_3< ...$ with $E_1$ the ground state energy. Let the associated energy eigenstates be $ψ_1,... | I guess there is not that much to grasp, unless you can really understand dark spots on an electron diffraction pattern. Very roughly explanation would be to interpret wave functions of a particle in a potential well as "standing waves", or as two interfering waves reflected from the walls of the well. Increasing the e... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186140",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 2,
"answer_id": 0
} |
Why do electric sparks appear blue/purple? Electric sparks tend to appear blue or purple or white in color. Why?
| Air is normally a bad conductor of electricity, but with enough voltage it can be converted to plasma, which is a good conductor. In a plasma, the electrons constantly bind to and leave atoms. Each time an electron binds to an atom, it emits the energy in light. As a result, the plasma glows the color of a photon with ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186199",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "49",
"answer_count": 3,
"answer_id": 0
} |
If I shoot a hockey puck on ice, is the force of me shooting it applied throughout its travel, or is it a one time force? For example, if I take a slap shot on a hockey puck, from what I understand, the forces acting on the puck are friction, the normal force, and the puck's weight. And, since I'm not constantly either... | Yes, you are thinking about it correctly. No force is required to keep the puck in motion. This is an important idea in physics. It is actually a common misconception among physics students that a force is required to keep an object in motion, so it is good you do not have this misconception.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186297",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Are spherical coordinates distances or angles? I've become confused about spherical coordinates when dealing with electric fields.
The way I always understood spherical coordinates is something like the below picture. To define a vector, you give it a distance outwards (r), and two angles to get a final position. Below... | For an electric field usually you have three components like $(E_x, E_y, E_z)$ in Cartesian coordinate system. Now you want to rewrite the same vector in a spherical coordination, what you should do is as follows:
first you write the vector like the electric field as $\mathbf{E}=|E|\mathbf{e_r}$ where $|E|$ is given by... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186388",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
True randomness? I am a physics high-school student so my knowledge is not very deep on the subject.
We started learnning about quantum mechanics and on some processes that my teacher described as random. I began to think on the concept of randomness and question it, thinking how can a process or an outcome be determin... | Bell proved that, if there exists such an unknown cause, then it surely must violate special relativity (Information must travel faster than the Speed of Light).
Taken from Wikipedia
"Realist interpretations of quantum mechanics are possible, although, such interpretations must reject either locality or counter-factual... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186477",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 3,
"answer_id": 1
} |
Obtaining a copy of Hawking's Ph.D thesis - Properties of Expanding Universes Due to its popularity, I am interested to know the 4 chapter titles and topics covered in S.W. Hawking Ph.D, Properties of Expanding Universes. I also ask this because that thesis is hardly available.
| Every thesis submitted for a PhD in Cambridge is archived at the Cambridge University Library. They should be able to get you a copy (for a fee).
See http://www.lib.cam.ac.uk/deptserv/manuscripts/dissertations.html
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186539",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 2,
"answer_id": 1
} |
What about a surface determines its color? Light falls on a surface. Some wavelengths get absorbed. The other are reflected. The reflected ones are the colors that we perceive to be of the surface.
What is the property that determines, what wavelengths are reflected and what are absorbed? Is it electronic configuration... | Paul G. Hewitt has a great non-mathy description of this in his book Conceptual Physics.
Strike a tuning fork and it vibrates at a characteristic frequency, its Natural Frequency. The tuning fork might put out other frequencies but they are dampened faster than the target frequency. Dampening sucks energy out of vibra... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186642",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 6,
"answer_id": 4
} |
Is there any tension in a massless spring that connects two free falling bodies in different horizontal planes? Two bodies A and B of same mass $m$ are attached with a massless spring and are hanging from a ceiling with a massless rope. They are in same vertical plane but not in same horizontal plane.
Now the string t... | The answer is that it depends on how your initial spring loaded mass is moving. But, the fascinating (but not too fascinating once you phrase it like this) part is that until the compression wave from the top interacts with bottom out on the slinky the dynamics of the bottom half won't change.
If we assume it was at r... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186736",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 1
} |
Why do momentum and position have the same direction in space in the Heisenberg Uncertainty Principle? It is known that $\Delta p \Delta x \geq \frac{h}{4\pi}$. I read that the two uncertainties must be along same axes. Why is that so?
| On the one hand, the uncertainty principle is in fact a postulate of quantum mechanics, so the question why it is postulated the way it is does not make a lot of sense. On the other hand, few thing can be said with respect to the question.
First, the exist a more general form of the uncertainty principle usually refere... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/186860",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What is the physical interpretation of second quantization? One way that second quantization is motivated in an introductory text (QFT, Schwartz) is:
*
*The general solution to a Lorentz-invariant field equation is an integral over plane waves (Fourier decomposition of the field).
*Each term of the plane wave satis... | In the statistical mechanics of the grand canonical ensemble, one needs to allow for
superpositions and mixtures of of states with different particle number. Thus one is
naturally led to considering the tensor product of the $N$-particle spaces with arbitrary $N$. It turns out (and is very relevant for nonequilibrium ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/187098",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "32",
"answer_count": 3,
"answer_id": 0
} |
On energy levels and emission of photons This is a very basic question but I cannot seem to find the answer anywhere.
