Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
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What is the relationship between entropy and work? Can someone explain the relationship between entropy and work? I've been reading my textbook and looking online but I feel like I'm missing something. Can someone explain it in layman's terms :)
| Let's frame this question in terms of a heat engine (Carnot engine).
Here is a diagram I made for a class when teaching this stuff.
Heat flow $\dot{Q}$ has an associated entropy flow $\dot{Q}/T$. The job of a thermodynamic engine is extract/filter as much useful work as possible from a flow of energy and entropy.
To a... | {
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Where extra stored energy of Capacitor go ? Suppose We have a capacitor with capacitance $\ C $ and charge $\ Q $ . So total stored energy is $$ E=\frac{Q^2}{2C} $$ Now if I connect a capacitor with same capacitance parallel with it then current will flow until voltage across both capacitor become same and this case ... | Connecting two perfect capacitors like that would be like connecting two perfect but different voltage sources; you would get a hypothetical explosion.
In real life, every capacitor has inductance and resistance. So, as the current built up between the two capacitors, you'd heat up the wire between the capacitors as we... | {
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What does the g mean after the isotope given? I'm familiar with notation such as Sc-44m standing for the meta stable state of Sc-44.
What does Sc-44g mean? There are a few examples of this notation; here's one:
http://iopscience.iop.org/0031-9155/60/17/6847/pdf/0031-9155_60_17_6847.pdf
Many thanks
| The answer is already on page 2 of your link above:
"Among the large number of radionuclides of medical interest, Sc-44 is
promising for PET imaging. Either the ground-state Sc-44g or the metastable-state Sc-44m can be used for such applications, depending on the moleculeused as vector."
So the metastable state Sc-44m ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Why don't lakes have tides? There's a tidal effect that we can clearly observe in oceans, which is the effect of gravity from the Sun and the Moon. If gravity affects everything equally, why don't lakes have tides?
| You probably got voted down cause this can easily be google searched, but the simplest way to explain it is that a tide happens because the lunar tug on one side of the ocean is measurably more than on the other side of the ocean and as the earth rotates the tidal "bump" follows the moon so you get 2 high tides and 2 l... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/202788",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
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Books on waves with Fourier Transforms There are many waves and oscillations books out there that also include Fourier analysis but very few give the subject a thorough treatment, they just pass it in a few pages. If anybody has any sources(particularly books) that have Fourier analysis and particularly Fourier Transfo... | The FourierTransform.com is a website maintained by an enthusiast. The site is not peer-reviewed, but it looks as though it might provide helpful explanations.
Here's a link which provides some basic introduction to the Fourier transform. And here is another link to class notes provided by Prof. Carlton M. Caves for a... | {
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Why is a relativistic quantum theory of a finite number of particles impossible? In Dyson's book Advanced Quantum Mechanics , he said "These two examples (the discovery of antimatter and meson) are special cases of the general principle, which is the basic success of the relativistic quantum theory, that A Relativistic... | Because a pair of particle and anti-particle can be created from the vacuum, it means that infinite number of pairs of particle and anti-particle can be created from the vacuum. So when you consider relativistic quantum theory it's impossible to only consider a finite number of particles.
When you calculate Feynman dia... | {
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What do elements of CKM matrix imply? In CKM matrix, there are 9 elements, e.g. Vud = .974, Vus = .227 ,Vub = .004. The sum of these 3 elements is greater than one, so they cannot represent the probability of an up quark to transform in an interaction/decay into down, strange, bottom quark respectively by emitting W+ b... | https://en.wikipedia.org/wiki/Cabibbo%E2%80%93Kobayashi%E2%80%93Maskawa_matrix
$$ \begin{bmatrix} d^\prime \\ s^\prime \\ b^\prime \end{bmatrix} = \begin{bmatrix} V_{ud} & V_{us} & V_{ub} \\ V_{cd} & V_{cs} & V_{cb} \\ V_{td} & V_{ts} & V_{tb} \end{bmatrix} \begin{bmatrix} d \\ s \\ b \end{bmatrix} $$
The d,s,b quarks... | {
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Rockets and distance I am trying to create an equation which allows for me to change the aspects of the rocket so i can calculate the distance traveled vertically. My idea is for a rocket that only moves vertically; with this i can calculate the amount of time it would take to make it past the first Lagrange point. ... | You are assuming constant thrust $T$ during flight, presumably until the rocket runs out of propellant.
You are also assuming (or neglecting) any air drag to be zero. The resulting drag force could be very significant at high speeds and assuming your rocket is launched from the Earth's surface.
With those limitations i... | {
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What's the difference between the work function and ionisation energy? In a particular textbook, the work function of a metal (in the context of the photoelectric effect) is defined as:
the minimum amount of energy necessary to remove a free electron from the surface of the metal
This sounds similar to ionisation ene... | Ionization energy is the energy required to remove the outermost shell electrons of an isolated atom of an element (gaseous phase). We define ionization energy when we have a single atom.
On the other hand, the work function is the energy required to remove the outermost shell electrons of a metal's surface atoms when ... | {
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When does the concept of electric field in classical electrodynamics fail, and QED is needed? It is really hard to find reference to when the traditional concept of electric wave, especially TEM wave, fails, and needs to be replaced by quantum electrodynamics.
So when does the concept fail? At high frequencies of elec... | The failure of classical electrodynamics has to occur when we are talking about very low intensity light (such the wave is describing one photon) or when the frequency of the classical waves is very high (because a photon can decay to an electron-positron pair but an EM wave cannot).
| {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
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Rectangular potential barrier Take the usual rectangular potential barrier, that is:
$$V(x)=0 \: \text{if} \: x<0 \: \text{or}\: \: x>a$$
$$V(x)=V_0 \: \text{if} \: 0\leq x \leq a.$$
I've looked at several notes and books and in everyone of them the books supposes, in the third region ($x\geq a$), a solution to the tim... | To put it another way: typically with this sort of setup we want there to be an incoming wave from the left, $\psi_0 = e^{-i \omega t + i k x},$ which is partially "reflected" into an outgoing wave to the left $\psi_\ell = r e^{- i \omega t - i k x},$ and partly "transmitted" to an outgoing wave to the right, $\psi_r =... | {
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How to Vary the wavelength of UV CFL? I have a $12$ $V$ $DC$ operated UV $[CFL]$(http://en.wikipedia.org/wiki/Compact_fluorescent_lamp) with $365$ $nm$ wavelength. I need to vary this wavelength in the $250-300-350-400-450-500$ $nm$. Please guide me regarding how to proceed with it.
| Mount the lamp on the outside of a really fast spaceship.
If you want a longer wavelength, fire up the engines of the spaceship and get it to travel away from you, so that the light is red-shifted.