Say we have an atom in ground state. Its first energy level is 2 eV. An incoming photon of energy 2.5 eV hits an electron in the atom (with the lowest energy level) which is excited and moves up one en... | The probability of a photon of energy $E$ — or corresponding wavelength $\lambda = h c / E$ — being absorbed by an atom and bringing an electron from level $i$ to level $j$ is given by the cross section $\phi_{ij}(\lambda)$, which is a sharply peaked function of wavelength (a so-called Lorentzian function). For instanc... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/187196",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Why do higher modes propagate more in the cladding of an optical fiber than lower modes? I am trying to understand the theory of inter-modal dispersion in optical fibers. It seems quite obvious that if higher modes have a greater angle of incidence in the fiber than lower modes, the path length of higher modes through ... | For optical fibers with cylinder geometry there are an orbital angular momentum (OAM) mode number $\ell\in\mathbb{Z}$, and a radial mode number $m\in\mathbb{N}$. The average radial position grows with $m$. A fixed OAM $\ell$ leads to a centrifugal term in the radial equation, which diminishes the number of radial modes... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/187285",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Are Fluted Barrels More Rigid Than Standard Ones? This seems to be quite a debated topic in the shooting community and is something I'm not sure about.
A fluted barrel is a barrel that has grooves milled into it to reduce weight, help it cool faster and supposedly to make it less flexible (see picture below)
I don't s... | A 'more rigid' barrel is actually a relative question. If two barrels start with the exact same profile and then one is fluted, and only the resistance to bending is measured, then the difference is extremely small. One barrel will be significantly lighter, but generally not more rigid since the rigidity is largely a f... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/187370",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Turning points of particle
A particle of mass $m$ and energy $E<0$ moves in a one-dimensional Morse potential:
$$V(x)=V_0(e^{-2ax}-2e^{-ax}),\qquad V_0,a>0,\qquad E>-V_0.$$
Determine the turning points of the movement and the period of the oscillation of the particle.
I have started learning for my exam and... | nephente's answer solves for the period. My answer is just made to make you see how to go on from the point you were, solving:
$$ e^{-2ax}-2e^{-ax}=\frac{E}{V_0} $$
We make the change $y=e^{-ax}$, which yields:
$$y^2-2y-\frac{E}{V_0} = 0$$
Second grade equation. Solve for $y$, and have in mind that $E/V_0$ is a negativ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/187622",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Force of an ideal spring Suppose you have an ideal spring (constant of the spring $k$) attached to a uniform disc of radius $R$ as in the picture below:
The force $F$ in red is from the spring.
My question is the following:
How should I decompose the force $F$ into its $x$ and $y$ components??
My intuition would tell ... | You have a correct answer, but I'm not sure if you have it for the right reason. Hooke's law states that
$$ \vec{F}_{\text{spring}} = -k\vec{r}$$
where $\mathbf{r}$ is the displacement of the spring from its equilibrium length (which you have not provided in the problem). If you take the equilibrium length to be 0, the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/187726",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
How can we be sure the Maxwell speed distribution equation is always a rational number? The Maxwell speed distribution equation is given as
$$f(v) = 4\pi \biggl(\frac{m}{2\pi kT}\biggr)^{3/2}\exp\biggl(-\frac{mv^2}{2kT}\biggr)v^2.$$
The left hand side gives the fraction of molecules and is a rational number. But the ri... | A few issues with this argument. First of all, you're clearly right that the RHS can yield an irrational/transcendental number. It would require insane mathematical coincidence for this not to be so. (It's really hard to have $e^x$ be algebraic, for instance.) And there's nothing in the formula about how many particles... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/187781",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why do our ears pop? Have you ever been on a train going through a tunnel or plane and your ears pop?I was wondering why this happens and I know it relates to pressure but don't know exactly the reason
| When a train rushes through a tunnel, it tries to push the air out of the way, but the narrow confines of the tunnel force the air to be compressed in front of the train, as though the train were a piston in an air compressor.
The compressed air tries to find extra volume wherever it can, and since the train does not m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/187905",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 1
} |
How can there be entropy change in this system? How can there be an entropy change in this system?
Suppose if I have a system consisting of liquid water, $1\, \mathrm{kg}$ at $290\,\mathrm{K}$, I stir it, and do say, $10\, \mathrm{J}$ of work on it, I can calculate the temperature change of the system given that:
$$U =... | There are two points here:
*
*Those equations above from $dU=\delta Q+dW$ are for gas (ideal gas), not liquid. You cannot use them to calculate the entropy of liquid.
*Work done in the first law of thermodynamics is defined as: $dW=pdV$ (quasistatic process), where p is generalised force (pressure) and V is general... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/188153",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 1
} |
Explanation of Michelson Interferometer Fringe Shift I have been working on an experiment where 2 glass microscope slides are pinched together at one end (so that there is a "wedge" of air between them) and placed in the path of a laser in one leg of a Michelson interferometer. When I move the glass slides (fractions o... | At a guess, the effect rises from the fact that your interferometer is not properly aligned. The presence of linear, rather than circular, fringes suggests that there is an angular misalignment. Then moving the wedge causes a lateral shift in the intersection point of the beam and the angled slide, which results in a s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/188247",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 4,
"answer_id": 1
} |
Force between two point charges moving parallel to each other When we observe two point charges moving parallel to each other we can see two forces acting on each of the charges:
*
*the Coulomb force
*the magnetic force ($\mathbf{F}=q\mathbf{v}×\mathbf{B}$) (similar to the force between two parallel wires with the ... | If the charges are moving parallel to each other, in the reference frame of of the one charge, the charges aren't moving. So the magnetic field is zero and the force is $\bar F=q \bar E $. For the charge to be still, the force must be zero, and thus the electric field also. But if the electric field can't be zero, then... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/188388",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Obtaining quantum Hamiltonian for charged particle from path integral formulation I was working on Shankar 8.6.4, which is about obtaining in one dimension the Hamiltonian operator of a charged particle from the path integral formulation.
First, I get the propagator over a time slice $\epsilon$, which is
$$
U(x,\epsilo... | As Tom pointed out, I made a mistake with assuming that the operator PA is -ih(∂A/∂x) when it should be -ih(∂A/∂x+A∂/∂x). This means that there is a (1-α) coefficient for the AP operator.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/188448",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Is this definition of orthohelium and parahelium incorrect?
"One electron is presumed to be in the ground state, the 1s state. An electron in an upper state can have spin antiparallel to the ground state electron ($S=0$, singlet state, parahelium) or parallel to the ground state electron ($S=1$, triplet state, orthohe... | With such a two-electron spin wave function, both electrons cannot be in the 1s state due to the electron parity. (The wave function should be antisymmetric relative to exchange of two particles.) When both electrons are in the 1s orbital, the only possible spin function reads
$$\frac{1}{\sqrt{2}} \left( |\uparrow \dow... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/188543",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 1
} |
What would put a harddisk drive (HDD) under 350G's of force? I always see the label and it says 350G's withstandable. What would put this over 350G's? Is it even possible to hit 350Gs of force to a hard drive?
| You are not the first person to ask this question.
https://superuser.com/questions/925826/what-would-put-a-hdd-under-350gs-of-force claims that 350 g of force is slightly more than a soccer player kicking a football. What this means is that you basically can kick your case, and it shouldn't brick your hard drive. It mi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/188673",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "27",
"answer_count": 4,
"answer_id": 0
} |
Relationship between phase velocity and group velocity using the de Broglie postulates If I have to show that the group velocity of a free particle is twice the phase velocity, is the following argument correct (avoiding to use the wave function and the momentum operator):
For a particle with energy $E$ and momentum $... | This is a perfectly correct derivation that uses the correspondence principle nicely: we can identify the group velocity with the classical velocity because a classical particle corresponds to a quantum particle whose wavefunction is a sharply peaked wavepacket, whose velocity is the group velocity.
If you want to do i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/188783",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Explosion in a sphere and the Gravitational field outside Take a hollow sphere and conduct a process on the inside, which transfers mass into kinetic energy (e.g. we let a big nuclear bomb detonate or something like that). For simplicity, assume that this will happen in a spherically symmetric way. How will the gravita... | According to Birkhoff's theorem, any spherically symmetric gravitational field should be produced by a mass at its center. Conversion of mass to kinetic energy inside a spherical shell should not affect the gravitational field outside the shell. Even if the interior of the shell becomes an intense hot group of photon... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/188869",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Are wavelength and the distance same thing? Can you clarify for me the following question: are wavelength and distance same?
I know wavelength is measured in terms of distance but when we have a look at the two equations:
$$
c=f\,\lambda\\
v=d/t
$$
it actually explains the same thing where $v=c$=velocity and $1/t$ i... | A wavelength is a particular distance, corresponding to the length travelled during a period, which is a special time. Since $v=d/t$ holds good for the distance $d$ travelled by a constant velocity object over any given time interval $t$, a fortiori this relationship holds for the special, particular time known as the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/189121",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 5,
"answer_id": 0
} |
Is net work and total work same? According to my text book
Total Work = Delta Kinetic Energy = KEf - KEi
But then work is defined to be
dot product of Force (vector) and Displacement (vector).
Also to my knowledge work is positional.
So, if we assume an object running in circle, and it completes one cycle,
Is it correc... | I am trying to go to a bit basic level. The formula work=Force*Displacement works only if the force is constant and not changing its direction or magnitude. When an object moves in circle,the force continuously changes its direction. So to calculate it we have to use integral of F with dl,assuming that force remains co... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/189190",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 3
} |
Why is probability of finding the electron at a certain point when one of the slits is closed $|\Psi|^2 $ & not $|\Psi|^2 dx$? Let in a given physical condition, the wave-function to a particle be assigned as $|\Psi (x_i,0,0,t)|^2 dx$.