If you want a shorter wavelength, do the same but with the spaceship travelling towards you, so that the light is blue-shi... | {
"language": "en",
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Help me understand Gauss law Suppose I have the following, the gaussian surface is the drawing in the middle. So charge enclosed is zero, and then eletric field must be zero since the area of the gaussian surface is not zero. But, clearly the eletric field is not zero in the middle, because if you put a charge there it... | If the charge between the two surfaces is $0$ (zero) initially, that makes ${q}/{\epsilon} = 0$ and Gauss Law states that the Electric Flux is directly proportional to charge inside it, i.e.,
$\phi = \dfrac{q}{\epsilon} = \displaystyle\oint\vec{E}.\vec{dA}$ ,
Here $\vec{E}.\vec{dA}$ represents the dot product of the di... | {
"language": "en",
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Question Regarding torricelli's theorem/Law I recently studied about bernoulli's equation/principle.
After the derivation of the said equation , my book gave some applications of the principle, which include torricelli's theorem/law.
In deriving torricelli's law from bernoulli's principle, the pressure at the opening ... | I suppose that the best way to answer this question is by using an analogy. Take a glass of water(at room temperature) and place it in a refrigerator. What happens to the water? It cools down. Now you take this glass of cold water and keep it back outside. What happens now? The temperature of the water comes back to th... | {
"language": "en",
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Where is the event horizon in a black hole? At the beginning I thought that the event horizon coincides with the surfaces, but then making a new name when you could just call it surface would seem a bit pointless.
Then where is the event horizon? Is it inside or outside the black hole?
Notice that I have a really basi... | An event horizon is a "sphere-shaped surface of influence" of inescapable gravity from/towards the black hole. It is the point where the object (victim?) cannot overcome the gravity of the black hole, and will be sucked into it. This is also the point where, theoretically, you would vanish to an outside observer, since... | {
"language": "en",
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Stone dropped from a moving train
*
*This may look like a stupid question, but it is really getting to me. Imagine a train moving with an acceleration $a$, and a person drops a stone from the window. To an observer on the ground, the stone follows a parabolic path, as it is a projectile with initial velocity the same... | The path of the stone for the observer on the train should be a straight line as, when releasing the stone, it had the velocity same as train. However, air resistance may affect the stationary path. For the observer on ground it should be parabolic due to the action of gravity on the moving stone. The net acceleration ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/206033",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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When I take a Gaussian surface inside an insulating solid sphere, why does the outer volume have no effect on the electric field? Say I try to find the magnitude of the electric field at any point within an insulating solid sphere. I know that in the case of a conductor, the electric field within it is 0. However, I ha... | Who says the outside field doesn't effect.The Gauss's law gives the net field due to entire charges inside or outside the Gaussian surface only the charge taken is what inside.
| {
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"timestamp": "2023-03-29T00:00:00",
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What is the wavefunction of the Young Double Slit experiment? I have never seen the wavefunction for this experiment and would like to know how to derive it using the Schrodinger equation. I specifically want to see how the electron wave function leaves the source, then goes through the slits, and produces the characte... | Well, there are many things you could do. You could:
*
*consider two gaussian beams (the linked article is for electrodynamics)
*apply some paraxial approximation (which would be more appropriate to treat electrons with a high forward momentum)
*do a cheap/cheesy symmetric point source approximation using Green's ... | {
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Fluid speed and fluid density How does fluid density affect fluid speed?
Basically I am trying to figure out if, with all other quantities remaining constant, would an increase in fluid density cause the fluid speed to increase/decrease?
For example, would water and honey have different fluid speeds in a pipe, because ... | Continuity equation for $\textit{steady}$ flow, in which properties remain uniform over cross-section, is $\rho A v=$constant. If area remains constant along the flow then $v ~\alpha~ \frac{1}{\rho}$. For unsteady flow the statement is more complicated (read up compressible flows).
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/206867",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Coffee Straw Physics When I put my little, cylindrical coffee straw into my coffee, the liquid immediately rises about half a centimeter up the straw without provocation. This is also the amount of coffee that the surface tension of the coffee will allow to stay in the straw when removed from the liquid in the cup.
Kee... | The liquid rises due to surface tension. In this case the adhesion between liquid and cup material Is higher than cohesion between liquid molecules. So it is higher than liquid in cup.
I think the liquid in the straw that remains after removing will be lesser than the liquid that you saw rising while in the cup. Try fo... | {
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Why don't non-Hermitian operators with all real-eigenvalues correspond to observables? Suppose you could construct an operator that was non-Hermitian but had all real eigenvalues or could at least be restricted in a way to create only real eigenvalues, why would this operator not correspond to an observable quantity?
| 1) If all the eigenvalues of an operator are real, then it is Hermitian. You can see this by writing the operator (call it A) in the eigenvector basis. Then A has all real eigenvalues along its diagonal and zeros everywhere else. Therefore, $A^\dagger = A$ which means it is Hermitian.
2) Many of the operators that w... | {
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Physical interpretation of the creation operators in string theory? Is there any way to describe phsycially which each creation operator $a^{(i)+}_{n}$ in string theory does to the ground state string?
Here would be my guess (although it is likely to be totally wrong):
You can consider the ground state string as a str... | The states of string theory are quantum states. They represent a "vibration" of the string in the same sense that a particle in standard QFT represents a "vibration" of the quantum field.
That is, they do not represent actual "physical" vibration at all. In particular, the states do not describe actual physical positio... | {
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How big are clouds? How big are clouds? When I look up into the sky I have no frame of reference, so I don't know if they are 200 feet or 2 miles across. When I am in a plane looking out at a cloud, I try to use the wing as reference but I still don't have a good reference point, because the clouds are just a large whi... |
I don't know if they are 200 feet or 2 miles across
Clouds are fractal. Cloud particles can be a few dozens of micrometres, and big tropical cyclones can be thousands of kms across. That's a range of more than 10 orders of magnitude! That's why clouds are a pain to represent in models — it is simply impossible to ... | {
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Velocity of an Electron as it Passes through a Uniformly Charged Ring I've been presented with a problem in which an electron is placed a certain distance x from the center of a positively charged ring and allowed to move freely. The ring has a known charge density λ. I am tasked with finding the velocity of the electr... | The first thing is that $U_e = q_e * V$. Then, electrostatic potential $V= \int {E.dr}$. For a ring, $E= \frac {kqx}{(x^2 + R^2)^{3/2}}$. The energy conservation otherwise is spot on.
| {
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Do relativistic events need to match if accounted for time dilation and length contraction? To explain the question let me give you a short example.
In the scenario there are two references frames A and B.
A consists of a x'=1 Ls (lightsecond) long pole in the positive x direction. At t=0 a flash is generated at its or... | I think that your calculation is correct in that it would take $3.73$ seconds for the light pulse to reach the end of the pole according to B's perspective. However, that number is not the time dilation factor. For $v=0.866$, the factor should be equal to about $2 (= \frac{1}{\sqrt{1-(v/c)^2}})$.