Now, at the double-slit experiment , the probability of finding the particle at an... | Given the wavefunction $\psi(x)$, the probability to find any particle within an interval $[x_0,x_0+\Delta x]$ is
$$ P([x_0,x+\Delta x_0]) = \int_{x_0}^{x_0+\Delta x} \lvert\psi(x)\rvert^2\mathrm{d}x$$
i.e. $\lvert\psi(x)\rvert^2$ is not a probability, but a probability density that has to be integrated over a set of n... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/189468",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Typo in Griffiths' electrodynamics We are referring to the second equality in equation (9.24) in section 9.1.3 Boundary Conditions: Reflection and Transmission.
$$ \frac{\lambda_1}{\lambda_2} = \frac{k_2}{k_1} = \frac{v_1}{v_2}. \tag{9.24}$$
Shouldn't this be $ \frac{k_2}{k_1} = \frac{v_1}{v_2} \frac{\omega_2}{\omega_1... | This is not a typo. In this situation both waves must have the same frequency, $\omega_1=\omega_2$, so the extra factor is simply unity. The equality of both frequencies is due to the fact that, at the interface itself, the waves must remain in step for all time, which can only happen if they oscillate at the same freq... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/189591",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Difference between a reversible change and a reversible process? Question
In thermodynamics what is the difference between a reversible change and a reversible process?
Additional information
I am new to the topic of thermodynamics and getting confused about the difference, if any, between a reversible change and a re... | This is simply about words. A process can cause a change.
For example: A (reversible) adiabatic process can cause a (reversible) temperature change.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/189808",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
How is the Lagrangian defined in GR?
*
*Reading about the Schwarzschild metric in general relativity I see that sometimes $$L=g_{\mu\nu}\dot{x}^{\mu}\dot{x}^{\nu}$$ and sometimes $$L=\sqrt{g_{\mu\nu}\dot{x}^{\mu}\dot{x}^{\nu}}.$$
Which is the right way?
*Also how is the energy $E$ defined as $$E=-\frac{\partial{L}}... | The correct way is to define the reparametrization-invariant action
$$ S[X] = \int d\tau \sqrt{g_{\mu \nu} (X(\tau)) \cdot \frac{dX^{\mu}}{d\tau} \frac{dX^{\nu}}{d\tau} }. $$
Note that the choice of $\tau$ is arbitrary. The system has a large group of gauge symmetries - those are reparametrizations of the worldline (di... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/189949",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 1,
"answer_id": 0
} |
How and when are the relativistic corrections applied to GPS satellites? It is known that there is a need to correct the onboard clocks to reduce the time difference from 38μs to 50ns. Where is relativity playing its role here? Why cant the clocks be simply synchronised with the ground clocks through telecommunication?... |
Where is relativity playing its role here?
This part is well explained in Wikipedia: Effects of relativity on GPS
Why cant the clocks be simply synchronised with the ground clocks
through telecommunication?
They can, and in fact that's exactly part of what the GPS Control Segment does: measuring the satellite clo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/190155",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 1
} |
Quantum entanglement and the big bang Prior to the Big Bang all matter was compressed into a point of high density. Why isn't all matter already entangled?
| A particle can only be maximally entangled with exactly one other particle.
If it helps, you can think of being maximally entangled as having a perfect relationship between two particles rather than either particle having a perfect property in the slightest.
If you had a perfect spin up (in a particular direction) then... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/190274",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 0
} |
Can we find actual rest mass of things on Earth Earth moves around the Sun and the Sun moves around the galaxy and the galaxy moves with unknown speed and direction. We have speed so the mass of us all altered.
Can we know the real rest mass? If so, can we deduce our speed in the universe?
| We do have a rough idea of the relative speed and direction of our galaxy, with respect to the other galaxies around us, the so called local group.
In general relativity, which is our best theory of the universe to date, there is no such thing as absolute speed, as it depends on which frame of reference you use to meas... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/190558",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "16",
"answer_count": 5,
"answer_id": 1
} |
Find the force needed to accelerate body to a certain velocity for a certain time with respect to drag force So, the problem is straightworward when we suggest that air resistance force is constant:
$$ \vec F = \frac {\vec V_1 - \vec V_0} {t} m / b $$
$$ \vec V_0, \vec V_1 - \text {initial and final velocities respect... | If your object has mass $M$ and you want to accelerate it with acceleration $a$ to a specific end-velocity $v$ you have to keep in mind that the energy
$$e = \frac{M\cdot v^2}{2}$$
and also
$$e = F\cdot x$$
where $x$ is the distance over which the force $F$ which equals $M\cdot a$ is applied. Knowing that you can sol... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/190637",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Weather Meridians Are there meridians that effect cloud movement? While observing radar images of moisture bearing cloud movements (at my location in Florida), I've noticed a splitting of the cloud formations a majority of the time. It has become apparent and predictable.
| No, meridians won't affect cloud movement because there is no meridian which is special due to any geophysical feature. Latitudes may have an effect due to the 23.5$^o$ tilt of the Earth. The 23.5$^o$N and 23.5$^o$S latitudes mark the maximum latitudes for the Sun to be directly overhead, and the Equator marks the gre... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/190750",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Is it possible for man to break Earth into 2 parts? Many countries have extreme devastating nuclear weapons. Also they have weapons in very large numbers.
I want to ask that
Is it possible for man to break earth into 2 parts with the help powerful weapons like nuclear weapons or any other technology?
| No, because the vast majority of the planet has a molten interior and where it is not in the liquid phase it is held in solid phase by the internal pressure. You could maybe disperse it into space with a big enough bomb, but not actually break it into two parts.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/190844",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Why is the introduction of a quantization volume necessary for quantization of the EM field I have been working through the quantization of the electromagnetic field, and every source I find introduces a quantization volume with periodic boundary conditions in the process, in which we fit the general solution of $A(\bo... | One reason for the box is the Fourier expansion of field in stable macroscopic condition (thermal radiation, cavity oscillations) works well only for finite volume. For infinite volume, the Fourier integral of such stationary field is problematic, because the field function is not L2 integrable.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/190948",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
$\phi^4$-theory: nested two-loop contribution _8_ Wherever I see calculations of two-loop contributions to the $\phi^4$ propagator (such as Peskin, page 328, on the bottom), only the sunset diagram (aka the Saturn diagram) is considered, but not, say, the two-loop diagram involving a loop on top of a loop (looks like t... | As far as I know, I think you mean this diagram,
where,
And the divergent part of $I_1$ is $I_1^{\text{div}}=-\frac{m^2}{8\pi^2\epsilon}$, where $\epsilon$ is from $d=4-\epsilon$. At least we can see that $I_1^{\text{div}}\frac{\partial I_1^{\text{div}}}{\partial m^2}$ will have a divergent term like $1/\epsilon^2$. ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/191028",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
Advantages of a deadblow hammer? A deadblow hammer is a type of mallet with a hollow head filled with shot or sand. When you hit with it, say on an anvil, the mallet does not rebound, but just falls flat and heavy.
I don't understand the advantage of this from a physics point of view. Force is force, right? Normally w... | The hammer can be thought as a mean to deliver enough energy to the nail to deform the underlying material and let it penetrate deeper.
Ideally, all the energy the hammer gets from your arm as kinetic energy is transferred to the material and results in its (hopefully) permanent deformation, but as always, that's not ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/191324",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Cylindrical capacitor in an electric circuit I've come across a tricky question and would appreciate some hints or explanations as to why the given solution is the way it is. The question reads as follows:
A coaxial cable consists of a wire with radius $a$ (the core of the cable), which is wrapped with insulating mate... | I think the solution is incorrect.
If the current becomes zero after an infinite time, the potential of the rod and the wrapper would become equal ( because they are connected by a wire of $R$ resistance, and if $I=0$, the $V$ across the wire $=0$. )
Also, your diagram seems incorrect. You assume that there are diffe... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/191424",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
Does isotropy imply homogeneity? This question comes from exercise 27.1 in Gravitation by Misner, Thorne and Wheeler. They required the following:
Use elementary thought experiments to show that isotropy of the universe implies homogeneity.
I know homogeneity as the universe is the same everywhere at a given time, a... | This is closely related to the fact that in a Euclidean space, coordinate translations can be generated by performing two successive rotations around different points, as isotropy is essentially rotation invariance and homogeneity translation invariance. Suppose we have a rotation $R(\vec{r}_0)$ respect to $\vec{r}_0$ ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/191543",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 0
} |
Electric Motors: why do they draw more current when stalled, and less when moving? I'm familiar with how an electric motor works, and also familiar with what stall current and free current refers to in an electric motor. I'm also somehow familiar with the notions of energy being transformed from electromagnetic to kine... | When a motor moves it also acts as a generator and the current trough the windings is given by the difference of the external voltage and the induced voltage. When the motor stands still, though, the generated voltage is zero and the windings will draw the max. current they can based on their DC resistance.
In other w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/191628",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Coulomb collision I was reading an article by N. Bohr and came upon the following problem (the following wording is actually taken from a book by Thompson - Conduction of Electricity Through Gases):
Let $M_1, M_2$ be the masses of the corpuscles $A$ and $B$ respectively; we shall suppose the velocities of the collidin... | I believe b is the impact parameter. So b should be the perpendicular distance between the asymtote you drew and the fixed ion. The Wikipedia link has a picture that is pretty clear, although the picture in the link is illustrating Coulomb REPULSION, while your problem involves Coulomb attraction..
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/191749",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Are there limits for the speed of sound? A maximum or a minimum only? The speed of sound in materials of various states of matter differs a lot.
But does it have fundamental limits?
*
*Is there a maximal possible speed of sound?
*Is there a minimal possible speed of sound?
*Is the speed of sound in a material... | Fresh out of the oven: Physicists have discovered the ultimate speed limit of sound. Quote:
Sound is a wave that propagates by making neighbouring particles
interact with one another, so its speed depends on the density of a
material and how the atoms within it are bound together. Atoms can
only move so quickly, and t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/191817",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "16",
"answer_count": 5,
"answer_id": 4
} |
Is magnetic reconnection reconcilable with magnetic field lines neither starting nor ending? According to Maxwell's equations, magnetic fields are divergence-free: $\nabla \cdot \mathbf{B} = 0$. If I understand this correctly, this means that magnetic field lines do not start or end. How can we reconcile this with magn... | Magnetic reconnection comes from a cartoon picture of what magnetic field line motion may portray. This is not based on any physical law. Field lines are not real entity - just a means to display the lines of force when magnetic field is present in space. Field line motion is non-unique also, which is a fundamental fla... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/191942",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 3,
"answer_id": 2
} |
Force acting on a dipole placed in a non-uniform electric field For an electric dipole is placed in a non-uniform electric field, why does the net force act in the direction of increasing electric field?
| This is best understood by approximating the dipole as a pair of finite charges $\pm q$ separated by a finite distance $d$. In a uniform electric field, the electrostatic forces on each of the charges will cancel out exactly, but in a non-uniform one the forces on the two will be slightly different, leading to a slight... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/192120",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 3,
"answer_id": 0
} |
Is air infiltration a type of convective heat transfer (convection) I have a building / physics question...