So where did your rea... | {
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What is the relation between energy levels of hydrogen atom in Bohr's solution to that of Dirac solution In Dirac solution for hydrogen atom, the energy levels are calculated as positive
\begin{equation}
E=\frac{mc^{2}}{R(t)\sqrt{1+\frac{z^{2}\alpha^{2}}{\left(n+\sqrt{\left(j+\frac{1}{2}\right)^{2}-z^{2}\alpha^{2}}\ri... | I have found the answer myself here
Energy in Dirac model $E_d$ is related to energy in Bohr's model $E_b$ as
$E_b \approx E_d - m_ec^2$
where $m_e$ is mass of electron and $c$ is speed of light. The answer above is not useful.
| {
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Is there a rigorous definition of 'much greater than'? I have encountered $\gg$ in many physics text books where it's used as a relation between constants or functions but in none of the text books I have read is it properly defined anywhere.
If $A \gg B$ where $A$ and $B$ are constants, or $f(x) \gg g(x)$ does this s... | It is a symbol and an idea used in mathematics too. But the important part is just that $B$ is 'ignorable' relative to $A$. This depends on the level of precision that is being used experimentally. If you're working to a precision of 1 part in 100, then $B$ should not effect the answer to that level of precision. I... | {
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How can the unstable particles of the standard model be considered particles in their own right if they immediately decay into stable particles? How can the unstable particles of the standard model be considered particles in their own right if they immediately decay into stable particles?
It would appear to a layman su... | You're question is interesting because it is connected to the notion of elementary particle. As mentioned by anna v, the elementary particles (fermions) of the standard model have very specific properties under the symmetry of the standard model ($SU(2)_L\times U(1)_Y \times SU(3)_c$): they lie in the fundamental repre... | {
"language": "en",
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Are white noises always Markovian? Are white noises always Markovian? I am a bit confused about it. As white noise always has a constant power spectrum， its auto correlation function must contain a delta function of time. Thus the correlation time of the noise vanishes. But I don't know whether they can be called Marko... | Mathematically, the answer to your question is yes. The dynamics of a physical system that is driven by pure white noise, with constant power spectrum up to arbitrary high frequencies, will be perfectly Markovian.
However, as CuriousOne points out, it is essentially impossible to verify that a physical noise process i... | {
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Why can't be the EPR experiment simplified? Alice measures the spin of her electron on the x axis. She now knows the spin value of Bob's electron on the x axis at time T0. Bob measures the spin of his electron on the z axis. He now knows the spin value of Alice's electron on the z axis at time T0.
The two meet up and s... | Under your assumption of simultaneously well-defined x and z value, you reach predictions which are inconsistent with quantum theory. This is exactly what leads to Bell's inequality which is (experimentally!) violated by quantum theory, see https://en.wikipedia.org/wiki/Bell%27s_theorem or the explanation in Preskill's... | {
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Models for populations of decay products I'm looking to create a population model for the specific nuclides in a neutron spallation source. The source is a target (Tantalum clad Tungsten) which is being bombarded with protons, and in turn is producing neutrons. The change in a specific population of a Nuclide is going ... | These sorts of calculations are part of the design process for spallation sources and are done quite carefully. The tool of choice is MCNP, which has been extensively benchmarked over many decades. MCNP is maintained by folks at LANL, which happens to have a tantalum-clad tungsten spallation target; if you are on-sit... | {
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"url": "https://physics.stackexchange.com/questions/208832",
"timestamp": "2023-03-29T00:00:00",
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Does $v^2=v_0^2-2gh$ work if the positive axis is up and the initial velocity is down? In the situation where the positive axis is up, the acceleration due to gravity is $g$, and the velocity is represented by the equation $v^2=v_0^2 - 2gh$. This works great if the initial velocity is upwards. But say, what if the init... | You are looking at a specific application of a more general formula
$$v_q^2-v_{oq}^2=2a_q(q-q_o),$$
where
*
*$q$ is the coordinate direction,
*the $v$ terms are velocity components along the $q$ axis,
*$a_q$ is the constant acceleration component along the $q$ axis,
*and $q$ and $q_o$ are the positions along ... | {
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Derivation of $E=h\nu$? I found this question here and the answers got me thinking.
Take a case where $\Psi(x,t)$ is the linear combination of two eigenvectors of a charged particle:
$\Psi(x,t)=c_1\psi_1e^{-iE_1t/\hbar}+c_2\psi_2e^{-iE_2t/\hbar}$.
At $t=0$ the wave function is:
$\Psi(x,0)=c_1\psi_1+c_2\psi_2$.
The prob... | As pointed out in zeldredge's answer, what is missing is the connection to the photon. However, I do think you can fix this problem. If you couple a system that exhibits natural oscillations at certain frequencies to an external force that oscillates at some frequency $\omega$, then you get a resonance when $\omega$ ma... | {
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What will I see if launch a thing into a black hole? Suppose that I launch a thing into a black hole from a secure distance, this black hole is secure at 2 meters and is floating over my yard, doesn't matter.
What will I see?
Will I see that the thing increases their speed and falls quickly into the hole?
Or will I see... | You'll see the object at first accelerate towards the hole (under gravity) and then slow more and more as it approaches the event horizon. It will asympotically freeze in place at the event horizon and then gradually shift redder and redder until it disappears.
This is assuming that the black hole is big enough that t... | {
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Sound when traveling faster than sound I was wondering, if I am running at the speed of sound while playing music on my iPod will I be able to listen to my iPod while running at the speed of sound? or we cant hear anything while running at the speed of sound.
| Well..
1) If you were running at the speed of sound, you probably wouldn't be for long. The human body isn't designed to handle those kinds of stresses.
2) Assuming you're listening to the iPod using ear buds (in your ear) You can probably think of the air between the seal on the ear bud and your ear drum as isolated... | {
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Does an ion thrust engine consume more energy as it speeds up? This question goes to a very basic non-understanding of mine that I have had in the back of my mind for ages - I just read the following here:
ion thrusters are capable of propelling a spacecraft up to 90,000
meters per second (over 200,000 miles per ho... | That page is not well written.
The 90km/s speed is the exhaust velocity of the engine.
It is not the maximum speed of the spacecraft.
There is no maximum speed of the spacecraft, short of the speed of light.
They make the mistake again when they say:
"While a chemical rocket's top speed is limited by the thermal
capabi... | {
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Definition of a line charge with Dirac delta function Is the following statement correct for a line charge distribution $λ(x)$?
$$ρ(\mathbf r)=λ(x)δ(y)δ(z)$$
If yes - what does it say?
| The factor δ(y) indicates that the charge distribtution is non-zero only for y=0, i.e. on the zx plane; likewise, δ(z) that it is non-zero only on the xy plane. Thefore, the product δ(y)δ(x) Indiactes that the charge distribution in non-zero on the intersection of zx and xy planes, i.e. the x-axis. Then, the function λ... | {
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Is it possible to mix a drink with a non-standard phase of ice? Would it be possible to safely cool and drink a glass of water with anything else than the Ih form of ice?