A major source of heat loss in homes and buildings is infiltration through cracks (warm air from inside seeping out). Wondering if this falls in the category of convection as a mode of heat transfer?
| My guess, if the source of heat is in the centre of the room, is a mixture of radiation directly to the walls, plus convection of the air to the walls.
When the heat gets to the walls, it's conduction through the walls, unless there are holes, cracks or vents in the walls, when it's convection of air through the walls... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/192210",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Simple Harmonic Motion in Special Relativity I was trying to see what results I would get if I were to incorporate relativistic corrections into the case of a harmonic oscillator in one dimension. I thought that if the maximum velocity of the oscillating body were to approach relativistic velocities, the measurement of... |
I was trying to see what results I would get if I were to incorporate relativistic corrections into the case of a harmonic oscillator in one dimension. [...]
$ \gamma^3~m_0~\ddot{x} = -k x $
Since your question is specificly about harmonic motion, we might (instead) insist on this solution
$$x[~t~] := x_{\text{max}... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/192426",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Do electrostatic fields really obey "action at a distance"? In an electromagnetic theory class, my professor introduced the concept of "action at a distance in physics".
He said that:
If two charges are at some very large distance, and if any one of the charge moves, then the force associated with the charges changes ... | The force is not propagated instantly. It takes time for the information to get from one point to another.
You can treat that as an instant if you are working with small enough distances and velocities, but it's not. If you'll ever study field theory you'll meet retarded potentials that are just this: the field propaga... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/192527",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 4,
"answer_id": 0
} |
Do neutrinos refract? The most benign of interactions is refraction. While neutrinos rarely interact with matter in a sense like the photoelectric effect, does that mean that they don't refract either?
| Neutrinos are weakly interacting quantum mechanical point particles, with very small mass.
Refraction is a classical mechanics phenomenon, happens to waves traveling in a medium and it is a collective synergy of many photons impinging on the field of the atoms and molecules of the medium. Individual photons are not ref... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/192820",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 1,
"answer_id": 0
} |
Eigenstates of sum of creation and annihilation operators Does the operator $a+a^\dagger$ have eigenstates? If yes, what are they?
| No, it has not discrete spectrum (on $L^2(\mathbb{R}^d)$). In fact $a+a^*$ is proportional to the position operator (or the momentum one, depends on your definition of $a$ and $a^*$; by the usual one the position operator $x$ is proportional to the real part $a+a^*$ and the momentum $p$ to the imaginary part $\frac{1}{... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/192964",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Free fall into circular motion If I'm on a roller coaster free falling from height $h$ and then suddenly start going into horizontal motion with a radius $r$ of turn what is the $g$-force I experience?
I worked out the equation like this but am not sure if it is correct:
*
*(1) instant velocity of free-fall $v=\sqrt... | You're good.
Yeah, you can pretty much assume that it's a constant velocity, as long as $h \gg r.$ As I recall, the expressions involved are extremely simple as long as you don't try to figure out exactly what's happening in time: actually solving the Euler-Lagrange equations gives you some sort of $\int d\theta / \sqr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/193042",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 6,
"answer_id": 3
} |
What happens if the load on the electrical generator exceeds its generation power? And why? What happens if the load on the electrical generator exceeds its power generation? and why?
To be more precise, suppose we have a standard induction generator operating at frequency $\nu=50\:\mathrm{Hz}$ and voltage $V_0$, and r... | As the current drawn by the load increases the torque opposing the motion of the prime mover on its coils increase. This opposing torque is a result of the force acting on the coil since it's a current carrying conductor moving in a magnetic field. Hence its rpm reduces and so does its voltage output.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/193276",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 4,
"answer_id": 0
} |
Why are bandstructures plotted only along certain symmetry points? Why is it that bandstructures are usually represented along certain symmetry points ? What determines these symmetry points ?
| 3-D bandstructures are still very useful. (2 dimensions for x and y and 3rd for Energy).
For example, 3-D bandstructure plots allow us to see the Dirac cones that are found in the bandstructure of Graphene
Note that the x and y direction cover every single point for a given z (which in this case is kz = 0). So in this... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/193347",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
With a machete, why is a diagonal cut more effective than a right angle one? When cutting back some thick growth in the garden a question that always nagged me. Why is cutting diagonally seemingly more effective than cutting at right angles? Part of the answer is obviously to do with the ease of cutting vertically as o... | Simple answers:
When you cut diagonally, less material is being moved aside during a given bit of time. The time of the cut is longer, the force the same, therefore you are applying the same force to less material in a given moment. Therefore it's easier to cut the longer arc, even though the force is unchanged.
Al... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/193455",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Why are angles dimensionless and quantities such as length not? So my friend asked me why angles are dimensionless, to which I replied that it's because they can be expressed as the ratio of two quantities -- lengths.
Ok so far, so good.
Then came the question: "In that sense even length is a ratio. Of length of given... | Because length is relative, but angle is absolute.
(There is such a thing as a maximum angle against which you can compare, but not a maximum length.)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/193684",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "55",
"answer_count": 12,
"answer_id": 7
} |
Does body weight affect the speed when going downhill on a mountain bike? We know heavier objects fall faster when dropped at certain height. I was wondering if I am going downhill on my mountain bike without any peddling, will I travel faster or slower because I am fat?
| Heavier objects do not fall faster per se. But for heavy objects the influence of the air resistance will be smaller, if they have a similar surface area compared to the light objects.
The answer depends on the properties of your tyres and the road. But on an even road the air resistance will typically dominate once yo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/193839",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Do black holes violate the uncertainty principle? If black holes have mass but no size, does that imply zero uncertainty in position? If so, what does that imply for uncertainty in momentum?
| General relativity is a classical theory. The Heisenberg uncertainty principle does not apply to it.
The research frontier in physics now exists in quantizing gravity and unifying it with the other three forces (strong , weak, electromagnetic). Once that is done the solution for the black hole will become a probability... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/193954",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Minus sign in the time ordering operator The time ordering operator is usually defined as
$$\mathcal{T} \left\{A(\tau) B(\tau')\right\} := \begin{cases} A(\tau) B(\tau') & \text{if } \tau > \tau', \\ \pm B(\tau')A(\tau) & \text{if } \tau < \tau'. \end{cases}$$
The minus sign applies when $A$ and $B$ are fermion operato... | Perhaps the easiest way to see that there should be a Grassmann sign factor $(-1)^{|A| |B|}$ in the definition of time ordering
$$\tag{1} {\cal T} \left\{ A(t_A) B(t_B)\right\}
~:=~ \theta(t_A-t_B) A(t_A) B(t_B)
+ (-1)^{|A| |B|} \theta(t_B-t_A) B(t_B) A(t_A), \qquad $$
is to go to the classical limit $\hbar\to 0$. He... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/194073",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
Did the Sun form around a solid core? When Jupiter formed I assume like the other planets it started as tiny clumps of matter that eventually came together, became gravitationally bound and then eventually captured a lot of gas. I've also heard it was capable of collecting a lot of solid ice due to its distance from th... | Star formation isn't completely answered, but it is well believed that a solid core is not necessary. However if the sun did form around a planetary-sized solid core we would not know the difference. Due to the very high temperature of the sun, the result is not meaningfully different from colliding with planetary bodi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/194257",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
"answer_count": 5,
"answer_id": 0
} |
Where does particle borrow energy from to tunnel? Where does particle borrow energy from to tunnel? It is implied that particle can borrow energy and leaped over to the other side wherever that is, the shorter the gap the more energy it borrows my question is where does a particle borrow its energy from and what criter... | On a quantum level, particles don't really have "momentum". They're waves. The way the Schrodinger equation works, they move faster if they have a shorter wavelength. So we defined momentum based on the wavelength. Kinetic energy also is part of the whole conservation of energy thing, so we have a very good reason to d... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/194309",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 4
} |
Why must heat supplied in the definition of entropy be reversible? Can't it be irreversible after all it is a state function? The definition of entropy contains the term $Q_\text{rev}$ which means the heat supplied or taken out reversibly. I thought yes it can be after all only the initial & final states are important ... | Suppose you start with a system in some state $P_1, V_1, T_1$ and you add some quantity of heat $\Delta Q$ to it so the system changes to a different state $P_2, V_2, T_2$. The final state will depend on how you added the heat $\Delta Q$. Adding the heat $\Delta Q$ in a reversible process will result in different value... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/194478",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Is double-slit experiment dependent on rate at which electrons are fired at slit? I am a mathematician and I am studying string theory. For this purpose I studied quantum theory. After reading Feynman's book in which he described the double-slit experiment (Young's experiment) I was wondering if I send one electron per... | Yes, the electron is discribed not by a path, like a macroscopic object, but by a wavefunction. And if an undisturbed electron (we better say an undisturbed wavefuction associated with an electron) goes through the slit it, just like a normal wave, interferes with itself, producing an interference pattern that will bec... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/194570",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 5,
"answer_id": 3
} |
Where does the $(\ell + x)^2\dot\theta^2$ term come from in the Lagrangian of a spring pendulum? I am reading some notes about Lagrangian mechanics. I don't understand equation 6.9, which gives the Lagrangian for a spring pendulum (a massive particle on one end a spring).
$$T = \frac{1}{2}m\Bigl(\dot{x}^2 + (\ell + x)^... | Velocities in the kinetic part of Lagrangian
The variable $\;x\;$, that represents the displacement of the string from its position at rest, has been replaced by the variable $\;s\;$ in order not to be confused with the coordinate $\;x\;$ of a Cartesian system.
The velocity of the particle $\:\mathbf{v}\:$ is anal... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/194629",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 0
} |
Is Veritasium's "Shadow Illusion" caused by Image formation or Diffraction? I was watching this video on the YouTube channel Veritasium. In this episode, the host shows people paper containing holes of different shapes in the middle. So there is a paper which has a hole in the shape of a triangle, another one containin... | It's a pinhole camera image of the sun - as DJohnM's comment said.
My question is: Aren't 'lenses' required to converge the rays to make
an image? How can a hole in the centre of a cardboard form 'images'.