Here and here you can see that some alternative forms of ice have a higher density than water, hence they would sink.
Would it be possible to have th... | As you can see from the phase diagram plot in the first link you provided, the only other ice phase which is stable at atmospheric pressure is ice XI, and its density is about the same as that of the most familiar ice phase (ice Ih). The other denser ice phases that you see on the phase diagram are only stable at press... | {
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What has the potential energy: the spring or the body on the spring? Particles have gravitational potential energy due to its position in the gravitational field. We say the particle has potential energy and not the Earth (the body doing the work). Why is it not the same with a spring doing work on a body?
It is my und... | Potential energy is just energy stored in a static state -without motion. So a spring can have potential energy, and so can a body attached to the spring that's in a gravitational field. So for this type of system (undamped harmonic oscillator in a gravitational field) potential energy is not strictly defined for the s... | {
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Why isn't the acceleration at the top point of a ball’s journey zero? When I shoot a ball vertically upward, its velocity is decreasing since there is a downward acceleration of about $9.8\,\mathrm{ms}^{-2}$.
I have read that at the top most point, when $v = 0$, the acceleration is still $9.8\,\mathrm{ms}^{-2}$ in th... | At the topmost point, the velocity vector is a null vector whereas the acceleration vector has constant magnitude $-9.8\,\mathrm{m/s^2}$ and constant direction downwards i.e. towards the centre of earth.
| {
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Can tidal forces significantly alter the orbits of satellites? I would assume that there are other larger, more significant, forces acting on artificial satellites, but can tidal forces drastically alter the orbit of a satellite over time?
I was thinking this could especially be an issue for a satellite in geostationa... | Yes, of course the tidal forces affect the orbits. In the case of the Earth/moon
system, Earth's day used to be 18 hours, and when the tidal slowing got
to our current 24-hour solar day(23 hours fifty-odd minutes sidereal)
the angular momentum went into the Lunar orbit, and the moon is more distant
nowadays.
That 18-... | {
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Conservation of momentum in a baseball Conservation of momentum: A thought experiment.
A baseball is placed on top of a baseball holder, the kind used to train young batters.
A batter hits the stationary ball perfectly horizontal, sending it flying through the air in a relatively straight line, during which time it wil... | Provided the bat delivers exactly horizontal momentum impulsively to the second ball, it will not travel as far due to its initial downward velocity, as you say. Dissipating the downward momentum doesn't make much sense in the scenario you described.
| {
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How is normal force distributed along the surface of contact? Help me settle this argument.
A mass $m$ is placed on a thin diving board.
The base of the diving board has mass $M>>m$.
Does the board tip over?
I drew the following FBD and concluded there is no net torque.
My friend thinks the normal force $N$ will be ap... | I think that you're making this problem more complicated than it has to be in order to simply determine if the assembly will tip over or not. You don't really need the spatial distribution of the forces being exerted by the table or ground on the assembly. All you need to note is that if the pivot point is at x=D1 then... | {
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Power and frequency of photons and its relationship with biological safety I understand that x-rays are more dangerous than radio waves because they are of higher energy, since they have higher frequency. However, it’s less dangerous to stand near a radio station with a higher power output than to be near an x-ray mach... | There are two types of energy involved, and the blurring of this distinction is cause of a huge number of misunderstandings.
Light comes in discrete packets called photons. The energy of each photon is proportional to the frequency of the light. On top of that, a light beam can have any number of photons in it, and thi... | {
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Central force law An object has an orbit in polar coordinates as $r(\theta) = a\theta^2$ (where $a$ is constant).
Assuming the central force is directed towards the origin $r=0$, how can I know which central force law lead to such an orbit? And how to find $r$ and $\theta$ as function of time?
| Perhaps I can help.
For any object in orbit, the Earth exerts a force on the object and the object exerts a force on the Earth.
So we know from $F=ma$ that:
$$∑ F_θ=ma_θ$$
Using polar coordinates {r,θ}, this equation becomes:
$$ 0=2\left(\frac{dr}{dt}\right)\cdot \left(\frac{dθ}{dt}\right)+r\left(\frac{d^2θ}{dt^2}\righ... | {
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Where does the force appear when considering object interactions in another reference frame? Imagine I am sitting on an asteroid with my buddy and drinking a beer. When the bottles are empty we throw them simultaneously in opposite directions perpendicular to the asteroid's movement. What will happen?
From the logical ... | The technical answer is that the force "comes" from your buddy and you, who spend (chemical) energy to throw the bottles.
If we consider an idealised scenario, though, where a system of three glued points breaks suddenly down, then I will argue that there is actually no change in the momentum => no need of force:
1) Be... | {
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How does thin film interference work? So thin film interference is when light is half-reflected half-refracted then the reflected and refracted wave interfere with each other to produce another color.
What I don't understand is that the waves are off-sync; not on top of each other. So why do they interfere? And doesn't... | Everything is explained in wikipedia at "thin film interference". What do you mean by "off-sync" ? In classical images like the wikipedia one (see below) only one ray is drawn, but in practice there are an infinity of parallele rays, so superimposition do occurs.
But if by "off-sync" you mean there is a phase differen... | {
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Can the mass of an object be changed by adding opposing magnetic fields? Apparently it does. Or is this voodoo physics. If not what is really happening in this case?
https://www.youtube.com/watch?v=8N2TS3VReTA Boyd Bushman Changed the mass of a black box object, rock. He took two black box rocks. One rock he added two ... | It's obviously wrong: mass don't change. Now the effects of mass might be tilted by some other forces.
Moreover, the speed of free fall is not related to mass as well (at 1st order).
| {
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Feynman propagator for arbitrary values of the gauge parameter $\zeta$
For the choice $\zeta = 1$ the Lagrangian can be brought into a particularly simple form upon integration by parts in the action integral. Equation$$\mathcal{L}' = -{1\over4}F_{\mu\nu}F^{\mu\nu} - {1\over2}\zeta(\partial_\sigma A^\sigma)^2$$with $\... | \begin{align}
\mathcal{L}&= -\frac{1}{4} F^{\mu\nu}F_{\mu\nu} -\frac{\zeta}{2}\left( \partial\cdot A\right)^2 \\
&=-\frac{1}{2} \partial _\mu A_\nu\partial^\mu A^\nu +\frac{1}{2} \partial _\mu A_\nu\partial^\nu A^\mu -\frac{\zeta}{2}\left( \partial\cdot A\right)^2 \\
&=-\frac{1}{2} \partial _\mu A_\nu\partial^\mu A^\nu... | {
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Physics:Buoyant force and scale readings Sorry this might seem like a dumb question, but I'm having trouble understanding the concept behind buoyant force and scale readings.