No - all that is required is an aperture (hole) to restrict the range of rays that reach the screen to form an... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/194709",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Why under red light the additive synthesis of primary colors gives white where one primary color is absent? Under White Light
Under Red Light
For instance, red & green under red light gives white.
| To add to WhatRoughBeast's answer, this is because you are not really viewing the image under pure red light. The "red light" you used to create the image has small amounts of green and blue wavelengths, causing the white areas to appear more "white" than the red areas, even though they are both just slightly different... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/195113",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Does anti-matter increase or decrease in entropy over time? Antimatter is matter going backwards through time. From the perspective of a matter-based observer does antimatter:
*
*Increase in entropy (and therefore decrease in entropy in its own time)
OR
*
*Decrease in entropy (and therefore increase in entropy ... | Antimatter increase in entropy over time. We can verify this with a thought experiment. Take ten positrons. Put five in one side of a chamber with a barrier and then the other 5 on the other side of the barrier in the same chamber. The chamber and barrier are also made of antimatter. The positrons repel each other and ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/195238",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 0
} |
The Makeup of the Pentaquark Why is it that when they have the artist's rendition of the Pentaquark it shows two downs, two ups, and one anti-strange quark? Is this or is this just for show? Follow up to this question: if this configuration is just for show, what is the Pentaquark truly made of?
| The recent high-energy experiments have found the true pentaquark model of the proton: four quarks and an anti-quark. In fact these quarks may be highly-energetic electrons. Proton is pentaquark.
https://www.researchgate.net/publication/340741231_The_Geometry_of_the_Proton_and_the_Tetryen_Shape
https://vixra.org/abs/17... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/195310",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 4,
"answer_id": 3
} |
Is the event horizon of black holes visibly sharp, or blurry? As you come in closer to a black hole, how do you see the event horizon? Is it always like a clear-cut surface? Or it only looks clear-cut from a distance, but as you come closer to the black hole, you start seeing it's a blurry layer, and everything around ... | Calculating what you would see as you fell into a black hole is straightforward but tedious. Fortunately there are lots of sites that have done this for you. Actually, if you've been to the cinema recently the film Interstellar does a pretty good job of it.
Less spectacularly, have a look at this site that has videos o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/195427",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
"answer_id": 1
} |
Engine fuel consumption vs power Something that has bothered me for a long time is why a 600 hp engine uses more fuel per kilometer than a 80 hp engine. Let pretend I have two equal cars (same shape same weight etc) except for the engine, one is 600 hp and other is 100 hp. If I manage the throttle to accelerate both wi... | It’s all about engine's efficiency. According to wikipedia:
gasoline engine's efficiency = 1/(BSFC × 0.0122225)
(Also Actual efficiency can be lower or higher than the engine’s average due to varying operating conditions.)
To calculate BSFC (Brake specific fuel consumption) use the formula: BSFC=r/P (where: r is the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/195479",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 5,
"answer_id": 4
} |
Force division of moving pulleys? I am a second grade at a middle school and I was reading a physics workbook to prepare for a test. And I was solving pulley problems and one problem made my brain stop.
The problem asked me what would the minimum force of F would be when the weight of the pulleys were 30N. I checked t... | So within a cord/string there is a property called tension which is a measure of the force exerted along the string. If the string stretches homogeneously (the same at all parts of the string) then it turns out that this tension is the same at all parts of the string: you pull with force $m$ Newtons, then everywhere y... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/195566",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Is Gauss' law valid for time-dependent electric fields? The Maxwell's equation $\boldsymbol{\nabla}\cdot \textbf{E}(\textbf{r})=\frac{\rho(\textbf{r})}{\epsilon_0}$ is derived from the Gauss law in electrostatics (which is in turn derived from Coulomb's law). Therefore, $\textbf{E}$ must be an electrostatic field i.e.,... | You need to watch what you mean by the ambiguous term "derive", which can mean either "was derived historically" (i.e. was motivated by or is a derivative of, in the non-mathematical sense) or "is derived logically/mathematically".
Historically, I think you are correct that $\boldsymbol{\nabla}\cdot \textbf{E}(\textbf{... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/195842",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 5,
"answer_id": 0
} |
Why do semiconductors remain neutral outside the depletion region? Why there is a sharp cut off of the charged region outside the depletion region, like on this image?
For example why don't electrons on the conduction band in the n-type side rush towards the positively charged area making the whole piece positively cha... | From your response to @JonCuster:
The question is. when the pieces are in equilibrium why don't the remaining charge carriers distribute themselves uniformly in each half.
The remaining charge carriers within the depletion region are not electrons but semiconductor ions (both positive and negative) locked into their ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/195928",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 3
} |
Where does the equation $p=\frac{1}{c}\sqrt{T^2 +2mTc^2}$ come from? Where does the relativistic formula
$$p~=~\frac{1}{c}\sqrt{T^2 +2mTc^2}$$
come from? What is the derivation from Einstein's formula? $T$ is the kinetic energy $m$ is the mass $p$ is the momentum.
| Hint: $T = E - E_0 = m\gamma c^2 - mc^2 = mc^2(\gamma -1)$ and $p = |\vec p| = m\gamma |\vec v| = m\gamma v$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/196030",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.