Suppose I have a beaker filled with water, and the beaker is placed on a measuring scale.
a) If I then place a ball into the beaker, the ball fl... | Buoyant force has no effect on the interaction between the beaker and the scales it stands on.
The scales will register a weight which is the sum of the weights of the beaker, water and ball.
Consider that if the ball is at least partially immersed so that some part is below the level of water, it has displaced some w... | {
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Have David Wolpert's findings really “slammed the door” on scientific determinism? I recently read an article describing how mathematician/physicist David Wolpert's research closed the door on scientific determinism. I have huge doubts about the implied conclusion, considering the fact that a result like this would hav... |
any input on whether or not this result actually demonstrates that the world can't be deterministic,
The universe can be deterministic. Full stop. And there can't be a way to show it isn't, since the determinism can itself apply to the methods you use to test it. So you shouldn't get super excited about the universe ... | {
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Specific heat capacity and temperature, 0 K? I've found similar threads like this, but with no clear answer. I understand that the specific heat capacity of a substance increases with temperature, because the vibrational nodes and rotational movements of the atoms are quantized (I assume that the increase is because th... | Short answer: at absolute zero there is one - and only one - energy state available to each particle. Any attempt to change the state of even a single particle introduces energy into the system and you're no longer at absolute zero. Put another way: if there were two energy states allowed for a particle, the number o... | {
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How can the atmospheric pressure be different in distinct points at the same altitude? From an hydrostatic point of view, the pressure in a fluid should be the same at the same depth/altitude.
Obviously, in our atmosphere that does not happen. I am guessing that the main reason is the fact that the atmosphere cannot be... | The air moves in great swirls.
In places where the air is being warmed from below it moves up.
That causes air to be sucked in from below, and spread out at the top.
What it sees as the reason to be sucked in is a lower pressure pulling it.
When any fluid is pulled in to a center, its angular momentum is conserved (and... | {
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For a diatomic molecule, what is the specific heat per mole at constant pressure/volume? At high temperatures, the specific heat at constant volume $\text{C}_{v}$ has three degrees of freedom from rotation, two from translation, and two from vibration.
That means $\text{C}_{v}=\frac{7}{2}\text{R}$ by the Equipartition... | A diatomic molecule will have 7 degrees of freedom at high temperatures. However, the ratio of specific heats that you cited is for diatomic molecules around room temperatures, which have 5 degrees of freedom.
| {
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Relative Time from a far place I have read that whenever we see the sun we are seeing the sun as it was before 8 minutes ago. Meaning if the sun were to somehow go dark we would not know until 8 minutes after.
Now I became curious, say a civilization 4.7 billion light years away was looking at us through a very advanc... | It is like mailing a letter to a friend via cargo ship. The ship takes a week to cross the ocean and deliver the letter. If your friend is just now getting the letter, he is reading about how you were a week a ago.
It is the same with light. Light takes 4.7 billion years to travel a distance of 4.7 billion light years... | {
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What happens to Goldstone bosons in the Higgs potential after symmetry breaking? When the gauge symmetry of our Lagrangian breaks spontaneously through the Higgs mechanism, we usually find that $n$ Higgs degrees of freedom become massless through the vacuum expecation value (vev), where $n$ is the number of broken gene... | First, note that, strictly speaking, there is no such thing as spontaneous symmetry breaking in Higgs mechanism. I mean, that below and under the Higgs scale (i.e., at scale, at which non-zero Higgs VEV appears) the lagrangian can be rewritten in a gauge invariant way. How is it possible? The answer is that there are d... | {
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Gradient, divergence and curl with covariant derivatives I am trying to do exercise 3.2 of Sean Carroll's Spacetime and geometry. I have to calculate the formulas for the gradient, the divergence and the curl of a vector field using covariant derivatives.
The covariant derivative is the ordinary derivative for a scalar... | This problem is really nicely adressed is Weinbergs Gravitation and Cosmology, chapter 4 ig I remember correctly. There is basicalky one issue which leads to confusion:
In physics orthogonal coordinates are used, for example spherical or cylindrical. This leads to a diagonal line element.
This allows to normalize the n... | {
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Thermal state vs equibilibrium state Could someone explain what's the difference between a thermal state and an equilibrium state? Or is it even the same?
| Thermal states are used to explain the thermal equilibrium state, where the system attains the same temperature as that of the bath and will not exchange any more heat with the bath. This situation can be explained using the quantum mechanical equivalent of the canonical ensemble (that represents possible states of the... | {
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Measuring the moment of inertia of a flywheel using simple pendulum motion I've seen a method for experimentally determining the moment of inertia of a flywheel and I'm not sure whats the reasoning behind it. You attach a small weight $m_1$ to the flywheel's edge, it's important that $m_1 \ll M $, $M$ being the mass of... |
The force $m_1g\sin\theta$ provides the moment $Rm_1g\sin\theta$ ($R$ is the radius of the flywheel), so the equation of motion becomes:
$I\ddot\theta+Rm_1g\sin\theta=0$.
For small $\theta$, then $\sin\theta \approx \theta$, so we get:
$\large{\ddot\theta+\frac{Rm_1g}{I}\theta=0}$.
The solution of this classic DE is:
... | {
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Will this rope break due to the tidal forces or not? While I was thinking about how tidal forces can make objects float at the surface of a planet orbiting a massive object like a black hole, the fact that any material on the Earth isn't held together by gravity only, but also by chemical bonds which give it its tensil... | Between points A and C
The earth can be considered as a wall and thus to break off the 'wall' a relative acceleration with respect to the wall is a must , If the tensile strength is 10N and mass 10 kg then the rope must move at a relative acceleration of 1m/s^2 or
Acceleration(a) - Acceleration(c) = 1
| {
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How to derive end-correction value relationship for open-ended air columns? According to Young and Freedman's Physics textbook, in open-ended air columns like some woodwind instruments, the position of the displacement antinode extends a tiny amount beyond the end of the column.
UCONN's website states that the end cor... | This is actually a fairly involved calculation that was done by Levine and Schwinger in 1948. If you are interested , the reference is H.Levine and J. Schwinger, "On the radiation of sound from an unflanged circular pipe", Physical Review 73:383-406
I'll not attempt to replicate that calculation here but will try to de... | {
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Why are HCP materials brittle while FCC materials are ductile? Why are hexagonal close packed materials brittle, While face centered cubic is ductile. Is it related to crystal planes?
| Yes and how close the planes are packed and of course their geometries.
See some good answers below.
Source : http://www.researchgate.net/post/What_actually_makes_a_material_ductile_or_brittle
We may understand brittleness/ductility of solids from its bonding nature. In every solid, the constituent atom/ions are held b... | {
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Friction of a rolling cylinder I was wondering why friction vectors are drawn differently regarding a cylinder rolling on a surface and a cylinder rolling down an inclined surface. Since friction is responsible for the rotational motion shouldn't it be always pointing in the same direction (given that the cylinder is r... |
Since friction is responsible for the rotational motion
That's an assumption. Rotational motion could be caused by something else (like a drive shaft, or an electric motor).
For a rolling object, what friction does is bind together the rotational motion and the lateral motion.
The second case is easier to analyze. T... | {
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Can we predict throwing a dice? What happens if we throw a dice from same position, with same force, by creating a vacuum environment on earth? Will it be predictable now i.e. will the dice have same results all the time?
If answer to the question is no, I have another question, Why in quantum mechanics we say a partic... | In a pure Newtonian model, you can indeed make a prediction of the outcome if you know the inputs. But in this case there is a good deal going on. Rotation of the dice, how the dice leave the throwing surface, linear velocity, angle of impact to the landing surface, the coefficient of restoration from the landing sur... | {
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Relation of orbital speed and eccentricity Earth's eccentricity is 0. 0167 and speed at perihelion is 30.3 Km/s and at aphelion 29.3 with a difference of +/- 1. 0164 wrt average orbital speed
*
*Is this a coincidence or are the variations of speed directly related to eccentricity?
*can we calculate the time elapsed... | No it's not a coincidence.
The linear eccentricity, $c$, is the distance from the centre of the ellipse to either of the foci. This diagram shows an orbit with this marked - for clarity I've made the orbit very eccentric:
The eccentricity that you quote is defined as:
$$ e = \frac{c}{a} \tag{1} $$
where $a$ is the sem... | {
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Gauge theory for mathematicians? I'm looking for a textbook or set of lecture notes on gauge theory for mathematicians that assumes only minimal background in physics. I'd prefer a text that uses more sophisticated mathematical concepts like principal bundles and connections, and categorical language whenever convenien... | I have been writing something in this direction in section 1 of the book Differential cohomology in a Cohesive topos (pdf). Have a look, just focus on section 1 and ignore the remaining sections on first reading.
The survey-part is presently also appearing as a series on PhysicsForums. See at Higher prequantum geometry... | {
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Streamlines and line of flow of fluid particles
*
*Can the streamlines of a fluid particle show the position of the particle at a time(using the streamlines)?
*I know that streamlines cannot intersect because at a specific instant the particle reaching the intersection will have two different directions of motion (i... | Two particle cannot overcome each other because if it crosses then the direction of particles will be two different tangents at 1point which is not possible
| {
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Constructing Killing tensors from Killing vectors Background:
After reading about Carter constant and symmetries in GR, I became interested in Killing tensors.
I tried reading this paper by Alan Barnes, Brian Edgar and Raffaele Rani, discussing conformal Killing tensors. I have some trouble understanding the crux of th... | Killing tensors created by just product of two Killing vectors is only in the trivial case. In a non-trivial case this is not possible such as in finding the Carter constants. This is all I know as I'm also a beginner.
| {
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GR: Pseudo Riemannian or Riemannian? Is General Relativityy described by Pseudo-Riemannian manifold or Riemannian manifold? I cannot understand the vast difference between the two manifolds. In books, General Relativity is looked as a pseudo-Riemannian manifold, though I am not sure after reading some threads on the we... | In relativity (both special and general) one of the key quantities is the proper length given by:
$$ ds^2 = g_{\alpha\beta}dx^\alpha dx^\beta \tag{1} $$
where $g_{\alpha\beta}$ is the metric tensor. The physical significance of this is that if we have a small displacement in spacetime $(dx^0, dx^1, dx^2, dx^3)$ then $d... | {
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Why, in quantum field theory, is $\hat{a}(p)|0\rangle=0$? My Quantum Field Theory lecturer just said that $\hat{a}(p)|0\rangle=0$ because the vacuum state contains no particles. Now, according to Wikipedia,
"according to quantum mechanics, the vacuum state is not truly empty but instead contains fleeting electromagne... | In field theory, there are two vacua. The non-perturbative vacuum $|\Omega\rangle$ and the vacuum of the free theory $|0\rangle$. The wikipedia article makes reference to $|\Omega\rangle$ in terms of $|0\rangle$ and its excitations.
The true vacuum is annihilated by the (dressed) annihilation operators, and can be thou... | {
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Did I calculate this voltage correctly? I have just started to learn physics.
Please forgive me if I am completely wrong or something, I have just turned 14 haha.
I am trying to learn about how to work out voltage for my year 10 exams.
Here's what I have so far:
I got such a large value of 30000V so I think I migh... | There is an error in your multiplication.
$$
(9 \times 10^{9})(20 \times 10^{-9}) \neq 90\times20
$$
$$
(9 \times 10^{9})(20 \times 10^{-9}) = (9\times20)\times10^{0} = 9\times20 =180
$$
This means that your answer is off by a factor of 10. Without the error your answer should be 3kV.
| {
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Particle anti-particle annihilation and photon production This is just a conceptual question I guess. The annihilation of a particle with a finite mass and its anti-particle cannot lead to the emission of only one photon, and this is due to the conservation of energy and linear momentum.
However, how could this be show... | As dmckee says in a comment, the proof is ridiculously simple. Suppose we work in the centre of momentum frame so the total momentum is zero. The particle comes in with some momentum $p$ and the antiparticle comes in with the opposite momentum $-p$, and the two annihilate.
Suppose the annihilation produced a single pho... | {
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How to start an artificial gravity? I understand how artificial gravity in space stations works. It is by normal force the wall exerts on the foot.
But I wonder how to start it in the first place. I just learned about centrifugation in a centrifuge. To start, the side-wall of the tube produce a tangential acceleration... | An astronaut dressed in a magnetic suit inside a room or environment of copper or similar element in characteristics of conductivity or alloy (copper chamber) is a human magnet (magnet man) than will face a force that is opposed to his displacement inside the "chamber". If one of six sides or of all the sides than comp... | {
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(How) can we determine mid-point on Earth's orbit? The eccentricity of earth's orbit varies with time, but at present time its eccentricity is roughly mean e (0.0167).
The position of equinoxes is far more complicated than I thought and it is not at mid-point, can someone explain how you determine with a certain accura... | If you just want to find the dates for various events, the the following link provides a list of sources for astronomical calculators of various types.
http://www.midnightkite.com/index.aspx?URL=Software
The US Naval Office also is a good source of this sort of astronomical information.
http://aa.usno.navy.mil/index.ph... | {
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Quantum Dot Confinement - Band Gap I understand that the quantum confinement effect is observed when the wavelength of an electron is smaller than the wavelength of a material. I have also read that Quantum Dots have energy band gaps, because of the semiconductor material that makes them. For an exciton to be created a... | Confinement means that the possible electron states are discrete in energy, rather than continuous, NOT that the electron cannot move between these states. Creation of an exciton is an electron transition from a state in the valence band to one in the conduction band, induced by the optical field. This is similar to el... | {
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What is Pressure Energy? While deriving Bernoulli's Theorem, our teacher said that the sum of KE, PE and Pressure Energy per unit volume remains constant at any two points.
$$P + \rho g h + \frac{\rho v^2}{2} = \text{Constant}$$
In this, he stated that the first term is Pressure Energy per unit Volume. What exactly is ... | This site: Bernoulli Equation also uses the term "Pressure Energy". The pressure energy per unite volume is measured in N x m / m^3 = N / m^2. So this pressure energy per unite volume is in fact a pressure. Instead of the word "pressure" you can use the expression "pressure energy per volume". They are equivalent.
| {
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Is work a form of energy or means of transfer of energy? What actually is energy? Is it a property associated with matter or just a number? By doing work are we changing the energy of the body or converting one form of energy into other which is already present.
| I suppose that one way to look at energy is that it is a convenient tool for book-keeping tool since it is a conserved quantity. It's actually amazing that there is something that we call "energy" which can exist in so many different forms (e.g., kinetic energy, gravitational potential energy, electrostatic energy, ele... | {
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How does inhalation work? In school, we learn that during inhalation, the diaphragm expands, causing air to get sucked into our lungs. You can feel this suction by putting your hand over your mouth while inhaling.
Why is that? Does the expanded capacity of the lungs cause the air outside my body to rush into my body to... | You are correct that your chest muscles are in fact pulling the lungs "open," which creates a pressure differential and draws air into the lungs. When the muscles relax, the chest cavity collapses to its original state, expelling the air (not 100% of it!). You may have heard of a "collapsed lung" injury. What happens... | {
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Mercury-in-glass thermometer Question from my textbook: Jason says 'The mercury in the thermometer can be replaced by coloured water. The thermometer will function well after recalibrating using a similar method to calibrating a mercury-in-glass thermometer." Comment on his idea.
The answer is NO, but why?
Please use s... | A mercury-in-glass thermometer is traditionally calibrated by measuring two temperatures: the freezing point of water (0°C), and the boiling point of water (100°C).
There's an obvious reason why this won't work for a water-in-glass thermometer: the water in the thermometer will freeze when when you stick it in the ice ... | {
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Why only higher energy photons can affect atoms even though their speed is same? I understood that a baseball moving at the cosmic speed limit can do lot of harm, but since photons are not comparable to a baseball so irrespective of their high speed they do not rip anything in their way.
Now I want to know that why eve... | It's because photons are massless and so they can and must move at the speed of light with any none zero amount of momentum. This means that even though they move at the cosmic speed limit they can have very little amounts of momentum.
| {
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High Power/Range Electromagnet Pretty new to electromagnetism but my team and I are working on a project so have some questions before we spend more money doing R&D. We are looking to create a fairly small 10-20lb electromagnet. What we need this to be able to do is create down force from a range of say 3-4 feet.
So ... | The size of a magnetic field is on the order of the size of the magnet that creates it. You can focus the field so that it comes out one side of the magnet, but once the field emerges into air it will immediately spread out in order to minimize its energy. For example, it's impossible to make a magnetic jet that throws... | {
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Complex scalar field theory For the complex scalar field theory
$$L = -\partial_{\mu}\phi^{*}\partial_{\mu}\phi - m^{2}\phi^{*}\phi + J\phi^{*}+J^{*}\phi,$$
*
*Why is there no factor of 1/2 in the lagrangian like in the real scalar field?
*Can we say $ Y = 0$ (renormalization) because we know the two-point functio... | Without the factor $1/2$ for a complex field all observables constructed out of the Lagrangian in the standard way vie Noether theorem, like the energy $H:= \int T_{00} dx$ or the momentum $P_i = \int T_{i0} dx$, turn out automatically to be the ones of a system of identical particles of two types, {\em proper partic... | {
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Why is Gibbs Free Energy not equal to 0? Its definition itself makes it 0 Gibbs Free Energy is defined as
$$G = H - TS$$ and the change in $G,$
$$\Delta G = \Delta H - T\Delta S,$$ (isothermal and isobaric)
Now at constant pressure, $q = \Delta H$
and Entropy is defined as , $\Delta S = \dfrac{q}{T}$ which mean... | The only common situation I'm aware of in which a system can undergo both an isothermal and an isobaric process at the same time is when the system is undergoing a phase change at constant pressure. For instance, we could be considering a system comprised of liquid water and steam in phase equilibrium.
For a quasi-stat... | {
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Why will you float in free fall with 0 relative acceleration? Imagine you are in an elevator that is falling freely to the surface of the earth. You begin to float. However you and the elevator have equal acceleration. Why is it that you begin to float with respect to the elevator?
|
Imagine you are in an elevator that is falling freely to the surface of the earth. You begin to float. However you and the elevator have equal acceleration. Why is it that you begin to float with respect to the elevator?
You do not really float, you simply travel at the same speed as the elevator.
| {
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Why don't galaxies orbit each other? Planets orbit around stars, satellites orbit around planets, even stars orbit each other. So the question is: Why don't galaxies orbit each other in general, as it's rarely observed? Is it considered that 'dark energy' is responsible for this phenomenon?
| galaxy come in many different sizes: some of the small-er ones do rotate ["orbit"] around the edge of a large galaxy ... one can also visualize galaxy-clusters, in which the entire cluster rotates .....
| {
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Why aren't all objects transparent? I know that for an object to be transparent, visible light must go through it undisturbed. In other words, if the light energy is sufficiently high to excite one of the electrons in the material, then it will be absorbed, and thus, the object will not be transparent. On the other h... | You say:
For a non-transparent object like a brick, when the light is absorbed by an electron it will eventually be re-emitted.
but this isn't true. In a solid the excited state can decay by transferring energy to lattice vibrations instead of emitting a photon. This means the energy of the incident photon is convert... | {
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What is the difference when we measure torque/angular momentum about a point and about an axis? When do we measure torque about an axis and when do we measure torque about a point? What's the difference between measuring torque about an axis or a point.
I tried searching this on google but did not get satisfactory answ... | Everything in classical mechanics, momentum, angular momentum, torque, velocity etc. is measured about a point. Period. You can be sort of a Newtonian Nazi and complain that it is wrong to talk about torque about an axis and you'll be correct but here it means a completely different thing but in common language, we oft... | {
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Sliding and rolling friction I heard that friction depends only on the normal force but not on the contact area. Let's take a cube and a sphere which are of same weight (then normal force will also same ) but the force needed to move these two objects is different, why?
| Whenever one applies a sideways force trough the center of gravity of an object, that force has two components: 1) a direct force that tries to overcome friction and slide the object, and 2) a torque that uses friction to produce a rotation of the object by lifting its center of gravity over the leading edge.
A short,... | {
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How viscosity and velocity relate in a pipe? Assume we have a pipe that there is a fluid stream in it. By increasing the velocity of the fluid the resistance will increase either (because of the viscosity I think). My question is how are these to parameters depending to each other and what is the formula?
| With hydrodynamics we normally find that at low shear rates the flow is limited by the viscosity of the liquid while at high shear rates it's limited by inertial forces and the viscosity doesn't matter. This is the case for flow in a pipe. At low flow rates the pressure drop $\Delta P$ is related to the flow rate $Q$ b... | {
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How does one build a framework of sensors from nothing? I would like to understand how you can go from nothing to a framework of sensors you can use to try out things in physics/thermodynamics. That is, everything has to be bottoms up, nothing premade allowed (except maybe things like a lighter, screwdrivers, saws, ham... |
How does one build a framework of sensors from nothing?
For example, you can build a primitive ampere-meter using a compass like in the picture.
If you really want to get as primitive as possible see: The Science Notebook. The site teaches you even how to build a compass.
| {
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Bandwidth of laser pulses I am working on a practice test for my optics class. The professor does not provide the solutions for the test. I was just wondering if someone could tell me if I am doing this correctly because it seems kind of simple but I am unsure.
A laser emits short pulses of duration 14.0 ps separate... | About 30 GHz,
according to this.
Check "Bandwidth-limited Pulses".
Because of Fourier transform, the product of the temporal duration and spectral width is ≈ 0.44 for Gaussian-shaped pulses. See also this.
| {
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Does the direction of propagation of the natural light is perpendicular to the direction of electric and magnetic field making up natural light? I know that the direction of propagation of the wave(light) is perpendicular to the direction of electric and magnetic field in the situation of plane waves. And I want to kno... | Your text is rather muddled, but to answer the question: the Poynting vector is normally in the direction of propagation, which is to say the E and B fields are perpendicular to the direction of prop. This is always true in a vacuum, but it turns out that in various materials, the Poynting vector can be off-axis. ... | {
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Conflicts between Bernoulli's Equation and Momentum Conservation? The well known Bernoulli's equation states that
$P+\frac{\rho V^2}{2}=c$
However, a simple momentum conservation considering $P_1$ and $P_2$ acting on two sides, and velocity changes from $V_1$ to $V_2$, yields
$P_1+\rho_1 V_1^2=P_2+\rho_2 V_2^2$, which ... | The distinction between these two equations is that: $p+ \rho u^2=constant$, is valid only for 1D compressible flow while, $p+(1/2) \rho u^2=constant$, is valid for incompressible flow.
The difference arises because of the coupling of continuity and momentum equation in compressible flow. This coupling is absent for in... | {
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Power statement is valid for MW Or KiloWatts? If I can talk to someone and tell him that a new power plant inaugurated by Prime Minister will produce $60$ megawatts per hour, will it be true to use $\mathrm{MW}$ unit for Power?
| You're right that the unit "megawatt" is abbreviated MW.
However, as Aniket comments, watt itself means "energy per unit time", so saying that the power plant produces 60 MW per hour doesn't make sense.
In your comment, you question whether MW is a "basic unit". I'm not exactly sure what you mean by this, but the SI un... | {
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Force on a charged particle due to an uncharged infinite conducting plate How can I calculate the force on a charged particle due to an uncharged infinite conducting plate?
If there is a small object with positive charge placed above a metal plate, the object induces a negative charge on the surface of the plate faci... | This the kind of question that can be solved by the method of images.
Try placing a fictitious charge on the other side on the conducting plane. You should arrange it in such a way that the electrostatic potential is precisely zero on the surface of the conductor. If your case you put it at equal distance as the first ... | {
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Is there a relationship between kinetic energy of emitted electron and photoelectric current? I know that photoelectric current is dependent upon intensity of incident light. But it should also be dependent upon kinetic energy of emitted electron because mathematically $I=Q/T$. So if kinetic energy is more then time be... | You're wrong.
The current is limited by the number of electrons per second emitted from the metal surface. Once a steady state current is established the number of electrons per second received by the collecting electrode is the same as the number emitted per second from the metal surface and the speed the electrons tr... | {
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Is there a way to make a heat pipe that can transfer heat downward? Is there a way to make a heat pipe that can transfer heat downward? As in a 10 to 20 ft vertical pipe with 100°f at the top and 50°f at the bottom.
| You can circulate chilled water through the pipe which should distribute the temperature. Rate of circulation, along with the thermal properties of the pipe and embed media will control the precise temperature gradient - if thats even important for your application.
Lastly, the problem of steady state temperature dist... | {
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Compton effect in photo-electric? In photo-electric effect Einstein said that photons incidents on material and gives their energy which will gives kinetic energy to electrons. But i also want to know that why Compton's effect not works in this situation. In my view when photon incident on material it should eject a el... | Compton scattering occurs on free electrons, i.e. not in a bound state. The equivalent to a Compton scatter would be a scattering of a photon off the field of a solid, momentum and energy balance happening collectively with the total mass of the solid. In the best case it would be a whole atom that the photon would sca... | {
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How to keep a helium balloon between 1 to 5 meters above ground? (without it being tied) I understand that helium balloons rise because their density is less than air, so they can rise up to a point where the air surrounding it has the same weight as the balloon.
I was thinking to fill it with something like half air a... | If you are indoors, then...
*
*Seal all cracks or openings in the ceiling and in the walls above the mid-point. This includes the gaps around junction boxes and between drywall panels. Caulk and duct tape are your friends here.
*Extinguish any open flames and get rid of any possible spark sources. No smoking eithe... | {
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"answer_id": 4
} |
Dimensions of $\phi$ in scalar field theory On Srednicki page 90-91 (in printed edition) he derives that $$[\phi] = \frac{1}{2}(d-2) \tag{12.10}$$ from $${\cal L}=-\frac{1}{2}\partial^{\mu}\phi\partial_{\mu}\phi -\frac{1}{2}m^{2}\phi^{2} - \sum_{n=3}^{N}\frac{1}{n!}g_{n}\phi^{n}$$ and the fact that $[\partial^\mu] = +... | He uses that the action is dimensionless so that
\begin{align}
[ d^d x \left(\partial\varphi\right)^2] &= 0 \\
&=[d^d x]+2[\partial\varphi]\\
&= -d +2 + 2[\varphi]
\end{align}
using that $[dx]=-1$ and $[\partial\varphi] = [\partial] + [\varphi] = 1+[\varphi]$.
This gives $[\varphi] = (d-2)/2$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/221090",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
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